Synthesis and integration of noble gas radioisotope power systems

Abstract

A system to generate power in a low orbit environment may include an enclosure configured to maintain a noble gas radioisotope at a threshold pressure, wherein the noble gas radioisotope generates heat at the threshold pressure, a radiator configured to maintain the generated heat at a threshold heat, and a power converter configured to convert the maintained heat to electric power.

Description

SYNTHESIS AND INTEGRATION OF NOBLE GAS RADIOISOTOPE POWER SYSTEMSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisional application Serial No. 63 / 507,907 filed June 13, 2023, the disclosure of which is hereby incorporated in its entirety by reference herein.TECHNICAL FIELD[00021 Aspects of the disclosure generally relate to systems and methods for radioisotope power systems, noble gas isotopes and integration thereof.BACKGROUND

[0003] Current satellites require thrusters with on-board propellent and solar cells. This limits the amount of available surface area on the satellite, as well as creates drag. Further, very low earth orbit (VLEO) reduces the orbital debris threat by avoiding the overcrowded LEO space, which could rapidly fill with debris. In addition, VLEO orbits offer improved sensor resolution, faster data / comm links, smaller optical sensors, and enable other data link opportunities. However, gas composition, pressure, and temperature vary significantly with altitude in the VLEO range and aerodynamic drag at lower VLEO altitudes is a key concern. Even more, atomic oxygen (AO) is the dominant species for a large range of altitudes, and is a major constituent across all VLEO. AO is extremely reactive, rapidly degrades most polymers and metals and many ceramics.SUMMARY

[0004] A system to generate power in a low orbit environment may include an enclosure configured to maintain a noble gas radioisotope at a threshold pressure, wherein the noble gas radioisotope generates heat at the threshold pressure, a radiator configured to maintain the generated heat at a threshold heat, and a power converter configured to convert the maintained heat to electric power.

[0005] In one example, the noble gas radioisotope is one of Kr-85, Ar-39, Ar-42, or Xe-127.

[0006] In one embodiment, the enclosure comprises the radiator.

[0007] In another embodiment, the enclosure comprises shielding from the noble gas radioisotope.

[0008] In one example, the converter is a Stirling engine or a Brayton engine.

[0009] In one embodiment, the radiator conducts the maintained heat to the power converter.

[0010] In another embodiment, the environment comprises a threshold percentage of an element of the noble gas radioisotope.[OOH] An inlet for passive collection of ambient gas in a low orbit environment, may include a plurality of surfaces forming an air scoop for receiving a gas comprising atomic oxygen (AO), wherein the surfaces each include an outer surface configured to scatter a first set of particles of the gas in a first direction and a second set of particles of the gas in a second direction.

[0012] In one example, the outer surface includes a first coating and a second coating, the first coating having a higher AO resistance and a lower drag resistance than the second coating.

[0013] In one embodiment, the first coating specularly reflects the gas in the first direction towards an apex of the inlet and the second coating diffusely scatters the gas in a second direction towards the outer surface.(0014] In another embodiment, the outer surface comprises an array of angled-teeth structure, each tooth of the array forming a first side for the first coating and a second side for the second coating.

[0015] In one example, each surface forms a triangular shape configured to be coupled to an adjacent surface to collectively form the inlet and defining an opening at an apex thereof to receive at least a portion of the gas.10016] In one embodiment, each surface includes an inner surface including at least one trace to one or more electrodes.|0017| In another embodiment, the outer surface includes at least one slow etch layer and one fast etch layer.[0018| In one example, the at least one slow etch layer includes two slow etch layers and the at least one fast etch layer is arranged between the two slow etch layers.

[0019] In one embodiment, the at least one slow etch layer is exposed to the AO during the life of the outer surface more than the at least one fast layer.

[0020] In another embodiment, during the life of the outer surface, the at least one fast etch layer etches completely upon at least partial etching of the at least one slow etch layer coupled to the at least one fast etch layer to resettle the outer surface to a planar surface.[00211 An inlet for passive collection of ambient gas in a low orbit environment, may include an inlet forming an air scoop for receiving gas from a low orbit environment, wherein the air scoop includes an outer surface resistant to atomic oxygen (AO) erosion and configured to scatter particles of the AO and to collect the gas.

