Aerozine 50 (50% N₂H₄ + 50% (CH₃)₂NNH₂) is a hypergolic fuel blend propellant used as a fuel. Storable for years at ambient temperature. Typical ISP: 285–315 s (with NTO) seconds.
Aerozine 50 is a rocket fuel famous for one quiet superpower: it can sit in a tank for years, fully loaded, and ignite the instant it is needed. That reliability carried astronauts off the surface of the Moon.
Quick facts
- What it is: A liquid rocket fuel made of a 50/50 mixture by weight of hydrazine (the chemical N2H4) and unsymmetrical dimethylhydrazine, or UDMH.
- Who made it: Aerojet General Corporation, in the late 1950s.
- Density: About 0.903 grams per cubic centimeter (903 kilograms per cubic meter).
- Usual partner: Dinitrogen tetroxide (N2O4, also called NTO), which acts as the oxidizer, the chemical that supplies oxygen so the fuel can burn.
- How it lights: Hypergolic, meaning fuel and oxidizer ignite the moment they touch, with no spark or igniter.
- Storable: Both liquids stay liquid at room temperature, with no need for extreme cooling.
What it is and how it works
A rocket engine needs two ingredients: a fuel and an oxidizer (the substance that lets the fuel burn, the way air lets a campfire burn). Aerozine 50 is the fuel half of the pair, and dinitrogen tetroxide is the oxidizer half. This kind of two-ingredient system is called a bipropellant.
When the two liquids meet inside the combustion chamber, they catch fire on contact. No spark plug, no igniter, nothing to fail. That property is called hypergolic, and it is the heart of why engineers love this combination. The burning produces hot gas, which rushes out through the nozzle and pushes the rocket forward.
The 50/50 recipe is a careful compromise. Hydrazine alone burns with high energy but freezes too easily and is less stable. UDMH alone is steadier and easier to store but holds less energy and is less dense. Blending them lowers the freezing point and balances energy, density, and stability. Pure hydrazine freezes at about 2 degrees Celsius, while Aerozine 50 stays liquid down to roughly minus 7 degrees Celsius, so the blend is far less likely to freeze in a cold tank. In practice, engines run slightly fuel-rich, using a mixture ratio near 1.90 rather than the chemically exact 2.24, because that runs cooler and more efficiently.
Why it matters
Aerozine 50 solved a hard problem: how to keep a rocket ready to fire at a moment’s notice. Cryogenic fuels like liquid oxygen and liquid hydrogen must be kept supercold and slowly boil away, so a rocket cannot stay fueled for long. Aerozine 50 and its oxidizer are ordinary liquids that store for long periods without cooling or significant loss.
That meant Cold War missiles could wait fully fueled in their silos, and spacecraft engines could restart reliably far out in space with no ignition hardware to break. Where a single failed light-up could end a mission or cost lives, this dependability made Aerozine 50 the trusted choice.
The trade-offs are serious. Hydrazine and especially UDMH are highly toxic, and UDMH is considered a probable human carcinogen. The oxidizer, dinitrogen tetroxide, is corrosive and poisonous too. Ground crews handle these propellants in sealed protective “SCAPE” suits, and the industry has searched for safer replacements such as CINCH (Competitive Impulse Non-Carcinogenic Hypergol). Aerozine 50 is also less efficient than cryogenic fuels, so its value is storability and reliability, not peak power.
Where it has been used
- Titan II missile and rocket: Its first stage used the Aerojet LR87 engine burning Aerozine 50 and N2O4, producing about 956.5 kilonewtons (roughly 215,000 pounds of force) of thrust per chamber. The Gemini version carried NASA astronauts in the mid-1960s.
- Apollo Lunar Module: Both the descent engine (about 45 kilonewtons, used to land on the Moon) and the ascent engine (about 16 kilonewtons, used to leave it) burned this combination.
- Apollo Command and Service Module: Its main Service Propulsion System engine used Aerozine 50 and N2O4 for the critical burns to enter and leave lunar orbit.
- Titan Transtage and later Titan III and IV: Used the same propellant pair for orbital maneuvers and heavy-lift national-security launches.
Developed for the Titan II ICBM and later used on Titan III/IV and Apollo LM engines. Production ended with Titan IV retirement.
Better performance than UDMH alone, better stability than hydrazine, proven reliability
Extremely toxic, carcinogenic, environmental hazard, costly handling
