|Country of origin||France|
|First flight||24 December 1979|
|Application||Upper stage engine|
|Propellant||Liquid oxygen / Liquid hydrogen|
|Thrust (vac.)||64.8 kN (14,570 lbf)|
|Chamber pressure||3.7 MPa (37 bar)|
|Isp (vac.)||446 s (4.37 km/s)|
|Dry weight||165 kg|
The HM7B is a European cryogenic upper stage rocket engine used in Ariane rocket family. It will be replaced by Vinci as an upper-stage engine for Ariane 6. Nearly 300 engines have been produced to date.
The development of HM7 engine begun in 1973 on a base of HM4 rocket engine. It was designed to power a third stage of newly constructed Ariane 1, the first launch system for European Space Agency. Maiden flight took place on 24 December 1979 successfully placing CAT-1 satellite on the orbit. Introduction of Ariane 2 and Ariane 3 it become necessary to improve performance of the upper stage engine. It was achieved by extending engine nozzle and increasing chamber pressure from 30 to 35 bar increasing specific impulse and by this burn time from 570 to 735 seconds. Qualification tests were completed in 1983 and a modified variant was designated HM7B. It was also used on Ariane 4 upper stage where the burn time increased to 780 seconds, and since 12 February 2005 it's also used on the upper stage of Ariane 5 ECA.
The HM7B is a regeneratively cooled gas generator rocket engine fed with liquid oxygen and liquid hydrogen. It has no restart capability: the engine is continuously fired for 950 seconds in its Ariane 5 version (780 s in the Ariane 4). It provides 62.7 kN of thrust with a specific impulse of 444.2 s. The engine's chamber pressure is 3.5 MPa.