How SpaceX Streamlined the Raptor Engine

If you’re studying this, there’s an excellent likelihood you’ve seen this well-known picture 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 total Starship stack in 2023. Since then, it’s continued to enhance, going from the tangle of pipes and wires you may see on the Raptor 1 to the graceful, streamlined design of the Raptor 3, which first flew in May of this 12 months.

The evolution is so dramatic that many of us initially believed that it wasn’t actual; Tory Bruno, the then-CEO of house launch firm United Launch Alliance, tweeted that there was “no must exaggerate this by displaying {a partially} assembled engine,” which was adopted by SpaceX president Gwynne Shotwell tweeting an image of the Raptor 3 firing successfully:

This streamlining has come alongside significant good points in efficiency, with the Raptor 3 offering about 35% extra thrust than the Raptor 1.

I wished to higher perceive how this evolution truly occurred. What, particularly, did SpaceX change that allowed it to go from the tangle of wires and pipes to the svelte, streamlined engine on the appropriate?

There turned out to be much less element obtainable right here than I hoped. SpaceX doesn’t publish any official Raptor schematics, and nobody has performed a teardown of a Raptor engine. But due to the occasional remark from Elon Musk, and the speculations of a military of SpaceX followers, we will get some concept of what the foremost adjustments have been.

The Raptor engine is a “full-flow staged combustion” (FFSC) engine. What precisely does that imply?

A rocket engine works by throwing mass (“propellant”) out of a rocket nozzle — as a result of Newton’s Third Law (“for each motion there’s an equal and reverse response”), this pushes the rocket within the different path. The extra mass you may throw out, and the quicker you throw it, the extra thrust your rocket engine will produce.

The easiest way to do that is to easily fill a tank filled with pressurized gasoline, and vent a number of the gasoline out. The venting gasoline propels the rocket, in the identical manner that letting the air out of a balloon pushes the balloon. This form of rocket, known as a “chilly gasoline thruster,” is commonly used for making minor changes to a spacecraft’s place or orientation. Cold gasoline thrusters are discovered on, amongst different tools, NASA’s Manned Maneuvering Unit, and the management thrusters for SpaceX’s Falcon 9 rocket.

These engines are easy and dependable, however restricted; there’s solely a lot thrust you may virtually get from a chilly gasoline thruster. So how can we get extra thrust? One apparent manner is as an alternative of a single propellant, we will use two: take a gasoline (like kerosene) and an oxidizer (like oxygen) after which burn them collectively in a combustion chamber, creating sizzling gasoline that escapes out the again of your rocket nozzle. Now as an alternative of simply utilizing the vitality saved within the pressurized propellants, we’re additionally utilizing the vitality saved of their chemical bonds, and changing that vitality into thrust. An engine that makes use of the stress of the propellant tanks to power gasoline and oxidizer into the combustion chamber known as a pressure-fed engine. The engine used for the ascent stage of the Apollo Lunar Module was such a engine: it used nitrogen tetroxide (N2O4) as an oxidizer and a gasoline known as Aerozine 50, each pressurized.

But now you’ve got an issue. To stop the burning gases within the combustion chamber from flowing again into the gasoline and oxidizer strains (ravenous the combustion chamber of incoming propellant), the stress within the combustion chamber must be decrease than the stress within the gasoline and oxidizer tanks and features. But for a compact, highly effective rocket engine, we wish to have very excessive pressures within the combustion chamber. We may cope with this by rising the stress in our propellant storage, however this shortly will get impractical: the upper the pressures, the thicker and heavier our storage tanks and propellant strains have to be with the intention to preserve them with out rupture. A greater answer is to retailer the propellants at low stress, after which feed them by way of a pump that will increase their stress earlier than they attain the combustion chamber. This is the usual option to construct a robust rocket engine, and nearly each engine used for placing a rocket into orbit makes use of a pump of some sort to pressurize the gasoline and oxidizer.

Now we now have a brand new downside: due to the amount of fluid they have to deal with and the quantity of stress they have to add, these pumps require an unlimited quantity of energy to function. The turbopump on the F-1 engine used on the Saturn V, for example, required round 41 megawatts of energy, barely lower than the ability the S8G nuclear reactor delivers to the propeller shaft of an Ohio-class submarine. One manner to supply this energy is with a battery — Rocket Lab’s Electron rocket has an engine with a battery-powered electrical pump — however the extra frequent technique, employed by nearly each massive booster rocket, is to make use of the rocket’s personal propellant as an influence supply, burning a small quantity of gasoline and oxidizer to drive a turbine that in flip drives the pump.

There are totally different ways in which a system like this may be configured. The easiest is to route a small quantity of gasoline and oxidizer to a turbine, after which vent the ensuing exhaust. This is called a gas generator cycle, and it’s what’s used on the F-1 engine, in addition to SpaceX’s Merlin engine.

