AIAA 92-3636 US. Space Station Freedom Supplemental Reboost System (SRS) Design and Operational Impacts

Brian A. Winters Propulsion and Fluid Systems McDonnell Douglas Space Systems Company - Space Station Division Huntington Beach, CA

Donna M. Winters Fluid Systems Design McDonnell Douglas Space Systems Company - Space Station Division Huntington Beach, CA

AI AA/S AE/ASM E/AS E E 28th Joint Propulsion

Conference and Exhibit July 6-8, 1992 / Nashville, TN

./' U.S. SPACE STATION FREEDOM SUPPLEMENTAL REBOOST SYSTEM (SRS)

DESIGN AND OPERATIONAL IMPACTS

Brian A. Winters* ~ and - Donna M. Winters5 McDonnell Douglas Space Systems Company - Space Srarwn Division

Hum.n@on Beach, California

Current designs of the U.S. Space Station Freedom Propulsion System require the capability to perform reboost, attitude control, and other orbit adjustment maneuvers as part of normal operations. In order to reduce the quantity of hydrazine consumed by the Primary Propulsion System, the propulsion system includes the capability to propulsively dispose waste gases that are generated by various station systems and experiments. ?he impulses generated by the Supplemental Reboost System are used to Offse t orbital decay due to annospheric drag and reduce propellant requirements. The additional impulse results in operational cost savings by reducing resupply requirements and solves the logistics problem of handling and disposing gaseous waste products. Waste gases from various sources are collected and routed to storage hardware

' located on the station VUSS. At approximately two day intervals, resistojet modules are activated to propukively vent the gases at conditions that do not violate contamination or microgravity requirements. This paper presents the designs of the SRS including schematics of the waste gas collection architecture. various system and component characteristics. and methods of system operation. Estimates of waste gas quantities, propellant savings. and operational altitude impacts are provided. Qualitative impacts due to the operation of the SRS are examined and discussed to underscore the viability and importance of this system to economical operation of Space Station Freedom.

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McDonneIl Douglas Space Systems Company was awarded the Space Station Freedom (SSF) Work Package 2 (WP-02) contract by NASA / JSC in 1988. Included in the contract was the responsibility of designing, manufacturing, and assembling all components of the SSF propulsion system which is utilized for station uajectory and orientation adjustments. Assuring economic operation of the Propulsion System is critical for meeting SSF

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' mission objectives over its 30 year life span. .J $ EnginedScientist Specialist, Propulsion and Fluid

Systems (AIM Member) Copyright GI 1992 American Institute of Aeronautics and Astronautics. Inc. All rights reserved.

Operation of SSF in Low Earth Orbit &EO) results in appreciable orbital decay caused by interaction with Earth's upper atmosphere. This reduction in orbital altitude must be periodically compensated for by firing thrusters to increase the energy (altitude) of the orbit. While the propulsion system provides thrust to complete other maneuvers such as attitude control and collision avoidance maneuvers, maintanence of operational orbital altitudes presents the greatest consumption of delivered propellant. Reducing the quantity of propellant that needs to be delivered to the station via the shuttle results not only in a net increase in deliverable payload capability, but a significant reduction in operating cost as well.

SSF propulsion functions are supported by two systems, the Primary Propulsion System (PPS) and the Supplemental Reboost System (SRS). The purpose of the PPS is to provide thrust for attitude control and orbital velocity correction during all flight phases. Attitude Control System (ACS) functions provided by the PPS include three axis control, orientation, and Control Moment Gyro (CMG) desaturation and backup. Delta orbital velocity (AV) thrust is provided by the PPS to compensate for atmospheric drag, adjust orbit characteristics (eccentricity and inclination), and perform collision avoidance maneuvers to avoid impacts with detectable orbital debris.' The PPS consists of reboost and attitude control thrusters, propellant storage, thermal conditioning. and elecmnic conmls. Modules are serviced on the ground and are delivered periodically by the Space Shuttle to replace modules near depletion? Reducing the amount of consumables that need IO be delivered (such as hydrazine) would result in significant operational cost savings and increase payload delivery efficiency to the station.

