missions where speed is not essential.The design solutions.The rapid conceptual designs help to validate impact of the large total power requirement seen in the crude tradespace models,further clarify design Table 5 can be minimized by managing power use.Not issues,and add detail and credibility to the designs of running the manipulator and the full thruster set all at viable vehicles.The combined set of models represents once,and trading thruster power(and hence impulse) a capability that can be exercised to look at the specific vs.solar panel size results in panels not much bigger needs of customers (by identifying their utilities): than those required for the chemical propulsion designs exploring the possibilities of specific missions(by (see Figs.11 and 12). designing vehicles for them,and understanding their position in the overall tradespace)and investigating the GEO/LEO Tenders impact of specific technologies (by adding them to the A family of tender missions was developed based on tradespace and/or design analyses,and seeing the research of target satellite population densities.All of results). the tender missions use storable bipropellant systems for reduced cost and complexity.Each tender lives in a A number of lessons that should be widely applicable to heavily populated orbit and is capable of performing this class of vehicle were learned during the study. five or more missions involving moving or disposing of First,the unfriendly physics of high delta-V missions satellites near that orbit.The result of the tender study (the "rocket equation wall)make carrying out these was a line of similar vehicles with different fuel loads missions with chemical propulsion systems depending on the delta V requirements of the desired problematic.Even if a design of this type looks orbit.These designs are discussed in the companion feasible,it will be very vulnerable to unforeseen paper increases in mass fraction and/or mission requirements and it may be very expensive.Higher specific impulse A note on model consistency and accuracy systems show promise,although caveats are in order. The differences between the results of the detailed ICE The electric propulsion systems examined appeared to and very simple MATE analyses were remarkably offer the best overall mix of performance and cost,but small.Calculated masses differed by only a few at the expense of speed.The postulated missions are percent.The only exceptions were the chemical fuel somewhat outside the current range of experience with one-way GEO tug designs,due to their extreme fuel these technologies-it was assumed,for example,that loads.These differences did not affect the points made getting operating lifetimes beyond those of current here.Power was not calculated by the MATE model systems would be feasible.Nuclear systems look Delta-V was calculated differently by the ICE and interesting if there is need for very high-capability MATE models,with the ICE model taking into account systems with quick response times;they are the only the details of the mission including rendezvous technology studied that can meet such a need.They are maneuvers and the masses of target vehicles,but the always expensive however,and the costs quoted here results were consistent given this difference.A check do not include any technology development.Also,the of the ICE models'Theoretical First Unit (TFU)plus policy and/or political issues surrounding this launch costs against the simple MATE cost model again technology were not addressed here.Methods for showed remarkable agreement (within 15%in all quantifying the costs of policy choices were recently cases).The ICE model also included development and studied by Weigel,"and could be applied to this case. engineering cost outputs,but these were not used due the wide variation in technological maturity between The comparison of the performance of current and near the different propulsion systems considered,which the future propulsion systems give hints as to the potential model made no provision for. value of other technologies applied to this problem.A high-lsp,high impulse system without the large mass The above comparison,along with sensitivity study penalty of a nuclear system would be ideal:solar carried out for the MATE analysis,and the relative thermal,or stored-solar-energy systems (e.g.flywheel simplicity of the calculations,help verify that the storage)might be worth investigating for this purpose models are accurate predictors of their outputs,for the On the other hand,the good results with existing often estimated or parametric inputs used.The model electric propulsion options make other low thrust (e.g. results should therefore be useful for ranking and solar sail)technologies less interesting,unless there is a discussion,but the values given in all cases should be very large demand for delta-V.The trade-off between taken to be estimates with accuracy appropriate for Ip,impulse,and total delta-V was found to be very concept evaluation and comparison only. sensitive to user needs.Thus,any further discussion of the value of various propulsion systems needs to take DISCUSSION place in the context of the needs of a real user or at least a more completely specified desired capability The tradespace analyses clarify the challenges of designing spacetug vehicles.Visualization of the many An issue that was relatively insensitive to user needs possible solutions to the problem of moving mass was the high penalty for dry mass on the tug vehicle around in near-earth orbits reveals key constraints and The higher capability (higher observation and trades,and concentrates attention on a set of viable manipulator mass)vehicles showed large cost penalties 0 American Institute of Aeronautics and Astronautics10 American Institute of Aeronautics and