Tank and method for propulsion system

Abstract

A spacecraft propellant tank and methods of forming the same are disclosed. The propellant tank includes a tank body defining an internal volume configured to store a chemical propellant at an internal pressure substantially lower than a pressure required to deliver the propellant to a spacecraft thruster. The reduced internal pressure permits selection of materials, wall thicknesses, and structural parameters that differ from those of conventional high-pressure propellant tanks. In certain embodiments, the tank body is formed from aluminum or aluminum-based alloys and may have non-axisymmetric or conformal geometries configured to efficiently utilize available spacecraft volume. Methods of forming the tank include selecting design parameters based on a low operating pressure, forming one or more wall sections, and joining the wall sections to define a sealed propellant storage volume. The disclosed tank and methods enable reduced mass, improved manufacturability, and enhanced integration flexibility for spacecraft applications.

Description

Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669TANK AND METHOD FOR PROPULSION SYSTEM CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of priority of U. S. Provisional Patent Application No. 63 / 751,459, entitled “PROPULSION SYSTEM AND TANK, THERMAL MANAGEMENT AND CONTROLLER FOR PROPULSION SYSTEM,” filed January 30, 2025. The entire contents and disclosures of this patent application is incorporated herein by reference in their entirety.BACKGROUNDField of the Invention

[0002] The present disclosure relates to spacecraft propellant storage systems and, more particularly, to conformal multi-wall tanks fabricated using friction stir welding of machined plate and / or rolled or bent sheet metal structures, as well as automated burst-validation systems and methods for assessing the structural performance of such tanks.Background of the Invention

[0005] Spacecraft propellant tanks traditionally are designed to minimize dry mass, or mass of the material of the tank excluding its propellant, over other parameters. Additionally, spacecraft propellant, traditionally tanks are built with much longer-standing techniques, principally Gas Tungsten Arc Welding (GTAW, informally TIG). The combination of these factors has resulted in spacecraft propellant, tanks fabricated through spinforming, deep-drawing, or machining from custom forgings or large custom-ordered billets. These processes are high cost, require specialized equipment, rely on highly skilled manual operators and limit the ability to adopt unconventional or conformal geometries that maximize propellant mass fraction. Spacecraft propellant tanks that instead optimize at the vehicle- or system-level, or instead maximize propellant mass fraction, allow for a different approach to tank design that maximizes available volume for propellant. In conjunction to this different design approach, novel assembly techniques

[0003] Spacecraft propellant tanks traditionally rely on fabrication methods such as spinforming, deep-drawing, custom forgings, or machining from large billets. These processes increase cost, require specialized equipment and limit the ability to adopt unconventional or conformal geometries that maximize propellant mass fraction. Pump-fed propulsion systems,Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669operating at relatively low tank pressures, allow for alternative tank architectures based on machined plate assemblies joined by friction stir welding. However, the structural performance of such designs, including weld penetration behavior, weld strength, and local stress concentrations, do not match those found in tanks produced using the aforementioned conventional tank production methods. Accordingly, a need exists to construct tanks duplicating flight weld geometry and material properties and subjecting the same to burst testing in order to validate structural models and establish performance margins.SUMMARY

[0004] According to first broad aspect, the present disclosure provides a propellant tank comprising: a conformal multi-wall structure defining an inner volume and an outer volume, wherein the structure is formed from machined plate sections joined by friction stir welding to form friction stir welds along the structure, wherein the structure includes the friction stir welds formed along longitudinal weld seams, circumferential weld seams, and / or T-joint geometries, wherein the tank is configured to withstand an internal burst pressure of 1.5 times the maximum expected operational pressure (MEOP).

[0005] According to a second broad aspect, the present disclosure provides a method of making a conformal propellant tank comprising: forming a plurality of wall panels; arranging the wall panels to conform to an internal volume of a spacecraft bus; and joining the wall panels to form a sealed tank body configured for low-pressure operation, wherein joining the wall panels comprises friction stir welding adjacent wall panels along edge interfaces.

[0006] According to a third broad aspect, the present disclosure provides a method of making a spacecraft propellant tank comprising: forming a tank body from a metallic material; joining portions of the tank body using at least one welded seam selected based on low-pressure operation; and configuring the welded seam to withstand an internal pressure exceeding a nominal operating pressure of the tank.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0008] FIG. 1 illustrates a finite element analysis result for an exemplary tank assembly according to one embodiment of the present disclosure.

[0009] FIG. 2 illustrates a perspective partial cutaway side view of an exemplary tank assembly according to one embodiment of the present disclosure.

[0010] FIG. 3 illustrates exemplary weld joints according to one embodiment of the present disclosure.

[0011] FIG. 4 illustrates an exemplary tank assembly operatively coupled to a pressurization system according to one embodiment of the present disclosure.

[0012] FIG. 5 illustrates multiple components of custom weld tooling according to one embodiment of the present disclosure.

[0013] FIG. 6 graphically illustrates a pulsed pressurization system fluid network analysis output (bottom), solenoid valve pulsed pressurization system captured performance data (top), and fluid network model in GFSSP (inset) according to one embodiment of the present disclosure.

[0014] FIG. 7 illustrates the disclosed subscale tank integrated into an exemplary burst test process according to one embodiment of the present disclosure.

[0015] FIG. 8 illustrates an exemplary plumbing and instrumentation diagram for a burst test fluid system according to one embodiment of the present disclosure.

[0016] FIG. 9 illustrates test camera video frames before (left), during (middle) and immediately after tank rupture (right) according to one embodiment of the present disclosure.

[0017] FIG. 10 illustrates a tank plunge weld micrograph (left) and similar section of FEM (right) according to one embodiment of the present disclosure.

[0018] FIG. 11 illustrates a slice of an exemplary tank from an axisymmetric finite element model (FEM) according to one embodiment of the present disclosure.DETAILED DESCRIPTION OF THE INVENTIONDefinitions

[0019] Where the definition of terms departs from the commonly used meaning of the term, applicant intends to utilize the definitions provided below, unless specifically indicated.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0020] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of any subject matter claimed. In this application, the use of the singular includes the plural unless specifically stated otherwise. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. In this application, the use of “or” means “and / or” unless stated otherwise. Furthermore, use of the term “including” as well as other forms, such as “include”, “includes,” and “included,” is not limiting.

[0021] For purposes of the present disclosure, the term “comprising”, the term “having”, the term “including,” and variations of these words are intended to be open-ended and mean that there may be additional elements other than the listed elements.

[0022] For purposes of the present disclosure, directional terms such as “top,” “bottom,” “upper,” “lower,” “above,” “below,” “left,” “right,” “horizontal,” “vertical,” “up,” “down,” etc., are used merely for convenience in describing the various embodiments of the present disclosure. The embodiments of the present disclosure may be oriented in various ways. For example, the diagrams, apparatuses, etc., shown in the drawing figures may be flipped over, rotated by 90° in any direction, reversed, etc.

[0023] For purposes of the present disclosure, a value or property is “based” on a particular value, property, the satisfaction of a condition, or other factor, if that value is derived by performing a mathematical calculation or logical decision using that value, property or other factor.

[0024] For purposes of the present disclosure, it should be noted that to provide a more concise description, some of the quantitative expressions given herein are not qualified with the term “about.” It is understood that whether the term “about” is used explicitly or not, every quantity given herein is meant to refer to the actual given value, and it is also meant to refer to the approximation to such given value that would reasonably be inferred based on the ordinary skill in the art, including approximations due to the experimental and / or measurement conditions for such given value.

[0025] For purposes of the present disclosure, the term “ANSI / AIAA S-080A-2018 (Reaffirmed 2024)” refers to a key American Institute of Aeronautics and Astronautics (AIAA) standard that sets baseline requirements for the design, fabrication, testing, and operation of metallic pressure vessels and pressurized components used in space systems, like spacecraftAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669and launch vehicles. It covers everything from tanks and lines to cryostats and batteries, ensuring safety and reliability for storing liquids and gases in space.

[0026] For purposes of the present disclosure, the term “dynamic loading” refers to any force that changes over time, varying in magnitude, direction, or both, unlike a static load which is constant and stationary, and it's crucial for designing structures, machines, and components that. move.

[0027] For purposes of the present disclosure, the term “envelope expansion test” refers to an initial phase of flight testing, for example, focusing on expanding an aircraft's flight envelope, which refers to the range of flight conditions within which the aircraft may safely operate.

[0028] For purposes of the present disclosure, the term “feed pressure” refers to the pressure of the propellant is at the entrance the thruster at its inlet.

[0029] For purposes of the present disclosure, the term “finite element method” (FEM) refers to a method for numerically solving differential equations arising in engineering and mathematical modeling. Typical problem areas of interest may include the traditional fields of structural analysis, heat transfer, fluid flow, mass transport,, and electromagnetic potential. Computers may be used to perform the calculations required. With high-speed supercomputers, better solutions can be achieved and are often required to solve the largest and most complex problems. FEM is a general numerical method for solving partial differential equations in two-or three-space variables (i.e., some boundary value problems). To solve a problem, FEM subdivides a large system into smaller, simpler parts called finite elements. This is achieved by a particular space discretization in the space dimensions, which is implemented by the construction of a mesh of the object: the numerical domain for the solution that has a finite number of points. FEM formulation of a boundary value problem finally results in a system of algebraic equations. The method approximates the unknown function over the domain. The simple equations that model these finite elements are then assembled into a larger system of equations that models the entire problem. FEM then approximates a solution by minimizing an associated error function via the calculus of variations. Studying or analyzing a phenomenon with FEM is often referred to as finite element analysis (FEA).

