The HM -7 rocket engine was developed for the high-energy hydrogen / oxygen third stage of the Ariane 1 and had 60 kN thrust. It was, as is the current HM -7B, not wiederzündbar.
The first flight took place on 24 December 1979. In another test flights, the already made on this flight experience could be confirmed that the engine has a higher specific impulse than did calculated, whereby the payload capacity of Ariane 1 was above plan.
During the long production time of the Ariane 4, HM -7B engine was developed, the thrust increased and the mixing ratio was changed from 5:1 to 6:1 to allow the third stage to carry more fuel. The name of the engine, however, was not changed. Because of the comparatively sophisticated H2/O2-Technik the HM -7B engine was the most unreliable engine of the Ariane -1-4 missiles. Only after the fifth failure of one engine of HM-7/HM-7B-Reihe the error during several months of the start pause could be eradicated. Since then, the HM -7B engines fly flawlessly.
Due to the now high reliability of this engine and the fast availability, it was decided to incorporate it in performance enhancement program of the Ariane 5 Ariane 5 ECA, to which it was again increased slightly to 70 kN thrust and specific impulse. Moreover, it was at the max now. 970 sec long burning time adjusted ( Ariane 4 only max. 780 sec ).
The Ariane 5 ECA, however, was originally planned as a short-term interim solution until the even stronger Ariane 5 ECB is ready to use with the new Vinci engine. The Ariane 5 ECB has been, for lack of even heavier satellite, postponed indefinitely. According to current plans, the now -called Ariane 5 ME ( Midlife Evolution ) otherwise modernized Ariane 5 version 2016/2017 shall take off on its maiden flight. Would go up this version about 2017 service, the Ariane 5 will remain ECA and the HM -7B in the service.