HM7B

The HM7B is a European rocket engine that spent more than four decades giving Ariane rockets their final push into space. For over 40 years it lit the top of the rocket and placed satellites into precise orbits, retiring in 2023 as one of the most reliable upper-stage engines ever flown.

Quick facts

• Maker: Snecma, now Safran Aircraft Engines, of France
• Propellants: liquid oxygen (LOX) and liquid hydrogen (LH2), mixed at roughly 5 parts oxygen to 1 part hydrogen
• Vacuum thrust: about 62-65 kilonewtons (around 14,000 pounds of force)
• Specific impulse: about 444.6 seconds in vacuum (some sources cite about 446 s) — a measure of fuel efficiency, and a very high figure
• Chamber pressure: roughly 35-37 bar
• Combustion temperature: about 3,200 K
• Dry mass: about 155-165 kilograms; roughly 2.0 m tall and 1.0 m across
• Burn time: about 950 seconds on Ariane 5, 780 s on Ariane 4, about 735 s on Ariane 2/3
• Restart capability: none — one continuous burn
• First flight: 24 December 1979 (as the HM7); HM7B qualified in 1983
• Track record: about 300 engines built, 130-plus successful flights, retired 2023

How it works

The HM7B is a cryogenic engine, meaning it runs on supercold liquid propellants. It burns liquid hydrogen as the fuel and liquid oxygen as the oxidizer (the substance that lets the fuel burn). When the two combine and ignite, they produce a fast jet of exhaust that pushes the rocket stage forward.

To force those propellants into the engine, the HM7B uses a “gas-generator” cycle. A small amount of the fuel and oxygen is burned in a separate little chamber called the gas generator. That burst of hot gas spins turbopumps — pumps driven by turbines — which in turn shove the main flow of propellant into the combustion chamber at high pressure. In this cycle the turbine’s exhaust is simply dumped overboard rather than fully reused. That trims efficiency slightly, but it keeps the design simple and dependable.

The engine is also regeneratively cooled. Before the cold hydrogen is burned, it first flows through channels wrapped around the chamber and nozzle, carrying away heat so the metal does not melt — a bit like coolant running through a car radiator, except the coolant here is the fuel itself. Because a hydrogen-and-oxygen flame produces the highest practical exhaust speed of any common propellant pair, the HM7B reaches a very high specific impulse of about 445 seconds, making it extremely fuel-efficient in the vacuum of space. It fires once, continuously, for roughly 9 to 16 minutes depending on the rocket, and cannot be restarted.

Why it matters

The HM7B was the workhorse that gave Europe its own reliable, independent path to orbit for over 40 years. Its job was the final orbital insertion burn — the push that drops a satellite into the exact transfer orbit it needs — so its efficiency directly set how much payload an Ariane rocket could carry. On the Ariane 5 ECA, the HM7B-powered ESC-A upper stage helped lift the rocket’s capacity toward geostationary transfer orbit to around 10 metric tons, helping Ariane 5 dominate the market for launching heavy satellites. Reliability across 130-plus flights also built the commercial reputation of Arianespace. Its main limits — no restart and an older, lower-pressure design — are exactly what pushed its replacement by the newer Vinci engine.

Where it was used

• Ariane 5 ECA: powered the ESC-A cryogenic upper stage, the most recent version flown until the 2023 retirement.
• Ariane 4: powered the H10 third stage during its longest operational era, with about 780-second burns.
• Ariane 2 and Ariane 3: powered the third stage, with about 735-second burns.
• Ariane 1: used the original HM7, the direct predecessor from which the uprated HM7B was developed (first flight 24 December 1979).
• Vinci (successor): the roughly 180 kN restartable hydrogen/oxygen engine that replaced the HM7B on Ariane 6, with nearly triple the thrust and the ability to reignite multiple times in flight to deploy several payloads to different orbits.