AJAA- -=--- d a*-== I AlAA 91-1929 Design and Operation of the U. S. Space Station Freedom Propulsion System J. S. Morano McDonnell Douglas Space Systems Company - Space Station Division Huntington Beach, CA R. A. Delventhal NASA-Johnson Space Center Houston, TX

AI ANS AE/ASME 27th Joint Propulsion Conference June 24-26, 1991 / Sacramento, CA

For permlsslon to copy or republish, contact the American Institute d Aeronautlcs and Astronautlct 370 L'Enfant Promenade, S.W., Washington, b.C. 20024-2518

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DESIGN AND OPERATION OF THE u. s. SPACE STATION FREEDOM PROPULSION SYSTEM

Joseph S. Morano* McDonnell Douglas Space System Company-Space Station Division

Hunn'ngton Beach, California

Rex A. Delventhal** NASA -Lyndon B. Johnson Space Cenler

Houston, Texas

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The propulsion functions for the U. S. Space Station Freedom (SSF) are accomplished by two separate systems, the Primary Propulsion System and the Supplemental Reboost System (SRS). The Primary Propulsion System includes self-contained hydrazine modules for station reboost, attitude control and contingency maneuvers. These Propulsion Modules contain reboost and attitude control thrusters, propellant storage, thermal conditioning and electronic controls. The modules are serviced on the ground and launched on the Space Shuttle as replacements for the on-orbit modules which have expended their propellant The expended modules are returned to the ground for reservicing and subsequent reuse. The Supplemental Reboost System includes a Waste Gas Assembly and Resistojet Modules which are used for r e h s t maneuvers only. The Waste Gas Assembly contains waste gas storage, compressors and dryers and the Resistojet Modules contain multi-propellant resistojet thrusters, electronic pressure regulators and power conditioning equipment

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McDonnell Douglas Space Systems Company - Space Station Division (MDSSC-SSD) was awarded the Space Station Freedom Work Package Two (WP-02) conuact by NASNJohnson Space Center which includes the design, fabrication and delivery of the SSF Propulsion System. Many configurational changes have been made to the Space Station since the award of the WP-02 conuact. The latest major configurational change occurred in January 1991, when the main Space Station seUctural assembly changed from an on-orbit erectable truss assembly to a ground assembled and tested Pre-Integrated Truss assembly (PIT). The PIT is launched in the Space Shuttle Orbiter in several different "segments" which are robotically joined on-orbit. The major Propulsion System changes that resulted from this reconfiguration included a new concept for attaching the

* Manager, Propulsion & Fluid Systems, A I M Member ** System Development Manager, AIAA Member

This paper is declared a work of the U. S. Government and is not subject to copyright protection in the United States.

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Propulsion Modules to the truss assembly, integrating a smaller Waste Gas Assembly into the central PIT segment, and relocating and reducing the number of Resistojet Modules from four to two. Another major change to the program was that the SSF would be built in stages. Currently, the Space Station configuration is essentially complete when it reaches the Permanently Manned Configuration (PMC) stage. The Supplemental Reboost System is an exception in that while currently manifested as a "post-PMC item, it is included in the Propulsion System design.

Since the Primary Propulsion System is required early in the assembly process, portions of it are manifested on the first two Mission Build (MB) flights. The Primary Propulsion System being built by MDSSC includes eight Propulsion Modules (PMs), eight Propulsion Module Attach Assemblies (PMAAs), electrical power and data utilities and the software required to conml and monitor the propulsion opwations. The starboard PMAAs are included in PIT Segment S3 which is manifested on MB-1. The first two PMs are included on MB-2 and attached to the PMAAs brought up on the previous flight. The second two PMs are brought up on MB-9 and the port PMAAs are manifested on MB-IO in Segment P3. At PMC, there will nominally be four to six PMs installed on SSF depending on logistical and operational considerations. Each will be attached to one of the PMAAs which are located just in-board of the Solar Alpha Rotary Joints, both port and starboard, on the upper and lower sections of PIT segments S3 and P3. The locations of six of the Propulsion Modules are shown in Figure 1 (the 7th and 8th PM would be located next to the upper port and upper starboard PMs). A guide to the segment nomenclature is included in the Figure 1 insert.