[0022] In one example, the outer surface forms an angled-tooth structure, each tooth forming a first side for receiving a first coating and a second side for a second coating.|0023| In another example, the outer surface includes at least one slow etch layer and one fast etch layer, wherein the at least one slow etch layer is exposed to the AO during the life of the outer surface more than the at least one fast layer.BRIEF DESCRIPTION OF THE DRAWINGS[0024| The embodiments of the present disclosure are pointed out with particularity in the appended claims. However, other features of the various embodiments will become more apparent and will be best understood by referring to the following detailed description in conjunction with the accompanying drawings herein.

[0025] FIG. 1 illustrates a perspective view of an example power system for a low orbit environment.

[0026] FIG. 2 illustrates a view of an example noble gas radioisotope power system.

[0027] FIG. 3 illustrates a block diagram of a power conversion process for the power system of FIG. 1.[0028| FIG. 4 illustrates a side view of an example air scoop.

[0029] FIG. 5 illustrates a top view of the air scoop.

[0030] FIG. 6 illustrates a side view of an example outer surface of the air scoop.

[0031] FIGs. 7A-D illustrate various examples of the surface having an angled-tooth structure.

[0032] FIG. 8 illustrates a side view of an example self-healing surface for a low orbit environment .

[0033] FIGs. 9A-F illustrate a progression of the surface of FIG. 8 through a mission.DETAILED DESCRIPTION

[0034] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

[0035] There is a need for new technologies that can efficiently and safely power satellites in very low earth orbit (VLEO). Such orbit is typically at an altitude below 450 KM, where a low earth orbit (LEO) is at an altitude between 450-2000 km A VLEO orbit is advantageous because it reduces the orbital debris threat by avoiding the overcrowded LEO space, which could rapidlyfill with space junk. In addition, VLEO orbits offer improved sensor resolution, faster data / comm links, smaller optical sensors, and enable other data link opportunities. Existing satellites often require thrusters with on-board propellent and solar cells. These satellites are therefore limited by the amount of available surface area on the satellite, the location of the sun, and on-board fuel. It could be advantageous to develop alternatives to existing satellites that are powered to operate in VLEO orbit but do not require solar cells and / or on-board propellent — such systems would enable robust and long term operation. To enable long duration VLEO vehicles, power systems should ideally have a sufficiently high power to weight ratio to overcome drag and supply power to the propulsion system as well as any other electronics or payloads. An ideal power system would not be dependent on the location of the sun like conventional solar-based power units for space vehicles.[0036| Further, in VLEO environments, atomic oxygen can be present. Atomic oxygen can cause erosion of spacecraft components, due to erosion and other degradation mechanisms. A spacecraft can be coated in an atomic oxygen resistant, low drag coating to enable longevity in the environment and reduced drag. While atomic oxygen can be detrimental to components of a spacecraft, it can be desirable to capture air or gas in the environment surrounding the spacecraft. In certain cases, it can be advantageous to capture atomic oxygen surrounding the spacecraft, or other particles or combinations of particles of the ambient environment.

[0037] Finally, low drag coatings and materials should ideally be well understood through experimental testing and aerodynamic modeling to ensure that the atmosphere / satellite interactions meet all VLEO mission requirements. These surfaces should ideally reduce atomic-oxygen erosion and help ensure that the power, propulsion, and thrust systems overcome the required drag for the satellite. Successful solutions will enable significant improvements beyond the state of the art and start a new field of research for VLEO satellites in multiple technology areas since all three technologies will be required to enable long duration VLEO satellites. The systems disclosed herein describe technical solutions to these and other problems. Disclosed herein is a noble gas radioisotope power system to provide higher power density, higher reliability, and reduced mass as compared to conventional VLEO power systems. Further, the low-drag and high atomic oxygenresistant coatings for spacecraft in VLEO or other environments are disclosed. An inlet to capture VLEO environmental gases is also provided.|0038| FIG. 1 illustrates a perspective view of an example system for operation in a very low earth orbit environment. The system 100 can include a noble gas radioisotope power system 103, a payload 110, a thruster 112, and / or an air scoop 114. The system 100 may include a power system 103 comprising a radiator 102, a pressure tank 104, a power converter 106, a radiation shield 108. The power system 100 may be divided into the noble gas radioisotope power system (RPS) subsystem (i.e., the pressure tank 104 including one or more radioisotopes, the power converter 106 and / or a heat sink(e.g., the radiator 102), and an air scoop subsystem. The RPS includes the power converter 106, the pressure tank 104, and the radiator 102. The radiator 102 can include or be a part of an enclosure surround all or part of the system 100. The pressure tank 104 may include more than one tank and may be designed with optimal materials for maximum pressure and aspect ratios. The pressure tank 104 is configured to maintain a noble gas radioisotope at a threshold pressure, where the noble gas radioisotope generates heat at the threshold pressure. The threshold pressure can be between 10-100 MPa. In some cases, the threshold pressure can be different for different noble gases. For example, the threshold pressure may be between 40-60MPa for Ar-39 and 10-30MPa for Kr-85.[00391 The RPS may be configured similar to a traditional radioisotope thermoelectric generator (RTG) in function and design. However, the RPS further uses inert noble gas, such as Kr-85 or Ar-39, to generate heat, where traditional systems may use Pu-238. However, other noble gasses may be used, such as H-3, Am-241, etc.

