LE-7A

The LE-7A is the powerful Japanese rocket engine that lifts the H-IIA and H-IIB rockets off the launch pad. It burns ultra-cold hydrogen and oxygen to send satellites and space probes on their way.

Quick facts

• Type: Cryogenic liquid rocket engine (uses very cold liquid fuel) running a staged combustion cycle
• Propellants: Liquid hydrogen (LH2) as fuel, liquid oxygen (LOX) as oxidizer
• Maker: Mitsubishi Heavy Industries (MHI), with the Japan Aerospace Exploration Agency (JAXA), built in Japan
• Used on: The first (core) stage of the H-IIA and H-IIB rockets — one engine per H-IIA core, two on the H-IIB core
• Thrust in vacuum: About 1,098 kN with the long nozzle (roughly 112 metric tons of force, or about 247,000 lbf)
• Chamber pressure: About 12.0 MPa (around 1,740 psi)
• Specific impulse: About 440 seconds in vacuum (long nozzle) — a measure of fuel efficiency
• Dry mass: About 1,780 to 1,800 kg
• Burn time: Roughly 390 seconds on the first stage
• First flight: 2001, on the debut of the H-IIA (development began in 1994)

What it is and how it works

An engine like the LE-7A works by burning fuel (hydrogen) with an oxidizer (oxygen, which the engine carries because there is no air in space) and shooting the hot gas out the back to push the rocket forward. Both propellants are stored as liquids so cold that hydrogen must be kept below about minus 253 degrees Celsius.

To feed those liquids into the engine fast enough, two separate turbopumps — spinning pumps driven by hot gas — pressurize the hydrogen and the oxygen and force them in. Here is the clever part, called a staged combustion cycle. The engine first burns its propellants in small chambers called preburners, deliberately using extra hydrogen and only a little oxygen so the flame stays cool enough for the machinery. That produces hot gas that spins the turbines driving the pumps. But instead of being thrown away after doing its job, that gas is routed into the main combustion chamber, where the rest of the oxygen is added and everything burns completely.

Because almost none of the propellant is wasted just running the pumps, this cycle reaches high chamber pressure and high efficiency. Think of it like a stove that captures the heat normally lost up the chimney and uses it to cook too. The nozzle (the bell-shaped exit) is regeneratively cooled: cold hydrogen flows through channels in the nozzle wall to keep it from melting, then gets burned afterward. The LE-7A came in a short-nozzle and a long-nozzle version, the longer one squeezing out slightly more thrust and efficiency in space.

Why it matters

The LE-7A is an upgraded version of the earlier LE-7 engine that flew on Japan’s original H-II rocket. Engineers kept the same basic design but rebuilt it to cost less, be more reliable, and be easier to manufacture, while producing about the same thrust. Staged combustion is wonderfully efficient but mechanically demanding — high pressures and very fast turbopumps make such engines hard and expensive to build, so simplifying it was a real achievement.

That effort paid off. The H-IIA flew dozens of missions with only a single failure (Flight 6 in 2003), and that failure was traced to a solid rocket booster nozzle burning through, not to the LE-7A itself. For two decades the engine was Japan’s main route to orbit and beyond, and it is the direct ancestor of the LE-9 engine built for the next-generation H3 rocket.

Where it is used and notable examples

• H-IIA: A single LE-7A powers the core stage; it became Japan’s primary orbital launcher after debuting in 2001.
• H-IIB: Used two LE-7A engines on a larger core, mainly to send the HTV “Kounotori” cargo ship to the International Space Station.
• Hayabusa2 (2014): JAXA’s asteroid sample-return mission to Ryugu rode an H-IIA lifted by the LE-7A.
• Akatsuki / Venus Climate Orbiter (2010): Launched on an H-IIA core stage driven by the LE-7A.
• H-IIA Flight 6 (Nov 29, 2003): The LE-7A ran normally, but a solid booster’s nozzle burned through and the booster failed to separate — the only H-IIA failure, and not the engine’s fault.