Torch Igniter

A torch igniter is a small device that lights a rocket engine the way a long match lights a campfire — except this “match” is itself a tiny rocket engine. It fires a jet of flame into the big engine to get combustion going.

Quick facts

• Also called an augmented spark igniter, or ASI — the two names are used interchangeably in the engineering literature.
• It works by injecting a small flow of propellants (the fuel and oxidizer an engine burns) into a tiny chamber and lighting them with an electric spark plug.
• It can be re-lit many times, simply by refilling propellant — unlike single-use igniters.
• The energy needed for reliable ignition can reach roughly 1% of the main engine’s thermal (heat) power, depending on engine size.
• Common propellant pairs include gaseous oxygen or nitrous oxide as the oxidizer, with hydrogen, methane, ethane, propane, or propene as the fuel.
• NASA developed a hydrogen-oxygen torch igniter at its Lewis (now Glenn) research center in the late 1970s.

What it is and how it works

Lighting a large rocket engine is hard. You cannot just touch a flame to it — the propellants pour in by the ton, and the heat needed to ignite them is enormous. A torch igniter solves this by being a miniature rocket engine whose only job is to light the big one.

First, a small flow of fuel and an oxidizer (the chemical that supplies oxygen so the fuel can burn), usually as gases such as gaseous oxygen and gaseous methane or hydrogen, is injected into a small chamber called a prechamber. An electrically driven spark plug — the same idea as the one in a gasoline car — fires a spark across a gap. That spark ionizes a sliver of the propellant stream, meaning it strips electrons from the molecules, and creates a small “kernel” of burning gas. The kernel grows into a stable flame anchored inside the prechamber.

That flame then jets out of the igniter’s exit nozzle as a “torch” into the much larger main combustion chamber — or into a preburner or gas generator, smaller chambers that burn some propellant to drive an engine’s pumps. There the torch ignites the full propellant flow. In staged-combustion engines like the RS-25, the torch lights the preburners first, and the hot preburner exhaust in turn lights the main chamber.

The spark fires only during startup. Once the main combustion is self-sustaining, the igniter switches off. Because the spark can fire again on command, the engine can be re-lit in flight.

Why it matters

Ignition is one of the most failure-prone moments in a rocket engine, so the choice of igniter is critical. Torch igniters are restartable, relatively cheap, clean, and stable, and they reuse the engine’s own propellants instead of consumable cartridges or exotic chemicals. That makes them the standard for human-rated and reusable engines, and a key enabler of in-space restarts: landing burns, orbital insertion, and missions that fire the engine several times.

For deep-space ambitions like the Moon and Mars, this self-sufficiency matters even more. Spark-based ignition removes any dependence on third-party chemicals — such as the hypergolic fluids TEA-TEB, which ignite on contact but are toxic and cannot be made off Earth. A vehicle with torch igniters can relight its engines without resupply.

There are trade-offs. Hypergolic igniters need no ignition hardware at all but use toxic chemicals. Pyrotechnic igniters are simple and energetic but work only once. A torch igniter adds some plumbing, a spark system, and control complexity — in exchange for reusability and independence.

Notable examples

• RS-25 / Space Shuttle Main Engine (Rocketdyne): augmented spark igniters light both preburners and the main combustion chamber. The spark plugs are dual-redundant (two are fitted so one can fail), commanded by the engine controller during startup, and shut off after about 3 seconds once combustion sustains itself. The preburner ASI is a cylindrical chamber on the combustor centerline, with a conical dome holding two angled oxygen injection ports and two spark plugs set 90 degrees apart.
• J-2 engine (Saturn V S-II and S-IVB upper stages): used a hydrogen-oxygen augmented spark igniter — one of the classic human-rated torch-ignited engines.
• SpaceX Raptor (methane-oxygen, Starship and Super Heavy): uses spark-based torch igniters for its preburners. SpaceX moved away from TEA-TEB chemical ignition to a fully integrated spark-torch system, to drop its dependence on outside fluids and enable Moon and Mars relights.
• Research and student engines: torch igniters running gaseous oxygen with gaseous methane or propane have been built and tested for sub-scale liquid-oxygen/methane engines, and hydrogen-oxygen torch igniters have lit ABS-and-oxygen hybrid rocket motors in the lab.