Advanced propulsion system applications to upper stages

The conventional upper stage user has a range of vehicles to choose from, however, they all use chemical combustion for the propulsion process. To gain a step function in the overall stage performance, systems where the specific impulse (/sp) ranges between 700 and 5000 seconds are reqUired. Two such systems are the direct thrust nuclear and electric propulsion. A specific upper stage design, using these advanced systems, is very much mission dependent, however, certain characteristics apply to each, Le., the nuclear thermal systems can have a relatively high thrust, while the electric systems prOViding very low thrust. Thus, the transfer time, fast versus slow, becomes a key trade issue for a ,given payload delivery. Some interesting fallout from the analyses are the transfer time reductions and operational flexibility achieved when these systems are used in a hybrid mode or are integrated with the payload. The figure of merit used in these analyses is the Initial Mass in Low Earth Orbit (1MLEO) since that can be related to total program cost and the available capabilities of existing or near term planned launch systems.