The Supplemental Reboost System (SRS) suppons the PPS by providing a means of reducing reboost propellant requirements by generating impulse through the

5 Engineering Schedules Analyst - Senior, Propulsion and Fluid Systems

- disposal of waste gases produced by normal station operations and experiments. In addition to providing this reboost impulse, operation of the SRS alleviates the problem of disposing waste gases by other means such as overboard venting which creates contamination problems or returning gases to Earth, complicating logistics and increasing operational costs. Once installation on the station is complete, operation of the SRS requires no support from regular SRS logistics flights.

S B S ksivl

The SRS is similar to other station "disuibuted" systems dedicated to a panicular function, traversing most of the length of the station, servicing both internal and external locations. A vast network of fluid lines is provided to collect the waste g w from the various sources and route them to storage and disposal hardware locared on the he-Integrated Truss (PIT). Figure I provides an overview of the Permanently Manned Configuration (PMC) station and denotes the locations of the major subsystems of the SRS.

SRS Svstem Sum mary

The SRS is not planned for operation until after the fist phase of station assembly has been completed and is therefore denoted as a "Post-PMC" item. Since first operation of the system will not occui until after the 17 Mission Build (MB) flights, most of the major SRS hardware assemblies will not be delivered or installed until later in the station's construction. Fluid lines and support structures are provided in the main PIT segments as "scars" in preparation for later installation of SRS assemblies.

In order to be able to service all systems on SSF that generate gaseous waste by-products. the SRS separates collected gases into two categories; Mixed Waste Gases (MWGs) and Reducing Waste Gases (RWGs). Separation of the two types of gases prevents undesirable (or potentially explosive) reactions from occurring. Seperate, dedicated hardware in each subsystem is provided for each type of gas except for disposal hardware which utilizes vacuum purging to assure all of each type of gas is adequately vented prior to introducing the other gas type.

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Figure 1 - Space Station Freedom SRS Overview

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The overall architecture of the SRS can be. divided into three groups of subsystems that support the individual phases of SRS operation.

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Coliection. Collection hardware includes all fluid lines and components (valving and sensors), both internal and external, that route waste gases from sources to storage hardware located on the PIT. This hardware is integrated into the elements prior to delivery to orbit,

Storage hardware includes the Waste Gas Assembly (WGA) located on the PIT. The WGA is an On- Orbit Replaceable Unit (ORU) that contains compressors, tanks, and the associated hardware needed to store the gases prior to being transferred to subsequent hardware for disposal. The WGA is delivered after PMC has been reached and is robotically installed.

Disposal hardware includes the Resistojet Module (TUM) and the fluid lines that connect the storage hardware located on the central PIT segment to disposal hardware located at both port and starboard propulsion attach locations. During assembly, the tubing is integrated into the PIT segments prior to delivery, however the resistojet module is delivered following PMC and is robotically installed.

W Table 1 provides estimated compositions of waste

gases to be collected and disposed annually during normal station operations once the SRS has been activated. Estimates of specific impulse are also provided for each constituent as well as annual levels of delivered impulse. These estimates for specific impulse and delivered impulse include a factor to account for nozzle and heater efficiencies (-85%) and can vary due to total gas composition.'

tem WGm

The function of the Waste Gas Collection Subsystem (WGCS) is to collect gases generated at various station locations and route them to storage hardware for containment until an appropriate disposal time has been ceached

The WGCS consists of the fluid lines throughout the station that originate from systems and experiments located within pressurized modules as shown in Figure 2. The waste gases are provided by the Environmental Control and Life Support System (ECLSS) and various experiments in the laboratory modules. Throughout the entire WGCS, gas pressures are maintained below 14 psi which assures that any leaks in the system will prevent gases from reaching the controlled internal environment of the habitable

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WASTE GAS

Air AT

CH4 co co2

Freon H2~ - He Kr

Mixed NZ

9 Xe

TDTAL

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Table 1 - Estimated Waste Gas Inventory Characrensucs

QUANTITY hnkcar)

REDUCING MD;zD WASTEGAS WASTEGA!

