(U) The Advanced Upper Stage system would replace the current Inertial Upper Stage (IUS) and the Centaur upper stage in delivering satellites to their operational orbits. Today's upper stages are all based on chemical propulsion; typically a combination of expendable and integral stages is employed (e.g., the Centaur/IABS for placing DSCS-3 satellites into geosynchronous orbits). Because upper stage capabilities drive launch vehicle performance requirements, upper stages exert considerable leverage on the overall cost of spacelift. Advanced upper stages could reduce spacelift costs by allowing a given payload to be delivered with a smaller launch vehicle or by increasing the payload capacity of current launch vehicles.
(U) A future Advanced Upper Stages would utilize more efficient propulsion systems than today's upper stage systems. These systems falling into three main categories: advanced chemical propulsion, electric propulsion, and thermal propulsion.
(U) Advanced chemical propulsion systems include new solid propellant and cryogenic fuels with higher Specific Impulses (Isps) than currently used chemical systems. This increase in Isp obtainable by advanced chemical propulsion is small; however, developing these upper stages and integrating them into current launch vehicles/spacecraft is simple. Thus, some increase in payload capability is achievable with minimal effort.
(U) Electric propulsion systems utilize either solar or nuclear energy to accelerate a gas for thrust. Significantly higher Isps can be obtained with these systems, however, thrust levels are low, and trip times are long (trip times for chemical propulsion on are on the order of hours while electric propulsion systems take more than 30 days.). The Russians have long been using electric propulsion thrusters for stationkeeping, and more commercial companies are going to electric thrusters, so development of an electric propulsion orbit transfer vehicle (OTV) would be relatively simple.
(U) Thermal propulsion utilizes thermal energy (solar or nuclear) to heat a working fluid which then expands through a nozzle. Thermal propulsion also yields Isps higher than chemical propulsion but lower than electric. The thrust of thermal systems falls in between the other two technologies, so the trip times are in between as well. Development and integration of a thermal propulsion system would be more difficult than a chemical or electric system because the technology is not as mature.
(U) To be supplied.
|IUS||Inertial Upper Stage (IUS)|
|Upper Stages||Upper Stages|
|National Security Space Road Map||Integrated System Road Map|
|SPACE FORCES SUPPORT||SPACE FORCES SUPPORT|
(U) National Security Space Road Map Team, NSSA, Open Phone: (703) 808-6040, DSN 898-6040.
(U) 21 November 1997
(U) Road Map Production Date: 23 June 2001