[0030] For purposes of the present disclosure, the term “flight weld geometries” refers to specific thicknesses, tolerances and finishes of a weld, weld land and surrounding parent material components that are joined in the welding process, designed to the requirements forAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669use in spacecraft. Flight weld geometries may refer to a weld joint configuration, interface profile, and resulting cross-sectional weld shape that is specifically engineered, qualified, and certified for use in flight hardware. Such geometries are selected to satisfy structural load requirements, pressure containment performance, fatigue resistance, thermal cycling tolerance, and nondestructive inspection criteria associated with aerospace operating environments. Flight weld geometries may include, without limitation, full-penetration butt joints, lap joints, stepped interfaces, scarf joints, reinforced crown or root profiles, and multi-pass or tool-path-controlled solid-state weld formations. As used herein, flight weld geometries may refer to a weld joint profile and interface configuration formed in a flight-rated component using a solid-state or fusion welding process, including friction stir welding, wherein the geometry is configured to produce a defect-free weld nugget, controlled heat-affected zone, and mechanically robust joint capable of withstanding pressurization, cryogenic operation, launch loads, and orbital thermal cycling.

[0031] For purposes of the present disclosure, the term “friction stir welding” (FSW) refers to a solid-state joining process that uses mechanical stirring action to plastically deform materials, create minimal heat, and join materials without melting them. It's a continuous operation that can join a variety of materials, including, for example, aluminum, copper, titanium, and stainless steel. In some disclosed embodiments, FSW uses a non-consumable tool to join two facing workpieces without melting the workpiece material. Heat is generated by friction between the rotating tool and the workpiece material, which leads to a softened region near the FSW tool. While the tool is traversed along the joint line, it mechanically intermixes the two pieces of metal, and forges the hot and softened metal by the mechanical pressure, which is applied by the tool, much like joining clay, or dough. It may be used on wrought or extruded aluminum and particularly for structures which need very high weld strength to produce welds often stronger than the base material(s). FSW is capable of joining aluminum alloys, copper alloys, titanium alloys, mild steel, stainless steel and magnesium alloys. In addition, joining of dissimilar metals, such as aluminum to magnesium alloys, has been recently achieved by FSW. In some disclosed embodiments, friction stir welding is performed with a rotating cylindrical tool which has a profiled pin (also known as a probe) having a diameter smaller than the diameter of its shoulder. During welding the tool is fed into a butt joint between two clamped workpieces, until the probe pierces into the workpiece and its shoulder touches the surface of the workpieces. The probe may be slightly shorter than the weld depth required, with the tool shoulder riding atop the work surface. After a short dwellAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669time, the tool is moved forward along the joint line at the pre-set welding speed. Frictional heat is generated between the wear-resistant tool and the work pieces. This heat, along with that generated by the mechanical mixing process and the adiabatic heat within the material, cause the stirred materials to soften without melting. As the tool is moved forward, a special profile on the probe forces plasticized material from the leading face to the rear, where the high forces assist in a forged consolidation of the weld. This process of the tool traversing along the weld line in a plasticized tubular shaft of metal results in severe solid-state deformation involving dynamic recrystallization of the base material. The solid-state nature of the FSW process, combined with its unusual tool shape and asymmetric speed profile, results in a highly characteristic micro- structure. The solid-state nature of FSW leads to several advantages over fusion welding methods, as problems associated with cooling from the liquid phase are avoided. Issues such as porosity, solute redistribution, solidification cracking and liquation cracking do not arise during FSW. In general, in accordance with disclosed embodiments, FSW has been found to produce a low concentration of defects and is very tolerant to variations in parameters and materials.

[0032] For purposes of the present disclosure, the term “friction stir welds” (FSW) refers to welds produced by friction stir welding (FSW).

[0033] For purposes of the present disclosure, the term “internal burst pressure” refers to the maximum internal pressure a pipe, vessel, or component can withstand before it ruptures, cracks, or permanently fails, leading to a leak or catastrophic breach, determined by material strength, wall thickness, diameter, and temperature. It's a critical safety limit, significantly higher than normal working pressure, ensuring a safe buffer against sudden pressure spikes and allowing for safe operation in demanding industrial, chemical, or hydraulic systems.

[0034] For purposes of the present disclosure, the term “maximum pressure” refers to the highest pressure a system, device, or material can safely withstand or is designed to handle, varying by context from Maximum Allowable Working Pressure (MAWP) in vessels to peak systolic pressure in the body, representing the upper limit before failure, damage, or health risk. It's crucial for safety and design, often set by standards or manufacturers, and differs from normal operating pressures.

[0035] For purposes of the present disclosure, the term “MEOP” refers to the highest pressure a system or component is expected to experience during normal operation. In some specified contexts, MEOP may refer to the highest internal pressure that is expected to occurAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669within a pressure vessel, tank, or fluid system during any normal, nominal, off-nominal, or contingency operating condition over an operational lifecycle or mission profile, including transient conditions such as startup, shutdown, thermal excursions, pressurization events, and dynamic flow transients, but excluding pressures intentionally applied during proof testing, qualification testing, or burst testing.

[0036] For purposes of the present disclosure, the term “MMPDS” refers to Metallic Materials Properties Development and Standardization. It is a widely used, industry-government collaborative handbook containing statistically based design allowables for metallic materials and fasteners, primarily used in aerospace structural design.

[0037] For purposes of the present disclosure, the term “monoprop tank” refers to a storage vessel designed for spacecraft and satellites to hold a single liquid propellant, most commonly hydrazine, which decomposes to create thrust. These tanks may be generally pressurized with inert gas (e.g., helium or nitrogen) to force the liquid into a catalyst bed, providing reliable, compact propulsion for reaction control systems (RCS).

[0038] For purposes of the present disclosure, the term “non-axisymmetric” refers to an object lacking rotational symmetry; instead of looking the same when spun around an axis (like a perfect cylinder or sphere), its shape or properties change as it rotates, often exhibiting unevenness, lumps, or distinct patterns in different direction. Exemplary embodiments may include a lopsided ball versus a perfect one, or a spinning top with a flat side wherein the flat side makes it non-axisymmetric.

[0039] For purposes of the present disclosure, the term “ON-pulsing and OFF-pulsing” refers to control strategies used to manage a spacecraft's velocity and attitude (orientation) during maneuvers.

[0040] For purposes of the present disclosure, the term “ON-pulsing” refers to firing thrusters that are normally inactive in short, discrete bursts. It may be used for attitude control or small orbital corrections. Thrusters may be commanded to turn " ON" for a fixed duration to generate a specific impulse bit and then return to an " OFF" state. Applications may be employed in satellite formation flying or for precise pointing in deep-space missions.

[0041] For purposes of the present disclosure, the term “OFF-pulsing” refers to turning off thrusters that are already firing continuously (to change a spacecraft’s velocity). It may be used to generate control torques (rotation) while the main propulsion system is performing a primaryAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669burn. By momentarily cutting power or fuel to a specific subset of thrusters, the system creates an imbalance in thrust, allowing the spacecraft to rotate without needing a separate set of dedicated control thrusters. This method allows a single propulsion system to handle both velocity changes and attitude maintenance simultaneously.

[0042] For purposes of the present disclosure, the term “pressurant” refers to a gas (or sometimes a vapor) used to increase pressure within a system, typically to force a liquid out of a tank, maintain a liquid's state, or control fluid flow in pipelines, common in rocketry (e.g., like helium or self-generated gas) and industrial systems. It may be regarded as a working fluid that applies force, acting like a piston to move other fluids, ensuring they stay liquid or move where needed.

[0043] For purposes of the present disclosure, the term “pressurant tank” refers to a high- pressure storage vessel used in liquid propulsion systems, e.g., in spacecraft and satellites, to hold inert gas (the “pressurant”) for the purpose of expelling propellants.

[0044] For purposes of the present disclosure, the term “propellant” (PSIG) refers to any substance that produces thrust or force when expelled, enabling vehicles like rockets to move, or driving products out of aerosol devices, functioning as either a fuel / oxidizer mix (rockets), a compressed gas, or a liquefied gas to create pressure, with key types including solid (e.g., for boosters), liquid (e.g., liquid hydrogen / oxygen), and aerosol (e.g., nitrogen, propane) propellants. It works by expelling mass backward (relying on Newton's Third Law for propulsion) to propel an object forward, whether through combustion (e.g., rockets) or pressure release (aerosols).

[0045] For purposes of the present disclosure, the term “psig” (PSIG) refers to pounds per square inch gauge, and may be regarded as a unit used to measure pressure relative to atmospheric pressure. Thus, PSIG measures gauge pressure, which uses atmospheric pressure as its reference point. This is the pressure above or below the local atmospheric pressure.

[0046] For purposes of the present disclosure, the term “spacecraft” refers to a vehicle or device engineered to operate beyond Earth's atmosphere, either with or without a crew; in a controlled flight pattern. Disclosed embodiments may use rockets to carry astronauts, cargo, or instruments to their destination, or be the destination itself.

