System and method for densifying liquid oxygen

Abstract

A system and method for preparing and supplying a densified liquid oxidant to a spacecraft launch facility is provided. Liquid oxygen streams obtained from co-located air separation devices for liquid preparation are densified in an integrated densification system of two refrigeration stages. The first refrigeration stage is a nitrogen-based inverse Brayton cycle refrigeration cycle that provides refrigeration to the second refrigeration stage. The second refrigeration stage is a refrigerant circuit comprising helium and / or neon that densifies the liquid oxygen to a temperature between about 70 Kelvin and 57 Kelvin. The integrated densification system may also be configured to densify liquid methane or other propellants used in a spacecraft.

Description

Technical Field

[0001] The present invention relates to a system and method for liquefying a liquid oxygen stream for use as an oxidizer for a space vehicle, and more particularly, to a two-stage integrated densification system configured to densify liquid oxygen to a temperature as low as between about 70 Kelvin and 57 Kelvin. The first refrigeration stage is a nitrogen-based reverse Brayton cycle refrigeration cycle configured to provide refrigeration to a second refrigeration stage based on helium and / or neon. Background Art

[0002] Space vehicle launches require large amounts of liquid fuel and liquid oxygen to generate the propulsion force needed to get the vehicle into space. Minimizing the volume and weight of the fuel / oxygen tanks is crucial for the feasibility of rocket design. It is desirable to pack all the required fuel into the smallest possible volume. Typically, cryogenic liquids are prepared and stored in tanks, maintaining equilibrium with the vapor at the liquid boiling point temperature. Supercooling the liquid below its boiling point produces a denser fluid, which requires a smaller tank to hold the same mass or equivalent volume. The limit of the densification level is solidification at the triple point of the material.

[0003] For decades, cryogenic refrigeration systems have been used in many rocket or space applications for purposes such as propellant or oxidizer densification. For propellant or oxidizer densification, removing sensible heat from a liquid propellant or oxidizer (such as liquid oxygen) increases the density of the liquid, which depends entirely on temperature since liquids are generally considered incompressible. Challenges associated with propellant densification faced by the space industry include reducing the operating and capital costs of propellant densification and reducing the time taken to reach the desired temperature.

[0004] Supplying liquid oxygen to a launch platform at a launch facility may require transporting more than 40 liquid oxygen trailer trucks into the launch facility, and then densifying the liquid oxygen at the launch facility, and a similar or even greater number of nitrogen-based refrigerant truck loads are required for such densification. Compared to liquid oxygen trailers, current oxidizer densification typically requires more liquid nitrogen trailers to be transported by truck from an external source to densify the liquid oxygen, which increases the overall logistics burden of transporting liquid refrigerant trucks to the launch facility. Liquid nitrogen densification refrigerant is only used for cooling and is thus considered a consumable in the process. Embodiments of cryogenic-based liquid oxygen densification systems starting with a liquid oxygen stream are shown and described in U.S. Patents 10,808,967 and 11,293,671.

[0005] Such existing systems require the preparation of liquid oxygen at an air separation unit and the transportation of the liquid oxygen by truck to the launch facility. Similarly, gaseous nitrogen is liquefied at the air separation unit, and the resulting liquid nitrogen is also transported by truck to the launch facility. At the launch facility, part of the trucked-in liquid nitrogen is used to densify the trucked-in liquid oxygen. The ratio of liquid nitrogen to liquid oxygen to be densified used at the launch site is approximately 2:1. This supply arrangement faces two challenges, namely the transportation challenge of transporting large amounts of liquid oxygen and even larger amounts of liquid nitrogen from the air separation unit to the launch facility, and the resulting waste nitrogen that is likely to be vented to the atmosphere at the launch facility.

[0006] Alternatively, gaseous oxygen and gaseous nitrogen can be transported from the air separation unit to the launch facility, where the gaseous oxygen and gaseous nitrogen are liquefied and then the liquid oxygen is densified. An embodiment of a liquid oxygen densification system starting with a gaseous oxygen stream is disclosed in U.S. Provisional Patent Application 63 / 478,547, filed on January 5, 2023, the disclosure of which is incorporated herein by reference. While this supply arrangement represents an improvement over conventional launch site supply arrangements, it also faces logistics challenges, namely the construction, maintenance, and operation of the required pipeline.