While the Supplemental Reboost System (SRS) is a post-PMC system, portions are manifested prior to PMC to reduce the complexity of a later retrofit. The fluid utility lines between the Waste Gas Assembly (WGA) and the Resistojet Modules (RJM), along with the WGA and RJM

UPPER PORT PROPULSION MODULE

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S - Starboard M - Midships

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UPPER STARBOARD PROPULSION MODULE

LOWER PORT PROPULSION MODULES

\ SSF COORDINATES

LOWER STARBOARD PROPULSION MODULES

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Figure 1. Space Station Freedom at PMC

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attach structures. are included within the PIT segments. The two WMS are planned to be launched along with the WGA on a post-PMC Space Shuttle flight in March 2000. The two subsystems of the WGA, the Mixed Waste Gas (MWG) and Reducing Waste Gas (RWG) subsystems, are incorporated into one cargo element which is then installed in PIT Segment M1. The two WMs are installed on-orbit on the aft face of PIT Segments S3 and F'3 between the PMAAs. Also included in the SRS are two roughing vacuum pumps and internal waste gas utility lines which are located in Resource Nodes 1 and 2. The locations of the ,

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SRS hardware are presented in a simplified SSF layout in Figure 2.

P r i m a n , n Svstem

The Primary Propulsion System must meet several principle requirements. It must provide "thrust for attitude conml torque and orbital velocity correction during all flight phases", "utilize a modular approach with no fluid connections between modules" and "shall not require EVA for propellant re~upply".~ The attitude control torque

configuration. A fluid schematic of the Assembly Build Propulsion Module is presented in Figure 4.

w The Primary Propulsion System uses a monopropellant grade of hydrazine @er spec MIL-P-26536) as its propellant. The pressurant is gaseous nitrogen. The pressurization scheme is a simple blowdown system with an initial to final pressure ratio of 3:l. The nominal pressure range of a fully loaded PM is between 375 psia and 125 psia. Each PM contains six identical propellant/ pressurant tanks in series with the upstream two tanks filled with pressurant and the four downstream tanks filled with hydrazine. The nominally Figure 2. SRS Hardware Locations

includes three axis control for reorientation, momentum management device backup as well as desaturation backup, and accommodation of attitude disturbances that exceed the primary momentum management system capability. Control Moment Gyros (CMGs) are the primary momentum management device. The orbital velocity correction includes atmospheric drag compensation, orbit adjustments and collision avoidance maneuvers from ground trackable orbital debris. The orbital velocity correction is more commonly known as the "reboost" function. A majority of the Primary Propulsion System propellant is used to reboost the SSF to an altitude such that atmospheric drag will cause the station to decay in orbit to the proper rendezvous altitude to meet with the Space Shuttle Orbiter.

full PM propellant capacity is 6720 Ibm of hydrazine w i i approximately 6400 Ibm as usable propellant. During thruster firings, as the propellant load decreases and the ullage increases, both the system pressure and the thrust level will decrease. A major trade study was performed to compare the blowdown system with its variable thrust, to several systems with constant thrust, and the blowdown system was chosen primarily on its simplicity and increased reliability.

The design of the PM incorporates a self-supported structure while in the Space Shuttle Orbiter payload bay, meaning that the module structure includes longeron and keel trunnions for duect attachment into the payload bay. This self-supporting structure was chosen over other designs by an integrated rrade study which examined several criteria such as launch weight, cost, and installation operations. The structure itself went through several trade studies with the final selection being an aluminum structure. machined

An illustration of a Propulsion Module is shown in Figure 3. It satisfies its requirements for thrust by the various Rocket Engine Modules (REMs). The PM is a modular design and the umbilical mechanisms contain only data and power cables. The grapple fixtures shown allow for robotic replacement of modules for propellant resupply.