484.7 1100.0

1778.0 0 . 3

2482.0 57.7 21.9

7 . 0 0.9 18.5

104.0 43.1

99.0 1900.5

55.0 209.9 37 .6

4421.0 3979.1 Delivered Xmpulse

(Ibf-scc/yr Average Delivered Force

(Continuous Ibf

SPECIFIC IMPL'E

SIdM'F IS_

130 9 0

132 121 I O ? 485 284 62 123 132 350 123 50

SPECIFIC W U E @ 9 W F ,Wl

167

103

415

113

106

467.271 1572,494

0.0148 I 0.0181 I

WASTEGAS STORAGE SUBSYSTEM

Figure 2 - Waste Gas Collection Subsystem

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modules (at 14 psi). WGCS fluid lines terminate at the storage hardware located at the center of the PIT assembly. RWG WGCS tubing will be 3/8" OD 316L Stainless Steel to achieve the best comprimise between weight, cost. and material compatibility. MWG WGCS tubing will be 3/8" OD Inconel 625 and, while not as weight or cost efficient as Stainless Steel, provides maximum resistance to corrosion and stress corrosion cracking caused by some of the MWG constituents.

Currently under consideration to be included in the design of the WGCS is the addition of two Roughing Vacuum Pumps (RVPs). These will be located in Node 1 just prior to the bulkhead penemtiom in order to generate a sufficiently low enough vacuum source to draw the gases from the modules. If incorporated, line sizes upstream of the RWs will increase to 3/4" to address flow rate and pressure drop requirements.

Waste G- m (WGSSJ

The function of the Waste Gas Storage Subsystem (WGSS) is to accept waste gases from the WGCS, store them at high pressures, and deliver them for disposal at specified intervals. Disposal is initiated by reaching a maximum storage pressure or loo0 psi or by conmining gases for no longer than a period of approximately two days. The WGSS contains identical hardware to support the storage of both mixed and reducing waste gases. A schematic representing a typical WGSS assembly is provided in Figure 3. WGSS hardware required for storage of both types of waste gases are mounted together as a single unit in the Waste Gas Assembly (WGA). The WGA is subsequently installed as an ORU in the center PIT segment as shown in Figure 4.

Gases enter the WGSS provided by the WGCS and pass though a low pressure compressor. A filter is placed just prior to the compressor to elimate any particulates or

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Figure 3 - Waste. Gas Storage Subsystem (WGSS) Schematic

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CHARACTERISTIC Weight (Ibm) PeakPowaRequired (W) Estimated Operating Life

RWG MWG 60 75 300 400

87.700 21,900

Maximum Flow Rate

Dimensions (Length, width, hei ht

11 x 14 x 8 11 x 14 x 8

Prior to entering the storage tanks, the gascs arc processed through a dryer to eliminate any moisture lhal might have condensed from the gases as a resull of the increase in pressure. Maximum storage capability of the tanks is reached once pressures approach loo0 psi.

Relief valves are located throughoul the assembly to prevent over-pressurization during the collection cycle. Hardware is arranged so that a single failure could prevent the WGSS from completing its function and is therefore zero failure tolerant

Temperahue sensors are provided to monitor stored gas temperatures prior to transfer to the disposal hardware. Allowing gas transfer at sufficiently low temperatures could result in the condensation of the gas, possibly damaging disposal hardware. Funher design maturity will determine the need for heaters in the WGSS design to prevent condensation during venting.

Collection of gases is performed continuously over a 2 day period. even during the disposal process, assuming that maximum allowable pressures have not been reached. Optimization of system operations require studies to determine desirable settings and deadbands for the pressure switches to activate and shutdown the high pressure compressor.

Figure 4 - Waste Gas Assembly (WGA) Installation

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RWG). After being filtered, pressures are regulated between 20-200 psi prior to being distributed to the four Weight Obm)

WGCS WGSS WGDS Total 199 820 401 1420

&OM WGSS

Figure 5 - Resistojet Module Schematic

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SSF-SRS

The incorporation of the SRS into SSFs overall propulsion system architecture is intended to provide an operational cost savings by reducing the quantity of hydrazine that needs to be resupplied in order to mainlain acceptable operating altitudes. These savings can be highly dependant on various factors including solar activity, time periods between shuttle rendezvous, and variations in station physical characteristics (mass and drag area).