[0040] The use of noble gasses is preferable due to their short biological residence times, lack of chemical reaction with the human body, and reduction of radiological risk in the case of any sort of accidental release of the isotopes. Noble gasses have relaxed limits by 6-8 orders or magnitude relative to existing nuclear fuels.

[0041] Kr-85, Ar-39, Ar-42, or Xe-127 may be candidate isotopes for smaller systems (ex. l-50Wt), while Kr-85, Ar-42, Xe-127 may be a candidate isotope for larger space systems (ex. l-50kWt). Use of such candidate isotopes can enable greater heat generation and lifespan that otherconventional radioisotope thermoelectric generators. Such systems may also include, for example, remote power stations. It should be appreciated that other noble gas isotopes can be used.

[0042] The noble gases can be contained in the pressure tank 104. In some cases, the pressure tank can be made of a material to withstand the environment while maintaining a threshold pressure of the noble gases contained therein. In some cases, the environment can include a VLEO orbit, a planetary atmosphere or surface, a LEO orbit, undersea, among other environments which exhibit extreme pressures or temperature. In some cases, the environment can include atomic oxygen (AO).

[0043] The pressure tank 104 material can include Ti 6A1-4V, Ti 6A1-4V, Inconel 625, Inconel 718, Inconel 738, Hastelloy X, Hastelloy X, RENE 41, RENE 41 among others. In some cases, the pressure tank 104 can be spherical, pill-shaped, cubical, or any shaped container by which to contain the noble gas. The pressure tank 104 can have a yield strength. In some cases, the pressure tank 104 has a yield strength of 400-500 MPa at 500-700 °C. In some cases, the pressure tank 104 has a yield strength of 150-250 MPa at 700-1100 °C.

[0044] In some cases, the pressure tank 104 includes shielding. The radiation shield 108 can protect one or more parts of a spacecraft or other vehicle housing the RPS from radiation, heat, or other which may harm other components. The radiation shield 108 can include a shielding material such as tungsten, aluminum, among others, or a combination thereof. In some cases, a thickness of the shielding can vary across the shield in accordance with requirements of components of the system 100.

[0045] The radiation shield 108 may shield the payload 110 from radiation created by the pressure tank 104 as well as maintain the generated heat at a threshold heat. The shield 108 may be conductive to maintain heat to the power converter 106. The shield 108 may be approximately 3 cm of metal such as Tungsten backed by Aluminum. In the example of Kr-85, the half-life may be 10.73 years with a decay of 100% 100% , end point 687 keV; 0.434% coincident y at 0.514 MeV. The specific power may be 0.581 Wth / g, mass needed for 7 kWth, at 10 yr: 22.95 kg.

[0046] Betas all shielded by PV, dose all comes from y|0048[ dose(t, d, 1) = 4>(t, d) *Yunshielded(1) * aSi, aSifrom CEPXSYo

[0049] By maintaining the noble gas at a threshold pressure, the noble gas can offput heat. In some cases, this heat can be dissipated, routed, or otherwise maintained by the radiator 102 for various components of the system 100 and / or a vehicle containing the system 100. In some cases, the radiator 102 includes fins, tubing, heat pipes, fluid loops, or conductive materials to maintain, dissipate, and route the heat generated by the pressurized noble gas.|0050| The radiator 102 can maintain a temperature of various components of the system 100 or a vessel including the system 100 or subsystems thereof. For example, the radiator 102 can maintain the pressure tank 104 at the threshold temperature or within a threshold temperature. In some cases, the radiator 102 can adjust the threshold temperature 102.

