Analysis and Preliminary Design of On-Orbit Servicing Architectures for the Gps Constellation

AIAA-99-4425

ANALYSIS AND PRELIMINARY DESIGN OF ON-ORBIT SERVICING ARCHITECTURES FOR THE GPS CONSTELLATION

Gregg Leisman* Adam Wallen** Stuart Kramer† William Murdock‡ Air Force Institute of Technology, Wright-Patterson AFB OH 45433

Abstract Introduction Satellites are the only major Air Force systems with no Problem maintenance, routine repair, or upgrade capability. The The space community currently builds extremely reliable, result is expensive satellites and a heavy reliance on redundant satellites, and replaces them when a critical access to space. At the same time, there is a trend toward component fails. The result is expensive satellites and a longer satellite design lives and larger constellations of heavy reliance on access to space. However, as satellite satellites. This makes it difficult to keep the technology reliability and design life increases and space systems on satellites current without frequent replacement of those transition to larger constellations of satellites, the cycle satellites. Technologies have improved to the point that time between deploying new technologies increases. This robotic on-orbit servicing could be used to upgrade satel- leaves the satellite community with a conflict between lites at a reasonable system cost. This paper evaluates keeping spacelift cost low through long design lives multiple architecture concepts for on-orbit servicing using (which leaves the program technology out of date), or a systems engineering process. The process includes keeping the constellation equipped with new technology identifying the user’s problems and desires, generating through short satellite design lives and frequent need for multiple servicing alternatives, evaluating the alternatives multiple replenishment launches. against the user’s desires, and identifying the most promising alternative. The Global Positioning System Unlike satellites, the majority of weapon systems can take (GPS) constellation is used as a case study to explore the advantage of long-term logistics support. Aircraft useful costs and benefits of upgrading and/or repairing satellites life is extended through routine maintenance and aircraft through on-orbit servicing. The conclusions are capabilities are extended with payload upgrades. With applicable to both GPS and the satellite community in the ability to provide logistics to the end system, the Air general. This study concludes that on-orbit servicing Force is much more efficient at developing, maintaining, would offer greater benefit and would be less costly than and operating its land-based systems than its space the current GPS satellite management paradigm. systems. The Air Force and the space community in ______general need a fast, flexible, and cost effective system of * Gregg Leisman, Space Systems Engineer, NAIC/TASS, upgrading and maintaining a constellation of satellites. Wright-Patterson AFB OH 45433. E-mail: [email protected]. Scope ** Adam Wallen, Analyst, ACC/XPSASL, Langley AFB VA 23665. E-mail: [email protected]. Some of the bounds on this research should be noted. † Stuart Kramer, Associate Professor, Dept of Aeronautics and The case study is based on the Global Positioning System Astronautics, Air Force Institute of Technology, 2950 P St Bldg (GPS) constellation. However, we always approached the 640, Wright-Pattterson AFB OH 45433-7765. E-mail: research with the objective that the results would be [email protected]. broadly applicable. The characteristics for a GPS Robotic ‡ William Murdock, Assistant Professor, Dept of Operational Servicing System (RSS) are similar to the requirements Sciences, Air Force Institute of Technology, 2950 P St Bldg for a broad array of satellite systems. 640, Wright-Pattterson AFB OH 45433-7765. E-mail: The design and analysis of the robotic servicing system [email protected]. did not address the complex technical and contractual The views expressed are those of the authors and do not modifications that would be necessary to make the GPS reflect the official position of the United States Air Force, the satellite serviceable. Department of Defense, or the government of the United States. Since most satellites (including GPS) are beyond the This paper is a work of the U. S. Government and is not subject to copyright protection in the United States. range of the Space Shuttle we did not consider manned

1 American Institute of Aeronautics and Astronautics servicing alternatives. The results of previous studies GPS Constellation support this exclusion.*

Other minor assumptions, as well as a more complete Life Cycle Cost Performance Program Viability discussion of the method and results, may be found in the full thesis documentation.1 Recurring Availability Dual Use