Another choice is to burn a number of the gasoline and oxidizer to drive the pump, however then route the burned propellant by way of the primary combustion chamber together with the unburned gasoline/oxidizer. This is called “staged combustion,” and the smaller combustion chamber used to drive the pumps known as the “preburner.” This is a extra complicated association than a gasoline generator cycle, but it surely’s additionally extra environment friendly, for the reason that sizzling exhaust that drives the turbopump isn’t simply wastefully vented.

For most staged combustion engines, solely a portion of the propellant will get routed by way of the preburner: the remainder will get routed round and goes on to the combustion chamber. But SpaceX’s Raptor makes use of a specific kind of staged combustion often called “full move.” In a full-flow engine there are two preburners, one which drives the gasoline pump and one which drives the oxygen pump. All the propellant will get routed by way of the preburners: in considered one of them, a small quantity of oxidizer is burned within the presence of a considerable amount of gasoline (leaving a lot of the gasoline unburned), whereas within the different a small quantity of gasoline is burned within the presence of a considerable amount of oxidizer (leaving a lot of the oxidizer unburned). The fuel-rich and oxidizer-rich exhaust streams then each enter the primary combustion chamber, the place they’re burned collectively.

A full-flow staged combustion engine could be very complicated, and previous to the Raptor solely two had been constructed, neither of which efficiently flew on a rocket. The Russians developed an FFSC engine within the Nineteen Sixties, the RD-270, but it surely by no means flew, and the US constructed a part of an FFSC engine known as the Integrated Powerhead Demonstrator within the Nineties and 2000s, but it surely was by no means developed right into a full engine. (When SpaceX started engaged on the Raptor in 2012, it obtained some of the equipment used on the Integrated Powerhead Demonstrator.) But an FFSC engine has a number of benefits, considered one of which is (theoretically) reliability: as a result of a lot mass flows by way of the generators driving the pumps, the generators can run cooler and at decrease stress, making them (in concept) extra dependable. If you’re an organization betting closely on reusable rockets, a extra dependable turbine is clearly enticing.

SpaceX is, understandably, tight-lipped about lots of the specifics of its superior rocket expertise, and there’s much less official data on the precise particulars of how the Raptor operates than you would possibly hope. Nobody has opened up a Raptor engine to point out what’s inside, and far of the knowledge that exists comes from Elon Musk’s tweets or his feedback in Everyday Astronaut interviews.

There are, nevertheless, plenty of SpaceX fanatics on the market, a lot of whom do issues like obsessively photograph each Raptor engine leaving the manufacturing facility, and these people have put plenty of effort into speculating about how the Raptor engine works. The most helpful output of this hypothesis, for me, is the varied fan-made schematics that present how the Raptor engine is believed to function. These schematics aren’t official — they’re made by SpaceX fanatics piecing collectively data from varied sources — so that they should be taken with a big grain of salt. But they’re nonetheless a helpful place to begin for getting an concept of how folks suppose varied variations of the Raptor engine have labored.

To begin, let’s have a look at some schematics of model 1 of the Raptor. The image below was created by Elisei Maslov, a Russian propulsion engineer, in 2019.

Another helpful Raptor model 1 schematic is the one beneath, created by NASASpaceFlight forum member “hisdirt” in December 2019. This is especially helpful as a result of it calls out the varied engine elements on an precise 3D mannequin of the engine (modeled in Revit, of all issues).

You can see in these schematics the fundamental elements of a full-flow staged combustion engine: the oxygen pump, turbine, and preburner are on prime of the engine, whereas the gasoline pump, turbine, and preburner are the meeting on the aspect. You can even see how the gasoline flows by way of the surface of the nozzle earlier than going again into the preburner — this cools the nozzle so it doesn’t soften from the warmth of the rocket exhaust, and is called regenerative cooling.

The subsequent schematic, made by NASASpaceFlight customers “Livingjw” and “HVM” in February of 2022, is from Wikipedia, and it reveals model 2 of the Raptor. This is much less detailed than Maslov’s schematic — it mainly simply reveals the move of oxygen and methane — but it surely reveals the identical fundamental association: oxygen pump meeting on the highest, gasoline pump meeting on the aspect.

And this schematic, posted by Twitter consumer “TheSpaceEngineer” in January 2025, reveals model 3 of the Raptor.

Assuming that these schematics aren’t grossly deceptive, we will see that the foremost structure hasn’t modified between model 1 and model 3 of the engine. It’s nonetheless a full-flow staged combustion engine, and nonetheless has the fundamental association of the oxygen pump, turbine, and preburner meeting on the highest, with the gasoline pump, turbine, and preburner mounted to the aspect, with gasoline getting used to regeneratively cool the nozzle. And you may see that plenty of the smaller strains are the identical on variations 1 and three. Both present nitrogen strains used for purging (clearing propellant out of the system), each present gasoline and oxidizer strains going to the igniters, and each have gasoline and oxidizer strains carrying gaseous propellant again to the tanks to maintain them pressurized as they empty (this is called autogenous pressurization).