[0051] In some cases, the radiator 102 can improve or maximize a temperature differential. For example, the radiator 102 can generate a temperature differential across the power converter 106.|0052] By developing a system that includes an efficient and safe noble-gas power system, the larger system’s apparatus and structure can deviate from existing VLEO devices in ways not permitted with known power systems. In some instances, these noble-gas power systems can be micro power systems. Selection between Kr-85 and Ar-39, or other noble gasses, can depend on the size of the satellite to ensure the lifetime requirements are met. Kr-85 is more thermally dense than Ar-39 (0.58 vs 0.04 Wth / kg) but has a comparatively shorter half-life (11 years vs 269 years). Second, the converter 106 may include a conversion system such as a Stirling engine, Brayton engine, Strayton, photovoltaic, and / or thermophotovoltaic, for thermal to electrical conversion at higher efficiencies than traditional thermoelectric approaches.

[0053] As explained, the power converter 106 may be a Stirling power converter, in one example. The power converter 106 may also be thermoelectric, thermoradiative / photovoltaics (to prevent radiation losses), and / or achieve certain Stayton or Brayton cycles. Stirling power converters may be used due to the space requirements and efficiencies. Other converter variables may include netoutput power, hot side temperature and cold side temperature. Due to the type of engine or converter, thermal management variables may also be considered, such as the radiator area, drag vs. RPS power output, and whole body vs. isolated wings. In one example, a SUNPOWER Stirling converter may be used.

[0054] The payload 108 may be approximately 2.1m away from the PV centerline in one example. The dose survival requirements may be 100 krad(Si). The expected electronics dose over the mission life, 65.8 krad(Si), is less than 100 krad(Si).10055] The power system 100 may include a thruster 112 at one end of the enclosure 102 and an air scoop 114 at the opposite end of the thruster 112. The thruster 112 may be a radioisotope powered Hall thruster, among others. The air scoop 114 is described in more detail herein and is configured to capture ambient air, and in most cases atomic oxygen (AO). As explained herein, AO may be the dominate species for a large range of altitudes and is a major constituent across VLEOs. AO can be reactive and may degrade certain polymers and metals, as well as some ceramics. The air scoop 114 may receive air particles or atoms and prevent such atoms from scattering out of the air scoop to prevent unnecessary damage. The air scoop 114 is discussed in more detail with respect to FIGs. 4-6.

[0056] FIG. 2 illustrates an example noble gas radioisotope power system 103 including the pressure tank 104 arranged between a pair of power converters 106, similar to the arrangement of FIG. 1.10057] FIG. 3 illustrates a block diagram of a power conversion process 300 for the power system 100 of FIG. 1. The process 300 may begin at block 302 where a noble gas is selected to pressurize the pressure tank 104. At block 304 the tank 104 may pressurize and generate heat. The converter 106 may receive the heat at block 306 (hot side heat transfer) and may convert the heat to electricity at block 308. In one example, the converter 106 may transfer more than lOkW of heat. Concurrently, the materials selected may allow for thermal management of the generated heat at block 310 and the enclosure 102 may function as a radiator or heat sink at block 312. This may allow for the cold side heat transfer. The enclosure may allow for emissivity of 0.9, in one example, and may operate at approximately 1 0-200 degrees Celsius.10058] Due to the large amount of heat generated, the enclosure 102, as well as the other components of the system 100, may efficiently control the delta temperature and may combined multiple approaches for doing so. In some cases, for certain converters, the maximum temperature difference across the converter may be:

[0059] Although not shown, the power system 100 may allow for bypass heat transfer via a bypass active thermal system (BATS). The bypass system may also allow for excess power if the converter 106 is not running. It is appreciated that the system 100 may limit the maximum temperature of the pressure vessel, as well as limit the maximum temperature of any electronics / payloads.