[0047] For purposes of the present disclosure, the term “spacecraft bus” refers to the core structure and support, system of a spacecraft, providing essential “life support” functions likeAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669power, thermal control, navigation, communication, and data processing, allowing the missionspecific payload (e.g., cameras or telescopes) to operate in space. Embodiments may include the infrastructure that carries the entirety of the spacecraft, acting as the main body and framework for all the components, from propulsion to structural support, enabling the scientific mission.

[0048] For purposes of the present disclosure, the term “spinforming'’ (metal spinning) refers to a manufacturing process that shapes flat metal discs or tubes into axially symmetric parts (e.g., cones, hemispheres, cylinders) by rotating them at high speeds on a lathe and pressing them against a rotating mold (mandrel) with tools, creating hollow, round components without removing material, often for cost-effective, low-to-medium volume production.

[0049] For purposes of the present disclosure, the term “'specific impulse” (Isp) refers to a key measure of rocket engine efficiency, showing how much thrust is generated per unit of propellant consumed over time, often expressed in seconds (s) by normalizing exhaust velocity by Earth's gravity, meaning a higher Isp indicates more efficient fuel use for longer thrust, similar to better gas mileage for a car. It’s calculated as thrust divided by propellant weight flow rate, and higher Isp allows a rocket to achieve greater velocity changes with less fuel. Thus, in terms of efficiency, Isp may quantify how effectively a rocket converts propellant into thrust.

[0050] For purposes of the present disclosure, the term “thruster” refers to a small rocket engine on a spacecraft, used to make alterations in its flight path or altitude.Description

[0051] While the invention is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and the scope of the invention.

[0052] Spacecraft systems, particularly chemical propulsion systems, have traditionally employed pressure-fed architectures, in which one or more propellants are stored in high- pressure tanks and delivered to thrusters by means of a dedicated pressurant gas, such as helium. In such systems, the propellant tank is required to withstand substantial internalAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669pressures throughout the entire mission lifecycle, including during ground handling, launch site preparation, ascent, and on-orbit operation. As a consequence, tank structural design has historically been driven primarily by pressure containment requirements rather than by considerations of mass efficiency or spacecraft integration.

[0053] To meet these pressure demands, conventional spacecraft propellant tanks are commonly fabricated from high-strength materials, including titanium alloys or composite overwrapped pressure vessels (COPVs). These tanks are typically configured with non- conformal geometries, such as spherical or cylindrical shapes, which are well suited for resisting internal pressure but are poorly optimized for efficient use of available spacecraft volume. In addition, the manufacture of such tanks is often costly and time-intensive, requiring specialized materials, complex fabrication processes, and long lead times associated with qualified aerospace suppliers. The resulting tanks therefore tend to be mass-inefficient, consuming a disproportionate share of the spacecraft’s mass budget relative to the amount of usable propellant they contain.

[0054] These limitations are increasingly problematic for modern spacecraft platforms, where payload capacity, maneuverability, and overall mission economics are highly sensitive to subsystem mass and volume. Moreover, high-pressure propellant tanks introduce added complexity and operational risk during launch site activities, where large quantities of energetic propellants are handled in close proximity to personnel. Such conditions necessitate extensive qualification testing, conservative safety margins, and rigorous certification procedures, further increasing cost and development timelines.

[0055] Additionally, conventional high-pressure propellant tanks inherently tie tank structural design to thruster feed pressure requirements, forcing designers to oversize and over¬ engineer tanks for pressure containment rather than functional integration. This coupling results in tanks that are heavier, more expensive, and less adaptable to nontraditional spacecraft geometries.

[0056] Furthermore, traditional tank designs are poorly suited for advanced propulsion architectures that seek to optimize system efficiency, thermal behavior, and packaging flexibility. In particular, known tank solutions do not adequately address the opportunity to leverage lower-pressure storage to enable alternative materials, welding techniques, and structural configurations while still meeting propulsion system demands.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0057] Efforts to improve conventional pressure-fed tank designs have generally focused on incremental enhancements, such as the use of alternative materials or localized structural reinforcement. While these approaches may provide modest improvements, they do not. fundamentally address the underlying tradeoffs between high pressure tolerance, low mass, manufacturability, and efficient integration with increasingly compact spacecraft buses. As a result, existing tank technologies remain constrained by design paradigms that prioritize pressure containment at the expense of system-level efficiency.

[0058] Accordingly, there remains a need for a spacecraft propellant tank that departs from conventional high-pressure design approaches and instead operates at substantially lower internal pressures, while still meeting safety and qualification requirements. Such a tank preferably enables conformal geometries that maximize propellant volume within the available spacecraft envelope, reduce overall mass, cost, and manufacturing lead time, and remain compatible with modern propulsion architectures without compromising maneuvering capability or system performance.

[0059] In view of the foregoing, there is a clear need for an improved spacecraft propellant tank that departs from conventional high-pressure paradigms and enables lightweight, low- pressure, conformal storage of propellants, while maintaining structural integrity, safety, and compatibility with spacecraft propulsion systems. The present disclosure addresses these and other deficiencies of the prior art.

[0060] The present disclosure relates to propellant tank systems employing conformal, multi-wall geometries fabricated using friction stir welding (FSVV) of machined aluminum plate components. The disclosed tanks provide high propellant mass fraction, low operating pressure capability, and reduced production cost by eliminating the need for traditional tank manufacturing methods such as spinforming or large-billet machining. A progressive build-test- analyze development approach validates structural integrity, including subscale burst-test units that replicate flight weld geometries. Burst testing demonstrates that the tank design exceeds predicted structural margins, enabling mass reduction and scalable production. Additionally, reaction force data collected from the disclosed subscale weld process is used to design appropriate tooling for generation of the disclosed full-scale tank assembly.

[0061] A conformal multi-wall propellant tank is provided having an inner and an outer wall formed from machined plate components joined using friction stir welding. The welded structure includes longitudinal weld seams, circumferential weld seams, and / or T-jointAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669geometries configured to withstand burst pressures significantly greater than nominal operating pressures. Subscale tanks replicating these weld geometries are constructed and validated using an automated burst-test system employing nitrogen pressurization, liquid fill masses, flow-restriction orifices, fail-safe solenoid valves, a data-acquisition subsystem, and a programmable controller. The controller executes predetermined pressure-gate sequences and captures synchronized pressure data and high-speed imagery to identify rupture initiation sites and validate structural models. The test data enables refinement of weld geometry, m ss-reduction strategies, and full-scale tank development.

[0062] Friction stir welding (FSW) enables joining of machined plate structures with minimal distortion, consistent weld penetration, and high joint strength. However, traditional tank geometries and T-joint weld configurations introduce stress distributions that cannot be reliably predicted using classical shell analysis. Because weld microstructure, penetration depth, and geometric imperfections can significantly affect burst behavior, subscale testing is required to establish weld knockdown factors and validate finite-element structural models.

[0063] Accordingly, the present disclosure also relates to propellant storage systems for spacecraft and, more specifically, to conformal multi-wall propellant tanks fabricated using friction stir welding of machined aluminum plate structures. The disclosure also relates to automated structural validation systems configured to perform controlled hydro-pneumatic burst testing.

[0064] Burst-test programs typically require specialized hydro-pneumatic facilities. To support rapid, iterative tank development, an automated pressure-test system is disclosed using solenoid valves, a precision pressure regulator, data-acquisition electronics, and programmable control software. Subscale tanks are subjected to controlled internal pressurization while pressure-time data, valve states, and high-speed imagery are recorded. Rupture locations are correlated with stress predictions, weld micrographs, and finite-element models.

[0065] The present invention is directed to a spacecraft propellant tank configured to store one or more chemical propellants at substantially lower internal pressures than those required by conventional pressure-fed propulsion systems. By decoupling propellant storage pressure from thruster feed pressure requirements, the disclosed tank enables a combination of structural, geometric, and manufacturing advantages that are not achievable with traditional high-pressure tank designs.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0066] In one aspect, the propellant tank is configured for operation at internal pressures that are significantly below those associated with pressure-fed architectures that rely on pressurant gases such as helium. Because the tank is not required to maintain high internal pressure to deliver propellant to downstream propulsion components, the structural requirements imposed on the tank walls, seams, and joints are correspondingly reduced. This allows the tank to be fabricated from lighter, more readily manufacturable materials, including aluminum or aluminum alloys, rather than higher-cost and higher- strength materials such as titanium or composite overwrapped pressure vessels.

[0067] The reduced pressure environment further enables the tank to be formed in non- traditional, conformal geometries that are tailored to the available volume within a spacecraft bus. Unlike spherical or cylindrical pressure vessels that are optimized primarily for uniform stress distribution under high pressure, the disclosed tank may be shaped to conform to surrounding spacecraft structures, panels, or internal cavities. In this manner, the tank maximizes usable propellant volume within a given spacecraft envelope, thereby improving volumetric efficiency and overall system packaging without increasing spacecraft size.

[0068] In certain embodiments, the tank includes one or more welded seams, joints, or interfaces configured to provide structural integrity sufficient for low-pressure operation while meeting applicable safety and qualification requirements for ground handling, launch site operations, and flight. Because the tank is designed for lower internal pressures, welding techniques and joint configurations may be selected to balance strength, manufacturability, and cost, rather than being driven exclusively by extreme pressure containment considerations. This facilitates faster manufacturing schedules and reduces reliance on specialized fabrication processes traditionally associated with high-pressure aerospace tanks.