b u l s i o n Modu le Desim

The Propulsion Module is a self contained structural element containing propellant and pressurant storage, Reboost and Attitude Control System (ACS) REMs containing the hydrazine thrusters, propellant distribution and isolation, ground servicing interfaces, micrometeoroid/ orbital debris ( W O D ) protection, passive thermal control, and avionics with its resident software. There are two PM configurations, the difference between them being the number of REMs on the module. The fmt four "Assembly Build PMs contain two additional REMs for reboost along the truss (Y) axis, while these REMs are omitted from the remaining four "Assembly Complete" PMs. During early assembly, SSF must be conaolled and reboosted with only two PMs insralled, so additional rhrusters are required. After the Assembly Build PMs have exhausted their propellant supply and they have been returned to the ground for their f is t reservicing, the additional REMs will be removed and all eight PMs will be of the Same Assembly Complete Figure 3. Propulsion Module

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Figure 4. Propulsion Module Schematic (Assembly Build Configuration) -

and mechanically assembled. Fastened to the PM structural The PM tank is made of titanium with an internally frames is an aluminum skin W O D shield. The design is mounted surface tension (metal screen) Propellant a double wall whipple shield with a 4 inch separation Acquisition Device (PAD). The tank envelope is a 38 inch between walls. This design yields lighter weight for the diameter and 59 inch long cylinder with ellipsoidal dome same protection and a slightly larger volume which is ends which has an internal volume of 27.3 k 0.1 cubic feet. negligible compared to the PM volume. The MWOD The PAD is comprised of four full length galleries and an aft shield is designed so that its skins are removable for collector which include Twill Double Dutch Weave (TDDW) replacement or enhancement during the PM operational lifetime.

promllant and R e The PM tank design provides storage for both gaseous nitrogen pressurant and

series such that for a nominal 3:l blowdown load, the upstream two tanks are full of pressurant and the downstream four tanks are full of propellant. A study was performed to determine if the pressurant tank should be a separate design, but the cost savings associated with one

weight increase with the common design. The tanks are mounted to the PM by polar boss attachments at each end of the tank longitudinal axis which allow for circumferential

its own individual tank checkout port used for tank integrity tests. A diagram of the PM tank is presented in Figure 5.

liquid hydrazine propellant. There are six tanks plumbed in ARRESOR SCREEN

versus two tank designs overshadowed the pressurant tank PAD ASSEMBLY

and longitudinal expansion when pressurized. Each tank has GALLERY SCREEN (40 PLCS)

Figure 5. Propulsion Module Tank ,

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gallery and arrestor screens. The screens prevent gas ingestion during outflow prior to tank propellant depletion in a micro-gravity environment. The tank is designed for nominal operations of 375 psia at 70" F and a maximum non-operating abort case of 438 psia at 140' F for a gas filled tank and IOo F for a propellant fdled tank. The tank design also incoqwrates a vertical drain port for ground expulsion of residual propellant. The estimated weight for the tank is 146 Ibm and the expulsion efficiency is estimated at 98% minimum.

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There are two different Rocket Engine Module (REM) designs for the PM; the Reboost REM and the ACS REM. All PMs have one Reboost REM and three ACS REMs. The Assembly Build PMs have an additional two ACS REMs which perform the reboost function along the SSF truss axis during early assembly. During this early assembly stage, SSF resembles an arrow with the Photo-Voltaic Arrays acting as the "feather". These additional ACS REMs, which reboost the station in this attitude, have been referred to as the "Arrow Mode Reboost" REMs. The Reboost REM is located on the PM so that it will be firing in the aft direction of the fully assembled station. One ACS REM is located adjacent to the Reboost REM (aft fuing) and one is located on the opposite side of the PM (forward firing). The third ACS REM is located opposite of the keel trunnion (Ref. Figure 3) and wiU either be fkng zenith or nadir depending on whether the PM is located on an upper PMAA or lower PMAA. The arrow mode REMs are currently baselined as starboard fuing, but the design also allows for port firing.

