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On-Orbit Propulsion and Methods of Momentum Management for the International Space Station

Samuel P. Russell, Victor Spencer1 and Kevin Metrocavage2

NASA Johnson Space Center, Houston TX 77058

Robert A. Swanson and Ulhas P. Kamath3

The Boeing Company, Houston, TX 77059

This paper describes the concept of operations and architecture of the International Space Station (ISS) on-orbit propulsion system and details the methods used for momentum management. The intent of this paper describes state of the art for the complex ISS currently being built in low Earth orbit. Construction and operation of the ISS propulsion systems are the result of international collaboration with the ISS partners although principle heritage lies with the United States and Russia. While design details are beyond the scope of this work, this paper will provide the reader with an understanding of how the ISS elements (and partners) work together to provide altitude and attitude control for the on-orbit ISS from hardware on-orbit to ground operations.

I. IntroductionINCE the first documented architectural design of a space station in 19291, it has been the dream of many to sustain a permanent human presence in space2. Initially funded by nations, Russia and the U.S. spent several decades competing for a sustained human presence in low Earth orbit3,4. It wasn’t

until the 1980’s that the two super powers began to openly collaborate to achieve this lofty goal5. This open collaboration lead to the current design for the International Space Station (ISS). As identified in the original 1929 publication, any large on-orbit construction project must be performed in stages using discrete modules that, when integrated, perform the basic functions required for human habitation including momentum management and debris avoidance.

The architecture of the ISS is based on heritage designs from both Russia and the U.S., and political division of responsibility to ensure the ISS partners are able to leverage successful experiences and capabilities to ensure joint venture success. While Russian hardware provides the propulsive elements of the ISS, U.S. provided hardware provides non-propulsive momentum management. Joint operations teams creatively implement the integrated designs to minimize unnecessary propellant consumption and ensure the vehicle is flown within the design limits and capabilities of the on-orbit hardware. The architectural elements that make up the propulsion system and the operational scenarios and methods used to fly the vehicle are the subject of this work; however, before these are addressed, it may be advantageous to discuss the concept of operations employed for momentum management of the ISS.

While the 1980’s saw the creation and review of a variety of designs, a Torque Equilibrium Attitude (TEA) approach was ultimately selected for the ISS. This decision lead to the need for active momentum management to ensure the attitude and altitude of the resulting vehicle is maintained. Due to the limited life nature of propulsive end effectors and the lessons learned from previous space stations, the ISS was designed to allow temporary service vehicles to provide the bulk of the propulsive support with on-orbit elements making up the balance. To reduce the need for propellant resupply, a suite of control moment gyroscopes are integrated into the ISS to maintain attitude control between propulsive events. Therefore,

1 ISS Propulsion System Manager, Engineering Division, EP4.2 ISS Attitude and Control Office, Mission Operations Directorate, DI3.3 ISS Propulsion Group Lead, Integrated Defense Systems, MC HB5-30.

S

45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit2 - 5 August 2009, Denver, Colorado

AIAA 2009-4899

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

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the joint operations team has several options available for flying the ISS from purely propulsive capability to purely gyroscopic capability, although, the majority of methods employ a combination of both.

II. On-Orbit Propulsion Systems

A. Architectural Overview As seen in Figure 1, the integrated on-orbit propulsion system is composed of a combination of

permanently attached modules and transient vehicles. The elements that make up the on-orbit propulsion system include Functional Cargo Block (FGB, Zarya or “Sunrise”), Service Module (SM, Zvezda or “Star”), Docking Compartment (DC1), Multipurpose Logistics Module (MLM), Mini Research Module (MRM), Progress, Automated Transfer Vehicle (ATV) and the Soyuz. Of these, MLM and MRM are not yet on orbit, and the Soyuz provides no contribution to ISS momentum control. The U.S. Space Shuttle is not shown in Figure 1, but is used to provide ISS attitude control and reboost during docked missions. Both the Progress and ATV are transient vehicles that supply propellant to the on-orbit system as well as provide propulsive support to the ISS during the relatively short 6 month stay. While technically capable of providing propulsive support, the Soyuz vehicle provides the solitary function of crew supply and return and carries no consumable margin for integrated attitude control.

