IRFNA (Inhibited Red Fuming Nitric Acid) (HNO₃ + ~13% N₂O₄ + ~0.6% HF) is a storable oxidizer propellant used as a oxidizer. Storable for months with proper containment. Typical ISP: 255–275 s (with UDMH) seconds.
IRFNA is a rocket oxidizer you can keep at room temperature for years and fire on short notice. That single trait made this orange-red, fuming acid a backbone of Cold War missiles and a few early space rockets.
Quick facts
- Type: Liquid oxidizer (the oxygen-supplying half of a two-part, or “bipropellant,” fuel system).
- Base chemical: Concentrated nitric acid (HNO3) loaded with dissolved nitrogen oxides.
- Typical recipe (US “Type IIIB”): about 83-84% nitric acid, 13-14% dissolved nitrogen oxides (NO2 / dinitrogen tetroxide, N2O4), 1.5-2.5% water, and about 0.6% hydrogen fluoride (HF).
- High-density version (IRFNA IV): roughly 54% nitric acid, 44% NO2, 1% water, 0.7% HF.
- Color: Orange-to-red, with toxic red-brown fumes.
- Storage: Stable at ordinary temperatures; needs no cryogenic (super-cold) cooling.
- First US military specification: 1954.
- Soviet/Russian equivalent: nicknamed “Melange.”
What it is and how it works
A liquid rocket needs both a fuel and an oxidizer. The oxidizer supplies oxygen so the fuel can burn inside a sealed engine chamber, or even in the vacuum of space where there is no air. IRFNA is that oxidizer.
The name spells out the chemistry. “Red fuming” comes from the dissolved nitrogen oxides (NO2/N2O4), which give the acid its red color and its choking red-brown fumes. Those same nitrogen oxides do useful work: they raise the acid’s oxidizing power and density, and they lower its freezing point, so it stays liquid in cold weather. “Inhibited” refers to the small splash of hydrogen fluoride (HF). The HF forms a thin, protective metal-fluoride layer on the inside of tanks and engine parts — a “passivating” coating, meaning it slows corrosion the way a layer of paint shields metal from rust. Without it, the acid would eat through its own container.
In flight, tank pressure or pumps push the IRFNA and a fuel into the combustion chamber. They react, and the hot gas rushes out the nozzle to make thrust. IRFNA is usually paired with a “hypergolic” fuel — one that bursts into flame the instant it touches the oxidizer, with no spark or igniter needed. Common partners include UDMH, hydrazine, aniline, and kerosene. That self-igniting behavior makes engines simple to start and restart reliably.
Why it matters
IRFNA bridged a gap that super-cold propellants could not. Cryogenic liquid oxygen is more energetic, but it boils away and cannot sit in a tank for long. IRFNA is energetic enough for real rocket performance yet stable at room temperature, so a missile or rocket stage can stay fueled and ready to launch on short notice. That combination of decent power and long storage made nitric-acid oxidizers — with IRFNA being the practical, corrosion-tamed form — a mainstay of mid-20th-century rocketry on both sides of the Cold War.
The drawbacks were real. IRFNA is severely toxic and corrosive, causes serious burns and eye, skin, and lung injury, and reacts violently with water. The HF inhibitor adds its own fluoride hazard. It also delivers less performance than the best alternatives. Over time, much of the field shifted to nitrogen tetroxide (NTO) or cryogenic oxidizers, leaving IRFNA mostly a historical and specialized-military propellant.
Where it is used and notable examples
- Kosmos-3M: This Soviet/Russian light orbital launch vehicle used IRFNA as its oxidizer (with UDMH fuel) and became the world’s most-launched light orbital rocket — the most prominent space-launch example of IRFNA in service.
- Aerojet engines and the Aerobee sounding rocket: A broad family of Aerojet liquid engines burned nitric-acid oxidizer with hypergolic amine or UDMH fuels, powering early US upper stages and high-altitude research rockets, including engine heritage behind Vanguard, Able, and Delta.
- US and Soviet missiles: IRFNA paired with aniline, hydrazine, UDMH, or kerosene was a standard storable-propellant recipe for field-ready, quick-launch strategic and tactical missiles.
- MGM-52 Lance: This US Army battlefield missile burned IRFNA with the hypergolic fuel UDMH in a throttleable engine — a clear example of why this chemistry appealed to ready-to-fire field weapons.
- Gelled IRFNA: US Army research developed thickened, NO2-rich “gel” versions to raise density and safety for advanced tactical propulsion, showing the chemistry’s continued military development.
Used in Nike Ajax, Corporal, and early tactical missiles. Largely replaced by NTO for most applications due to corrosion issues.
Storable, hypergolic, high density, relatively inexpensive
Extremely corrosive, toxic, attacks most metals and organics, lower Isp than NTO
