How SpaceX's Raptor works, one problem at a time

Compressed gas and a nozzle

A rocket throws mass out of the back, and that pushes the rocket forward (Newton's third law). The faster the mass leaves, the more push you get per kilogram.

Simplest version: fill a tank with compressed gas and let it out through a nozzle. That's a cold gas thruster. Falcon 9 uses little nitrogen thrusters like this for steering, and NASA's Manned Maneuvering Unit, the jetpack astronauts flew in the 1980s, worked the same way.

Tank pressure

Problem: the only energy you have is the pressure in the tank, and that's not much. Fine for steering, useless for getting to orbit.

Used on: Falcon 9 steering thrusters, the MMU jetpack

Burn something

Chemical bonds hold far more energy. Take a fuel (Raptor uses methane) and an oxidizer (liquid oxygen), push both into a chamber and burn them. The hot gas leaves the nozzle much faster than cold gas ever could.

The simplest way to push them in is tank pressure. That's a pressure-fed engine. The Apollo lunar module lifted off the Moon on one.

But propellant only flows from high pressure to low pressure. For propellant to flow into the chamber, the tanks have to be at higher pressure than the chamber. And you want the chamber at very high pressure, because that's how you get a small, powerful engine. Raptor runs at around 300 bar. Set a chamber pressure, then see what the tanks need to keep up:

Chamber pressure:

Tank pressure:

Steel wall a 9-metre-wide tank needs at that pressure:

Thin-wall estimate: stainless steel at about 500 MPa, no safety margin. Starship's real tanks run at a few bar with walls a few millimetres thick.

Problem: high chamber pressure means thick, heavy tanks. At Raptor's pressure the walls would be about 30 cm of steel.

Used on: Apollo lunar module ascent engine

Add pumps

Fix: keep the tanks at low pressure, and put a pump in each line that squeezes the propellant right before it enters the chamber. Light tanks, high chamber pressure. Almost every rocket that reaches orbit does this.

The catch is power. The turbopump on the Saturn V's F-1 engine needed about 41 megawatts, roughly 55,000 horsepower, to move its propellant. Something has to spin it.

Rocket Lab's Electron uses batteries and electric motors. That works for a small engine. For a big one the batteries get far too heavy.

Problem: you need tens of megawatts from something light.

Used on: every big orbital rocket. Electron with batteries.

Spin the pumps with a turbine

Fix: the rocket is already carrying an enormous amount of energy in its propellant. Tap off a little fuel and oxygen, burn it in a small burner called a gas generator, and let the hot gas spin a turbine. The turbine sits on the same shaft as the pumps.

One catch: burn fuel and oxygen at the perfect mixture and you get over 3,000 °C, which melts any turbine blade. So the gas generator runs lopsided, with lots of fuel and just a little oxygen, which keeps the gas cool enough for the turbine. Try it:

Gas temperature: .

The used gas gets dumped overboard through its own little pipe. On a Falcon 9 launch you can often spot it as darker, sooty exhaust next to the bright flames of the Merlin engines.

Problem: that dumped gas is propellant you paid for, and it barely adds thrust. More chamber pressure needs more pump power, which means dumping more. You hit diminishing returns fast.

Used on: Merlin (Falcon 9), F-1 (Saturn V)

Burn the turbine exhaust again

Fix: the turbine exhaust is still full of unburned propellant. Instead of throwing it away, pipe it into the main chamber and burn it there. The small burner is now called a preburner, and because the propellant burns in two stages, this is staged combustion.

Now nothing goes overboard, so you can push far more gas through the turbine, get far more pump power, and run at much higher chamber pressure. The Russian RD-180 works like this (the oxygen-rich version drawn here), and so did the Space Shuttle's main engine (a fuel-rich version).

Problem: one turbine, bathed in hot gas made mostly of one propellant, drives the pumps for both. Somewhere on that shared shaft, hot oxygen-rich gas sits right next to the fuel, kept apart only by seals. If a seal leaks, fuel meets hot oxygen inside the machinery.

Used on: RD-180 (Atlas V), RS-25 (Space Shuttle, SLS)

Full flow: give each side its own turbine

Fix: split it in two. One preburner gets all the methane and just a little oxygen. It makes hot, fuel-rich gas, which spins the methane pump. The other gets all the oxygen and a little methane. It makes hot, oxygen-rich gas, which spins the oxygen pump. Every drop of propellant passes through a turbine before it reaches the main chamber. That's what full flow means.

That solves the seal problem from step 5, and you get two more things on top:

No dangerous seals. Each turbopump only ever touches one propellant. Methane-rich gas spins the methane pump, oxygen-rich gas spins the oxygen pump.
Cooler turbines. A turbine's power is roughly how much gas flows through it times how much energy each kilogram gives up. Push all the propellant through and each kilogram can give up less, so the gas can be cooler for the same power. Cooler turbines last longer, which matters when you want to reuse the engine.
Gas meets gas. Both propellants reach the main chamber already hot and gaseous, so they mix and burn fast and completely. That helps run at very high pressure.

The price: hot oxygen-rich gas attacks metal like a cutting torch, so SpaceX had to develop its own alloy for that side of the engine. And two preburners that feed each other are hard to balance and hard to start. Before Raptor, only two full-flow designs had ever been built: the Soviet RD-270 in the 1960s, and the American Integrated Powerhead Demonstrator in the 2000s, a test rig for the front half of such an engine. Neither flew. Raptor first flew in July 2019, on Starhopper's 20-metre hop.

Used on: Raptor (Starship)

Now add the plumbing

The core engine is done. Most of the lines on the real schematics belong to the systems around it. Keep scrolling and they switch on one by one, or tap them yourself:

Regenerative cooling. The hot gas would melt the chamber wall, so the cold methane runs through channels inside the chamber and nozzle walls on its way to the preburner and carries the heat off. Deep dive ↓ Tank pressurization. As the tanks empty, a little warm gas goes back up to keep them pressurized. Using the propellant itself for this is called autogenous pressurization. Deep dive ↓ Igniters. Sparks to light the preburners and, on Raptor 1, the main chamber too. Purge, start and sensors. Inert gas (nitrogen on Raptor 3) to spin the turbines up at start and flush propellant out afterwards, plus pressure and temperature sensors everywhere. Deep dive ↓

With all four on, this is roughly the fan-made schematic from Brian's article. Almost every extra line there belongs to one of these systems.