Liquid Hydrogen (LH2)

Liquid hydrogen, or LH2, is the most efficient chemical rocket fuel in regular use. Cooled until it becomes a liquid and burned with oxygen, it squeezes more thrust out of every kilogram of propellant than any other practical fuel combination, which is why it powers some of spaceflight’s most famous engines.

Quick facts

• What it is: hydrogen gas chilled into a liquid so it can be stored compactly and pumped into an engine.
• Storage temperature: about -253 C (-423 F). It must be kept this cold (cryogenic) or it boils back into gas.
• Density: very low, around 0.07 grams per cubic centimeter, roughly one-tenth the density of water.
• Burned with: almost always liquid oxygen (LOX) as the oxidizer, the chemical that lets the fuel burn.
• Efficiency: a specific impulse of roughly 420-450+ seconds in vacuum (the RS-25 engine reaches about 452 seconds).
• Exhaust speed: about 4,462 meters per second in vacuum for LH2/LOX.
• Exhaust: essentially just water vapor.

What it is and how it works

Rocket engineers measure a fuel’s efficiency with a number called specific impulse, which is essentially how much push you get for each unit of propellant burned. The key to a high specific impulse is the weight of the exhaust: the lighter and faster the gas leaving the nozzle, the more efficient the engine. Hydrogen is the lightest element of all, so when it burns with oxygen the exhaust (water vapor) is low in molecular weight and races out extremely fast. That gives LH2/LOX the highest practical exhaust velocity and specific impulse of any common chemical propellant.

Inside an engine, the LH2 and LOX are stored separately, each kept cryogenically cold. Powerful pumps force them at high pressure into a combustion chamber, where they are ignited, and the hot gas is funneled out through a nozzle to produce thrust. Before reaching the chamber, the icy hydrogen is often routed through the engine and nozzle walls first, soaking up heat and keeping the metal from melting. This clever double duty is called regenerative cooling. Engines are also typically chilled down before ignition so the sudden arrival of ultra-cold propellant does not crack them through thermal shock.

The catch is density. Because hydrogen is so light and spread out, it needs large, heavily insulated tanks. The freezing liquid constantly boils off and its tiny molecules leak easily, so tanks must be topped off right up until launch, and the plumbing and launch pad operations are demanding. Hydrogen molecules can even seep into metal and weaken it, a problem called embrittlement.

Why it matters

LH2/LOX is the propellant of choice whenever performance per kilogram counts most, because a higher specific impulse means more payload delivered to orbit or deep space for the same mass of fuel. That makes it ideal for upper stages and efficiency-critical core stages. Its downsides (bulky tanks, boil-off, leak and handling difficulty, and lower thrust for its volume than denser fuels like kerosene) are why many first stages still use heavier fuels, while hydrogen dominates upper stages. Mastering liquid hydrogen is regarded as one of NASA’s landmark technical achievements. There is a safety upside too: a hydrogen spill disperses upward and dissipates quickly, unlike kerosene, which pools and burns.

Notable engines and vehicles

• RS-25 (Space Shuttle Main Engine): an LH2/LOX engine producing about 1,859 kN of thrust at liftoff (roughly 418,000 pounds of force) with a vacuum specific impulse near 452 seconds. It powered the Space Shuttle and now drives the core stage of NASA’s Space Launch System (SLS).
• RL10: the first liquid-hydrogen rocket engine to fly, first flown successfully on November 27, 1963 atop a Centaur upper stage. With a vacuum specific impulse up to about 470 seconds, it has powered Centaur, the Delta IV upper stage, and Vulcan and SLS variants.
• J-2: the LH2/LOX engine that flew from 1966 on the Saturn V second stage and the upper stages of Saturn IB and Saturn V.
• Vulcain: the hydrogen core engine of Europe’s Ariane 5, continued on Ariane 6.
• Others: Delta IV, Japan’s H3, India’s GSLV/LVM3, China’s Long March 5, and the BE-3U upper-stage engine of Blue Origin’s New Glenn.

First used on the Centaur upper stage in 1963. Powered the Saturn V upper stages, Space Shuttle main engines, and SLS core stage.

Highest Isp of any chemical propellant, clean exhaust (water vapor), non-toxic

Very low density requires huge tanks, extreme cryogenic handling, hydrogen embrittlement risk