[0007] However, to further improve launch operations and further reduce operating costs, there is a need to improve the existing supply model, particularly to reduce or eliminate the transportation of liquid nitrogen to the launch facility by truck. Accordingly, there is also a need to continue to develop an improved refrigeration cycle for densifying liquid oxygen at a location with an adequate supply of liquid nitrogen outside the launch facility and then transporting the densified liquid oxygen to the launch platform at the launch facility. SUMMARY OF THE INVENTION

[0008] The present invention features a system for preparing a densified liquid oxygen stream from a liquid oxygen stream. The system includes: (i) a first refrigeration stage configured to receive a nitrogen refrigerant and pass the nitrogen refrigerant through at least one first heat exchanger; and (ii) a second refrigeration stage configured to pass a second refrigerant containing helium or neon through a second heat exchanger, the second heat exchanger being configured to cool the second refrigerant containing helium or neon via indirect heat exchange with one or more streams of the nitrogen refrigerant. The second refrigeration stage is further configured to pass the second refrigerant containing helium or neon through a densification heat exchanger to subcool and densify the liquid oxygen stream via indirect heat exchange with the cooled second refrigerant containing helium or neon. The first refrigeration stage further includes at least four circuits, such as a first warm refrigeration circuit, a second cold refrigeration circuit, a residual refrigeration circuit, and one or more recirculation circuits. Within the first refrigeration stage, the nitrogen refrigerant passing through the first heat exchanger is divided into a first warm portion in the first warm refrigeration circuit, a second cold portion in the second cold refrigeration circuit, and a residual portion of the nitrogen refrigerant stream in the residual refrigeration circuit. Within the first warm refrigeration circuit, there is a warm turbine configured to expand the first warm portion of the nitrogen refrigerant stream to produce a medium-pressure warm exhaust; and within the second cold refrigeration circuit, there is a cold turbine configured to expand the second cold portion of the nitrogen refrigerant stream to produce a medium-pressure cold exhaust. The residual portion of the nitrogen refrigerant stream is also expanded to produce an expansion residual stream. At least a portion of the expansion residual stream, the warm exhaust, and the cold exhaust are recycled through a heat exchanger in one or more recirculation circuits to cool the nitrogen refrigerant stream, and the recirculated stream is compressed in one or more nitrogen recirculation compressors.

[0009] The present invention also features a method for supplying a densified liquid oxygen stream for a spacecraft launch, the method including the steps of: (i) preparing a liquid oxygen stream and a nitrogen refrigerant from an air separation unit; (ii) cooling the nitrogen refrigerant in a first refrigeration stage including a reverse Brayton refrigeration cycle and a first heat exchanger; (iii) cooling a second refrigerant containing helium or neon via indirect heat exchange with one or more streams of the nitrogen refrigerant in a second heat exchanger of a second refrigeration stage; (iv) subcooling and densifying the liquid oxygen stream via indirect heat exchange with the second refrigerant in the densification heat exchanger in the second refrigeration stage to produce a densified liquid oxygen stream; and (v) delivering the densified liquid oxygen stream to one or more launch platforms provided at a launch facility.

[0010] As described above, the first refrigeration stage is preferably a reverse Brayton cycle refrigeration stage, and step (ii) of the above method further includes the following steps: (a) dividing the nitrogen refrigerant into a first warm portion, a second cold portion, and a residual portion of the nitrogen refrigerant stream; (b) expanding the first warm portion of the nitrogen refrigerant stream in a warm turbine to produce a medium-pressure warm exhaust; (c) expanding the second cold portion of the nitrogen refrigerant stream in a cold turbine to produce a medium-pressure cold exhaust; (d) expanding the residual portion of the nitrogen refrigerant stream to produce an expansion residual stream; (e) recirculating at least a portion of the expansion residual stream, the warm exhaust, and the cold exhaust in one or more recirculation loops to cool the nitrogen refrigerant stream; and (f) compressing the recirculated warm exhaust, the recirculated cold exhaust, and the recirculated expansion residual stream in one or more nitrogen refrigerant cycle compressors. In many embodiments, one of the one or more streams of nitrogen refrigerant flowing through the second heat exchanger includes a split portion of the cold exhaust.

