Using the utility weightings for the"Response Time All the propulsion systems appear to hit a"wall"where Stressed"user(Table 2)results in Fig.7.The results costs increase sharply at little or no advantage in utility are clear;electric propulsion is eliminated from Examination of the designs on this wall reveal two very consideration.In the nominal case (Fig.4)electric different phenomena.The bi-propellant and propulsion appears at the"knee"of the Pareto front, cryogenically fueled systems are up against the limits of and would appear to give good utility for modest cost. the rocket equation.Each small increment in utility is but that conclusion will be very sensitive to the gained only by carrying a lot more fuel,most of which weighting given response time by an actual user. is used to push fuel around.The nuclear and electric Conversely,if the nominal weights and the delta-V systems,on the other hand,are limited only by the fact utility function from Fig.3 are used (representing a user that they achieve a high enough delta-V to score a 1.0 with a demand for very large delta-V)the result is Fig. on the delta-V utility,and there is simply no value in 8.Now,almost all the designs on the Pareto front carrying more fuel.If that limit is removed,both feature electric propulsion. systems show large advantages,as shown in Fig.8. A more detailed view of the lower right-hand corner of Also shown on Fig.9 are some specific designs capable the nominal Pareto front (from Fig.4)is shown in Fig of carrying out the mission mentioned in the 9.Only low-capability systems are shown.The lines introduction-moving from a LEO parking orbit to connect designs that differ only by fuel load carried. GEO transfer orbit,grappling a stranded target vehicle inserting it in GEO,and (optionally)returning to LEO. The biprop design is"on the wall",needing a very large 4000 fuel load to create the necessary delta-V.The 3500 cryogenically fueled design is not as bad,but is clearly +Biprop sensitive to the details of its design-slight increases in 3000 +Cryo manipulator mass etc.will send it too "up the wall." 2500 x Nuclear Neither chemical fuels can(without refueling)return a 73354 vehicle to LEO.The electric vehicles,both one-way 2000 数车 “tug”and round-trip“cruiser”do not have this problem 1500 The Electric Cruiser design,in fact,sits in the lower- right corner of the tradespace because it has maximized 1000 the delta-V utility,not because it is limited by physics. 500 To flesh out the vehicles briefly described here,and verify the reasonableness of the very approximate 0.0 0.2 0.4 0.6 0.8 1.D methods used in the tradespace analysis,conceptual Utility(dimensionless) designs for these vehicles were created using ICE. ICE METHOD Fig.7.Trade space for response time stressed user ICE is a way of streamlining the design process to make it more efficient.ICE addresses some of the major 4000 problems of spacecraft development including complicated interdisciplinary interfaces and inefficient 3500 time usage.Caltech's Laboratory for Spacecraft and +Biprop Mission Design has made several important 3000 +Cryo Electno contributions to the ICE method,including the 2500 ×Nuclea ICEMaker software that was used throughout the project. 2000 1500 ICEMaker is a parameter exchange tool that facilitates 448 sharing of information amongst the design team.The 1000 design problem is broken down into individual 500 modules,also known as“sheets"or“clients”which are linked together via the ICEMaker server.Users can 0 query the server to either send their latest numbers or 0.0 0.2 04 0.6 0.8 receive any recent changes made in other modules that Utility (dimensionless) affect their work.The querying process is manual preventing values from being overwritten without Fig.8.Trade space for user with large delta-V needs permission from the user.The combination of a human expert and a computational module is referred to as a chair.” 6 American Institute of Aeronautics and Astronautics6 American Institute of Aeronautics and Astronautics Using the utility weightings for the “Response Time Stressed” user (Table 2) results in Fig. 7. The results are clear; electric propulsion is eliminated from consideration. In the nominal case (Fig. 4) electric propulsion appears at the “knee” of the Pareto front, and would appear to give good utility for modest cost, but that conclusion will be very sensitive to the weighting given response time by an actual user. Conversely, if the nominal weights and the delta-V utility function from Fig. 3 are used (representing a user with a demand for very large delta-V) the result is Fig. 8. Now, almost all the designs on the Pareto front feature electric propulsion. A more detailed view of the lower right-hand corner of the nominal Pareto front (from Fig. 4) is shown in Fig. 9. Only low-capability systems are shown. The lines connect designs that differ only by fuel load carried. 0 500 1000 1500 2000 2500 3000 3500 4000 0.0 0.2 0.4 0.6 0.8 1.0 Utility (dimensionless) Cost (M$) Biprop Cryo Electric Nuclear Fig. 7. Trade space for response time stressed user 0 500 1000 1500 2000 2500 3000 3500 4000 0.0 0.2 0.4 0.6 0.8 1.0 Utility (dimensionless) Cost (M$) Biprop Cryo Electric Nuclear Fig. 8. Trade space for user with large delta-V needs All the propulsion systems appear to hit a “wall” where costs increase sharply at little or no advantage in utility. Examination of the designs on this wall reveal two very different phenomena. The bi-propellant and cryogenically fueled systems are up against the limits of the rocket equation. Each small increment in utility is gained only by carrying a lot more fuel, most of which is used to push fuel around. The nuclear and electric systems, on the other hand, are limited only by the fact that they achieve a high enough delta-V to score a 1.0 on the delta-V utility, and there is simply no value in carrying more fuel. If that limit is removed, both systems show large advantages, as shown in Fig. 8. Also shown on Fig. 9 are some specific designs capable of carrying out the mission mentioned in the introduction—moving from a LEO parking orbit to GEO transfer orbit, grappling a stranded target vehicle, inserting it in GEO, and (optionally) returning to LEO. The biprop design is “on the wall”, needing a very large fuel load to create the necessary delta-V. The cryogenically fueled design is not as bad, but is clearly sensitive to the details of its design – slight increases in manipulator mass etc. will send it too “up the wall.” Neither chemical fuels can (without refueling) return a vehicle to LEO. The electric vehicles, both one-way “tug” and round-trip “cruiser” do not have this problem. The Electric Cruiser design, in fact, sits in the lower￾right corner of the tradespace because it has maximized the delta-V utility, not because it is limited by physics. To flesh out the vehicles briefly described here, and verify the reasonableness of the very approximate methods used in the tradespace analysis, conceptual designs for these vehicles were created using ICE. ICE METHOD ICE is a way of streamlining the design process to make it more efficient. ICE addresses some of the major problems of spacecraft development including complicated interdisciplinary interfaces and inefficient time usage. Caltech’s Laboratory for Spacecraft and Mission Design has made several important contributions to the ICE method, including the ICEMaker software that was used throughout the project.7 ICEMaker is a parameter exchange tool that facilitates sharing of information amongst the design team. The design problem is broken down into individual modules, also known as “sheets”’ or “clients” which are linked together via the ICEMaker server. Users can query the server to either send their latest numbers or receive any recent changes made in other modules that affect their work. The querying process is manual, preventing values from being overwritten without permission from the user. The combination of a human expert and a computational module is referred to as a “chair