The HM7B engine is a gas generator liquid oxygen / liquid hydrogen engine that powers the Ariane 4 third stage. The HM7 engine built upon the development work of the 40 kN thrust HM4.  The HM7 development program began in 1973 as part of Europe's effort to develop an indigenous launch capability.    Final qualification of the HM7 engine occurred in 1979 and the engine went on to power the third stage of the Ariane 1.  SEP continued to perfect and upgrade the engine, increasing the specific impulse by 4 seconds by increasing chamber pressure and lengthening the nozzle.  The new engine, the HM7B, powered the third stage of the Ariane 2,3 and 4.  As of June 1st, 1995, SEP had produced 111 HM7B engines, with a cumulated total of 171,700 seconds of operation, including 47,400 in flight.