Preburner

Inside the most powerful rocket engines ever built sits a small, hidden chamber called a preburner. It never makes thrust on its own, yet without it those engines could not run.

Quick facts

• A preburner is the small first-stage combustion chamber in a staged-combustion rocket engine (a design where propellant is burned in two steps).
• It burns only a small portion of the propellant, deliberately “off-ratio” — with too much fuel or too much oxidizer — to make hot gas rather than thrust.
• That gas spins the engine’s turbopumps (pumps that force propellant into the engine at very high pressure).
• Three types exist: fuel-rich, oxidizer-rich, and full-flow (which uses one of each).
• Staged combustion was first proposed by Alexey Isaev in 1949; the first such engine, the Soviet S1.5400 (11D33), flew around 1959 on the Molniya rocket.

What it is and how it works

In a staged-combustion engine, the propellant is burned in two stages instead of one. First, a small fraction of the fuel and oxidizer is fed into the preburner and ignited at an intentionally unbalanced mixture ratio — the proportion of oxidizer to fuel. Burning it “off-ratio,” either fuel-rich (excess fuel) or oxidizer-rich (excess oxidizer), keeps the resulting gas cool enough that the metal turbine blades survive, while still producing a large volume of hot, high-pressure gas.

This gas is then blown across the turbines of the high-pressure turbopumps, spinning them like wind through a windmill. The turbopumps in turn pressurize and force the bulk of the propellant into the engine. Here is the clever part: instead of dumping that turbine exhaust overboard, the engine routes the preburner gas — which still contains plenty of unburned, vaporized propellant — into the main combustion chamber, where it mixes with the remaining propellant and burns completely to make thrust. Because nothing is thrown away, the engine is highly efficient. This is what makes the cycle “closed.”

In a full-flow design there are two preburners. A fuel-rich one drives the fuel turbopump and an oxidizer-rich one drives the oxidizer turbopump, so the entire flow of both propellants passes through the turbines. This lets the turbines run cooler and at lower stress, and removes the need for a seal separating the fuel and oxidizer turbines.

Why it matters

The preburner is the part that makes the staged-combustion cycle possible, and that cycle delivers the highest performance of any chemical rocket engine, because no propellant is wasted overboard — all of it ultimately burns in the main chamber. This efficiency, measured as specific impulse (how much push you get per unit of propellant), and the ability to reach very high chamber pressures make staged combustion the choice for many of the most capable engines ever built.

The trade-off is severe complexity. The preburner creates extremely harsh conditions for turbines and plumbing. An oxidizer-rich preburner is the hardest of all: hot, high-pressure oxygen can corrode or even ignite the metal it flows through, which is why such engines need exotic oxidation-resistant alloys and coatings. The preburner is also a key control point — engines like the RS-25 throttle and tune their mixture ratio by independently regulating their preburners.

Notable examples

• RS-25 (Space Shuttle Main Engine): Uses two independently controlled fuel-rich preburners burning liquid hydrogen and oxygen, producing about 1,859 kN (418,000 lbf) of thrust at liftoff. It now flies on NASA’s SLS rocket.
• SpaceX Raptor (Starship/Super Heavy): The first full-flow staged-combustion engine ever flown (2019), with both an oxidizer-rich and a fuel-rich preburner running on liquid methane and oxygen, each driving its own turbopump.
• RD-180 (Atlas V): A kerosene-and-oxygen engine using a single oxygen-rich preburner, with main chamber pressure over 250 bar. Kerosene engines favor oxidizer-rich preburners because a fuel-rich one would coke and soot up.
• RD-170/171: A four-chamber kerosene-oxygen engine whose single turbopump is driven by oxidizer-rich preburners; it flew on Energia and Zenit, and its modernized RD-171MV version now powers Russia’s Soyuz-5.
• S1.5400 (11D33): The first staged-combustion engine, flown on the Soviet Molniya rocket around 1959 — the historical origin of the preburner cycle.