[0069] The disclosed tank design also provides advantages in terms of mass efficiency and cost reduction. Lower pressure requirements permit thinner wall sections and reduced reinforcement, resulting in a tank that occupies a smaller fraction of the spacecraft mass budget relative to the amount of propellant stored. Additionally, the ability to use widely available materials and simplified manufacturing processes reduces both direct production costs and long lead times, which are common limitations of conventional propellant tank procurement.

[0070] In some implementations, the disclosed tank is configured to interface with a propulsion system in which propellant delivery pressure is generated downstream of the tank, such as by one or more pumps or other pressure-generating devices. In such configurations, theAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669tank functions primarily as a storage vessel rather than as a pressure vessel, allowing internal pressure to remain relatively low during operation. However, the invention is not limited to any particular propulsion architecture, and the tank may be employed in a variety of spacecraft systems that benefit from low-pressure, lightweight, and conformal propellant storage.

[0071] The tank may further be configured to participate in thermal management of the spacecraft, including acting as a thermal mass or heat sink. Because propellant tanks typically contain a substantial mass of fluid, the disclosed tank may be arranged to receive, distribute, or moderate thermal energy within the spacecraft, thereby assisting in maintaining propellant and surrounding components within desired temperature ranges. Such thermal interaction may be achieved through conductive coupling, fluid circulation, or other heat transfer mechanisms, without requiring the tank to serve as a high-pressure containment vessel.

[0072] Advantageously, the disclosed tank design addresses safety considerations associated with the storage of energetic propellants. Lower internal pressures reduce the stored energy within the tank, thereby mitigating risks during ground handling and launch site operations. The tank may be qualified through structural testing, including over-pressurization or burst testing, to demonstrate appropriate safety margins relative to its intended operating pressure, while still benefiting from reduced structural demands compared to conventional high-pressure tanks.

[0073] Accordingly, the invention provides a spacecraft propellant tank that departs from traditional high-pressure design paradigms and instead enables low-pressure operation, conformal integration, reduced mass, reduced cost, and improved manufacturability, while remaining compatible with modern spacecraft propulsion and operational requirements.

[0074] The disclosed propellant tank system includes a conformal multi-wall structure defining inner and outer volumes configured for use in pump-fed spacecraft propulsion systems.

[0075] In certain embodiments, a propellant tank according to the present disclosure is configured such that the tank shape is not principally governed by a surface of uniform curvature, nor by geometries in which curvature smoothly transitions between regions, including spherical, hemispherical domed cylindrical, or ellipsoidal domed cylindrical configurations. Instead, the propellant tank may incorporate non-axisymmetric geometries, planar wall segments, angular transitions, and locally sharp edges, thereby enabling the external tank profile to more closely conform to the internal envelope of a prismatic spacecraft bus.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0076] In this configuration, the presence of sharp edges and non-uniform curvature regions reduces the volumetric mismatch between the exterior surface of the propellant tank and the interior structural boundaries of the spacecraft bus, thereby significantly decreasing unused packaging volume. Accordingly, the disclosed propellant tank geometry improves volumetric efficiency, increases usable propellant storage capacity for a given spacecraft envelope, and enables more compact system integration relative to conventional pressure vessel geometries that rely on smooth, axisymmetric curvature profiles.

[0077] In certain implementations, the disclosed tank geometry may further facilitate integration of mounting interfaces, thermal management components, and structural attachment features without requiring secondary adapter structures, thereby reducing overall mass and system complexity while maintaining structural integrity and pressure containment performance.

[0078] FIG. 1 illustrates a representative finite element analysis (FEA) output for an exemplary tank assembly according to one embodiment of the present disclosure. As depicted, the tank assembly includes a tank section 100 comprising an outer wall 101 and an inner wall 102 configured in a multi -wall structural arrangement. A cap section 103 is mechanically coupled to the tank section 100, with the interface between the cap section 103 and the tank section 100 being formed by a friction stir weld region 104.

[0079] The illustrated FEA results represent a computational stress and deformation distribution across the tank assembly under an applied internal pressure loading condition representative of nominal or maximum expected operating pressure. As shown, localized stress concentrations and strain gradients are predicted to occur in the vicinity of the friction stir weld region 104 and along transitional geometry regions between the cap section 103 and the multi - wall tank structure. The analysis further demonstrates load transfer between the inner wall 102 and the outer wall 101, thereby illustrating the structural contribution of the multi-wall configuration to overall pressure containment, stiffness enhancement, and reduction of peak stress amplitudes within the tank assembly.

[0080] In certain embodiments, the finite element model may' incorporate material property definitions, weld zone mechanical properties, geometric tolerances, and boundary conditions corresponding to launch, ground handling, and on-orbit operating environments, thereby enabling predictive assessment of structural margin, factor of safety, and deformation behavior for the disclosed tank architecture. The disclosed exemplary tank assembly may be analyzedAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669against multiple load cases, including maximum expected operating pressure and launch loading.

[0081] FIG. 2 illustrates a perspective partial cutaway side view of an exemplary tank assembly 200 according to one embodiment of the present disclosure, in which internal structural features of the tank assembly are exposed for clarity. As shown, the tank assembly 200 includes an outer wall 201 and an inner wall 202 arranged in a multi-wall configuration that defines one or more internal volumes configured to contain propellant, pressurant, or other working fluids.

[0082] A cap section 203 is coupled to the tank assembly 200 and is configured to close an open end of the outer wall 201 and inner wall 202, thereby forming a sealed pressure vessel structure. In the illustrated embodiment, the cap section 203 is joined to the tank assembly 200 by a friction stir weld region 204 that extends circumferentially about the tank perimeter to provide a continuous, hermetic structural joint.

[0083] In certain embodiments, the outer wall 201 and inner wall 202 may be formed from machined plate, rolled sheet material, or extruded sections and may be fabricated from aluminum alloys, stainless steels, titanium alloys, or other high-strength metallic materials suitable for spaceflight pressure vessels. The multi-wall configuration shown in FIG. 2 enables improved load distribution, enhanced structural stiffness, thermal isolation, and increased resistance to crack propagation as compared to single-wall tank architectures.

[0084] Although illustrated as a cylindrical geometry with a domed cap section 203, the tank assembly 200 may alternatively be implemented using conformal, prismatic, elliptical, or non-axisymmetric geometries depending on packaging constraints and mission requirements, without departing from the scope of the present disclosure.

[0085] Friction stir weld region 204 may include friction stir welded seams having longitudinal weld seams and T-joint welds. Longitudinal weld seams extend along the interface between machined plate segments, and T-joints connect orthogonal wall panels. Cap section may be attached using additional weld seams, and bolt rings provide attachment for interfaces and fill ports. The tank assembly 200 may be formed by machining aluminum plate components, positioning them in specialized tooling assemblies, and performing controlled FSW operations to produce consistent weld penetration depths and joint profiles. Thus, the tank structure is manufactured from machined aluminum plate sections joined by friction stirAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669welding (FSW), forming longitudinal weld seams and T-joints with geometry corresponding to anticipated flight hardware.

[0086] Accordingly, tank assembly 200 may be disclosed as a propellant tank incorporating a conformal multi-wall structure consisting of machined aluminum plate sections joined by friction stir welding. The geometry allows propellant volumes to be integrated efficiently within the spacecraft structure, and the friction stir welded joints provide high structural continuity while reducing manufacturing cost and complexity. The longitudinal weld seams and T-joints are configured to reproduce the expected weld geometry of full-scale tanks, thereby allowing subscale validation of structural performance.

[0087] FSW provides high weld quality and repeatable penetration depth due to the solid-state joining process. Weld geometry includes a nugget zone at the center of the tool path and a heat-affected zone surrounding the weld. Weld seam profiles are confirmed using micrographs as shown, for example, in FIG. 9,

[0088] Machined tank panels may be secured in fixturing tooling assemblies including clamping fixtures, cylindrical rotational fixtures, and guide rails. These tooling structures ensure alignment of the weld path and support plate geometry during joining operations, as described below (e.g., see FIG. 5).

[0089] FIG. 3 illustrates exemplary weld joints according to one embodiment of the present disclosure. As shown, FIG. 3 shows T-weld micrograph 100, circumferential weld micrograph 301, tank cap parent material 302, FSW pin-dominated retreating weld region 303, FSW shoulder-dominated retreating weld region 304, FSW shoulder-dominated advancing weld region 305, FSW pin-dominated advancing weld-region, 306, inner wall parent materials 307, FSW pin retraction region 308 and tank outer wall parent materials 309.

[0090]

[0091]

[0092] FIG. 3 illustrates representative weld joint microstructures for an exemplary tank assembly according to one embodiment of the present disclosure. As shown, FIG. 3 includes a T-weld micrograph 300 and a circumferential weld micrograph 301, each depicting metallurgical interfaces formed between adjoining tank components.

[0093] In the illustrated embodiment, tank cap parent material 302 is joined to adjacent, structural members by a friction stir welding (FSW) process that produces distinct weld sub-Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669regions characterized by localized thermomechanical processing. These regions include an FSW pin-dominated retreating weld region 303, an FSW shoulder-dominated retreating weld region 304, an FSW shoulder-dominated advancing weld region 305, and an FSW pin- dominated advancing weld region 306. The spatial distribution of these regions reflects the rotational direction of the FSW tool and the relative material flow dynamics generated during weld formation.