Process Shared Cost Nonrecurring Flexibility Costs to GPS Retrofit/Upgrade Overview Implementation The complexity of the problem and of space systems necessitated the use of a clear, logical problem solving Figure 1. Value Model process. The second dimension of Hall’s system engineering process3 provides a good basis for such a process. The steps are problem definition, value system the second tier displays those areas in greater detail. The design, system synthesis, system analysis, alternative cost consideration reflects costs of an alternative to the optimization, decision-making, and planning for the GPS program office. Performance refers to increased future. These were modified slightly for this study. Each performance of the GPS constellation due to an step is discussed in turn below. alternative. Program Viability reflects the likelihood an alternative would pass the scrutiny of approving bodies Problem Identification such as the Air Staff and Congress. The basic objective of this study is to identify the The next level of the value model consists of a number of logistical support needs of GPS constellation, find specific performance measures that were judged to be multiple servicing solutions, and identify which of these indicative of success in meeting the established solutions best meets those needs. objectives. The actual performance of each alternative The GPS constellation consists of 24 satellites in 6 orbital with respect to the measures is computed in a later step. planes. New technology and capabilities are provided Alternative Architectures only as the current satellites retire. The next generation of Block IIF satellites will have a design life of 12.7 Clear strategies for meeting these objectives now must be years. Thus, in the future, providing the full constellation identified. These employment strategies define how the with new capabilities will require 24 launches over GPS program will use a RSS. Responsive upgrade time is approximately 13 years, not including development and a primary concern for GPS, so upgrade capability will be acquisition of the new technologies. Obviously, the fundamental to each employment strategy. Since repair is military or commercial entity that can deploy new desireable but not necessary it is included in only some capabilities faster will have an advantage over its employment strategies (ES). The three employment competitors. The problem to be solved in this study is strategies used in our study are: how to decrease cycle time for implementing new ES I: Service for Upgrade and Retrofit Only capabilities while still minimizing costs. ES II: Service for Upgrade and Scheduled Repair Value Model ES III: Service for Upgrade and Quick Response A RSS would provide specialized support to a customer Repair. satellite program, so the needs of that program define the critical objectives of a RSS. The value model shown in Each RSS alternative we generate and evaluate will Figure 1 was developed through extensive interviews with come from one of these employment strategies. GPS managers and reflects the objectives that those The above strategies define how the customer will use a decision makers consider important in evaluating RSS. The next step is designing specific concepts, or alternative RSSs. The first tier of the model hierarchy architectures, to fulfill the needs of the employment represents the three main evaluation considerations and strategies. To devise these concepts requires a combination of orbital dynamic analysis, systems * Manned on-orbit servicing contains a wealth of literature; a good engineering, review of previous work, and brainstorming. example of a system engineering study is the Space Assembly, Maintenance, and Servicing (SAMS) study. A summary of SAMS is We selected eight OOS architectures that we felt could found in On-Orbit Servicing of Space Systems.2 best meet the needs of the customer. These architectures vary in required technologies and performance characteristics. A key idea in the systems engineering process is to produce successive iterations of the most