So what modified? If we glance intently on the model 1 and model 3 schematics (remembering once more that these are unofficial and speculative), we will see a number of variations. Version 3 reveals the smaller strains bundled collectively in a “frequent umbilical,” and if we have a look at photographs of the Raptor 3 linked to a rocket we will see this:

The model 1 schematic additionally reveals helium getting used to spin up the gasoline and oxygen generators initially, whereas in model 3 these strains have been eradicated, and nitrogen is used as an alternative. (It’s not amazingly apparent to me if this evolution is appropriate — some Reddit commenters on the model 1 schematic said that helium wasn’t used — however there’s some evidence that it’s.) Per these schematics, the helium used to manage some valves current on model 1 can be absent on model 3, and the model 3 schematic reveals no helium strains in any way.

The model 3 schematic additionally notes {that a} warmth exchanger has been eradicated, one thing echoed by various folks on the NASASpaceFlight boards — this seems to be a gaseous oxygen warmth exchanger close to the preburner. There’s additionally a gasoline line going to the preburner that’s current on model 1 however is absent on the model 3 schematic.

The extra dependable supply of adjustments, after all, is statements by Elon Musk and SpaceX. A 2022 NASASpaceFlight article about SpaceX’s replace on the Starship progress noted that “every little thing from turbomachinery to chamber nozzle to electronics” was redesigned on Raptor 2, the turbopump had shrunk, and the preburner controllers “had been moved to packing containers relatively than being everywhere in the engine.” An Everyday Astronaut article about the identical replace additionally notes that “many valves had been mixed into valve plates.” In a latest response to a September 5 tweet by Yishan Wong, the previous CEO of Reddit, Musk ran down a short list of changes within the Raptor over time:

One massive class right here is sensors, and the varied wires and cables they require. From what I perceive, Raptor model 1 was largely a improvement engine, and thus required plenty of further sensors monitoring issues like temperature and pressure in varied elements of the engine to find out the way it was behaving. As the engine obtained dialed in, plenty of these sensors might be eradicated (although a few of them could have been moved internally).

Another factor Musk calls out is the primary chamber spark igniters. Igniters, per their title, ignite the gasoline and oxidizer in the primary combustion chamber. These had been current in model 1 of the Raptor, however by model 2 that they had been eradicated. Musk doesn’t say what they had been changed with, however this alteration could have one thing to do with the truth that the recent gasoline and oxidizer exhaust streams from the preburners in all probability want little or no encouragement to combust. (A NASASpaceFlight remark from 2019 wonders if most important chamber igniters are even obligatory, given the excessive temperature of the preburner exhaust streams.)

Musk additionally notes that plenty of bolted flange connections had been changed by welded connections, decreasing mass and joints that may leak on the expense of serviceability. This seems to be nonetheless happening, as early Raptor 3 photographs present a big bolted flange on the primary physique of the engine that in later photographs has been eradicated:

Another substantial change on Raptor 3 is that a lot of the piping hasn’t been eradicated, however moved internally, by means of 3D printing lots of the elements (Musk has beforehand famous that SpaceX “has the most advanced 3D metal printing technology in the world,” and in 2024 the corporate licensed the 3D steel printing expertise of Velo3D).

Close-up photographs of the Raptor 3, in reality, seem to point out 3D printing strains:

The objective of this internalization is the removing of extra elements, specifically the warmth defend and hearth suppression methods. The large mass of wire and pipes on model 1 and model 2 of the Raptor required a big, heavy warmth defend to guard it from the warmth of the rocket exhaust. Internalizing the varied propellant strains and including inner cooling methods permits this warmth defend and hearth safety system to be eradicated. In reality, the most important mass change from model 1 to model 3 of the Raptor comes from mass removing of “automobile aspect” engine {hardware}, which is sort of actually largely the warmth defend.

SpaceX’s Raptor engine has undergone spectacular evolution, going by way of a number of main variations, a dramatic improve in efficiency, and a dramatic lower in mass and exterior elements in a brief period of time. The elementary structure of the engine hasn’t modified — it’s nonetheless a full-flow staged combustion engine, utilizing the identical main elements — however the varied ancillary parts and elements supporting its operations have been modified considerably.

It’s price noting that whereas this streamlining has made the Raptor engine easier within the sense that varied particular person elements have been eradicated, it was nonetheless an (as Musk notes) enormously complicated job to get it to work, and there’s an enormous quantity of inner complexity added to the Raptor 3 that these photos don’t present. And the issues haven’t been fully ironed out: the primary launch try of Starship’s flight test 13 was aborted mechanically by the automobile’s flight software program at T-0 (proper earlier than liftoff) this previous July when a number of Raptor 3 engines failed to begin. So the image of a number of variations of the Raptor is a snapshot of a chunk of expertise that’s persevering with to evolve.