[0060] This system can be easily scalable by increasing the quantity of radioisotope and power production of the RPS to match a combination of the electrical power needs required for an efficient noble-gas-based power supply. System sizing analysis demonstrates that such a concept can be viable in a 100-450km altitude A key benefit of the power system 100 is that solar panels are not required, which would significantly reduce drag by at least fifty percent (50%). Table 1 below demonstrates potential energy surges the power system could experience if supplemental batteries were used. In some instances, the RPS can include one or more battery packs, such as Li- ion battery packs, that can be used to increase the output of the RPS.Table 11'00611 FIG. 4 illustrates a side view of an example air scoop 114. FIG. 5 illustrates a top view of the air scoop 114. The example air scoop 114 may be formed of a plurality of slabs 130, and inthis example, generally triangular slabs attached to adjacent slabs to form the air scoop 114. In some cases, the air scoop 114 can be conical, nearly-conical (e.g., having sides arranged in a conical manner), among other shapes and have an inner surface on the inside of the cone, and an outer surface formed on the outside of the cone. Generally, for VLEO systems, the ram surfaces of the vehicle or apparatus powered by the power system 100 will face atomic oxygen (AO) fluxes on the order of at least 1015- 1016cmV. In some instances, this vehicle or apparatus has an outer layer such as an enclosure, or as shown in FIG. 1, a superstructure / radiator. In the system 100 of FIG. 1, the outer layer includes the air scoop 114. In other instances, voltages or electronics could be embedded into the outer layer provided that the components are comprised of materials that meet the material requirements needed to eliminate drag and provide heat sinking capabilities. This outer layer may experience energy collisions in the range of 3-10 eV which can readily lead to damage of the vehicle or apparatus via both oxidation and mechanical erosion. Thus, the ram surfaces of these outer layers, including the outer surface of the slabs 130, should be resistant to both oxidation (e.g., atomic oxygen erosion) and etching via atomic impact. These AO resistant materials allow for selective AO scattering. In some cases, it can be desirable to specularly scatter the AO atoms. Specular scattering results in minimal momentum transfer between incoming gas atoms and surfaces or surface coating, both reducing aerodynamic drag and improving the ability of geometrically shaped inlets to collect gases in, for example, an instrument or receptacle. These materials are also useful for reducing the aerodynamic drag on LEO and VLEO craft through application to outer craft hull surfaces.

[0062] Such surface materials that have been applied to outer layers for AO resistance include polyimides, FEP Teflon, noble metals (Au, Pd, Pt), metal oxides (TiO2, SnO2, AI2O3), and SiO2. The oxide coatings can provide resistance to ablation as well as oxidation (being already oxidized) but thermal cycling can lead to the formation of defects in the coating, leaving the underlying materials unprotected as well as increasing drag on the outer surface. This can be addressed by having two AO resistant coatings on the outer layer - the first may be less AO-resistant, or induce more drag but be immune to thermal-cycle induced defects, and the second a more AO-resistant and lower-drag oxide. The first coating may be a polymeric coating that smooths out surface irregularities and provide significant thermal-cycle resistance, such as FEP Teflon or a polyimide.

[0063] FIG. 6 illustrates a side view of an example surface 152 of the air scoop 114 of FIGs. 4 and 5. In some cases, the surface of the air scoop 114 can be located on the inside of the scoop or within the scoop and exposed to the environment. The surface 152 may also be on the outer surface of the scoop 114 in some examples. The surface 152 can include one or more textured surfaces 152.

[0064] In this example, the surface 152 may be comprised of a textured surface to improve gas collection and may include at least one of a first material 154 and a second material 156. The first material 154 may include the less AO-resistant, diffusely scattering material, while the second material 156 may be a more AO-resistant, specularly AO reflecting. This textured surface may be formed via photolithography to produce regular 1 -dimensional arrays of lines on a photoresist followed by etching of those lines using an anisotropic etch process such as etching oriented Si with potassium hydroxide. In one example, the first material 154 may include silicon dioxide (SiO ). The second material 156 may be gold or highly oriented pyrolytic graphite. The materials may be deposited by means such as but not limited to physical or chemical vapor deposition on an oriented and etched crystalline substrate as described immediately above. Under such processes, the ruling and blaze angle are adjusted by the crystallographic cut and lithographically defined line spacing. When produced as a “master”, such structures may also be rapidly transferred to other materials or even to curved surfaces (such as is done with focusing x-ray gratings) by means of e.g. nanoimprint lithography. The result (after coating and dicing to a specific shape) has the pattern of the “shark-skin” surface.