[0094] Inner wall parent material 307 and outer wall parent material 309 are shown adj acent to the weld nugget region and heat-affected zones, thereby illustrating the metallurgical transition from base material microstructure to recrystallized stir zone microstructure. An FSW pin retraction region 308 is also illustrated, corresponding to the termination location of the FSW tool path and representing a region that may be locally processed or post-treated to achieve a continuous, hermetic pressure boundary'.

[0095] In certain embodiments, the illustrated weld joint configurations provide enhanced mechanical strength, reduced porosity, improved fatigue resistance, and superior leak tightness relative to conventional fusion welding techniques. The disclosed friction stir weld geometries further enable formation of continuous circumferential pressure-retaining joints and complex T-joint interfaces required for multi-wall tank architectures, while maintaining favorable grain refinement and residual stress profiles suitable for spacecraft pressure vessel applications.

[0096] Although specific weld morphologies and microstructural regions are illustrated, it will be appreciated that tool geometry, rotational speed, traverse speed, plunge depth, and axial force may be varied to tailor weld penetration depth, nugget geometry, and mechanical properties without departing from the scope of the present disclosure.

[0097] In certain embodiments, subscale tank assemblies may be fabricated using substantially the same friction stir welding parameters, material thicknesses, joint geometries, and structural features as corresponding flight-qualified tank designs. Prior to pressurization testing, representative weld coupons and sectioned joint samples may be subjected to metallurgical inspection using etching, optical microscopy, and microstructural analysis, as illustrated in FIG. 3, to verify weld integrity, grain structure, and defect absence. The fabricated tank assemblies may further undergo preliminary leak-check procedures, as illustrated in FIG.4, to confirm pressure boundary' continuity prior to structural loading.

[0098] FIG. 4 illustrates an exemplary tank assembly test configuration according to one embodiment of the present disclosure, in which a tank assembly 400 is operatively coupled toAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669a pressurization system 410 configured to apply controlled internal pressure to the tank assembly for structural qualification, leak verification, and performance characterization. Pressurization system 410 may include a pressurant supply source, one or more pressure regulation devices, and a fluid delivery line fluidly coupled to an inlet port of the disclosed tank assembly.

[0099] As shown, the pressurization system 400 is fluidly coupled to a cap section 402, which is affixed to an outer wall 401 of the tank assembly. In the illustrated embodiment, cap section 402 is joined to outer wall 401 by a friction stir weld 403, thereby forming a hermetic pressure-retaining interface configured to withstand applied internal test pressures.

[0100] In certain embodiments, the pressurization system 400 may be configured to supply a controlled pressurant medium to the tank assembly, including compressed gas or liquid test media, and may include pressure regulation components, flow control devices, and instrumentation for monitoring internal pressure, temperature, and pressurization rate. The pressurization system 400 may further be configured to execute proof pressure testing, maximum expected operating pressure (MEOP) validation, burst testing, and cyclic pressurization / fatigue testing and leak integrity verification for the tank assembly to characterize structural performance and safety margins of the disclosed tank architecture.

[0101] The use of subscale tanks fabricated with flight-representative parameters enables correlation of experimental test data with analytical and finite element models, thereby providing predictive validation of structural performance, weld joint behavior, and load distribution characteristics of full-scale tank assemblies.

[0102] Following verification, the tanks are integrated into an automated burst-test apparatus. The automated burst-test system may include a nitrogen gas source, a regulator, flow-restriction orifices, solenoid valves arranged to pressurize or vent individual tank volumes, and a data-acqui sition system configured to log pressure, valve state, and timing information at high rates. A programmable controller executes sequential pressure gates that incrementally increase pressure within the inner or outer volume, depending on test configuration. Deionized water is added to the tank volumes in measured masses to achieve controlled ullage volumes and to maintain the stored energy within prescribed limits. High-speed video cameras positioned around the tank record structural behavior during pressurization.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0103] During testing, the controller increases pressure until rupture occurs. In one representative test, the inner tank volume withstood more than nine hundred pounds per square inch before rupture initiated at the longitudinal weld seam, followed shortly by the outer wall. Post -test inspection and refined finite-element analysis confirm that the longitudinal weld region — not the T-joint — constitutes the critical location for burst initiation. These results support mass-reduction strategies and confirm that the tank architecture provides ample structural margin relative to operating pressures.

[0104] To validate the structural performance of the tank architecture, subscale tanks are constructed that generally maintain the same weld thicknesses, penetration depths, and material properties as flight designs. These subscale units are subjected to controlled hydro-pneumatic burst testing using a custom-built automated test apparatus featuring computer-controlled solenoid valves, pressure regulators, and synchronized high-rate data acquisition. Thus, in some disclosed embodiments, subscale versions of the tank are constructed and tested using a burst-validation system that includes a nitrogen supply, pressure regulator, flow restriction, pressurization and vent solenoids, a burst test stand, and a data acquisition module. A test controller executes sequential pressure-gate steps, providing command traces while pressure transducers measure real-time structural response. High-speed video cameras document weld deformation and rupture onset.

[0105] During testing, the inner and outer tank volumes are filled with deionized water to precise mass targets, ensuring appropriate ullage and safe stored-energy conditions. Sequential pressurization gates are executed, with high-speed video positioned around the tank and pressure instrumentation capturing structural behavior up to rupture.

[0106] A finite-element structural model including inner wall elements, outer wall elements, weld mesh regions, and symmetry boundaries correlates with weld micrographs showing nugget zones and heat-affected zones. Test results confirm that rupture typically initiates at longitudinal weld seams, enabling refinement of tank geometries and weld parameters.

[0107] A first disclosed test demonstrates that both inner and outer chambers withstand significant internal pressure without bursting. A second disclosed test, pressurizing only the inner chamber, produces a controlled rupture at approximately 942 psig, initiating at the longitudinal weld seam. Post-test inspection and refined finite element analysis correlate failure to predicted stress regions, confirming the conservative strength of T-welds previously believed to be critical.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0108] These results demonstrate that the tank structure provides significantly greater pressure capacity than required for flight, supporting design optimizations including weight reduction and weld-geometry refinement. The subscale test program establishes validated analytical models for subsequent full-scale tank manufacturing.

[0109] In certain embodiments, the disclosed conformal multi-wall propellant tank architecture is configured to operate across a wide range of internal pressures, including low-pressure storage conditions and elevated operational pressures associated with spacecraft propulsion systems. By way of non-limiting example, experimental testing of a monopropellant-compatible subscale tank fabricated using friction stir welded plate and sheet metal structures demonstrated sustained operation at a maximum expected operating pressure (MEOP) of approximately 900 pounds per square inch gauge (psig), thereby establishing an upper-bound performance capability for representative embodiments of the disclosed tank architecture.

[0110] At the lower end of the operational spectrum, the disclosed tank configurations are capable of accommodating internal pressures at or near atmospheric pressure, including approximately 1 atmosphere absolute (0 psig gauge), thereby enabling compatibility with ground handling, low-pressure storage, and system integration scenarios. In some embodiments, the disclosed tank has an average wall thickness within a range of about 0.5 millimeters to about 8 millimeters.

[0111] In one representative implementation, the subscale tank was initially designed for a nominal internal design pressure of approximately 400 psig, with a corresponding target burst pressure of approximately 1.5 times the design pressure in accordance with conventional aerospace structural margin practices. Subsequent structural testing resulted in achieved internal pressures exceeding 900 psig, including burst pressures approaching approximately 940 psig. While such elevated pressure capability was not a primary design objective for the representative subscale configuration, the results demonstrate substantial structural margin and scalability of the disclosed conformal multi-wall tank architecture.

[0112] Accordingly, in certain embodiments, the tank may be configured to withstand internal burst pressures exceeding at least about 1.5 times the maximum expected operating pressure, and in some implementations exceeding more than double the MEOP, depending on material selection, wall geometry’, weld configuration, and fabrication parameters. These demonstrated performance characteristics further indicate that the disclosed fabrication andAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669structural design approach is adaptable to a wide range of pressure classes, including lower- pressure spacecraft tank applications as well as higher-performance variants configured for increased operating margins.

[0113] Having described the many embodiments of the present disclosure in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. Furthermore, it should be appreciated that all examples in the present disclosure, while illustrating many embodiments of the invention, are provided as non-limiting examples and are, therefore, not to be taken as limiting the various aspects so illustrated.EXAMPLESExample 1

[0114] Introduction

[0115] Disclosed embodiments develop unique tank technologies that enable spacecraft systems to achieve higher propellant loads at lower cost than has been achieved with traditional tank methodologies. Additionally, the nature of these designs allow for rapid production by avoiding some of the more costly production methods required by conventional tanks (such as spinforming, custom forgings, and complex machining from large material billets). However, it is important to predict the maximum pressure and dynamic loading that these tanks can survive before a rupture occurs.

[0116] This approach begins with detailed mechanical analyses and hardware-analysis correlations on subscale tank units.