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Both REM designs contain mounting srmcture, thruster solenoid valves, internal manifolding, chamber pressure transducers, thermal conditioningjmonitoring, and thrust chamber assemblies. These thrust chamber assemblies (TCAs) include the catalyst beds and the thruster nozzles as a single ground replaceable unit. Since the catalyst is a life- limited material. the TCA must be removed to allow for catalyst replacement The pressure transducers will be used to monitor the thruster "roughness" as the determining factor for when the catalyst needs to be replaced. The TCA design incorporates simple removal and installation operations.

The Reboost REM contains three reboost thrusters which include the thruster valve and TCA. The thrust is nominally 55 - 20 Ibf with a nominal blowdown pressure cycle of 375 - 125 psia. The proposed SSF reboost thruster will be testing the state-of-the-art in terms of the desired propellant throughput 'and the Iength of single duration bums. Performance is also important to the Propulsion System resulting in a trade-off between performance and long life. The reboost thruster Specification requires a throughput of 1 . 5 ~ 1 0 ~ Ibf-sec, single duration

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Figure 6. Reboost Rocket Engine Module

bums of approximately 7.2 hours, and a minimum vacuum steady state specific impulse (Isp) of 234 Ibf-secflbm at 375 psia. Figure 6 presents a proposed Reboost REM.

The ACS REM contains two smaller ACS thrusters which also include the thruster valve and TCA. The thrust is nominally 25 - 9 Ibf with a nominal blowdown pressure cycle of 375 - 125 psia. The ACS thruster specification requires a throughput of 7 . 5 ~ 1 6 Ibf-sec. Performance is also important with the ACS thrusters which operate in both steady state and pulse modes. The ACS thruster specification requires a minimum vacuum steady state specific impulse of 232 Ibf-secflbm and a minimum vacuum pulse mode specific impulse of 180 Ibf-secflbm at 375 psia. The minimum impulse bit for the ACS thrusters is 2.0 Ibf-sec f 25% at 375 psia and 0.9 Ibf-sec f 25% at 125 psia. Figure 7 presents a proposed ACS REM.

a i d Valves The Primary Propulsion System contains two types of solenoid valves. The thruster solenoid valves that control the hydrazine thrusters are part of the

Figure 7. ACS Rocket Engine Module

REM design. These thruster valves require power to open and will close automatically once power is removed. The latching isolation valves are similar to the thruster valves, except that power is required to commandlopen the latching isolation valve and required to command/close the valve, but no power is required to keep the valve open or closed. Both valves are similar in that they operate on 120 Vdc. A trade study was performed to chose between 28 Vdc and 120 Vdc valves for the PM. Based on the weight, reliability and cost for load converters which are required for 28 Vdc valves, 120 Vdc valves were chosen2. The latching isolation valves are part of the PM Valve Module which is located in the proximity of the Reboost REM.

Primarv ProrrulsipnSvstem

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The Propulsion Module will go through several different operational scenarios during its lifetime. Propellant requirement analyses show that each PM will undergo over ten on-orbiUground servicing cycles. During each cycle, the PM will experience three main operations. interfacing with the SSF, the ground, and the vehicles used for PM transportation. The most critical are the on-orbit operations when the PMs are installed on the SSF at the PMAAs. Also of importance are the ground servicing operations that the PMs experience to prepare them for return to SSF. Due to thermal and safety concerns with liquid hydrazine, the operations during PM transportation from the ground to SSF and back again are also crucial. The PM lines and components which contain hydrazine must be kept from freezing during all operations.

- Multiplexer-Demultiplexers (MDMs) which contain the

Primary Propulsion Manager software are included in the PM design because of the various locations in which operations will be performed. There are two MDMs per module for redundancy and they are designed as on-orbit replaceable units (ORUs). All power and data to and from the PM valves and inswmenration go through the MDMs. The MDMs also interface with different systems at each location.