[0011] In a preferred embodiment, the second refrigerant includes a helium refrigerant, a neon refrigerant, or some combination of a helium and neon refrigerant with possibly small amounts of other gases (such as nitrogen and oxygen), which subcools the liquid oxygen stream through the second heat exchanger and produces a densified liquid oxygen stream. The system and method may also include a third refrigeration stage, which includes a third heat exchanger configured to densify a liquid methane stream to produce a densified liquid methane stream via indirect heat exchange with a split portion of the nitrogen refrigerant stream. Finally, in a preferred system for preparing a densified liquid oxygen stream, the first refrigeration stage and / or the second refrigeration stage may be disposed on a movable platform or skid and transported as needed to support the launch of a space vehicle at different locations.

[0012] In some other preferred embodiments, the expansion residual stream may be divided into a first expansion residual stream and a second expansion residual stream. Then, the first expansion residual stream is further expanded and then recirculated via the first heat exchanger as a low-pressure return stream to cool the first refrigerant stream. In such embodiments, the second expansion residual stream is involved in the second refrigeration stage as one of the one or more streams of nitrogen refrigerant flowing through the second heat exchanger to cool the second refrigerant containing helium or neon. Both the heated low-pressure return stream and the other heated nitrogen refrigerant streams flowing through the second heat exchanger return to the first refrigeration stage and are compressed in one or more first refrigerant recirculation compressors. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] It is believed that the systems and methods claimed will be better understood when considered in conjunction with the accompanying drawings, in which Figure 1 A schematic diagram of one embodiment of a system and method for densifying a liquid oxygen stream is shown. DETAILED DESCRIPTION

[0014] Steering Figure 1 The exemplary system 525 and associated method provide for the densification of a liquid oxygen stream. The exemplary system and method utilize two refrigeration stages, including a first refrigeration stage and a second refrigeration stage. The first refrigeration stage is preferably a nitrogen-based refrigerant arrangement, such as a reverse Brayton cycle refrigeration cycle, which provides refrigeration to the second refrigeration stage. The second refrigeration stage is a refrigerant circuit containing helium or neon, which contains a certain combination of helium and / or neon refrigerant and possibly a small amount of other gases (such as nitrogen and oxygen), and is used to further subcool and densify the liquid oxygen stream 540 to produce a densified liquid oxygen stream 545 having a temperature between about 70 Kelvin and 57 Kelvin. The liquid oxygen stream 540 is preferably received from a liquid oxygen storage tank (not shown) associated with an air separation unit for producing liquid (not shown) or can be obtained directly from an air separation unit for producing liquid.

[0015] The first refrigeration stage uses a high-pressure nitrogen refrigerant stream 555 at a pressure of about 50 bar(a), which is cooled by a first heat exchanger 550 that can be divided into a plurality of heat exchange sections E1, E2, E3. To cool the high-pressure nitrogen refrigerant stream 555, the first heat exchanger 550 is configured to include a first warm refrigeration circuit 552, a second cold refrigeration circuit 554, a residual refrigeration circuit 556, and one or more recirculation circuits 558, 559.

[0016] The nitrogen refrigerant stream 555 is referred to as the first refrigerant stream and is preferably divided into three parts when passing through the first heat exchanger 550, namely, a first warm part 562 of the first refrigerant stream passing through the first warm refrigeration circuit 552, a second cold part 564 of the first refrigerant stream passing through the second cold refrigeration circuit 554, and a residual part 566 of the first refrigerant stream passing through the residual refrigeration circuit 556. When leaving the first warm refrigeration circuit, the second cold refrigeration circuit, or the residual refrigeration circuit, the corresponding parts of the first refrigerant stream are recirculated in one or more recirculation circuits 558, 559.

[0017] The first refrigeration stage also preferably includes: a warm turbine 563 configured to expand the first warm part 562 of the first refrigerant stream to produce a medium-pressure warm exhaust 572 at a pressure of about 6 bar(a); and a cold turbine 565 configured to expand the second cold part 564 of the first refrigerant stream to produce a medium-pressure cold exhaust 574 also at a pressure of about 6 bar(a). The warm exhaust 572 and the cold exhaust 574 are recirculated in one or more recirculation circuits 558 to cool the nitrogen refrigerant stream 555.

