Integrated auxiliary propulsion for the Space Tug

This paper summarizes the results of a design study which considered several possible methods for applying the liquid-liquid cryogenic oxygen/hydrogen auxiliary propulsion concept to the Space Tug. This concept, which is based on maintaining the propellant in the liquid phase from storage to thruster inlet, is much simpler than previous oxygen/hydrogen systems which converted the propellant to vapor for distribution to the thrusters. The paper describes the selected liquid-liquid concept which is an integrated design that uses the Tug main propulsion tanks as a propellant source. The characteristics of this integrated system are compared with dedicated storable propellant systems. The integrated concept is found to have only slightly superior mission payload performance, but provides inherent vehicle versatility through the interchangeable availability of main and auxiliary propellant. Propellant interchange also was found to permit the auxiliary system to provide abort backup capability in the event of most main engine failures. This capability implies Tug program cost savings which more than offset the higher development costs of the integrated auxiliary propulsion system.