While the PM is on-orbit and installed on SSF, there are two different modes of operation. The most crucial mode is the "fuing" mode in which valves will be opened and hydrazine thrusters will be producing thrust. A wide variety of individual operations will be occurring during thi mode and there is a significant interface with the Guidance, Navigation and Control (GN&C) system during this period. Timewise though, while on-orbit. the PM will predominantly be in the "quiescent" mode, during which the operations include thermal control and monitoring as well as general instrumentation monitoring for health status checks. _-

The firing mode includes both reboost fving and ACS fving. In addition to the reboost thrusters, ACS thrusters will also be utilized during reboost fving for SSF attitude stability due to unequal reboost thrust moment arms. The firing command and control is through the GN&C/F'ropulsion (GNCP) software which is resident in shared Standard Data Processors (SDP). These SDPs are initially located on truss Segment S2 and later in the assembly sequence, they reside inside of the pressurized elements -Node 2 and Lab A.

Typical firing operations involve several different systems and organizations. The Space Station Control Center (SSCC), which continuously monitors the SSF status, will determine if a firing is required and will command GN&C to begin its calculations. GN&C will determine the present SSF position, the desired position and the fving sequence required to achieve it. The Propulsion System will determine the individual thruster status for GN&C to chose which thrusters to select. The Propulsion System will then activate thruster catalyst bed heaters to increase the catalyst temperature approximately one hour prior to a thruster firing. Thii reduces the thermal shock to the catalyst when hydrazine is fmt introduced. GN&C will command the thruster firing initiation and termination, while the Propulsion System will perform the actual thruster valve opening and closing. Throughout this Operation, other SSF systems will perform status checks and report the results to the SSCC.

' During the ground handling operations, the PM will be installed in a special handling fixture where it will remain until re-installed in the Orbiter payload bay. Most ground handling operations will take place in the Space Station Hazardous Processing Facility at NASA-Kennedy Space Center, Florida. In this building, the PM will be drained of residual propellants, examined for visible anomalies, any damaged or defective components will be replaced or repaired, Thruster Chamber Assemblies will be replaced if required, functional checkout tests, including leak checks. will be performed, and the PM will be loaded with pressurant and propellant in preparation for a return 10 SSF. The PM operations will involve instrumentation monitoring, valve cycling, interfacing with Ground Support Equipment (GSE), and propellant/ p u r a n t servicing.

O o e w During all of the aansportation operations, the main activities that the PMs will be performing are thermal control and environmental monitoring. There are several systems which will interface with the PM during the transportation operations, including the Space Shuttle Orbiter, the Shuttle Remote Manipulator System (RMS). the Mobile Servicing Center (MSC)

Payload ORU Accommodations (POA), and the Space Station Remote Manipulator System (SSRMS).

Typical launch operations include a series of robotic events between the ground and on-orbit operations. Once the PM is installed in the Orbiter payload bay, an umbilical will physically connect the PM to the Orbiter to supply the PM with heater power as required and transfer data between the two systems. After Space Shuttle Orbiter launch and dock to SSF. the RMS will attach to the PM at an Electrical Flight Grapple Fixture (EFGF). lift the module out of the payload bay and position it so that the SSRMS can attach at a separate Power Data Grapple Fixture 0. The SSRMS will then place the PM on the MSC POA where it will travel along the truss to the appropriate module location. The SSRMS will then remove the PM from the POA and attach the module to the PMAA. A typical return to Earth mces this sequence of events UI reverse order.

Primarv ProDulsion Svstem Near Term Milestones

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The Primary Propulsion System completed its Preliminary Design Review (PDR) in July 1990. A Delta PDR is currently scheduled for July 1991 to cover the configuration changes brought about by the January 1991 Restructuring. The himary Propulsion System will take part in the Delta PDR primarily in the PMAA design review. A Critical Design Review is currently scheduled for January 1993 with the First Element Launch targeted for November 1995. e

The Primary Propulsion System will undergo a system development test at the NASA-White Sands Test Facility (WSTF) in New Mexico. The test article, which includes non-flight structure and prototype components, is presently being built up for a delivery date to WSTF in March 1992. The testing is planned to begin in July 1992 and run through March 1994. A PM Development Unit comprised of high fidelity structure, mass/thermal simulated components and functional heaters is in the final design phase. This unit is planned to undergo thermal-vacuum and vibro-acoustic testing starting June 1992. A PM Structural Test Article will be fabricated in early 1993 for modal survey and static loads testing between March and November 1994. A PM Qualification Test Article which has a flight PM configuration with additional instrumentation is scheduled for environmental qualification tests from January to May 1994. After the completion of these tests, the Test Article will be shipped to WSTF for Static Hot Fire qualification tests between August 1994 and January 1995.