[0117] Progressive Development Approach

[0118] Disclosed embodiments began by designing and analyzing a full-scale propellant tank, capable of holding 1600kg of usable propellant. The main uncertainties lie in material thicknesses and strength knockdowns versus weld penetration depths. As such, prior data from past vendor experience was utilized to formulate an initial design. These weld geometries, materials, and weld parameters were then maintained on a smaller design, optimized against fabricator machine availability. The result is an appropriately -sized tank for a highly-capable ESPA-Grande spacecraft.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0119] Due to the diameter and length scaling of the tank, new predictions were generated for the tanks maximum pressure capabilities. These capabilities are then confirmed via burst pressure and pressure cycling. The resulting failure points and post-test inspections are then used to update the structural analysis and design data for the disclosed full-scale tank.

[0120] Additionally, reaction force data collected from the subscale weld process is used to design appropriate tooling for the disclosed full-scale tank.

[0121] Custom Tooling Development

[0122] As part of the disclosed tank design and manufacturing process, custom tooling has been developed to enable friction stir welding operations to be performed in a controlled, repeatable, and production-scalable manner. FIG. 5 illustrates multiple components of weld tooling that have been validated during fabrication of the disclosed tank assemblies.

[0123] As illustrated, FIG. 5 shows an in-process cylindrical wall longitudinal weld 500 being formed using a longitudinal weld fixture 501 that is configured to maintain alignment, concentricity, and dimensional tolerances of cylindrical wall segments during welding. A friction stir weld support jig 502 is positioned adjacent to the weld region to provide localized structural backing and reaction force support during tool engagement. A friction stir weld weld head and pin 503 is operatively coupled to a CNC machine and machine table 504 (background) and is configured to generate controlled rotational and axial forces required to form the friction stir weld joint.

[0124] Also shown is a circumferential weld fixture ring 505 configured to support and restrain cylindrical tank components during circumferential welding operations. A friction stir weld center support jig 506 is positioned within the tank interior to provide internal reaction support and to maintain geometric stability during welding. A powered rotary’ axis 507 is configured to rotate the tank assembly relative to the friction stir weld head and pin 503, thereby enabling formation of continuous circumferential weld seams with uniform tool engagement conditions.

[0125] Further illustrated is a T-weld friction stir weld fixture and support jig assembly 508 configured to support orthogonal joint geometries formed between a tank outer wall 509 and a tank cap 510. The T-weld friction stir weld fixture and support jig assembly 508 maintains joint positioning, perpendicularity, and contact pressure during friction stir welding to enable formation of structurally robust intersection welds.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0126] In certain embodiments, the same weld tooling components may be utilized across multiple tank design phases, including subscale development builds and full-scale production builds, with minor part substitutions or modular component exchanges to accommodate variations in tank diameter and length. The disclosed tooling architecture is configured such that adaptation between different tank geometries may be achieved through simple part swaps while retaining the same core fixture framework and friction stir weld support architecture. Accordingly, throughout progressive design iterations and fabrication of full-scale tank assemblies, the same underlying weld tooling technology may be employed to ensure manufacturing consistency, repeatability, and scalability.

[0127] Status

[0128] In accordance with disclosed embodiments, completed subscale burst tanks have been received and inspected. Furthermore, initial weld coupon qualification and leak checks have been completed, which indicate that all welds were successful. Feedback has been incorporated about weld fitups and tooling access, and improvements have been incorporated into the flight tanks.Example 2

[0129] Summary

[0130] Disclosed embodiments are committed to developing novel common-wall propellant tanks for scouts, rangers, and future spacecraft vehicles. The initial development effort focused on a subscale demonstrator tank that integrates welds from a potential flight tank design, but at a more reasonably size and cost. Disclosed embodiments decided to test the viability of the welds, geometries, and analysis via burst testing. A burst test apparatus was designed and built to perform the tank tests in-house. An automated hydro-pneumatic pressure test facility was designed, built, and activated. Subsequent testing of a subscale manufacturing prototype ruptured its inner chamber at 942psig. Resulting analysis and test data will aid the disclosed tank development roadmap going forward. This summary report reviews the motivation, development, test setup and test data for the first burst test conducted on a subscale demonstrator tank.

[0131] IntroductionAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0132] Disclosed embodiment propulsion systems are differentiated by high propellant mass fraction, low operating pressure, low-cost propellant tanks enabled by pump-fed thrusters. While the disclosed current baseline pumps and thrusters are an integration of externally developed components, the disclosed tank relies on a novel manufacturing process applied to a novel tank geometry of a proprietary design. This conformal, low pressure, common-barrel tank is manufactured by friction stir welding (FSW) of simple machined plate aluminum and allows for rapid production by avoiding costly production methods used by conventional tanks (such as spinforming, custom forgings, and complex machining from large material billets).

[0133] Motivation

[0134] The novel nature of the disclosed tank designs may create uncertainty in an ability to predict the maximum pressure and dynamic loading that these tanks can survive before a rupture occurs. To address this risk, multiple tanks were built with the same weld joints (same material, section geometry, and thicknesses) as are anticipated for spacecraft flight tanks, but at a smaller diameter and overall height to lend ease of manufacturing, inspection and test. Bringing this tank to structural failure via hydrostatic burst was intended therefore to characterize realistic weld knockdowns for a full-scale spacecraft tank.

[0135] Objective

[0136] In accordance with disclosed embodiments, technical direction to manufacture demonstrator tanks and complete a burst test was provided and achieved. This was to be completed for minimal cost, while producing high-quality time-history pressure data up to rupture, in a safe manner for all test personnel and capital equipment. This pressure data is intended to corroborate analysis techniques and design qualification for present and future tanks.

[0137] Test Planning and End Configuration

[0138] FIG. 6 graphically illustrates a pulsed pressurization system fluid network analysis output (bottom), solenoid valve pulsed pressurization system captured performance data (top), and a fluid network model in GFSSP (inset) according to one embodiment of the present disclosure. The illustrated data sets provide comparative visualization of modeled system behavior and experimentally captured performance metrics for a controlled pressurization architecture.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0139] As shown in FIG. 6, pressurization system performance data 600 and corresponding pressurization system analytical data 601 are overlaid to demonstrate correlation between measured system response and predicted fluid network behavior. A maximum pressure 602 and a delivered pressure 603 are further illustrated to indicate peak pressure limits and the effective pressure applied to the tank assembly during pulsed pressurization operation. The graphical representation enables evaluation of transient response characteristics, pressure ramp profiles, stabilization behavior, and pulse-to-pulse repeatability of the pressurization system.

[0140] In certain embodiments, system-level trade studies indicated that a traditional manual hydraulic pressurization scheme exhibited unfavorable performance characteristics when compared to a computer-controlled pneumatic pulsed pressurization system. Initial safety analysis and fluid network analysis, informed by solenoid valve pulsed pressurization system captured performance data (top) plotted in FIG. 6, demonstrated that a pressurization architecture employing commercial off-the-shelf (COTS) solenoid valves and a low-cost single-board computer (SBC) controller could conduct pressurization testing within applicable stored energy safety thresholds, including NIST NCNR PNNL-18696 safe stored energy limits, while simultaneously acquiring pressure data of sufficient resolution and accuracy to support downstream structural analysis validation.

[0141] By contrast, the manual hydraulic pressurization scheme was determined to be less capable of independently regulating pressure across multiple internal volumes and exhibited reduced controllability of pressure ramp rates and stabilization profiles, thereby limiting data fidelity and test repeatability. Accordingly, the pulsed pneumatic pressurization architecture illustrated in FIG. 6 provides improved pressure control, enhanced safety margin compliance, and superior data acquisition performance for structural test validation of the disclosed tank assemblies.

[0142] FIG. 7 illustrates the disclosed subscale tank integrated into an exemplary burst test process according to one embodiment of the present disclosure. As shown, FIG. 7 includes a pressure vessel (tank) Unit Under Test (UUT) 700 positioned within a controlled test environment and operatively coupled to a pressurization and monitoring architecture configured to execute controlled pressure ramping and structural failure characterization.

[0143] In the illustrated embodiment, an inner volume of the pressure vessel (tank) Unit Under Test (UUT) is fluidly coupled to a UUT inner volume pressure source 704 through a UUT inner volume pressure application valve 703, thereby enabling controlled introduction ofAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669pressurant media into the UUT inner volume. A UUT inner volume pressure sensor 702 is operatively coupled to the UUT inner volume to measure internal pressure during pressurization, hold, and burst phases. A UUT inner volume pressure vent valve 701 is further provided to selectively release pressure from the UUT inner volume for depressurization, safety relief, or post-test venting.

[0144] The UUT outer volume pressure is monitored and controlled through a UUT outer volume pressure application valve 707 and a UUT outer volume pressure vent valve 708, which regulate pressurant flow into and out of the UUT outer volume. A UUT outer volume pressure 706 is monitored to characterize differential pressure conditions across tank wall structures and to support evaluation of multi-wall tank behavior during the burst test process.

[0145] In certain embodiments, test sequencing and operational coordination of the pressurization system components are managed by a test controller 709, which may be configured to execute predetermined pressure ramp profiles, synchronized valve actuation, safety interlocks, and real-time data acquisition from the UUT inner volume pressure sensor 702 and the UUT inner volume pressure sensor 705. The test controller 709 may further log time-correlated pressure data for post-test structural analysis and failure mode evaluation.

[0146] The pressure vessel (tank) Unit Under Test (UUT) is supported by a UUT stool 711, which provides mechanical stability and positional alignment during testing. A spill containment 712 is positioned beneath and around the UUT to capture released fluid in the event of rupture or controlled draining. The UUT outer volume water drain 710 and the UUT inner volume water drain 713 are configured to remove residual test fluid following completion of the burst test process and to facilitate safe post-test handling and inspection.