Analvsis Methods 8

Various computational methods have been developed to estimate propellant savings due to SRS operations. The two methods discussed here. the Equivalent Hydrazine (EH) method and the Time Integration (TI) method, allow for simple Rough Order of Magnitude (ROM) calculations or complex, involved calculations with greater accuracy.

p The EH method for determining propellant savings examines the immediate, first order effects of operating the resistojets but ignores effects due to variations in altitude profile. The basis of the EH method is to determine the quantity of

, hydrazine that would impan an equivalent impulse on the - station utilizing the PPS at the average altitude during the decay period. Operation of the resistojets results in forces being applied to the station over a given period of time. From the definition of linear impulse :

where : I = Deliveaed Impulse (lbf-W) F =Thrust(lbf) t =Time(sec) Wf = Ropellant mass (Ibm) I,,, = Specific Impulse (lbf-secflbm)

The EH method assumes that the impulse generated by SRS operations over a given period of time replaces an equivalent amount of impulse that would have been generaled by the SSF PPS.

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This is a lather simplistic method buL i t does, as will bc seen, provide an excellent estimate of average propellant savings.

Time Intematio n ml Method A more accurate means of determining propellant savings is to examine the orbital motion of the station while under the influence of atmospheric drag and continuous thrust due to resistojet operations. From the Conservation of Angular Momentum for an assumed circular orbit :

(mvr), = (mvr) , + (Fr);-,Ar (5)

where : m = Mass of station (slugs) r =Radius of orbit (from Eanh center (ft) A t = Integration time step (sec) V =Circular orbital velocity (ft/sec)

p = !3mh Gravitational Constant

F = 1.4076~10'~ ft3/sec2 =Force impaned on station in orbit (lbf) = Atmospheric Drag - Resistojet Operations = qCdA - R

= Effective force due to resistojets (Ibf) CdA= Drag Coefficient * Drag kea (ft2) R q =Dynamic pressure @sf) = TV 1 2

i = Initial Condition f = Final Condition

For small A t (-6t). the first term on the RHS of the equation is negligible as compared to the second term, resulting in the following equation describing orbital decay:

Studies of propellant savings and impacts to operational altitude profiles have been performed by incorporating equation (7) into an orbital decay model and examining the effects of activating the SRS. While the model assumes that the resistojets are firing continuously, models duplicating the SRS operational cycle of two days have shown this assumption to be reasonable.

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-mal ,411 1- i ---,

Rendezvous with the shuttle to resupply consumables and deliver hardware occurs at periodic, specified intervals. In order to account for orbital decay following rendezvous, station altitude must be increased to achieve the next target rendezvous altitude. Resistojet operations tend to reduce the decay rate resulting from atmospheric drag. thus reducing the increases in altitude necessary for meeting the next shuttle rendezvous. This reduction in altitude adjusunent is where the propellant savings is realized.

The nominal operational (ie. no resisotjet firing) flight profile is used as an envelope for examining the possible alterations to rendezvous and reboost altitudes when attempting to maximize propellant savings due to waste gas disposal. The two extreme cases differing from the nominal operational scenario will be discussed; operations with constant rendezvous altitude and operations with constant reboost altitudes, as shown is Figure 6.

~ M R C p m a p m F J C / -

Figure 6 - SRS Operational Altitude Strategies

In order to measure the effectiveness of an altitude strategy, an efficiency factor was developed to compare the propellant savings with some baseline quantity. This Resistojet Operational Efficiency (ROE) is defmed as:

(8) Actual N2H4 Savings -

= Equivalent N2H4 Savings - I

As defined, the ROE factor is only relevant when the altitude strategy being examined falls within the nominal rendezvous and reboost altitudes. ROE can range from 0 for no SRS operations to values greater than 1.

C o n s t a n r v o u s Suateev. The Constant Rendezvous Altitude strategy utilizes the same target altitude for docking with the shutde but reduces reboost altitudes to achieve propellant savings (Ah, in Figure 6). As the SRS is operated during the decay period following reboost, the Constant Rendezvous altitude profile and the ' Nominal altitude profile converge to the rendezvous

altitude. This strategy is desirable from a payload manifesting standpoint since neglecting any system failures, no adjustment 10 rendezvous altitude is needed.