[0065] Some atoms traveling into and through the scoop 114 may be in an ionic state, and thus a magnetic shield can be used to re-direct these atoms from the sidewalls of the scoop to reduce wear. To enable magnetic field protection of an inlet, the AO-resistant coating must have low magnetic permeability. In some cases, Diamond Like Carbon (DLC) films have been shown to behave ferromagnetically when grown on certain substrates, and may be used to enable magnetic protection of the scoop. Once gas atoms have proceeded deeper into the air-scoop, specular reflection of gaseous species may no longer be desired and instead diffuse reflection may be desired to aid in gas concentration. This diffuse scatter can be accomplished either by determining and employing coatings with worse PES, by increasing the surface roughness, or both. Nanoporousdealloyed Au can provide highly tunable surface roughness and could be controlled spatially across a part. Given AO is highly AO resistant, dealloyed Au can enable spatially tuned specular reflection (including of AO) based on micro-topography such as ligand length and porosity.

[0066] Highly-oriented pyrolytic graphite (HOPG) has been shown to dramatically outperform equally flat Au and flatter SiO? for hyperthermal scattering AO and O2 in a concentrator geometry. This is attributed to the ‘roughness’ of the potential energy surface (PES) of the material, with covalently bonded HOPG having a much smoother PES interaction potential than Au or SiO2. DLC may have a more promising PES for hyperthermal scattering of AO than the traditional metal or semiconductor oxides employed for AO resistance, and its AO-resistance has been improved by doping. The design presented to enable hypothermal scattering of AO may be applied to additional AO-resistant materials, such as hydrogenated Ti-doped DLC, Pt, AhO3,and HfO2 to probe their PES roughness in AO and N2 interactions.

[0067] Referring still to FIG. 6, the surface 152 may form an array of angled-tooth structures in which lines of a saw-tooth structure are formed on a surface. In some cases, the texted surfaces 152 can include one or more angled-teeth, or other such protrusions / inclusions arranged in an array or pattern on one or more of the slabs 130. This may be referred to as “shark skin” and having “teeth.” The surface 152 may receive incoming forward scatter 160 from the environment. Upon abutting the surface 152, the incoming gas atoms may ionize, with the ions forward scattering 162 and non-ionized atoms 164 may stay within the scoop. One side of the tooth may include the first material 154, while the other side may include the second material 156. The first material 154 may be, as explained, an insulator, while the second material 156 may be conductive to enable in ion funneling and ionization of incoming neutral gas atoms. The surface 152 may be configured, therefor, to scatter a first set of particles in a first direction and a second set of particles in a second direction. The first coating or material 154 specularly reflects the gas in the first direction towards an apex of the inlet and the second coating or material 156 diffusely scatters the gas in a second direction towards the outer surface.

[0068] The back-side or outer surface 158 of the scoop 114 may include a wiring layer on which components such as resistors and capacitors may be arranged, or directly fabricated, for ease of operation and assembly of the scoop 114.

[0069] Minimizing drag at hypersonic velocities can include providing an effective air scoop 114. In some instances, providing an effective air scoop relies on minimizing inelastic scattering of incident gases in the concentrator. Combined low-drag and AO resistance can be approached by an AO-resistant surface coating that is extremely flat, however flatness is not the sole predictor of inelastic scattering. In some instances, thermoelectric cooling can be used to cool the air scoop as a means to increase the scoop efficiency. For example, a thermoelectric cooler that utilizes the Peltier effect to create a heat flux can be used to reduce the temperature of the air scoop thereby minimizing the inelastic scattering of incident gases. Modeling efforts have also demonstrated that controlling surface temperature can limit AO-induced material erosion, and thus a Peltier cooler can also be employed to further diminish AO erosion.

[0070] Additional efficiencies can be obtained by using the pressure vessel 104 that has a shape configured to reduce drag. For example, a spherical pressure vessel can reduce drag more effectively than pressure vessels having a different shape.

[0071] Heat transfer needs of the vehicle or apparatus’ structure can vary with position. For instance, maintaining high temperatures at the hot side of the Stirling generator is crucial, as is heat rejection and a low cold-side temperature, using electron-beam melting (EBM) additive manufacturing to produce complex geometries out of refractory alloys (including molybdenum Tm=2626°C and hastalloy Tm=2300-2470 °C) for nuclear power applications. Additive manufacturing of the vehicle or apparatus’ frame and mechanical components out of advanced materials opens the door to architecturally controlled heat management, such as in-built heat pipes with flow paths unattainable by traditional methods and optimized hollow structures near hot zones to mitigate losses. Heat rejection to the ambient can be enhanced by control over the surface coatings discussed above. A nanoporous AO-resistant oxide coating can be made via dealloying followed by thermal oxidation to provide extremely high specific surface area to enhance thermal emission.

