 The candidates were the Centaur, propelled by liquid hydrogen and liquid oxygen, the Transtage, propelled by hypergolic storable propellants Aerozine-50 and , and the Interim Upper Stage, using solid propellant.The DOD reported that Transtage could support all defense needs, but could not meet NASA's scientific requirements, the IUS could support most defense needs and some science missions, while the Centaur could meet all needs of both the Air Force and NASA.Development began on both the Centaur and the IUS, and a second stage was added to the IUS design which could be used either as an apogee kick motor for inserting payloads directly into geostationary orbit or to increase the payload mass brought to escape velocity.