[0018] More specifically, the medium-pressure cold exhaust gas 574 is first mixed with the medium-pressure recirculation flow 577 and then fed to the cold end of the exchanger section E3, where the mixed flow (together with the cold exhaust gas) is heated to provide refrigeration for cooling the nitrogen refrigeration flow 555. Similarly, the medium-pressure warm exhaust gas 572 is also first mixed with the medium-pressure recirculation flow 578 and then fed to the cold end of the exchanger section E2, where this mixed flow also provides refrigeration for cooling the nitrogen refrigeration flow 555. The mixed medium-pressure recirculation flow 579 (including cold and warm exhaust gases) is further heated in the heat exchange section E1 to produce a fully heated medium-pressure nitrogen recirculation flow 581, which is then compressed in one or more recirculation compressors 695.

[0019] The remaining portion 566 of the first refrigerant flow is further cooled in the heat exchange section E3 of the first heat exchanger and then preferably expanded using one or more expansion valves (including the first expansion valve 575). Before expansion, the remaining portion 566 of the high-pressure first refrigerant flow is at a pressure above its critical point such that little phase change occurs. However, once the remaining portion 566 of the first refrigerant flow expands or "flashes" to medium pressure, it can exist in a liquid or two-phase state. As described above, pseudo-liquefaction is advantageous.

[0020] Then, the medium-pressure liquid nitrogen flow 576 is subcooled in the nitrogen subcooler E4. The first portion 582 of the subcooled medium-pressure liquid nitrogen is further expanded or "flashed" in the expansion valve 585 to form a low-pressure nitrogen refrigerant flow 583 at a pressure of about 1.5 bar (a), which is used to subcool the medium-pressure liquid nitrogen 576 in the nitrogen subcooler E4. The currently boiled low-pressure nitrogen refrigerant flow 583 is recycled through the heat exchange sections E3, E2, and E1 via the recirculation loop 559, where it is further heated to about ambient temperature. Upon leaving the heat exchange section, the heated low-pressure return flow 586 is compressed in the multi-stage feed compressor 685 and mixed with the fully heated medium-pressure nitrogen recirculation flow 581. The mixed recirculation flow 587 is further compressed in the multi-stage recirculation compressor 695 to form the high-pressure nitrogen refrigerant flow 555. Make-up nitrogen 599 can be supplied at different points in the process depending on the pressure and phase of the nitrogen source.

[0021] The liquid oxygen flow 540, preferably obtained from a storage tank or directly from an air separation unit, enters the densification system at a relatively low pressure, which will allow its boiling point to match well with the condensation of the medium-pressure liquid nitrogen 576. In the current embodiment, the liquid oxygen flow 540 is at a pressure between about 1.3 bar (a) and 25.0 bar (a), and more preferably between 1.3 bar (a) and about 3.0 bar (a), and more preferably at about 2.3 bar (a). The liquid oxygen flow 540 may be at a temperature of about 81 Kelvin.

[0022] The liquid oxygen stream 540 is subcooled in the densification heat exchanger E6 and is thus densified to a temperature of from about 70 Kelvin to 57 Kelvin. A second refrigerant stream 595 containing helium, neon, or a combination of helium and neon is cooled. The second refrigerant stream 595 may also include small amounts of other gases such as nitrogen and oxygen.

[0023] The densification heat exchanger E6 and the second refrigerant stream 595 form part of a second refrigeration stage. As shown, the second refrigeration stage is configured to pass a second refrigerant stream 595 containing neon and / or helium through a second heat exchanger 590 and a densification heat exchanger E6. The second refrigeration stage further includes: a second refrigerant recycle compressor 591, which is disposed downstream of the second heat exchanger 590 and is configured to compress a second refrigerant feed 592 to produce a compressed second refrigerant 593; and a aftercooler 598, which is configured to cool the compressed second refrigerant 593 to ambient temperature.

