GoKawiilIf you’re reading this, there’s a very good chance you’ve seen this famous image of three iterations of SpaceX’s Raptor rocket engine. The Raptor engine was developed for SpaceX’s Starship spacecraft (the Falcon 9 and Falcon Heavy use the Merlin engine); it was first test-fired in 2016, first flew on Starhopper in 2019, and first flew on a Starship prototype in 2020 and on the full Starship stack in 2023. Since then, it’s continued to improve, going from the tangle of pipes and wires you can see on the Raptor 1 to the smooth, streamlined design of the Raptor 3, which first flew in May of this year.
The evolution is so dramatic that many folks initially believed that it wasn’t real; Tory Bruno, the then-CEO of space launch company United Launch Alliance, tweeted that there was “no need to exaggerate this by showing a partially assembled engine,” which was followed by SpaceX president Gwynne Shotwell tweeting a picture of the Raptor 3 firing successfully:
This streamlining has come alongside meaningful gains in performance, with the Raptor 3 providing about 35% more thrust than the Raptor 1.
I wanted to better understand how this evolution actually happened. What, specifically, did SpaceX change that allowed it to go from the tangle of wires and pipes to the svelte, streamlined engine on the right?
There turned out to be less detail available here than I hoped. SpaceX doesn’t publish any official Raptor schematics, and no one has done a teardown of a Raptor engine. But thanks to the occasional comment from Elon Musk, and the speculations of an army of SpaceX fans, we can get some idea of what the major changes have been.
How the Raptor engine works
The Raptor engine is a “full-flow staged combustion” (FFSC) engine. What exactly does that mean?
A rocket engine works by throwing mass (“propellant”) out of a rocket nozzle — due to Newton’s Third Law (“for every action there is an equal and opposite reaction”), this pushes the rocket in the other direction. The more mass you can throw out, and the faster you throw it, the more thrust your rocket engine will produce.
The simplest way to do this is to simply fill a tank full of pressurized gas, and vent some of the gas out. The venting gas propels the rocket, in the same way that letting the air out of a balloon pushes the balloon. This sort of rocket, called a “cold gas thruster,” is often used for making minor adjustments to a spacecraft’s position or orientation. Cold gas thrusters are found on, among other equipment, NASA’s Manned Maneuvering Unit, and the control thrusters for SpaceX’s Falcon 9 rocket.
Cold gas thruster diagram, via Wikipedia .
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