SVpplemental Reboost S y & i ~

The Supplemental Reboost System is manifested as a "PostPMC system and as such, most of the detailed design work is being deferred. Since the SRS components will reside in several flight elements which are manifested early in the assembly sequence, there is presently enough design definition to allow for proper scaning of those elements to accept these components when installed. A simplified schematic of the SRS is presented in Figure 8. More

Reducing Mixed Roughing Roughing Waste Gas WasteGas -1

Vacuum Pump r - - - - - Vacuum Pump I

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I I L - - - . . - -

Compressor I Compressor

Resistojets Resistojets

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Regulator I

L - - - - - - - - - - 1 Regulator

Figure 8. Supplemental Reboost System Simplified Schematic

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mm analyses will be performed as the design becomes more maturc.

i- The SRS must meet several principle requirements. It must "control and monitor the collection, conditioning, and storage of waste fluids discarded by flight elements and systems", "utilize resistojets to provide for disposal of waste fluids to produce reboost thrust" and "ensure compliance with microgravity and external contamination requirements".l The Waste Gas Assembly (WGA) is designed to meet the fmt requirement by accepting waste gasses from the users in the U. S. Laboratory, the Japanese Experiment Module (JEM), the European Attached Pressurized Module (APM), and the Environmental Control and Life Support System (ECLSS). The two subsystems of the WGA allow for the collection of both "oxidizing" and "reducing" SSF waste gasses, ensuring separation to prevent chemical reactions from occurring. The Resistojet Module @JhQ contains resistojets arranged so that while disposing of the waste gasses, the thrust will increase the velocity of the station in the reboost direction. The thrust levels of the resistojets and the nature of the plume will ensure compliance with the microgravity and external contamination requirements. Also included in the SRS are the roughing vacuum pumps, which act as the interface from the users to the SRS, and the Fluids Utility Distribution System (UDS), which transpons the waste gasses between the various assemblies.

&&tojet Module De- . . ..

The Resistojet Module is a self contained structural element containing resistojets, an electronic pressure regulator, control/iilation valves, power conditioning units (Po ,passive thennal controls. fluid distribution Lines and instrumentation. Gasses fmm both subsystems flow into the RJM where check valves prevent the back flow of one subsystem into the other. A relief valve with an overboard vent is located upstream of the resistojet valves to prevent damage to the system due to overpressurization if the electronic pressure regulator fails open. A more detailed schematic of the RIM is shown in Figure 9.

There are. four resistojets per RTM, each with its own X U . The quantity of resistojets was chosen as a balance between microgravity considerations. total bum times, redundancy and cost considerations. They are designed to accept a multitude of gasses, but onZy gasses, since their design does not take into account liquid flow in a microgravity environment. The resistojets are low thrust engines, producing faces in the milli-pound-force (mlbf) regime. The thrust range for a single resistojet varies from 50 mlbf to 240 mlbf with candidate reducing waste gasses and from 211 mlbf to 392 mlbf with candidate mixed waste

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RE515TOdET WWLE ISTARBOARE LOWER1

> Figure 9. Resistojet Module Schematic

gasses. The actual thrust level is dependent on the ratio of the waste gas constituents.

W e Gas Assmblv Desigl!

The Waste Gas Assembly is comprised of two subsystems which are identical in hardware design. One subsystem collects, conditions, and stores Co;? and possibly methane at a later stage, and the other subsystem collects, conditions, and stores a combination of oxygen and inert gasses. The WGA is a self contained structural unit which contains high pressure storage tanks, two-stage compressors. low pressure accumulator tanks, gas dryers, isolation valves, fluid distribution lines and instrumentation. Relief valves, connected to overboard vents, are located at the WGA inlet, at the low pressure accumulators and at the high pressure storage tanks. Except for the storage tanks and accumulators, all WGA components are designed to be ORUs. A more derailed schematic of the RWG subsystem is shown in Figure IO.