[0147] The illustrated configuration enables controlled execution of hydrostatic or pneumatic burst testing, differential pressure evaluation between inner and outer volumes, and acquisition of high-fidelity pressure data for correlation with analytical and finite element structural models, thereby supporting qualification and validation of the disclosed tank architecture.

[0148] FIG. 8 illustrates an exemplary plumbing and instrumentation diagram for a burst test fluid system according to one embodiment of the present disclosure. As shown, the burst test fluid system is configured to supply controlled pressurization media, regulate flow rates, monitor system performance, and provide automated safety and data acquisition functionality during structural qualification testing of a tank assembly.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0149] In the illustrated embodiment, the burst test fluid system comprises a dry nitrogen gas bottle pressure source fluidly coupled to a pressure regulator and one or more flow limiting orifices, each having a diameter of approximately 0,010 inches, to limit pressurization rate and stored energy release during testing. The system further includes a pressurization solenoid valve and a vent / depress solenoid valve configured to selectively introduce pressurant gas into the test volumes and to rapidly relieve pressure when required. Manual fill and drain valves are provided on the liquid volume, and manual fill pumps are provided for both inner and outer tank volumes to enable controlled introduction and removal of test fluid.

[0150] In certain embodiments, the burst test procedure includes filling tank volumes with deionized water using pumps HP-F001 and HP-M001 to target masses of approximately 13.90 kilograms for the inner volume and approximately 18.64 kilograms for the outer volume. At these mass fill levels, the remaining ullage volumes are approximately 12 cubic inches and 20 cubic inches, respectively, thereby maintaining stored energy levels within predetermined safety thresholds.

[0151] Each set of solenoid valves associated with each tank volume is configured to operate in a fail-safe mode such that removal of electrical power results in depressurization of trapped volumes and isolation of high-pressure gas sources. In the illustrated configuration, SV-F001 and SV-M002 are normally closed valves, while SV-F003 and SV-M004 are normally open valves, thereby providing redundant depressurization and pressure isolation functionality.

[0152] As further shown in FIG. 8, the burst test fluid system includes a tank 800 configured as a Unit Under Test (UUT), a pressure application and monitoring component 801, and an automated digital control and data acquisition systems component 802. The pressure application and monitoring component 801 is configured to regulate pressurant delivery, measure pressure and flow parameters, and provide feedback signals associated with system performance. The automated digital control and data acquisition systems component 802 is configured to coordinate solenoid valve actuation, execute predefined pressurization profiles, implement safety interlocks, and record time-correlated test data for subsequent structural analysis and failure characterization.

[0153] The illustrated plumbing and instrumentation architecture enables precise control of pressurization conditions, enhanced operator safety through automated depressurization andAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669isolation logic, and acquisition of high-fidelity pressure and system performance data to support correlation with analytical and finite element structural models.

[0154] In certain embodiments, the automated digital control and data acquisition systems component 802 may comprise a video controller, a test controller, and a high-rate data acquisition system (DAQ) configured to cooperatively manage system operation, monitoring, and data capture during pressurization and burst testing.

[0155] The DAQ may be configured to collect, digitize, time-align, and store sensor signals including pressure measurements, temperature readings, flow rates, strain gauge outputs, and valve state indicators. The DAQ may further provide real-time data streaming to the test controller and the video controller to enable synchronized correlation between visual observations and measured system parameters. The DAQ may be operated in parallel to simultaneously record commands, responses and sensors for complete test reconstruction, operated by two test personnel.

[0156] The test controller may be configured to execute, for example, predefined test sequences, including pressurization ramp profiles, dwell intervals, depressurization commands, and safety interlock logic. In certain embodiments, the test controller may further coordinate actuation of solenoid valves, pumps, and pressure regulation components, while implementing fault detection routines and emergency shutdown procedures. The test controller runs in-house- developed configurable valve sequencing software, written so that test configuration can be readily captured for anomaly detection & resolution and regression testing, as well as lending portability to run different tests when equipped with a different set of sensors and effectors.

[0157] The video controller may be configured, for example, to acquire, synchronize, and record visual data associated with the tank assembly and test environment, including high¬ speed video, time-stamped imagery, and event-triggered recordings corresponding to pressurization, deformation onset, or structural failure events. In some disclosed embodiments, the video controller was utilized to handle control of 4 GoPro HERO 12s recording the UUT at 2.7k resolution and 240 fps arranged around the tank to capture full coverage of all externally visible welds, as several potential initial rupture locations were possible.

[0158] Collectively, the automated digital control and data acquisition systems component 802 enables closed-loop test control, high-fidelity data capture, synchronized multi-sensor recording, and automated safety management for structural qualification and burst testing of the disclosed tank assemblies.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0159] Test Run

[0160] FIG. 9 illustrates test camera video frames before (left), during (middle), and immediately after tank rupture (right) according to one embodiment of the present disclosure. As shown, FIG. 9 includes a tank test unit 900 positioned within a pressurization environment and operatively coupled to a pressurization system 904. The illustrated sequence includes a unit before rupture 901, a unit during rupture 902, and a unit after rupture 903, thereby providing a time-resolved visual record of structural failure progression during burst testing.

[0161] In the illustrated embodiment, the unit before rupture 901 depicts the tank test unit 900 under elevated internal pressure prior to structural failure. The unit during rupture 902 captures the onset of material separation and rapid decompression associated with pressure boundary breach. The unit after rupture 903 illustrates the post-failure condition of the tank test unit 900 following depressurization and structural collapse.

[0162] FIG. 10 illustrates a tank plunge weld micrograph (left) and a similar section of FEM (right) according to one embodiment of the present disclosure. As shown, the micrograph includes a weld cross section, etched sample 1000, while the analytical representation includes a weld cross section, design 1001 generated from finite element modeling. The illustrated weld region further includes a tank inner wall 1002, a tank cap 1003, and a friction stir weld 1004, thereby enabling direct visual and analytical comparison between physical weld morphology and modeled structural geometry.

[0163] During a representative test run, regulator setpoints were established at approximately 1006 psig for PR-G002 and ambient pressure for PR-G001. An envelope¬ expansion test demonstrated that the press solenoid valves were capable of actuating against a pressure differential of approximately 1000 psi, thereby enabling reconfiguration of the test setup to pressurize only the inner volume of the tank test unit 900 while maintaining the outer volume at ambient pressure. In this configuration, SV-F003 was held open to maintain the outer volume at ambient pressure. New pressure gates were calculated based on inner -to-outer volume pressure differential values derived from prior test gates, and additional gates were introduced to increase the pressurization range of the inner volume up to approximately 985 psig. An ambient pressure condition may be regarded as approximately 15 psia.

[0164] In the illustrated test run, the inner tank reached approximately 942 psig prior to failure of the inner wall, resulting in a transient pressure spike within the outer volume and subsequent failure of the outer barrel. The burst event was directly recorded by a test camera,Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669as shown in FIG. 9, where rupture of the outer tank at its longitudinal weld is visually observable. Post-test inspection further confirmed the failure location, as correlated with the tank plunge weld micrograph (left) and the similar section of FEM (right) illustrated in FIG.10. The post-test condition of the inner tank barrel exhibited similar failure characteristics, indicating that failure likely occurred at a comparable weld location.

[0165] The combined use of synchronized video capture, microstructural weld analysis, and finite element modeling provides correlation between observed failure behavior and predicted structural response, thereby validating analytical models and supporting structural qualification of the disclosed tank architecture.

[0166] Analysis

[0167] Prior to execution of burst testing, the disclosed subscale tank design was modeled and discretized to evaluate structural capability and to predict potential failure location, timing, and mode during pressurization. Initial analytical emphasis was placed on the T-weld region joining the inner tank wall to the tank caps. In the illustrated embodiment, the finite element model (FEM) is constructed as a single continuous mesh comprising solid hexahedral elements having uniform material properties corresponding to AL 6061-T651 (Ref. MMPDS-2023, Table 3.6.2.0.b2). This modeling approach enables consistent representation of stress and strain behavior throughout the tank structure.

[0168] FIG. 11 illustrates a slice 1100 of an exemplary tank from an axisymmetric finite element model (FEM) according to one embodiment of the present disclosure. The FEM incorporates an idealized weld geometry representative of a friction stir weld configuration. Because weld regions are inherently complex and computationally intensive to model with high fidelity, particularly within exclusively linear analysis frameworks, the mesh of the inner tank wall 1103 and weld region 1105 is merged and continuous with the tank cap section 1102. In disclosed embodiments, weld region 1105 may comprise friction stir welding / welds

[0169] In alternative modeling conventions, the tank cap section 1102 and tank walls may be represented as contacting bodies rather than continuous material regions, thereby permitting separation and peeling behavior in the absence of a modeled weld. However, such approaches significantly increase computational complexity and resource requirements. Accordingly, the meshes were merged to represent a continuous structural joint. Failure to merge these meshes was observed in prior analysis iterations to produce stress singularities at the root regionsAttorney Docket No.: 118717-25167WO01 / Customer No.: 110669adjacent to the inner tank wall 1103, thereby compromising numerical stability and result interpretability.

