Upper stage / space tug - in production. Delta 4 second stage with hydrogen tank increased to 5.1 m diameter.

The upgraded cryogenic second-stage Pratt & Whitney RL10B-2 engine was based on the 30-year heritage of the reliable RL10 engine. It incorporated an extendable exit cone for increased specific impulse (Isp) and payload capability. The basic engine and turbo pump were unchanged relative to the RL10. The engine gimbal system used electromechanical actuators that increased reliability while reducing both cost and weight. The propulsion system and attitude control system (ACS) utilized flight-proven off-the-shelf components. The second-stage propulsion system produced a thrust of 24,750 lb with a total propellant load of 37,090 lb, providing a total burn time of approximately 700 sec. Missions requiring more than one restart were accommodated by adding an extra helium bottle for the additional tank repressurization.