A second approach would be to use a moderate-thrust (100-200 lb), modest-performance chemical propulsion thruster at 350-360 sec Isp, such as liquid oxygen (LOx)/monopropellant hydrazine using a cryocooler to keep the LOx from boiling away. Velocity changes of hundreds of feet per second can be achieved in minutes to hours, depending on the propellant mass available, for platforms of thousands of pounds, permitting position changes of thousands of miles per day. With higher thrust levels, maneuvering times could be quite rapid.
One important technology that would permit multiple maneuvers of critical assets would be an on-orbit refueling system to resupply the propellants while changing stations. The on-orbit refueling capability would enable the space asset to stay alive as long as everything kept working and to make as many rapid station changes as required. The capability for on-orbit docking and refueling will be demonstrated with hydrazine and high-pressure helium in space by the end of 2006.
A third approach to implementing rapid station changes would be to have a large (perhaps refuelable on orbit) space tug with high-performance electric propulsion for slow strategic moves or a high-thrust, modest-performance chemical system for responsive maneuvers. Or, a tug could have some combination of propulsion systems on board that would allow it to fly up, dock with a key space asset, and move it to the desired new operational location. The tug could then de-mate from the spacecraft and fly on to reposition other assets as needed. This approach would entail a small fleet of permanently based space maneuvering tugs that would have no function other than to rapidly maneuver space assets to new stations. The fleet would have its own dedicated guidance, navigation, and control and telemetry/command system. The tugs themselves could also be refuelable on orbit to extend their lifetimes.
Of course some combination of the above approaches for slow or rapid maneuvering and repositioning could be adopted to provide more robust and flexible capabilities, better survivability, and longer life.
Recommendation 5-1. DoD should support extensive basic research and technology projects for various in-space propulsion thruster concepts and for in-space electric power generation and energy storage. This fundamental long-range support need not be tied to any specific mission or platform requirement. The current range of technical opportunities is so great that progress will be directly proportional to annual