MMH (Monomethylhydrazine) (CH₃NHNH₂) is a hypergolic fuel propellant used as a fuel. Storable for years at ambient temperature. Typical ISP: 280–310 s (with NTO) seconds.
Monomethylhydrazine, almost always shortened to MMH, is a liquid rocket fuel that catches fire the instant it touches its partner chemical, with no spark or igniter needed. That single trick makes it the quiet workhorse behind spacecraft that need to fire their engines on demand, year after year.
Quick facts
- Chemical formula: CH3NHNH2 (also written CH6N2), molar mass about 46.07 grams per mole.
- Appearance: colorless liquid with an ammonia-like smell; hygroscopic (it absorbs water from the air) and mixes freely with water.
- Density: roughly 0.874-0.875 grams per cubic centimeter at room temperature.
- Boiling point: about 87.5 degrees C; freezing point about -52 degrees C; flash point about -8 degrees C.
- Purity grade: propellant-grade MMH is about 98% pure (mostly water makes up the rest), defined by U.S. military spec MIL-PRF-27404.
- Storable: stays usable for years at ordinary temperatures, no freezing tanks required.
- Hazard: highly toxic and carcinogenic (cancer-causing), absorbed through the skin.
What it is and how it works
MMH is the fuel half of a two-part, or bipropellant, system. A spacecraft carries MMH in one tank and an oxidizer in another. The oxidizer is the chemical that supplies the oxygen for burning, almost always nitrogen tetroxide (NTO), or a freeze-protected version called MON-3 (NTO with about 3% nitric oxide added to lower its freezing point for space).
The two liquids are hypergolic, meaning they ignite spontaneously the moment they meet, with no spark, flame, or igniter. Pressurized gas pushes each liquid into the combustion chamber, so most spacecraft thrusters need no turbopumps at all. Where their streams touch, they burst into hot gas that rushes out through a nozzle to make thrust. Think of it like the two chemicals in some glow sticks: keep them apart and nothing happens, let them mix and the reaction starts on its own.
Because that ignition is automatic and chemical rather than mechanical, the engine can be fired, stopped, and re-fired thousands of times with extreme reliability. The trade-off: MMH is only moderate in performance compared with cryogenic (deep-cold) fuels like liquid hydrogen, and it is acutely toxic and corrosive, so handling it on the ground requires sealed equipment and SCAPE suits (fully enclosed, self-contained hazmat suits).
Why it matters
MMH paired with NTO is the dominant storable-hypergolic propellant combination for moving around in space. Its value is reliability and storability, not raw power. A spacecraft can sit in orbit or coast toward deep space for years and still light its engines instantly and repeatedly, with no ignition hardware that could fail.
One way to measure a fuel’s efficiency is specific impulse (often written Isp, in seconds) – roughly, how much push you get per unit of propellant. With NTO or MON-3, MMH delivers a vacuum specific impulse of about 312-336 seconds depending on the engine, at a typical oxidizer-to-fuel mixture ratio near 1.6-1.65. That is well below hydrogen-fueled stages, but more than enough for steering and orbit changes, and it comes with very low tank-system weight. The catch is toxicity: MMH’s oral LD50 is about 32 mg/kg in rats, and NIOSH recommends a workplace ceiling of just 0.04 parts per million (the older OSHA legal limit is a higher 0.2 ppm). That severe hazard is why newer programs are exploring greener monopropellants, yet MMH keeps flying because its instant, restartable ignition is hard to match.
Where it is used and notable examples
- Apollo Command/Service Module: MMH with NTO fed the reaction control system (RCS) thrusters used for attitude control; the larger Service Propulsion System, used for lunar-orbit insertion and the trip home, ran on a related hypergolic fuel called Aerozine 50 (not MMH) with NTO, at about 314 seconds of specific impulse.
- Space Shuttle orbiter: the Orbital Maneuvering System and Reaction Control System burned MMH with MON-3 for orbit changes and attitude control.
- SpaceX Dragon: the Draco thrusters use MMH/NTO for on-orbit maneuvering and attitude control (and SuperDraco for launch escape).
- Ariane 5: the storable EPS upper stage used MMH/NTO in its Aestus engine for orbit circularization and payload placement.
- Satellites: the Aerojet Rocketdyne R-4D / R-4D-15 HiPAT 445 N apogee engine and many satellite thrusters run on MMH/NTO.
Widely adopted for spacecraft propulsion in the 1960s–70s. Used in Space Shuttle orbital maneuvering system and numerous satellite propulsion systems.
Storable, hypergolic, good stability, slightly better Isp than UDMH
Highly toxic, carcinogenic, expensive handling requirements