[0024] The second refrigeration stage further includes a second refrigerant turbine 596 disposed downstream of the second heat exchanger 590 and upstream of the densification heat exchanger E6. The second refrigerant turbine 596 is configured to expand the compressed second refrigerant 593 to produce an exhaust stream 597 that provides the refrigeration required to further subcool the liquid oxygen stream 540 in the densification heat exchanger E6 and to produce a densified liquid oxygen stream 545. A portion of the partially warmed second refrigerant stream 595 exiting the densification heat exchanger E6 is further warmed in the second heat exchanger 590 to produce a second refrigerant feed 592 to the second refrigerant recycle compressor 591. The partially warmed second refrigerant may be further warmed to ambient temperature or maintained slightly below ambient temperature to take advantage of power savings in cold compression in the second refrigerant recycle compressor 591. The method will have a make-up system on site to address potential refrigerant losses over time and / or to utilize liquid nitrogen from storage tanks for liquid densification assistance or backup.

[0025] Figure 1 The depicted embodiment optionally further includes an integrated configuration located between the first and second refrigeration stages. As shown, a split portion 674 of the medium pressure cold exhaust 574 is directed to an auxiliary warming passage 692 in the second heat exchanger 590 where it further cools the compressed second refrigerant stream 595 containing helium or neon via indirect heat exchange. Additionally, a split portion 868 of the medium pressure stream 668 is also directed to an auxiliary warming passage 892 in the second heat exchanger 590 where it also cools the compressed second refrigerant stream 595 containing helium or neon via indirect heat exchange. The warmed split stream 876 then mixes with the warmed split stream 676 and the mixed stream 976 is recycled back to the recycle compressor 695 where it recombines with the mixed recycle stream 587 and is further compressed in the multi-stage recycle compressor 695 to form a high pressure nitrogen refrigerant stream 555.

[0026] Figure 1 The illustrated embodiment also optionally includes an integrated liquid methane densification system 400. When the integrated liquid methane densification system 400 is used, a residual portion 566 of the high-pressure first refrigerant stream is preferably split into two parts, including a first high-pressure residual portion 666 and a second high-pressure residual portion 766. The first high-pressure residual portion 666 expands or "flashes" in an expansion valve 675 to a pressure of about 6.0 bar (a), while the second high-pressure residual portion 766 "flashes" in a second expansion valve 775 to a pressure of about 15.0 bar (a). The resulting medium-pressure stream 668 is directed to and received by a subcooler E4, where the medium-pressure stream is subcooled, while the resulting medium-pressure stream 768 is diverted to the liquid methane densification system 400. The warmed liquid nitrogen stream 770 exiting the methane subcooler E7 further expands or "flashes" in another expansion valve 875 to produce an expansion stream 884 having a pressure approximately equal to the second portion 584 of the subcooled medium-pressure liquid nitrogen.

[0027] As described above, Figure 1 the illustrated embodiments of the present system and method are particularly useful in the presence of an available supply of liquid oxygen and / or a nitrogen source. Thus, Figure 1 the illustrated embodiments are preferably co-located with an air separation unit for producing liquid.

[0028] Another aspect of the present system and method relates to the turbine configuration in the embodiments. The present system and method can be operated with a conventional configuration, where the warm turbine and the cold turbine are each directly coupled to a single-stage supercharger, preferably a single-stage recirculation compressor 695. The present system and method can also utilize a fully integrated "bridge" machine, which allows the compressor to absorb energy from the warm turbine and the cold turbine while decoupling the speeds of the compressor and the turbine. Linde Inc., a member of the Linde Group of Companies, has also developed an integrated gear machine or a combination of single machines that combines a compression stage with a high-efficiency radial inflow expander having up to four driven pinions in what is referred to as an integrated gear "bridge" machine. Linde's "bridge" machine is typically used in air separation equipment and generally has different frame sizes, such as the frame size that supports the "bull gear" or the drive gear size with a diameter between approximately 90 mm and 180 mm. Design studies have investigated the application of Linde's "bridge" machine to operably couple multiple radial inflow turbines and centrifugal refrigeration compression stages in a liquefaction system. Linde's "bridge" machine is fully encapsulated or integrated with a suitable PLC controller, control valves, safety valves, oil systems, etc., and can be easily equipped with an intercooler and / or a aftercooler. The hardware constraints and limitations of Linde's "bridge" machine are generally a function of the bull gear and drive assembly sizes. The use of the "bridge" type machine provides a very efficient system. When configuring the present system and method to utilize the "bridge" machine arrangement, it may be advantageous to configure the feed compressor 685 as a separate multi-stage centrifuge.