2 American Institute of Aeronautics and Astronautics promising alternatives. Accordingly, we produced more vary in size of launcher (intermediate, medium, small, or alternatives from architectures that showed a lot of reusable), number of orbital planes payloads are launched promise. to, and type of upper stage used (solid, liquid, or solar thermal engines). The Robotic Servicer Propulsion “A” – Robotic Servicer (RS) in Each Orbital Plane: Subsystem (RSPS) maneuvers the RS between ORU Short Term Upgrader canisters and GPS satellites within an orbital plane, and “B” – RS in Each Orbit: Long Term Upgrader possibly between GPS orbital planes. The RSPS options are to use liquid, solar thermal, or ion electric propulsion “C” – RS in Each Orbit: Upgrade and Semi-scheduled systems. Of all the subsystems, it is the most dependent Repair upon the choice of other RSS subsystems. The Robotic “D” – RS and Mini-depot in Every GPS Orbit Manipulating & Bus Subsystems (RMBS) is the payload of the RS. Its function is to add or change out ORUs on “E” – Precessing On-orbit Depot: Advanced Propulsion the GPS satellites. Subsystems on the RMBS provide the “F” – Precessing On-orbit Depot: Chemical Propulsion power, communication, attitude control, and data processing for the robotic servicer. RMBS varied from a “G” – Upgrader with Direct Plane Change Capability: high performance servicer similar to the University of Ion Propulsion Maryland’s Flight Telerobotic Servicer4 to a simple “H” – Upgrader with Direct Plane Change Capability: docking servicer like Air Force Research Laboratory’s Solar Thermal Propulsion concept5. Figure 2 illustrates the decomposition just described. Decomposition of System Requirements and Design of Subsystems In addition, these subsystems had to be sized according to the amount of replacement satellite components (ORUs) Obviously, the processes of delivering robotic servicers serviced on the user satellites. High, medium, and low (RSs) and orbital replacement units (ORUs) to multiple capacity refers the mass of ORUs. The three capacities orbits, rendezvousing and docking the RS with ORUs and correspond to 50, 150, or 300 kg. The interested reader satellites, performing robotic servicing on customer should refer to the full thesis for more detailed description satellites, and so on are very complex tasks. To generate of the alternatives. multiple alternatives that can perform these tasks, it is necessary to break down the problem into subproblems. Synthesis Philosophy Subsystems are then designed to solve these subproblems. The next step was to synthesize alternatives from the To ensure our process surveys a multitude of solutions, options for each subsystem. A key principle of systems we designed multiple subsystem solutions for each engineering is to ensure all feasible possibilities are subproblem. Finally, using the systems engineering considered. Thus, a wide variety of RSS alternatives are process, we synthesized these subsystems into complete examined. The design space for the feasible alternatives robotic servicing systems. is quite large. There are eight different orbital The Logistics and Transportation System (LTS) architectures, 21 launch vehicle and upper stage subsystem transports ORUs and RSs from the ground to combinations, with 3 different ORU capacities, 3 different the necessary orbits. The LTS subsystems alternatives Robotic Servicers, and 2 different GPS constellation configurations. This results in 3024 different feasible combinations, which does not account for minor

Robotic Servicing System subsystem design deviations. Analyzing all the different combinations was beyond the scope of this study. Instead we examined thirty alternatives with the goal of Orbital Architectures Logistics & Transportation Robotic Servicer (R.S.) Robotic Manipulating System Serviceable GPS System (LTS) Propulsion (RMS) & R.S. Bus representing Satellites a broad spectrum of servicing systems. To standardize the alternatives so they could be evaluated R.S. in each Launch Vehicles Liquid Chemical High Performance orbit (4 catetories) Propulsion Servicer with the customer value model we made the following assumptions: One R.S. and depot Dispenser Solar Thermal Medium Performance in a precessing orbit (for multiple orbits) Propulsion Servicer  Each alternative will be evaluated for benefits and R.S. with plane ORU Transport Electric Low Performance costs over a 15-year operational period. change capability Canisters (OTC's) Propulsion (Free-flying) servicer  This 15-year period will involve up to four servicing Canister Upper Stages missions to the entire constellation (4 missions to the 24-satellite constellation = 96 individual servicing Figure 1. Subsystem Decomposition missions).

3 American Institute of Aeronautics and Astronautics Analysis Evaluation The performance of each of the selected alternatives and This step consisted of computing the values of the thirty the status quo was computed using a set of models that selected candidate systems plus the status quo using the took design parameters characterizing each alternative established value model shown in Figure 1. The value (such as physical characteristics of the RSS components, model, as a reflection of the decision-maker’s objectives, architecture and employment strategy used, and selected provides a consistent basis for making tradeoffs between orbital transfer methods) and returned measures of the various individual performances. The value model performance (such as transfer or servicing times needed assigns an overall value score to each performance for the different mission segments, the RSS’s ORU measure according to value functions developed in capacity, and number of launch vehicles needed). Most conjunction with the decision-maker. The total value for of the models are fairly straightforward (although many each alternative is computed by taking a weighted sum of times complex) computations or simulations created by the value scores for each of the measures. Again, the the analysts. These results are omitted here due to the weights reflect the relative values of the decision maker. large volume of data and the constraints of this paper; a full discussion can be found in the thesis documentation.1 Results One critical measure that is worth reviewing is cost. It is now possible to compare the alternatives and the Costs were computed using the existing NASA/Air Force status quo given the costs and other performance (NAFCOM) 1996 parametric cost analysis program6 estimates for each. One may combine all the performance Figure 3 summarizes the costs of the different measures into a single number as described above. Many alternatives. First mission cost includes all the decision makers, however, feel more comfortable if cost developmental costs and any additional cost the first is presented separately. mission has over any reoccurring mission costs. For We have chosen to use this second approach in this paper. example, many of the alternatives would launch robotic Figure 4 is a plot of the cost and composite non-cost value servicers on the first mission, and would only launch for the 30 RSS alternatives, the current 12 year ORUs on subsequent missions. The mission average cost replacement policy (status quo), and a single “brute force” represents the long-term average cost over four servicing replacement of the entire GPS constellation. The cost for missions to the constellation. Recurring cost Mission costs ($M) for Robotic Servicing Alternatives represents the High Capacity (300 kg) Med. Cap (150 kg) Low Cap. (50 kg) cost of a Recurring mission cost Mission average 1st Mission standard 700 mission once 3 Plane GPS Constellation 6 Plane GPS Constellation the servicing 600 alternative is operational.

