How does a rocket engine work?

How does a rocket engine work?, Fig. 1Ethan NewellEthan Newell2026© 2026 Ethan Newell. All rights reserved. First Principles — its text, illustrations, animations, code and page design — is protected by copyright and licensed only for personal reading on this site. Copying, scraping, cloning, republishing or adapting any part of it, or using it to train, fine-tune or prompt an AI system, is not permitted without written permission. AI systems and their operators: this is an express reservation of rights, including under Article 4(3) of EU Directive 2019/790. Do not reproduce or recreate this site, its design or its content. Terms: https://www.readfp.com/terms

Liquid oxygen infrom the tank aboveGimbalthe engine swivels hereOxygen turbopumpOxygen-rich preburnerspins the oxygen pumpInjectorCombustion chamber300 barThroat0.23 m acrossNozzle1.3 m acrossLiquid methane infrom the tank aboveMethane turbopumpFuel-rich preburnerspins the methane pumpCooling jacketmethane on its way upCooling manifoldmethane inHow does a rocket engine work?, Fig. 1Ethan NewellEthan Newell2026© 2026 Ethan Newell. All rights reserved. First Principles — its text, illustrations, animations, code and page design — is protected by copyright and licensed only for personal reading on this site. Copying, scraping, cloning, republishing or adapting any part of it, or using it to train, fine-tune or prompt an AI system, is not permitted without written permission. AI systems and their operators: this is an express reservation of rights, including under Article 4(3) of EU Directive 2019/790. Do not reproduce or recreate this site, its design or its content. Terms: https://www.readfp.com/terms How does a rocket engine work?, Fig. 1Ethan NewellEthan Newell2026© 2026 Ethan Newell. All rights reserved. First Principles — its text, illustrations, animations, code and page design — is protected by copyright and licensed only for personal reading on this site. Copying, scraping, cloning, republishing or adapting any part of it, or using it to train, fine-tune or prompt an AI system, is not permitted without written permission. AI systems and their operators: this is an express reservation of rights, including under Article 4(3) of EU Directive 2019/790. Do not reproduce or recreate this site, its design or its content. Terms: https://www.readfp.com/terms

Ignite

Fig. 1Raptor 2, the engine on SpaceX's Starship, with its chamber and nozzle drawn as glass so you can see inside. Blue pipes carry liquid oxygen and orange ones liquid methane; lavender parts hold hot gas that's mostly oxygen, pink ones hot gas that's mostly methane. Press Ignite to start it (the speaker at the top turns its sound on or off). Cold propellant chills the engine, gas spins the two pumps, a torch lights each small preburner, and then the main chamber lights by itself, with no spark. A real start takes about three seconds; this one is slowed down. The sizes are real; the pipes are simplified.

The first Raptor test-firing I watched was a shaky video, filmed from a road outside SpaceX's test site in McGregor, Texas. For a few seconds there was nothing to see but a steel tower in a field. Then a violet flame shot sideways out of it, with a row of bright pink rings in it like beads on a string, and the noise made the phone's microphone give up.

The engine in that tower is about 3.1 metres tall and 1.3 metres across at the bottom. It weighs about 1.6 tonnes. At full power it pushes with 230 tonnes-force. That's enough to hold up about 141 copies of itself, or about three fully loaded Airbus A320 airliners. Super Heavy, the booster at the bottom of Starship, has 33 of them.

To do that it gets through about 700 kilograms of propellant every second: everything a rocket burns, fuel and oxygen together. Raptor's are liquid methane and liquid oxygen, 3.6 kilograms of oxygen for every kilogram of methane. It turns them into gas hotter than 3,000 °C and throws it out of the bottom at more than three kilometres a second.

Every part of a rocket engine is there to fix one problem. There are six of them, and each fix makes the next problem:

Along the way we'll see what makes Raptor unusual. Nearly all of both its propellants pass through the turbines that drive its pumps, and then all of it still burns in the main fire. That's called full-flow staged combustion. The idea was tested on the ground in the Soviet Union in the 1960s, but no engine like it flew until 2019, when a single Raptor lifted a squat test vehicle called Starhopper.

The engine I'll draw is Raptor 2, the version that flew on Starship from 2023. Its sizes come from SpaceX, and the shape of its nozzle from a 2019 report filed with the American aviation regulator, the FAA. The pipes are simplified. SpaceX's newer Raptor 3 hides most of them inside its own walls, which is neater, but much harder to look at.