Hydrazine (N2H4) (N₂H₄) is a hypergolic fuel propellant used as a fuel (or monopropellant). Storable at room temperature for years. Typical ISP: 220–240 s (monopropellant), 280–310 s (bipropellant with NTO) seconds.
Hydrazine is one of spaceflight’s most trusted fuels: a clear, water-like liquid that can sit quietly in a spacecraft’s tank for years and then fire a thruster on command. It has steered satellites and probes for more than six decades.
Quick facts
- Chemical formula: N2H4 (written H2N-NH2) — a compound made only of nitrogen and hydrogen.
- Appearance: a clear, colorless (“water-white”) liquid that looks much like water, with an ammonia-like smell. It is hygroscopic, meaning it readily absorbs moisture from the air.
- Density: about 1.004 grams per cubic centimeter (just slightly heavier than water).
- Propellant-grade purity: at least about 97% hydrazine, with the rest mostly water.
- Performance: as a monopropellant, a specific impulse (“Isp,” a measure of fuel efficiency) of roughly 220-240 seconds; as a bipropellant fuel, typically about 270-330 seconds.
- Hazards: toxic if swallowed, inhaled, or absorbed through skin; causes severe skin and eye burns; flammable; classified as a probable human carcinogen.
What it is and how it works
Hydrazine can release its energy in two different ways.
As a monopropellant (a single propellant that needs no separate oxidizer), liquid hydrazine is squirted into a small chamber packed with a catalyst — a material that speeds up a chemical reaction without being used up. The usual choice is iridium spread on alumina (aluminum oxide); the best-known commercial form is Shell 405, often used in an electrically pre-heated catalyst bed. On contact, the catalyst triggers a rapid, energetic breakdown of the hydrazine into hot ammonia, nitrogen, and hydrogen gas, reaching temperatures up to roughly 1,000 degrees Celsius (around 1,800 degrees Fahrenheit). That hot gas rushes out a nozzle to make thrust. Because no oxidizer or spark is needed, the thruster is extremely simple and reliable — think of striking a single match against a special strip, where the strip itself sets it off.
As a hypergolic bipropellant fuel, hydrazine (or its relatives — monomethylhydrazine/MMH, unsymmetrical dimethylhydrazine/UDMH, or Aerozine-50, a 50/50 blend of hydrazine and UDMH) is paired with an oxidizer, usually nitrogen tetroxide (NTO, also written N2O4). “Hypergolic” means the fuel and oxidizer ignite the instant they touch, with no igniter. An engine like this can be restarted simply by opening valves. Both modes use propellants that stay liquid at ordinary temperatures, so they can be stored aboard a spacecraft for years.
Why it matters
Hydrazine and its derivatives are the workhorse “storable” propellants of spaceflight. Their advantages are storability (liquid at room temperature, ready after years in the tank), restartability and reliability (ignition needs no spark and can be cycled thousands of times), and simplicity (a monopropellant thruster has very few parts). That makes them ideal for keeping satellites in position, controlling a spacecraft’s orientation, and nudging deep-space probes onto course. Many spacecraft use “dual-mode” systems that share one hydrazine supply between a larger bipropellant engine and small monopropellant thrusters.
The major trade-off is severe toxicity and carcinogenicity. Handling demands sealed protective suits, scrubbers, and elaborate ground procedures, and the chemical faces tightening regulation — the European Chemicals Agency (ECHA) lists it as a Substance of Very High Concern (on its REACH “candidate list” since 2011). That hazard and cost have spurred “green” replacements such as ASCENT (also called AF-M315E, a hydroxylammonium-nitrate blend), demonstrated on NASA’s Green Propellant Infusion Mission (GPIM), which packs more energy into each tankful with far lower toxicity. Europe’s LMP-103S is another. These are gradually appearing on some new small satellites, though hydrazine remains widely used.
Where it is used and notable examples
- Voyager 1 and 2: each carries 16 hydrazine monopropellant thrusters for attitude control. In 2024, engineers famously commanded Voyager 1 to switch to a less-clogged backup set of thrusters from over 15 billion miles away.
- Cassini (Saturn orbiter): used clusters of tiny hydrazine monopropellant thrusters (Aerojet Rocketdyne MR-103H, about 0.25 pounds-force / roughly 1 newton each, with electrically heated catalyst beds) for attitude control through its mission, including the Grand Finale.
- Mars “Sky Crane” (Curiosity and later Perseverance): used monopropellant hydrazine thrusters to hover during powered descent, then fly the descent stage away after lowering the rover.
- Space Shuttle Orbiter: its orbital maneuvering and reaction control systems used the hydrazine derivative MMH with nitrogen tetroxide as a hypergolic bipropellant.
- F-16 Fighting Falcon Emergency Power Unit: a non-rocket use — it stores hydrazine as H-70 (70% hydrazine, 30% water) to spin a gas turbine for emergency hydraulic and electrical power.
Used since the 1950s in military and space applications. Remains the dominant monopropellant despite decades of effort to find safer alternatives.
Storable, hypergolic ignition (no igniter needed), reliable, high-density impulse
Extremely toxic, carcinogenic, requires SCAPE suits for handling, expensive safety measures