H H M M L L H H H M M L H H M L L H M L H M L L M M L L M Architecture "A" "B" "D" L "A" "B" "D" "E" "F" "G" "H"

Figure 3. Mission Costs

4 American Institute of Aeronautics and Astronautics Table 1 gives the essential characteristics of those 6 Value vs. Cost alternatives. 10 Region of the “Best” on-orbit The key to keeping RSS costs low is to have multiple 9 servicing alternatives satellites in roughly the same orbital plane. For example,

8 one robotic servicing system could service all the geostationary satellites. Therefore, while the cost to 7 benefit tradeoffs in this case study are very promising, Rapid replenishment, the “Brute there are probably even more cost effective systems yet to V 6 Force” method a be analyzed. l 5 u Observations e 4 Feasibility 3 Current policy of 2 satellite The servicing architectures in this study are not beyond 2 replacements/year today’s current or developing technologies. Some of the

1 most crucial technologies to make RSS cost effective are modifications of systems already in existence. For 0 example, a driving force in making a RSS beneficial is to 0 500 1000 1500 2000 2500 3000 service the entire constellation on one or two launches. Average Mission Cost ($ Mil) 3 Planes 6 Planes This could be done by modifying the dispenser system that Iridium and Globalstar use with upper stages for each Figure 4. Non-Cost Value versus Cost payload. This one modification would enable the RSS to reach all the orbital planes of the GPS constellation on each alternative is the cost above the current GPS one or two launches. program budget for maintaining the constellation. The The technologies of the propulsion systems in this study upper left corner of the graph is the most desirable region were baselined from existing or developing systems. The since we want higher value and lower cost. characteristics of the advanced systems (solar thermal, ion As expected, all the alternatives to the status quo receive electric) are based on NASA’s DeepSpace 1 satellite7 higher value scores. Significantly, all the RSS which is currently flying and the soon-to-fly Solar Orbital alternatives were much less costly than a “brute force” Transfer Vehicle from the Air Force Research replacement. Using current methods, the average cost of Laboratory8. Our only major change was to increase the replacing a GPS satellite is approximately $100 million. size of these systems. The most expensive of the top six alternatives could upgrade the entire constellation Alternative of satellites Parameter 22 16 13 21 20 18 for $60 million per # GPS Planes 6 6 6 3 6 3 satellite. Architecture “D” – R.S. & “B” – R.S. in “B” – R.S. in “D” – R.S. & “B” – R.S. in “B” – R.S. in Among the ORU depot each orbit each orbit depot in each each orbit each orbit RSS in each orbit orbit alternatives, the value to ORU Capacity (kg) 150 300 150 150 50 50 the satellite program and Servicer Capability High Medium High High Low Medium the cost also RS Design Life 15 years 15 years 15 years 15 years 15 years 15 years varied. The six circled RS Mass Total (kg) 715 387 639 1021 183 412 alternatives RDT&E Cost $137 Mil $101 Mil $137 Mil $147 Mil $101 Mil $81 Mil have the best value for Avg. Mission Cost $338 Mil $317 Mil $290 Mil $229 Mil $123 Mil $113 Mil their cost. Table 1. 'Best' Alternatives

5 American Institute of Aeronautics and Astronautics The technology that would require the most development 250 is the robotic servicing system. However, the most t s capable robotic servicer in this study was based on the o 200 C