[0170] The illustrated slice 1100 represents the FEM configuration used throughout the analysis and test campaign and leverages the assumed axisymmetric characteristics of the tank geometry and applied pressure loads. The slice 1100 is assumed to exhibit mechanical behavior equivalent to that of a fully modeled three-dimensional tank geometry / . Boundary' conditions are applied at the planar faces used to generate the axisymmetric slice, such that nodes are permitted to translate along the plane while being constrained from motion normal to the plane.

[0171] Although bolted connections between the tank cap section 1102 and the tank cap reinforcement (stiffener plate) 1104 are not strictly axisymmetric in physical implementation, these interfaces are assumed to exhibit sufficient stiffness such that omission of bolt pattern asymmetry' does not materially influence predicted structural response. As further illustrated in FIG. 11, the inner tank wall 1103 and the outer tank wall 1101 are identified within the FEM geometry to illustrate load paths, stress distribution, and structural interaction between concentric tank walls under pressurization loading.

[0172] The disclosed axisymmetric FEM configuration enables efficient computational evaluation of structural performance while maintaining sufficient geometric and material fidelity to support correlation with experimental burst test results.

[0173] Separate pressure loads were applied to the inner and outer tank to simulate each burst test “gate.” Initial analysis showed likely failure near the T-weld of the inner tank. Stress on the weld was ignored. However, the first burst tank reached 750 psi in the inner tank without a leak or burst. Thus, a second analysis was performed to determine at what pressure the tank could be expected to burst but results continued to predict a failure in the weld at much lower pressures. After the second test resulted in a successful burst of the inner tank at approximately 950 psi, the analysis was refined to investigate the area of failure (the longitudinal weld of the inner tank) at the burst pressure.

[0174] Results

[0175] Two tests were conducted, with the first test pressurizing the inner volume to 743 psig and outer volume to 185 psig without inciting a rupture, and the second test rupturing the inner volume at 942 psig with the outer volume vented to ambient (approximately equivalent to a 1254 psig inner and 312 psig outer at an operational differential pressure ratio of 4.03:1).Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669The locus of rupture was determined to be mid-span of the longitudinal weld of inner barrel with outer barrel closely following (under 5ms). In one disclosed embodiment, the disclosed subscale tank has two separate sealed volumes within it, but they are a single component, a single pressure vessel, akin to “common wall” rocket launch vehicle main propellant tanks. Accordingly, disclosed subscale tanks may define two distinct sealed internal volumes formed within a unitary structure, such that the tank functions as a single integrated pressure vessel while providing fluid isolation between the respective volumes, analogous to common¬ bulkhead or common-wall propellant tank architectures employed in launch vehicle main propellant stages.

[0176] The analysis results, reinforced with hand calculations, showed that the stress in the inner tank wall was very near or beyond the material strength values listed in MMPDS. Thus, it made sense that the failure occurred at the longitudinal weld, a location of likely “imperfect” geometry and weakened material strength due to the friction stir welding process. This region of tank was initially overlooked during the investigation of the T-weld. Practical test and post¬ test finite element analysis show that the T-weld is much stronger than analytically predicted, which is less surprising in retrospect. Weld finite element analysis (FEA) is difficult and often leads to stress singularities, and traditional tank design avoids sharp internal corners for similar reasons. The stress in the wall is much simpler to predict and more reliable, thus was able to indicate when the tank wall could be expected to burst.

[0177] Conclusions and Recommendations

[0178] This test campaign produced a controlled rapture of the test article, while capturing high-rate, high-quality pressure data. This exercise will inform future tank design, analysis methods, and test procedures. While this subscale tank test article reached an unexpectedly high internal pressure of nearly 10 times what is needed for flight tanks - indicating that this tank is overbuilt (as is likely at a small scale) - this proves the feasibility design concept, welds and manufacturing methods used. Better understanding of the behavior of these welds will allow for mass reduction efforts. Continued use of the control cabinet as a generic fluid system controller is advised, with improvements and extensions enabling further tank testing and increased capability. The procedures used for the test worked well, with no safety issues thanks to test control conducted from a safe distance, multiple software and hardware ways to stop the test in case of emergency, and personal protective equipment (PPE) utilized by all test personnel,.Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669

[0179] Fracture mechanics and weld sensitivity studies are likely the next step to further understand the behavior and failure of this and future designs. The subscale burst test indicates that the finite element models created thus far have been sufficient to predict a region and pressure near which the inner tank wall burst, but FEA, both linear and nonlinear, can only get so far before a different approach is required to more accurately capture real-world behavior of weld joints. Vendor-supplied flaw distribution and size data for these weld processes, would provide critical insight into how the longitudinal welds may behave in future tanks. The results from analysis indicate there is high stress along the T-welds and while it can be ignored as a finite-element artifact to an extent, it cannot be forgotten. That stress still exists in some capacity and needs to be fully understood before mass reduction can begin.

[0180] All documents, patents, journal articles and other materials cited in the present application are incorporated herein by reference.

[0181] While the present disclosure has been disclosed with references to certain embodiments, numerous modification, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the present disclosure, as defined in the appended claims. Accordingly, it is intended that the present disclosure not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.

Claims

Attorney Docket No.: 118717-25167WO01 / Customer No.: 110669WHAT IS CLAIMED IS:

1. A propellant tank comprising:a conformal multi-wall structure defining an inner volume and an outer volume, wherein the structure is formed from machined plate sections joined by friction stir welding to form friction stir welds along the structure, wherein the structure includes the friction stir welds formed along longitudinal weld seams, circumferential weld seams, and / or T-joint geometries, wherein the tank is configured to withstand an internal burst pressure of 1.5 times the maximum expected operational pressure (MEOP).

2. The tank of claim 1, wherein the tank has a non-axisymmetric geometry.

3. The tank of claim 1, wherein the conformal multi-wall structure comprises aluminum or an aluminum -based alloy plate sections configured with consistent weld penetration depths and joint profiles that correspond to flight weld geometries.

4. The tank of claim 3, further comprising:one or more end plates affixed to the multi-wall structure via friction stir welds forming flight weld geometries disposed along a connection thereof.

5. The tank of claim 1, wherein a body of the tank has an average wall thickness of less than about 3 millimeters.

6. The tank of claim 1, wherein a body of the tank has an average wall thickness within a range of about 0.5 millimeters to about 8 millimeters.

7. The tank of claim 1, wherein the longitudinal weld seams, circumferential weld seams, and / or T-joint geometries are configured to withstand an internal pressure at least 1.5 times a nominal operating pressure of the tank.

8. The tank of claim 1, wherein a body of the tank is thermally coupled to at least one spacecraft component such that thermal energy is transferred between the tank and the spacecraft component.

9. The tank of claim 8, wherein the tank body acts as a thermal mass to moderate temperature of the propellant.Attorney Docket No.: 118717-25167WO01 / Customer No.: 11066910. The tank of claim 8, wherein thermal energy is configured to be transferred to the tank body from an electrically powered component.

11. The tank of claim 8, wherein thermal coupling is achieved via conductive interfaces or fluid-based heat transport,12. The tank of claim 1, where' in the tank structure is configured to withstand an internal burst pressure of at least about 900-940 psig.

13. The tank of claim 1, wherein the internal burst pressure exceeds at least about 2.0 times the MEOP.

14. The tank of claim 1, wherein the structure is configured to operate at internal pressures at or near atmospheric pressure, including approximately 1 atmosphere absolute corresponding to about 0 psig gauge.

15. The tank of claim 1, wherein a shape of the tank is not principally governed by a surface of uniform curvature nor by geometries in which curvature smoothly transitions between regions.

16. The tank of claim 1, wherein a shape of the tank comprises sharp edges such that a difference in volume between a typical prismatic spacecraft bus and a wall of the tank is significantly reduced.

17. A method of making a conformal propellant tank comprising:forming a plurality of wall panels;arranging the wall panels to conform to an internal volume of a spacecraft bus; and joining the wall panels to form a sealed tank body configured for low-pressure operation,wherein joining the wall panels comprises friction stir welding adjacent wall panels along edge interfaces.

18. The method of claim 17, wherein the tank has a non -axi sy metric geometry,19. The method of claim 17, wherein the tank body is formed from aluminum.Attorney Docket No.: 118717-25167WO01 / Customer No.: 11066920. The method of claim 17, wherein the plurality of wall panels are configured to define an inner volume and an outer volume.

21. A method of making a spacecraft propellant tank comprising:forming a tank body from a metallic material,joining portions of the tank body using at least one welded seam selected based on low- pressure operation; andconfiguring the welded seam to withstand an internal pressure exceeding a nominal operating pressure of the tank.

22. The method of claim 21, further comprising:forming the weld seam by friction stir welding.

23. The method of claim 21, wherein the friction stir welding produces friction stir welds formed along the weld seam and T-joint geometries.

24. The method of claim 21, wherein the tank is configured to withstand an internal burst pressure of at least 900 psig.

25. The method of claim 21, wherein the tank body has a non-axisymmetric geometry.

26. The method of claim 21, wherein the welded seam comprises friction stir welds having consistent weld penetration depths and joint profiles corresponding to flight weld geometries.

27. The method of claim 21, wherein the tank body is formed from aluminum.

28. The method of claim 21, further comprising:determining a target internal operating pressure below a thruster feed pressure; selecting a wall thickness based on the target internal operating pressure; and forming a sealed tank body.

29. The method of claim 21, wherein the tank body is configured to define an inner volume and an outer volume.

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