The fixed elements provide two basic functions, 1) the FGB serves as the propellant storage facility utilizing several externally mounted tanks and 2) the SM serves as the command and control hub using three internally mounted computers. The visiting Progress and ATV vehicles are directly integrated into the SM while U.S. systems remain virtually independent. As each on-orbit element was individually launched, each component was once a single spacecraft complete with a stand alone propulsion system. Of these, only the FGB thrusters have been permanently disabled.

B. Element Description On-orbit propulsion elements are located on the ISS to provide attitude control (pitch, yaw and roll) and

altitude control (reboost). In general, attitude control is provided by auxiliary control thrusters and altitude control is provided by vehicle orbital control thrusters (main engines in the case of Progress and ATV). Pitch and yaw are provided by either the aft Progress or ATV (shown in Figure 1 as ATV) if available, or SM attitude control thrusters. Currently, roll control is provided by the nadir oriented Progress (not shown in Figure 1) or SM attitude control thrusters. The MLM attitude control thrusters will primarily control roll once it is docked to the SM nadir port (shown in Figure 1). Altitude control, including debris avoidance and phasing maneuvers, are performed by either the aft vehicle orbit control thrusters or the SM orbit control thrusters. Both attitude maintenance and altitude changes can be performed by the U.S. Space Shuttle vernier auxiliary control thrusters.

Propellant resupply is a critical operation to long term support of the ISS. To account for periods where transient vehicles are not available, the relatively immense tankage of the FGB and the lesser tankage of the SM are serviced by both Progress and ATV vehicles. Since ISS propulsive support is provided by auxiliary class thrusters, the entire system is pressure fed. To facilitate resupply operations, bladder tanks are employed to prevent vapor entrainment in the pressurization systems, and compressors are used to reduce FGB tank pressures prior to a refueling operation.

Operational safety of ISS Propulsion Systems is managed by design controls and operational control of human exposure to the employed propellants or combustion by-products. The principle hazard of propellant leakage, which can degrade or damage sensitive surfaces and possibly harm human life through

Figure 1. Propulsion Elemenets of the Interational Space Station.

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suit expose, while external to the ISS, is controlled by two-fault tolerant mechanical isolation of on-orbit elements and single-fault tolerance mechanical isolation of visiting vehicles. The difference in control methodology is due in part to the transient and temporary nature of the visiting vehicle and the implied level of insight ground operators have into the health of the vehicle propulsion system. The current approach for transient vehicles is to provide two-fault isolation if leakage can result in loss of life or vehicle. The hazard of operational activation of propulsion effectors is controlled by system design, electrical isolation of critical system components, and operational sensitivity to potential plume fields and areas of plume impingement. Due to the determination of the ground crew and the awareness of the on-orbit crew, operational hazards are identified, communicated, and controlled to minimize risk to crew and vehicle.

III. Momentum Management International Space Station (ISS) attitude control uses a combination of Control Moment Gyroscopes

(CMGs) and Russian Segment (RS) thrusters. The set of four CMGs were designed to provide non-propulsive attitude control during quiescent operations. A controller known as a Momentum Management (MM) controller would allow the vehicle attitude to deviate slightly (3-4 degrees) while maintaining a specific momentum vector. However, dynamic operations such as robotic motion and operations supporting visiting vehicle traffic (i.e., docking and undocking of Space Shuttle Orbiter, Soyuz, Progress, etc.) requires RS thruster propulsive attitude control due to either the inability of the CMGs to maintain the desired momentum vector or because a tighter attitude threshold is required. There are many control handovers between the Russian propulsion system and the U.S. CMGs during ISS operations. During docking and undocking of visiting vehicles, the ISS goes into a free drift mode where the thrusters and CMGs are not used. After the events are complete, the thrusters maneuver the ISS to the desired attitude and then hand over control back to the CMGs. Cooperation between the Russian and U.S. systems and some innovative operational strategies have helped minimize propellant consumption and keep ISS operations efficient.