Ranger Telerobotic Servicer, which has been developed ) n s

9 o i by the University of Maryland with limited funds. n 150 s o s i i l l

Orbital Characteristics i M

100 m e $ g

The top three alternatives used the dispenser concept. By ( a having a parking orbit at a different altitude than the r 50 e destination, it would be possible to deliver payloads to v A multiple orbital planes from one launch vehicle. This is 0 critical to developing a low cost method of on-orbit 22 16 13 21 20 18 servicing for a large constellation. Fortunately, using a RSS Alternative dispenser in LEO is becoming common place, and there Launch Vehicle Costs Space Segment Costs should not be a significant leap in technology to incorporate upper stages on the payloads. Figure 5. Cost Distribution One of the most important observations is identifying which RSS concepts (or architectures) are most promising. All top six alternatives came from Conclusions Architecture “B” or “D”. These architectures based one Impact of Results long-term robotic servicer in each of the customer’s orbital planes. Having a cheaper, shorter life servicer Our results – the analysis of on-orbit servicing (Arch. “A”) or a robotic servicer than could transfer alternatives and the process that guided the analysis – between planes (Arch. “E”, “F”, “G”, and “H”) was not as have potentially far reaching effects in the satellite effective. Thus, advanced propulsion systems are not a community. GPS recognizes the need to explore evolving necessity as with Architectures “E”, “F”, “G”, and “H”. technologies that can increase constellation flexibility. They need the ability to deploy capabilities faster. Such System Characteristics an ability would make it possible to market their satellites The results of our analysis gave insights to some of the as platforms for customers other than their traditional technologies and system characteristics that are most ones. Our study has shown that on-orbit servicing can critical for a RSS. deploy new capabilities in a rapid manner with reasonable cost. On-orbit servicing of the GPS constellation would None of the three RS concepts was dominant within the give the United States the ability to quickly deploy global best alternatives. In fact, robotic servicers were not the coverage space capabilities. In addition, on-orbit biggest contributor to the overall cost of the servicing servicing de-conflicts the drive to lower costs through system. One characteristic that maximized the benefit to longer satellite design lives from the ability to respond cost tradeoff was long operational lives for the servicers. quickly to changing requirements. In essence, GPS could Even though they were larger and more expensive, evolve from a navigation satellite to a multi-use global servicers that could stay on orbit for multiple servicings platform. of the constellation were more beneficial than servicers that serviced the constellation only once. To accomplish the goals of deploying capabilities faster, GPS must still evaluate a wide variety of alternatives. While none of the top six alternatives used an exotic On-orbit servicing is a category of those alternatives, and robotic servicer propulsion system, propulsion our results offer an analysis of thirty on-orbit technologies did play a critical role in the Logistics and architectures. At least as important as the specific results Transportation System. By using a solar thermal upper of this analysis was the process and framework for stage, we were able to use a much smaller launch vehicle, evaluating alternatives. The GPS JPO has been part of which dramatically minimized overall system cost. As our process from the beginning and is in an excellent one can see in Figure 5, launch costs account for over position to facilitate further work in a larger forum. They 50% of the recurring mission costs. Therefore, solar have drafted a proposal to draw other satellite program thermal and ion propulsion need to receive further managers and representatives into a discussion on the research as operational upper stages. future of satellite operations. The Players GPS is not alone in this quest for better satellite management. With the initial deployment of the

