Triethylborane (TEB)

Triethylborane, usually shortened to TEB, is a colorless liquid that bursts into flame the instant it meets air. Rocket engineers use that violent eagerness on purpose: it is one of the most reliable ways to light a rocket engine.

Quick facts

• Chemical formula: (C2H5)3B, also written (CH3CH2)3B — a single boron atom bonded to three ethyl groups.
• Molecular weight: about 98.0 g/mol.
• Density: 0.677 g/cm3 (lighter than water).
• Melting point: -93 C. Boiling point: 95 C.
• Autoignition temperature: about -20 C (-4 F) — it catches fire on its own well below room temperature.
• Flame color: apple-green, a signature of boron compounds.
• Toxicity: acutely toxic if swallowed and causes burns; LD50 about 235 mg/kg in rats.
• Storage: sealed under inert nitrogen at all times, because any exposure to air ignites it.

What it is and how it works

TEB is an organoboron liquid — an “organoboron” compound is one built from a boron atom joined to carbon-based groups. It is pyrophoric, meaning it ignites the moment it touches oxygen at room temperature, with no spark, glow plug, or torch needed. The reason lies in its structure: the bonds between the boron atom and its three ethyl groups break easily, so the molecule reacts violently and instantly with the oxygen in air.

In rockets, TEB is almost never used alone. It is blended with a close cousin, triethylaluminium (TEA), to make a mixture called TEA-TEB, in which the aluminium compound makes up roughly 10–15 percent. The blend is hypergolic — it lights spontaneously on contact — with liquid oxygen.

During an engine startup, the liquid oxygen (the oxidizer, the chemical that feeds the fire) begins flowing into the combustion chamber. A small, precisely measured slug of TEA-TEB is then injected and ignites instantly on contact with that oxygen. The main fuel — typically RP-1, a refined kerosene — follows and is lit by the TEA-TEB flame, establishing steady combustion. Think of it as a chemical match that strikes itself: instead of trusting a spark, the engine relies on a liquid that simply cannot help but burn.

Why it matters

RP-1 kerosene and liquid oxygen do not ignite on contact with each other, so a rocket engine needs something to light them. Spark plugs and torch igniters can fail, and low-volatility fuels can be hard to light with a spark at all. A pyrophoric chemical that fires the instant it meets oxygen removes that failure mode, giving near-absolute ignition reliability.

That reliability is most valuable for engines that must light more than once — restarting a first stage for boostback and landing burns, or relighting an upper stage on orbit. The trade-offs are real: TEB is pyrophoric, toxic, and corrosive to tissue, must be kept under nitrogen, and the onboard supply is finite. An engine carries only enough for a limited number of starts, which sets a hard cap on how many times it can be relit in a single flight.

Where it is used

• Rocketdyne F-1 / Saturn V (Apollo): TEA-TEB was injected before liftoff to ignite the five F-1 first-stage engines.
• SpaceX Merlin / Falcon 9 and Falcon Heavy: TEA-TEB ignites the Merlin engines on both stages, including the in-flight relights for boostback, entry, and landing burns.
• Firefly Aerospace Reaver / Alpha rocket: also ignited by a TEA-TEB mixture.
• Lockheed SR-71 Blackbird (and A-12), Pratt & Whitney J58 engines: TEB lit the low-volatility JP-7 fuel for engine start and for each afterburner light. Each engine carried a nitrogen-inerted tank holding about 600 cc of TEB — enough for roughly 16 ignition “shots.” The green flash it produces was visible even in daylight.

Used by the SR-71 Blackbird’s J58 engines for ignition. Adopted by SpaceX for Merlin and later Raptor engine ignition systems.

Reliable ignition, simple injection system, proven in flight

Limited supply per mission constrains restarts, pyrophoric handling, produces boric acid residue