[0029] While the present invention's system and method for liquid oxygen stream densification have been described with reference to a preferred embodiment, it should be understood that various additions, changes, and omissions can be made without departing from the essence and scope of the present system and method set forth in the appended claims.

Claims

1. A system for preparing a dense liquid oxygen stream from a liquid oxygen stream, the system comprising: A first refrigeration stage configured to receive a first refrigerant and flow the first refrigerant through at least one first heat exchanger; And A second refrigeration stage configured to flow a second refrigerant through a second heat exchanger, the second heat exchanger being configured to cool the second refrigerant via indirect heat exchange with one or more streams of the first refrigerant, and being configured to flow the second refrigerant through a densification heat exchanger to subcool and densify the liquid oxygen stream via indirect heat exchange with the second refrigerant; Wherein the first refrigeration stage further comprises: A first warm refrigeration circuit, a second cold refrigeration circuit, a residual refrigeration circuit, and one or more recirculation circuits; Wherein the first refrigerant flowing through the first heat exchanger is divided into a first warm portion of the first refrigerant flow in the first warm refrigeration circuit, a second cold portion of the first refrigerant flow in the second cold refrigeration circuit, and a residual portion of the first refrigerant flow in the residual refrigeration circuit; A warm turbine configured to expand the first warm portion of the first refrigerant flow to produce a medium-pressure warm exhaust; A cold turbine configured to expand the second cold portion of the first refrigerant flow to produce a medium-pressure cold exhaust; An expansion valve for expanding the residual portion of the first refrigerant flow to produce an expansion residual flow; Wherein the warm exhaust, the cold exhaust, and the expansion residual flow are recirculated in the one or more recirculation circuits to cool the first refrigerant flow; and One or more first refrigerant recirculation compressors configured to compress the recirculated warm exhaust, the recirculated cold exhaust, and the recirculated expansion residual flow.

2. The system for preparing a dense liquid oxygen stream according to claim 1, wherein the first refrigerant comprises nitrogen, and the second refrigerant comprises helium or neon or both helium and neon.

3. The system for preparing a dense liquid oxygen stream according to claim 2, wherein the second refrigeration stage is a closed-loop refrigeration stage, and further comprises: A second refrigerant circulation compressor disposed downstream of the second heat exchanger and configured to compress the second refrigerant; And A second refrigerant turbine disposed upstream of the second heat exchanger and configured to expand the compressed second refrigerant.

4. The system for preparing a dense liquid oxygen stream according to claim 1, wherein: The expansion residual flow is divided into a first expansion residual flow and a second expansion residual flow; The first expansion residual flow is further expanded and then recirculated via the first heat exchanger as a low-pressure return flow to cool the first refrigerant flow; and The second expansion residual flow is one of the one or more streams of the first refrigerant flowing through the second heat exchanger to cool the second refrigerant.

5. The system for preparing a dense liquid oxygen stream according to claim 4, wherein the warmed low-pressure return stream is compressed in the one or more first refrigerant cycle compressors.

6. The system for preparing a dense liquid oxygen stream according to claim 4, wherein the warmed second expansion residual stream is recycled to the one or more first refrigerant recycle compressors.

7. The system for preparing a dense liquid oxygen stream according to claim 4, wherein the one or more streams of the first refrigerant flowing through the second heat exchanger further comprise a split portion of the cold exhaust gas.

8. The system for preparing a dense liquid oxygen stream according to claim 7, wherein the warmed split portion of the cold exhaust gas is recycled to the one or more first refrigerant recycle compressors.

9. The system for preparing a dense liquid oxygen stream according to claim 1, wherein the first refrigeration stage or the second refrigeration stage or both the first and second refrigeration stages are disposed on a movable platform.

10. The system for preparing a dense liquid oxygen stream according to claim 1, wherein the liquid oxygen stream is at a pressure between about 1.3 bar (a) and 3.0 bar (a).

