understand the possibilities and problems of the space her need,and/or the designer's perception of the of possible solutions-the tradespace.3 It was appropriate design space,resulting in a need to repeat developed at MIT from earlier work on information the analysis.The semi-automated nature of the systems analysis applied to space systems.Integrated computations allows this valuable exploitation of Concurrent Engineering (the CON in MATE-CON,but emergent understanding with little cost or time penalty. usually referred to on its own as ICE)is a method for Eventually,a design or designs from the trade space are rapidly producing preliminary designs in a "design selected for further consideration. room"environment.The system used in this study descends from work at JPL and the Aerospace In this study.a somewhat simplified version of the Corporation,by way of Caltech.'The overall MATE- MATE method was used.The method was adapted in CON system,along with other front-end design tools, response to difficulties including the lack of an was developed by a consortium of MIT,Caltech,and immediate customer and a very open design space.The Stanford.8 customer utilities were handled parametrically to understand the sensitivities of the tradespace to ranges Using MATE,several hundred possible space tug of,and changes in,user needs.The analysis was done vehicles are evaluated for their ability to move mass in at a high level,using low-fidelity models,but covering orbit and interact with targets.The resulting tradespace a large range of possible designs. is examined to clarify some of the fundamental difficulties with the space tug concept,understand the Attributes and Utilities sensitivities of the tradespace to uncertainties in users The capabilities of a space tug vehicle determined to be needs,identify the Pareto front of"good designs,and useful to a potential user include:(1)total delta-V find some design points that are promising for multi- capability,which determines where the spacetug can go purpose tugs.ICE is then used to create ten conceptual and how far it can change the orbits of target vehicles: designs for a range of hypothetical mission scenarios (2)mass of observation and manipulation equipment The ICE designs lend credibility to the crude MATE (and possibly spare parts,etc.)carried,which models,further clarify design issues,and provide a determines at a high level what it can do to interact with starting point for further development of missions of targets,referred to here as its capability,and (3) interest. response time,or how fast it can get to a potential target and interact with it in the desired way.Note that the This paper covers the MATE and ICE models created design of observation and manipulation equipment and to do the analyses.the MATE tradespace and its its corresponding software is outside the scope of this interpretation,and the conceptual design of four tug study-the equipment is treated as a"black box"with vehicles for a mission involving the rescue of a mass and power requirements Geosynchronous Earth Orbit(GEO)satellite stranded in a transfer orbit by the failure of its apogee motor.A These attributes are translated into a single utility companion paper looks at a variety of specific missions, function.In the absence of real users from which to suggested originally by this tradespace analysis,that collect more sophisticated functions,"it was decided concentrate on servicing groups of satellites in similar that a simple function that could be explored orbits. parametrically was most appropriate.The three attributes are assigned single-attribute utilities.These MATE METHOD are dimensionless metrics of user satisfaction from zero (minimal user need satisfied)to one (fully satisfied In MATE.user needs are defined in terms of the user).The utilities are combined as a weighted sum system's attributes,or capabilities of the desired system,rather than the characteristics of the desired The delta-V utility is shown in Fig.1.Delta-V is a space vehicle.These needs are expressed and continuous attribute calculated for each system quantified in utility metrics,often through the use of considered.Utility is assumed to increase linearly with Multi-Attribute Utility Theory.Then a design vector is delta-V.with diminishing returns above the levels selected,consisting of a very large number (hundreds to necessary to do Low Earth Orbit(LEO)to GEO hundreds of thousands)of possible systems that could transfers.Variations on this utility are shown in Figs.2 be used to meet the user needs.Simulation models are and 3,which show respectively the utilities of a GEO- used to calculate the attributes of the proposed systems centric user(large steps in utility for achieving GEO The systems are then evaluated against the users and GEO round-trip capabilities)and a delta-V-hungry utilities to understand which systems best satisfy the user(continued linear utility for very high delta-V). users'needs.The results,collectively referred to as the The manipulator mass(capability)attribute has discrete tradespace,can then be explored.This process consists values,assumed to correspond to increasing utility as of the search for not only optimal solutions,but also fo shown in Table 1.The response time of a real system understanding of design sensitivities,key trade-offs. would be a complex function of many factors;at the dangerous uncertainties,and vulnerabilities to changes level of the current analysis it is reduced to a binary in the market or national policy.Often these attribute /valued at one for high impulse systems,and understandings will change a user's perception of his or zero for low impulse ones. American Institute of Aeronautics and Astronautics2 American Institute of Aeronautics and Astronautics