[0072] Thermal power generated by the RPS needs to be rejected via radiation to space. Kr-85 requires a higher radiator temperature, because of the shorter half-life. Thermal challenge may be severe at lower altitudes. It may be possible to alleviate this by isolating the radiator surface from the rest of the vehicle or apparatus (in some instances this can be a spacecraft). Initial calculationssuggest that operation up to 225 km altitude can be possible but for higher altitudes, powered compression may be required, increasing the system complexity. The surface of the vehicle or apparatus needs to have very low drag and protection against AO to enable long duration missions. AO degradation and scattering should ideally be assessed to determine the impact, longevity, and feasibility of the magnetic shield / electrodes on intake efficiency. EBM could also facilitate scaled models for thermal characterization identified solutions for model validation. Table 2 illustrates an example proposed solution for using Ar-39 in the power system 100.

[0073] FIGs. 7A-7D illustrates various examples of the surface 152 having an angled-tooth structure. FIG. 7A illustrates the surface 152 that enables specular reflection of the incoming atoms. FIG. 7B illustrates the surface 152 having a mask, for example, made of Aluminum. FIG. 7B illustrates the surface 152 having a diffusely scattering material. FIG. 7D illustrates the surface 152 having a strip mask such as a wet etch. Masking or liftoff techniques may enable less direction material deposition techniques like sputtering and chemical vapor deposition (CVD).

[0074] While the surface 152 is described herein as being arranged on the inside of the scoop 114, the surface 152 may also be applied to the outside of the scoop. Additionally or alternatively, the surface 152 may be applied outside of the system or on other components to aid in reducing drag as well as protecting against AO.

[0075] FIG. 8 illustrates a side view of an example flat surface 152 of the air scoop 114. In this example surface 152, the surface 152 may include a plurality of etch layers. These etch layers may include at least one slow etch layer 166 and at least one fast etch layer 168. In the example shown in FIG. 6, a single fast etch layer 168 is arranged between two slow etch layers 166. However, this is for example and explanatory purposes. The number of layers may be determined or selected to survive a full mission life. In one example, the number of layers may exceed several hundred layers. The layers should be prepared to cover as large of an area as possible. In instances wherethe underlaying surfaces are curved or, smaller sections may be coated individually and tiled to produce the final geometry.|0076| During mission, the wear of the surface 152 may degrade and create a roughness to the exposed layer. The fast etch layer 168 may also degrade as the upper slow etch layer 168 roughens and act as a release layer that will release the upper slow etch layer 168 to maintain a planar surface. This is described in more detail with respect to FIG. 9.

[0077] The ratio of fast etch layers to slow etch layers may also vary. For instance, the surface layer may be the slow etch layer 166 for more than 95% of the mission life. The layer thicknesses of any layer that does not form volatile reaction products with atomic oxygen (e.g., Au, SiO2, other oxides or metals) should be limited to less than 100 micrometers to minimize size of solid space debris that is released. The thickness of the slow etch layer 166 may be based on the maximum roughness. In one example, the thickness may be five micrometers. The thickness of the fast etch layer 168 may be large enough to allow complete removal of the remaining slow-etch material and thin enough to be etched completely during use. Materials should also be selected for their coefficient of thermal expansion (CTE) and maximum temperature, as required by the operation and / or adjacent engineering materials. Notably, the surface 152 may create an ultra-flat layer of material (such as graphene, goldene, MXenes, etc.) that enhances specular reflectivity when paired with a high-z or low PES-roughness material.[0078| FIGs. 9A-F illustrate a progression of the surface 152 having multiple layers through a mission. As illustrated in FIG. 9A, a fast etch layer 168 may be arranged between an upper slow etch layer 166a and a lower slow etch layer 166b. The upper slow etch layer 166a is exposed to the atmosphere. As gas comes into contact with the surface 152, the upper slow etch layer 166a begins to roughen, as illustrated in FIGs. 9B and 9C. Eventually, the slow etch layer 166a roughens or erodes enough to expose a portion of the fast etch layer 168, which degrades at a significantly higher rate than that of the slow etch layers 166. FIG. 9E illustrates the complete etching of the fast etch layer 168 and the fall of the upper slow etch layer 166a to the underlying slow etch layer 166b. FIG. 9F illustrates the complete cycle where the upper slow etch layer 166a is absorbed by the lower slow etch layer 166b, or completely eroded, returning the surface 152 to a flat surface.