An integrated hade. study was perfmed to determine the optimum location of the WGA. It also examined whether it would be advantageous to manifest the pressure vessels pre- integrated with the truss segment. The previous baseline separated the RWG and MWG systems by locating them near the Primary Propulsion Modules. Available secondary suucture for mounting the systems led to that configuration. The trade study recommended the combined location on Segment Ml including the pressure vessels because of near term costing, design simplification, and the reduction of Orbiter-based Extravehicular Activity (EVA).

The Roughing Vacuum Pump (RVP) provides a vacuum some to the users while collecting waste gasses to transport to the WGA. There are. two RVPs located in Node 1, one for each subsystem. The MWG RVP recovers waste gasses from the experiment test chambers of the U.S. Lab, E M and APM, by reducing the test chamber pressure

Figure 10. Waste Gas Assembly Subsystem Schematic

from 14.7 psia to 10Torr (approximately 0.19psia). The RWG RVP recovers carbon dioxide from the U.S. Lab and US. Hab ECLSS COZ removal system.

Waste gasses are delivered to the inlet of the compressors at low pressures from the RW. The fmt stage

.,. ., of the compressor increases the pressure and transfers the waste gasses to the low pressure accumulators. When the accumulator reaches approximately 100 psia, the second stage of the compressor turns on and the waste gas is transferred through the gas dryers and to the high pressure storage tanks. The high pressure storage tanks are sized to accept the present estimates of waste gasses for two days prior to achieving the upper pressure limit of approximately 1000 psia. The details of the dryer design have not yet determined as to whether or not it will be regenerative. Further studies will be performed to tmde off development costs, operations and logistics to determine the optimum dryer design.

The SRS does not have any dedicated MDMs as a pan of its system, but rather shares MDMs with nearby systems. The Resistojet Module Manager software is resident in two shared MDMs located one each within Segments S3 and p3. The Waste Gas Manager software is resident in a shared MDM located within Segment MI.

The SRS is designed for storage of waste gas for up to two days at which time the gasses are expelled through the

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resistojets. The resistojet needs to remain at consmt temperature during operations in order to maximize the impulse received from the various waste gasses. To ensure constant resismjet temperatures, the mass flow rate will vary depending on the specific heat of the waste gas constituents. This is accomplished by the electronic pressure regulator which acts as a mass flow controller, adjusting the flow in order to maintain a constant high temperature at the resistojet. The two subsystems are emptied in series, allowing for the vacuum of space to clear the residual gas from one subsystem prior to any introduction of gasses from the other.

conclusion

The Space Station Freedom Propulsion System design reflects the optimal configuration which meets overall performance. safety, operations, and ground processing requirements. With the selection of state-of-the-art propulsion technology. the hpuls ion Module design is at a high state of maturity in preparation for derail design and system level tests at NASA-WSTF. Testing will soon begin to assess individual hardware. capabilities as well as to characterize system level performance. With the emphasis of minimizing ground refurbishment requirements, significant increases in the impulse life capability of the hydrazine thrusters is a development objective. Likewise, issues regarding turn-around handling of a hydrazine system will be addressed during the system level tests. Lessons learned from these activities will aid in the future design of other liquid propulsion systems and expand the technical infi-asastructure in the aerospace community.

References

1. NASA, "Space Station Projects Description and Requirements Document - JSC31OOO Vol. 3 Rev G", Space Station Projects Office, Johnson Space Center. April 4 1991 2. Darrow, R. J., Jr., "Hydrazine Propulsion Module Design Considerations for Interfacing with the U. S. Space Station and Space Shuttle", McDonnell Douglas Space Systems Company, Huntington Beach, California, AIAA-91-2221.27th Joint Propulsion Conference