understand the possibilities and problems of the space of possible solutions – the tradespace.3 It was developed at MIT from earlier work on information systems analysis applied to space systems.4 Integrated Concurrent Engineering (the CON in MATE-CON, but usually referred to on its own as ICE) is a method for rapidly producing preliminary designs in a “design room” environment. The system used in this study descends from work at JPL5 and the Aerospace Corporation,6 by way of Caltech.7 The overall MATE￾CON system, along with other front-end design tools, was developed by a consortium of MIT, Caltech, and Stanford.8 Using MATE, several hundred possible space tug vehicles are evaluated for their ability to move mass in orbit and interact with targets. The resulting tradespace is examined to clarify some of the fundamental difficulties with the space tug concept, understand the sensitivities of the tradespace to uncertainties in users needs, identify the Pareto front of “good” designs, and find some design points that are promising for multi￾purpose tugs. ICE is then used to create ten conceptual designs for a range of hypothetical mission scenarios. The ICE designs lend credibility to the crude MATE models, further clarify design issues, and provide a starting point for further development of missions of interest. This paper covers the MATE and ICE models created to do the analyses, the MATE tradespace and its interpretation, and the conceptual design of four tug vehicles for a mission involving the rescue of a Geosynchronous Earth Orbit (GEO) satellite stranded in a transfer orbit by the failure of its apogee motor. A companion paper looks at a variety of specific missions, suggested originally by this tradespace analysis, that concentrate on servicing groups of satellites in similar orbits. MATE METHOD In MATE, user needs are defined in terms of the system’s attributes, or capabilities of the desired system, rather than the characteristics of the desired space vehicle. These needs are expressed and quantified in utility metrics, often through the use of Multi-Attribute Utility Theory. Then a design vector is selected, consisting of a very large number (hundreds to hundreds of thousands) of possible systems that could be used to meet the user needs. Simulation models are used to calculate the attributes of the proposed systems. The systems are then evaluated against the users’ utilities to understand which systems best satisfy the users’ needs. The results, collectively referred to as the tradespace, can then be explored. This process consists of the search for not only optimal solutions, but also for understanding of design sensitivities, key trade-offs, dangerous uncertainties, and vulnerabilities to changes in the market or national policy. Often these understandings will change a user’s perception of his or her need, and/or the designer’s perception of the appropriate design space, resulting in a need to repeat the analysis. The semi-automated nature of the computations allows this valuable exploitation of emergent understanding with little cost or time penalty. Eventually, a design or designs from the trade space are selected for further consideration.9 In this study, a somewhat simplified version of the MATE method was used. The method was adapted in response to difficulties including the lack of an immediate customer and a very open design space. The customer utilities were handled parametrically to understand the sensitivities of the tradespace to ranges of, and changes in, user needs. The analysis was done at a high level, using low-fidelity models, but covering a large range of possible designs. Attributes and Utilities The capabilities of a space tug vehicle determined to be useful to a potential user include: (1) total delta-V capability, which determines where the spacetug can go and how far it can change the orbits of target vehicles; (2) mass of observation and manipulation equipment (and possibly spare parts, etc.) carried, which determines at a high level what it can do to interact with targets, referred to here as its capability; and (3) response time, or how fast it can get to a potential target and interact with it in the desired way. Note that the design of observation and manipulation equipment and its corresponding software is outside the scope of this study – the equipment is treated as a “black box” with mass and power requirements. These attributes are translated into a single utility function. In the absence of real users from which to collect more sophisticated functions,9 it was decided that a simple function that could be explored parametrically was most appropriate. The three attributes are assigned single-attribute utilities. These are dimensionless metrics of user satisfaction from zero (minimal user need satisfied) to one (fully satisfied user). The utilities are combined as a weighted sum. The delta-V utility is shown in Fig. 1. Delta-V is a continuous attribute calculated for each system considered. Utility is assumed to increase linearly with delta-V, with diminishing returns above the levels necessary to do Low Earth Orbit (LEO) to GEO transfers. Variations on this utility are shown in Figs. 2 and 3, which show respectively the utilities of a GEO￾centric user (large steps in utility for achieving GEO and GEO round-trip capabilities) and a delta-V-hungry user (continued linear utility for very high delta-V). The manipulator mass (capability) attribute has discrete values, assumed to correspond to increasing utility as shown in Table 1. The response time of a real system would be a complex function of many factors; at the level of the current analysis it is reduced to a binary attribute Vt, valued at one for high impulse systems, and zero for low impulse ones