Astronautics missions where speed is not essential. The design impact of the large total power requirement seen in Table 5 can be minimized by managing power use. Not running the manipulator and the full thruster set all at once, and trading thruster power (and hence impulse) vs. solar panel size results in panels not much bigger than those required for the chemical propulsion designs (see Figs. 11 and 12). GEO / LEO Tenders A family of tender missions was developed based on research of target satellite population densities. All of the tender missions use storable bipropellant systems for reduced cost and complexity. Each tender lives in a heavily populated orbit and is capable of performing five or more missions involving moving or disposing of satellites near that orbit. The result of the tender study was a line of similar vehicles with different fuel loads depending on the delta V requirements of the desired orbit. These designs are discussed in the companion paper. A note on model consistency and accuracy The differences between the results of the detailed ICE and very simple MATE analyses were remarkably small. Calculated masses differed by only a few percent. The only exceptions were the chemical fuel one-way GEO tug designs, due to their extreme fuel loads. These differences did not affect the points made here. Power was not calculated by the MATE model. Delta-V was calculated differently by the ICE and MATE models, with the ICE model taking into account the details of the mission including rendezvous maneuvers and the masses of target vehicles, but the results were consistent given this difference. A check of the ICE models’ Theoretical First Unit (TFU) plus launch costs against the simple MATE cost model again showed remarkable agreement (within 15% in all cases). The ICE model also included development and engineering cost outputs, but these were not used due the wide variation in technological maturity between the different propulsion systems considered, which the model made no provision for. The above comparison, along with sensitivity study carried out for the MATE analysis, and the relative simplicity of the calculations, help verify that the models are accurate predictors of their outputs, for the often estimated or parametric inputs used. The model results should therefore be useful for ranking and discussion, but the values given in all cases should be taken to be estimates with accuracy appropriate for concept evaluation and comparison only. DISCUSSION The tradespace analyses clarify the challenges of designing spacetug vehicles. Visualization of the many possible solutions to the problem of moving mass around in near-earth orbits reveals key constraints and trades, and concentrates attention on a set of viable solutions. The rapid conceptual designs help to validate the crude tradespace models, further clarify design issues, and add detail and credibility to the designs of viable vehicles. The combined set of models represents a capability that can be exercised to look at the specific needs of customers (by identifying their utilities); exploring the possibilities of specific missions (by designing vehicles for them, and understanding their position in the overall tradespace) and investigating the impact of specific technologies (by adding them to the tradespace and/or design analyses, and seeing the results). A number of lessons that should be widely applicable to this class of vehicle were learned during the study. First, the unfriendly physics of high delta-V missions (the “rocket equation wall”) make carrying out these missions with chemical propulsion systems problematic. Even if a design of this type looks feasible, it will be very vulnerable to unforeseen increases in mass fraction and/or mission requirements, and it may be very expensive. Higher specific impulse systems show promise, although caveats are in order. The electric propulsion systems examined appeared to offer the best overall mix of performance and cost, but at the expense of speed. The postulated missions are somewhat outside the current range of experience with these technologies—it was assumed, for example, that getting operating lifetimes beyond those of current systems would be feasible. Nuclear systems look interesting if there is need for very high-capability systems with quick response times; they are the only technology studied that can meet such a need. They are always expensive however, and the costs quoted here do not include any technology development. Also, the policy and/or political issues surrounding this technology were not addressed here. Methods for quantifying the costs of policy choices were recently studied by Weigel,11 and could be applied to this case. The comparison of the performance of current and near future propulsion systems give hints as to the potential value of other technologies applied to this problem. A high-Isp, high impulse system without the large mass penalty of a nuclear system would be ideal; solar thermal, or stored-solar-energy systems (e.g. flywheel storage) might be worth investigating for this purpose. On the other hand, the good results with existing electric propulsion options make other low thrust (e.g. solar sail) technologies less interesting, unless there is a very large demand for delta-V. The trade-off between Isp, impulse, and total delta-V was found to be very sensitive to user needs. Thus, any further discussion of the value of various propulsion systems needs to take place in the context of the needs of a real user or at least a more completely specified desired capability. An issue that was relatively insensitive to user needs was the high penalty for dry mass on the tug vehicle. The higher capability (higher observation and manipulator mass) vehicles showed large cost penalties