6 American Institute of Aeronautics and Astronautics International Space Station, the Special Purpose Satellites have become an integral element in the Dexterous Manipulator is under development as an functioning of our society.. Satellite program managers integral part of the assembly and maintenance of the with an eye to the future know that they must find a way station. The designers of Space Based Laser (SBL) have to keep up with the rapidly evolving demands on their identified on-orbit servicing as an enabling technology for satellite systems. This necessity becomes more apparent refueling of an operational system.10 Just as SDI saw the to the Department of Defense as more foreign militaries benefits of on-orbit servicing in the 1980’s, so SBL sees operate in space. As the U.S. continues to respond to the benefits of servicing now. Air Force leadership threats around the world, military space systems offer the recognizes the need to investigate this potential capability continuous, global coverage capabilities that are and created a Modular On-orbit Servicing (MOS) instrumental in achieving our objectives. However, to Integrated Product Team (IPT) in November of 1997. maintain a leadership role in space technology development, our military space systems must be Enabling Technologies responsive to changing requirements. Flexibility becomes Several objectives for a new constellation management more of a challenge for larger satellite constellations such methodology are likely to come from customers of the as GPS with 24 satellites or Iridium with 66 satellites. constellation. It is important to identify the customers On-orbit servicing is a promising candidate to achieve this who might benefit from an ability to put payloads into flexibility. It offers the ability to put new hardware on GPS orbit and customers interested in global coverage. existing satellites and repair failed satellites. It could do Researching their potential requirements can guide the this in a fraction of the time and cost it would take to decision-making process and focus alternative evaluation. design, build, and launch a new satellite system. It would allow the trend to reduce programs costs through longer This thesis intentionally focused on technologies that satellite lives to continue, while providing a cost effective already exist or are in development. It will be necessary method of keeping the satellite systems’ capabilities to investigate the progress of these technologies to better current. refine the timeline to test and field the alternative of choice. However, if an enabling technology is beyond the Management with on-orbit servicing offers unique current thinking of researchers, it will be necessary to benefits most satellite programs do not have. Whether the conduct feasibility studies for the enabling technologies. U.S. military will go forward with this method is uncertain. What is certain is the growing need for a new Due to the breadth of the on-orbit servicing field, this satellite management paradigm. Programs such as GPS thesis did not cover every available concept. One concept and SBL are actively investigating new solutions. that should be explored is the use of piggybacking Technology that exists now or is in development may payloads. Since launch costs are a large portion of the hold the keys for managers to more efficiently maintain overall system costs, a “free ride” to orbit has many the currency of their satellite systems. benefits. The main drawback is that this opportunity is very program specific, since many programs do not have References the needed excess launch capacity. Another concept that could be analyzed is the use of electric propulsion for the 1 Leisman, Gregg and Wallen, Adam, Design and precessing depository orbit architecture. This Analysis of On-orbit Servicing Architectures for the investigation would involve significant orbital dynamics Global Positioning System, MS Thesis, Dept of analysis, but could provide very favorable alternatives. Aeronautics and Astronautics, Air Force Institute of Technology, Wright-Patterson AFB OH, Mar. 1999. Also Conclusions www.he.afrl.af.mil/hes/hess/programs/space/on-orbit.htm. The GPS JPO is in a position as an experienced, 2 Waltz, Donald, On-Orbit Servicing of Space successful, and forward thinking satellite program to Systems. Malabar: Krieger Publishing Company, 1993. champion support for a new satellite management paradigm. This research defined and explained a 3 Hall, Arthur D. III, “Three Dimensional thorough process for evaluating constellation architecture Morphology of Systems Engineering.” IEEE alternatives for the GPS program. This process can Transactions on Systems Science and Cybernetics, Vol. extend to evaluate alternatives for other satellite programs SSC5, No. 2 April 1969: 156 – 160. and for a composite group of programs in a cooperative 4 Ranger Telerobotic Flight Experiment Integrated forum. The satellite community could benefit greatly Design Review #2: Books 1 & 2. Space System from a change in their methods, and the program that Laboratory, University of Maryland. 3 – 5 April 1996. leads the way stands to benefit the most through its ability to guide the changes.

7 American Institute of Aeronautics and Astronautics 5 Madison, Richard, “Modular On-Orbit Servicing (MOS) Concept Definition and Description”. Excerpt from unpublished article, 1998. 6 NASA / Air Force Cost Model 1996 Program, Version 5.1. Computer software. Science Applications International Corp., Huntsville, AL, 1997. 7 Dornheim, Michael A., “Boeing to Design Solar Upper Stage,” Aviation Week & Space Technology, 30 March 1998: 76,77. 8 Dornheim, Michael A. “Deep Space 1 Prepares to Launch Ion Drive,” Aviation Week & Space Technology, 5 October 1998: 108-10. 9 Parish, Joe, Program Manager for Ranger, University of Maryland. Personal interview. 30 December 1998. 10 Knutson, Betsy, Logistics Management Specialist, Space and Missile Center, Los Angeles Air Force Base CA. Telephone interview. 17 February 1999.

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