[0079] Accordingly, a highly efficient power system configured to operate in various environments where solar power may be challenging such as underwater, LEO, or VLEO is described herein.

[0080] Computing devices described herein generally include computer-executable instructions, where the instructions may be executable by one or more computing devices such as those listed above. Computer-executable instructions may be compiled or interpreted from computer programs created using a variety of programming languages and / or technologies, including, without limitation, and either alone or in combination, Java™, C, C++, C#, Visual Basic, Java Script, Perl, etc. In general, a processor (e.g., a microprocessor) receives instructions, e.g., from a memory, a computer-readable medium, etc., and executes these instructions, thereby performing one or more processes, including one or more of the processes described herein. Such instructions and other data may be stored and transmitted using a variety of computer-readable media.

[0081] While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.

Claims

WHAT IS CLAIMED IS:

1. A system to generate power in an environment, comprising: an enclosure configured to maintain a noble gas radioisotope at a threshold pressure, wherein the noble gas radioisotope generates heat at the threshold pressure; a radiator configured to maintain the generated heat at a threshold heat; and a power converter configured to convert the maintained heat to electric power.

2. The system of claim 1, wherein the noble gas radioisotope is one of Kr-85, Ar-39, Ar-42, or Xe-127.

3. The system of claim 1, wherein the enclosure comprises the radiator.

4. The system of claim 1, wherein the enclosure comprises shielding from the noble gas radioisotope.

5. The system of claim 1, wherein the converter is a Stirling engine or a Brayton engine.

6. The system of claim 1, wherein the environment comprises a threshold percentage of an element of the noble gas radioisotope.7 The system of claim 1, wherein the environment is one of a VLEO environment, LEO environment, or undersea environment.

8. The system of claim 1, wherein the system is operable at an altitude of 100- 450km.

9. An inlet for passive collection of ambient gas in a low orbit environment, comprising: a plurality of surfaces forming an air scoop for receiving a gas comprising atomic oxygen (AO), wherein the surfaces each include an outer surface configured to scatter a first set of particles of the gas in a first direction and a second set of particles of the gas in a second direction.

10. The inlet of claim 9, wherein the outer surface includes a first coating and a second coating, the first coating having a higher AO resistance and a lower drag resistance than the second coating.

11. The inlet of claim 10, wherein the first coating specularly reflects the gas in the first direction towards an apex of the inlet and the second coating diffusely scatters the gas in a second direction towards the outer surface.

12. The inlet of claim 10, wherein the outer surface comprises an array of angled-teeth structure, each tooth of the array forming a first side for the first coating and a second side for the second coating.

13. The inlet of claim 9, wherein each surface forms a triangular shape configured to be coupled to an adjacent surface to collectively form the inlet and defining an opening at an apex thereof to receive at least a portion of the gas.

14. The inlet of claim 13, wherein each surface includes an inner surface including at least one trace to one or more electrodes.

13. The inlet of claim 9, wherein the outer surface includes at least one slow etch layer and one fast etch layer.

14. The inlet of claim 13, wherein the at least one slow etch layer includes two slow etch layers and the at least one fast etch layer is arranged between the two slow etch layers.

17. The inlet of claim 13, wherein the at least one slow etch layer is exposed to the AO during the life of the outer surface more than the at least one fast layer.

18. The inlet of claim 7, wherein during the life of the outer surface, the at least one fast etch layer etches completely upon at least partial etching of the at least one slow etch layer coupled to the at least one fast etch layer to resettle the outer surface to a planar surface.

19. An inlet for passive collection of ambient gas in a low orbit environment, comprising: an inlet forming an air scoop for receiving gas from a low orbit environment, wherein the air scoop includes an outer surface resistant to atomic oxygen (AO) erosion and configured to scatter particles of the AO and to collect the gas.

20. The inlet of claim 19, wherein the outer surface forms an angled-tooth structure, each tooth forming a first side for receiving a first coating and a second side for a second coating, and wherein the outer surface includes at least one slow etch layer and one fast etch layer, wherein the at least one slow etch layer is exposed to the AO during the life of the outer surface more than the at least one fast layer.

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