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Since the station is continually flying at an alutude lower than the Nominal profile, the overall average operating altitude is lower during this decay period which results in an increase in the observcd atmospheric density. Results from propellant consumpuon analyses show that the ROE for this strategy is approximately 0.85. This indicates that the increase in observed density is "consuming" 15% of the propellant savings due to SRS operations.

Another undesirable effect from utilizing Constant Rendezvous altitudes is the impact due to SRS failures. Since the station is generally at a lower altitude using this strategy, the only means of achieving the target altitude with an SRS failure is to reconfigure the station to reduce drag (which could impact station operations or experiments) or perform a "mini" reboost (which consumes additional hydrazine). If achieving the target altitude is not critical or does not represent a minimum operaling altitude limi4 this impact may be negligible.

Consm nt Reboost S t r a w The Constant Reboost Altitude strategy utilizes the same reboost altitude as the nominal condition but increases the rendezvous altitude (Ah2 in Figure 6). As the SRS is operated during the decay period following reboost. the Constant Reboost Altitude profde and the Nominal altitude profile diverge.

Since the station is continually flying at an altitude higher than the Nominal profile, the overall average operating altitude is higher during this decay period which results in a decrease in the observed atmospheric density. Results from analyses show that the ROE for this strategy is approximately 1.20. This indicates that the decrease in observed density has "recovered" an additional 20% of propellant savings due to SRS operations. This significant increase in propellant savings also results in conditions

' that require very little propellant to complete r e b s t .

Unlike the Constant Rendezvous Altitude strategy. this strategy will not require any additional actions to address violations of minimum altitude requirements in the event of SRS failure. If a failure were to occur at any point in the decay period, rendezvous would still occur at a higher altitude than the nominal condition, preserving any minimum altitude requirements that could be in effect

The drawback to this strategy is that payload manifesting becomes more difficult. Rendezvous altitudes will become more difficult to predict due to composition and quantity of waste gases disposed and the status of SRS operations. If a shuttle flight is weight-limited, any increases in rendezvous altitudes become less desirable.

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Based solely on a propellant conservation point-of- view, the Constant Reboost altitude strategy inherently provides the greatest propellant savings due to the reduction in average observed atmospheric density. Figure 7 shows the impact on the altitude envelope. of SRS operations while rendezvousing at minimum altitude requirements of 180 days of orbital decay to 150 nm. The cyclic nature of the results is indicative of the predicted 11 year cycle of solar activity.

While both strategies reduce the A h required for reboost, the change is more significant for the Constant Reboost altitude strategy. It is evident from the figure that the increase in rendezvous altitude (Ah3 is greater than the decrease in reboost altitude (Ah,) which is due to decreased observed atmospheric density. Pmpellant savings at any point in time is also a function of the slope of the nominal altitude profile (whether the next rendezvous altitude is above or below the current cycle). The reductions in altitude increases (Ah) are also proponional to realized propellant savings.

v 260

250

240

210

200

190

The propellant consumption esumates corresponding to these altitude profiles are shown in Figure 8. These propellant estimates are based on nominal rendezvous at the minimum altitude requirement of 180 days of decay to 150 nm and utilizing the impulses provided in Table 1 for SRS operations. In addition IO nominal propellant requirements, this chart shows propellant requiremenu based on savings calculated from the EH method and the two limiting strategies from the TI method.

The propellant savings estimated from the EH method is seen directly between the limiting TI strategies verifying the accuracy in determining average propellant savings. Propellant savings for the 11 year solar cycle range from 29.6% to 41.0% off the nominal (no SRS operations) case.

It should be noted that while only the two limiting altitude strategies were presented in the data, a wide range of possible flight profiles can be followed between hem. Any profile selected should be based on a trade between maximizing propellant savings, reducing payload manifesting impacts. and assuring the integrity of minimum altitude requiremenls.

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Figure 7 - Impacts to Altitude Pmfiles Due to SRS Operations

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