A. Momentum Vector Matching Orbiter dockings were originally performed using an “Attitude Hold” controller with CMGs utilizing

RS thrusters only to desaturate the CMGs, if necessary. CMG saturation occurs when the gyroscopes lose control of the momentum vector due to external torques. The CMGs can only absorb a certain magnitude of momentum. When the CMG momentum vector reaches its thresholds, it is designed to request a desaturation, or a “desat”. A desat uses RS thruster firings to realign the set of CMG spin axes in opposing directions in an effort to maintain attitude control. A single desat would typically use less than one kilogram of propellant, however, in cases where holding a specific attitude or maneuver was required, multiple desats would be necessary over a period of time (i.e., attitude hold prior to Orbiter dockings), potentially using dozens of kilograms.

Operational constraints have been imposed since the 2002 failure of an on-board CMG, in an attempt to limit the CMG gimbal rate, essentially eliminating desats. Desats were known to impart a high rate on the gimbals, which is believed to have contributed to the hardware failure of the CMG. These operational constraints have required the sole use of RS thrusters for maneuvers (with a few exceptions) as well as attitude hold periods when the Orbiter is not present. The Orbiter has been used during mated operations to maneuver the ISS/Orbiter mated vehicle stack. However, all Orbiter dockings since 2002 have been performed under RS thruster control.

Due to the varying configurations of the ISS, it is difficult to estimate the propellant usage comparison if we had not been required to use RS thrusters in such a capacity. However, the engineering and operations teams have come up with several techniques and operational measures that have proven to reduce propellant consumption during ISS dynamic operations. One method involves positioning the set of CMG inner and outer gimbals so that the resultant momentum vector matches the targeted momentum vector defined by the Torque Equilibrium Attitude (TEA) for that specific vehicle stage.

B. Torque Compensation Method When momentum vector matching method is not in use and RS thrusters are controlling the ISS, the

CMG momentum vector is maintained constant with respect to the inertial coordinate system and does not provide any torque to the ISS. However, it was determined that the CMG gimbals could be positioned such

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that the orbital motion of the ISS can be used to create a beneficial additional torque. This additional torque would be beneficial when holding attitudes that deviate from the TEA. Previously during periods of extended RS attitude control, a maneuver to the Minimum Propellant Attitude (MPA) was performed to minimize excess propellant usage caused by holding attitudes that deviated from the TEA. A maneuver to and from the MPA typically costs 10-15 kg of propellants. The additional torque produced by CMG positioning would compensate for the expected external torques and thus reduce the amount of propellant required to maintain vehicle attitude by eliminating the maneuvers to/from the MPA.

The torque compensation method has proven to provide significant propellant savings. It has reduced the propellant consumption during an Extra-Vehicular Activity (EVA) from an average of 24 kg to about 8 kg. Similar results have been observed in other dynamic activities. For example, a 50% to 75% savings has been seen for Progress vehicle undocking and up to 80% reduction in propellant usage during ISS software uplinks. The total estimated propellant savings is about 150 to 200 kg per year.

C. U.S. Thruster Only Controller In an effort to help restore the operational capability of the CMGs to perform attitude maneuvers under

U.S. control, the engineering and operations teams developed and demonstrated what became known as the U.S. Thruster Only (USTO) controller. The USTO controller was successfully tested during ISS Stage operations by performing an attitude maneuver from the ISS X-axis Perpendicular to Orbital Plane (XPOP) reference frame to the X-axis in Velocity Vector (XVV) reference frame in December 2004.

The controller gets its “U.S. Thruster Only” name because it is used while the U.S. Segment (US) is responsible for attitude control of the ISS, but uses RS thrusters instead of just CMGs. The USTO controller has the same basic composition of the originally developed “Attitude Hold” controller. The primary difference is that the USTO controller sets the torque command being sent to the CMGs to zero. In doing so, the CMGs experience no gimbal rates directly associated with attitude control. Instead of being sent to the CMGs, the torque commands required for attitude control to the desired attitude are processed and sent to the Russian Segment as a momentum request via Reaction Control System Assist (RCS Assist) logic, a function originally designed to provide torque over and above the CMG torque limit if excess control torque was required.

While RS thrusters are primarily used, the CMGs do not remain idle. The desat threshold is set to zero in USTO controllers, which prompts an infinite loop of desat requests to the Russian Segment (approx. every 18 seconds) in addition to the control torque commands. As a result of the desat requests, the CMGs actually do provide a small amount of “feed forward” torque, which drives the CMG gimbal angles toward the desired momentum vector of the momentum management controller. This process significantly reduces the transients expected upon momentum management startup, hence protecting the CMGs.