11. The system for preparing a dense liquid oxygen stream according to claim 1, wherein the one or more first refrigerant recycle compressors are configured to compress the recycled warm exhaust gas, the recycled cold exhaust gas, and the recycled first refrigerant return stream to a pressure greater than about 50 bar (a).

12. The system for preparing a dense liquid oxygen stream according to claim 1, wherein the medium-pressure warm exhaust gas and the medium-pressure cold exhaust gas are at a pressure between about 5 bar (a) and 10 bar (a).

13. The system for preparing a dense liquid oxygen stream according to claim 12, wherein the pressure of the low-pressure return stream is lower than the pressure of the warm exhaust gas and the cold exhaust gas.

14. The system for preparing a dense liquid oxygen stream according to claim 1, further comprising a third refrigeration stage, the third refrigeration stage including a third heat exchanger configured to densify a liquid methane stream by indirect heat exchange with a split portion of the expansion residual stream to produce a densified liquid methane stream.

15. A method for supplying a densified liquid oxygen stream for a space vehicle launch, the method comprising the steps of: (i) preparing a liquid oxygen stream and a nitrogen refrigerant from an air separation unit; (ii) cooling the nitrogen refrigerant in a first refrigeration stage including a first heat exchanger; (iii) cooling a second refrigerant containing helium or neon in a second heat exchanger of a second refrigeration stage by indirect heat exchange with one or more streams of the nitrogen refrigerant; (iv) subcooling and densifying the liquid oxygen stream by indirect heat exchange with the second refrigerant in a densification heat exchanger in the second refrigeration stage to produce a densified liquid oxygen stream; (v) transporting the densified liquid oxygen stream to one or more launch platforms disposed at a launch facility.

16. The method according to claim 15, wherein the first refrigeration stage is a reverse Brayton cycle refrigeration stage, and wherein step (ii) of the method further comprises the steps of: Dividing the nitrogen refrigerant flowing through the first heat exchanger into a first warm portion of the nitrogen refrigerant stream, a second cold portion of the nitrogen refrigerant stream, and a residual portion of the nitrogen refrigerant stream; Expanding the first warm portion of the nitrogen refrigerant stream in a warm turbine to produce a medium-pressure warm exhaust; Expanding the second cold portion of the nitrogen refrigerant stream in a cold turbine to produce a medium-pressure cold exhaust; Expanding the residual portion of the nitrogen refrigerant stream to produce an expanded residual stream; Recycling at least a portion of the warm exhaust, the cold exhaust, and the expanded residual stream in one or more recirculation loops to cool the nitrogen refrigerant stream; Compressing the recycled warm exhaust, the recycled cold exhaust, and the recycled expanded residual stream in one or more nitrogen refrigerant cycle compressors.

17. The method according to claim 16, wherein the second refrigeration stage is a closed-loop refrigeration stage, and further comprises a second refrigerant recirculation compressor disposed downstream of the second heat exchanger and configured to compress the second refrigerant; and a second refrigerant turbine disposed upstream of the second heat exchanger and configured to expand the compressed second refrigerant.

18. The method according to claim 16, further comprising the steps of: Dividing the expanded residual stream into a first expanded residual stream and a second expanded residual stream; Further expanding the first expanded residual stream to produce a low-pressure return stream; Recycling the low-pressure return stream through the first heat exchanger to cool the nitrogen refrigerant stream; Wherein the second expanded residual stream is one of the one or more streams of the nitrogen refrigerant flowing through the second heat exchanger to cool the second refrigerant containing helium or neon.

19. The method according to claim 18, wherein the heated low-pressure return stream is compressed in the one or more nitrogen refrigerant cycle compressors.

20. The method according to claim 18, wherein the heated second expanded residual stream is recycled to the one or more nitrogen refrigerant recirculation compressors.

21. The method according to claim 16 further comprises the following steps: Diverting a portion of the cold exhaust as one of the one or more streams of the nitrogen refrigerant flowing through the second heat exchanger to the second heat exchanger.

22. The method according to claim 21, further comprising the arrangement of recycling the heated diverted portion of the cold exhaust to the one or more nitrogen refrigerant recirculation compressors.

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