The propellant saving benefit of this method is minimal, but not insignificant. The availability of the USTO controller helps reduce the necessity for a US to RS attitude control handover during certain ISS operations (such as venting, vehicle safing, etc) that would require an Attitude Hold controller. Elimination of the attitude control handovers minimize the number of commands to be sent to the vehicle, thereby reducing the command error risk, while also saving some propellant usage. However, the actual amount of propellant used for a maneuver under USTO control and RS thruster control are very similar.

D. Zero Propellant Maneuver Perhaps the most significant method of performing attitude maneuvers in an effort to minimizing

propellant usage is via a non-propulsive attitude maneuver, known as the “Zero Prop Maneuver (ZPM)”. This method was developed by Draper personnel and was initially executed by the ISS Flight Control Team (FCT) in the Mission Control Center - Houston (MCC-H) in November 2006.

An attitude maneuver from the +XVV (+X axis in Velocity Vector) to +YVV (+Y axis in Velocity Vector) reference frame was demonstrated and proven successful. A series of attitude maneuver and attitude rate command pairs were calculated to transition the ISS from an initial to final state, with respect to attitude, rate, and momentum state all within vehicle momentum and CMG gimbal rate constraints. Once the software-generated commands were verified, the time-tagged commands were manually uplinked to the ISS Command and Control (C&C) Multiplexer De-Multiplexer (MDM) on-board computer prior to the desired maneuver time. Once uplinked, the time-tagged rate and attitude update command pairs executed (one second apart) beginning at the desired maneuver start time. Using the CMGs, the ISS maneuvered slowly towards the target attitude using the updated rate commands set. This 90-second cycle was repeated until the ISS achieved the desired attitude—taking approximately 1.5 to 2 hours to complete the entire

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maneuver. The demonstration proved that the ISS attitude maneuvers could be performed with sufficient CMG momentum, without the use of RS thrusters. This capability may also be used during mated operations with the Orbiter.

The ZPM method of performing an attitude maneuver takes significantly longer than the alternate USTO or RS thruster-only methods. Performing a +XVV to +YVV maneuver using RS thrusters takes approximately 15 mins (not including the time on RS attitude hold prior to and after the maneuver). A similar maneuver on USTO would take about 30 mins (not including approx 30 mins of attitude hold on USTO prior to incorporating the non-propulsive momentum management controller). However, in addition to low CMG gimbal rates (< 1 deg/sec), the benefit of performing the entire maneuver without expending any propellant far outweighs any impacts the additional maneuver duration time a ZPM would impose. Using this method saves a significant amount of propellant over the life of the ISS, minimizes contamination and erosion of sensitive surfaces such as solar arrays and maintains positive cycle life margins in propulsion system components such as propellant tank bellows/diaphragms, compressors and thrusters. To date, only a few maneuvers have been executed using ZPM. However, continued success and evaluation may prompt the operations team to standardize this method of maneuvering.

IV. Propellant Accounting & Budgeting

A. Propellant Accounting Due to the criticality of providing sufficient propellant when needed, it is essential for ISS operators to

track and monitor propellant quantity within each available tank. Although many in situ devices have been used, such as Linear Translation Transducers, Radio Frequency (RF) Quantity Gauging and Flow-meters; the most accurate long term method has proven to be the more analytical methods of Burn-time Integration (BTI) using known firing durations and expected performance ratios and Pressure, Volume, Temperature (PVT) methods using simple and reliable vehicle health measurements. Utilizing telemetry received from the ISS vehicles and elements, the ground operators calculate on-orbit propellant quantities and ensure sufficient margins exist for both planned and contingency operations ranging from phasing adjustments for visiting vehicles, to debris avoidance, to changes in the Earth’s atmosphere. Since PVT has proven to be the most accurate method of tracking on-orbit propellant usage, this method is briefly discussed in more detail.

PVT uses measured pressure and temperature changes in the pressurization system combined with known tank volumes to calculated changes in liquid volume. The basic equation used is the ideal gas equation:

MRzmTPV

Where P is tank pressure, V is the tank volume, z is the compressibility factor of the gas, m is the mass of the gas, T is the absolute gas temperature, R is the Universal Gas Constant and M is the molecular mass of the gas.

With a known mass of the total pressurant in the system, the pressurant volume in the propellant tank is determined by using the measured pressure and temperature data in the PVT expression. Onboard propellant volume is then determined by subtracting this pressurant volume from the propellant tank volume. As this method provides only an indication of change, distributed pressurant and propellant quantities are not directly computed but the error is known to be small.

B. Propellant Budgeting

Currently, it is difficult for the ISS Flight Control Team to gauge how much propellant consumption for various activities is appropriate. Recently, an operations flight rule was implemented regarding ISS propellant management that defines four categories to which propellant would be managed. These categories will eventually be used as part of a process to budget the use of ISS propellant, however specific guidelines for the following propellant budgeting is still under negotiations between International Partners.

‘Category 1’ propellant would encompass the ISS Program “Reserve”, defined as the minimum amount of propellant to be held for the preservation of the ISS in a contingency where further propellant delivery would be unavailable. The quantity of propellant that is budgeted as Category 1 propellant, accounts for events that would be required to maintain the ISS for approximately a year. Such activities include reboost

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propellant to maintain an average altitude of 278 kilometers and attitude control capability for approximately one year. Other activities include the propellants required to support a docking attempt with the Orbiter or Progress vehicle, a 1 m/s debris avoidance burn, and any beta dependent attitude changes necessary to maintain ISS thermal and power system requirement.

Outside the ISS reserve, a propellant budget would also be effective for nominally planned ISS attitude control activities. This would be known as ‘Category 2’ propellant. Activities such as standard attitude maneuvers, visiting vehicle arrival and departure, Orbiter-mated activities that require ISS attitude control and other nominal activities such as attitude control for planned software transitions and updates. The quantity of propellant budgeted for this category would be decided on an ISS Increment basis and agreed upon well in advance of the start of the associated Increment.

The third propellant management category would encompass activities that are not essential for ISS operations, but planned by the Flight Control Team. The ‘Category 3’ propellant, or discretionary propellant, would only be allocated on an Increment basis if it does not impact the availability of Category 1 and 2 propellant - as well as the portions of Category 4 propellant allocated for expected long-term altitude maintenance.

Category 4 propellant encompasses the quantity of propellant determined to be required for ISS altitude maintenance, debris avoidance maneuvers, and failure cases that require propulsive attitude control. It also includes propellant that is needed to support long-term program viability.

While these four categories have been defined, the actual process for budgeting the propellant is still in work. Once finalized, this methodology is expected to serve as a reference guide for an efficient propellant management strategy, which can then be used by the ISS Program and Flight Control Teams for planning and execution.

V. Conclusion

The ISS Propulsion System has proven to be a robust implementation of legacy hardware and evolving operational strategies. While the architecture is rigid, the modular nature of the assembly lends itself to an evolving political and scientific need while continuing to develop the means to sustain a continued human presence in space. While no system escapes without some problems, issues with the robust hardware have been minimal and ranged from contamination induced under performance to combustion by-product contamination of pressure sense tubes and sensitive surfaces. While risk of premature system failure and operational safety are managed through design of fault tolerance systems and operational awareness, operational risks and program reserves are managed through creative flying modes. Utilizing novel techniques such as Zero Prop Maneuver on a spacecraft as complex as the ISS saves a significant amount of propellant, minimizes contamination and erosion of sensitive surfaces such as solar arrays, and maintains positive cycle life margins in propulsion system components. Finally, accountability and budgeting of the limited consumable resource on orbit bears consideration by the system designer and a novel strategy with reliable accounting method are proposed.

VI. References

1 Herman Noordung, Das Problem der Befahrung des Weltraums - der Raketen-Motor (The Problem of Space Travel - The Rocket Motor), 1929 2 A History of Space Stations, NASA Facts, IS-1997-06-ISS009JSC, 1997 3 B.J. Bluth and Martha Helppie, Soviet Space Station Analogs, Second Edition, NASA-CR-180920, 1986 4 Leland F. Belew, Skylab, Our First Space Station, NASA-SP-400, 1977 5 David S. F. Portree, MIR Hardware Heritage, NASA RP-1357, 1995