The Insane Engineering Of the SpaceX Raptor Engine

The SpaceX Raptor engine is pushing engineering to new heights. And every technical aspect of these monster engines is groundbreaking.

So, we’re going to dig into the engineering side of these engines. But, the depth of our technical dive won’t be much because of the many complexities. Even so, our overview will be enough to open your eyes. You’ll quickly understand how insanely complex yet amazing these engines are.

Just as a warning though, this is a LONG article. It’s the only way I can do justice to this awe-inspiring brilliant engine. And if you’re on this page, I know you want to soak in as much Raptor knowledge as possible. I don’t blame you!

But before we go any further, let’s set the stage for this discussion. This reading will require some basic understanding of rocket science. You need to know how two substances enter a combustion chamber in a controlled way. And the devil lies in the details if you want to wrap your mind around all the hairy concepts.

Now, don’t get intimidated and close your browser. You don’t need to be a rocket scientist or even an engineer to make sense of this content. All you need is a quick lesson on rocket basics. So, if you need a brush-up, I suggest you watch the below videos in the listed order first. They’re short and easy to understand.

  1. Basics of how rocket propellants in a gas-generator power cycle work by Lescis.
  2. A detail of how rocket propellants and rocket engine plumbing work by Scott Manley.
  3. Overview of how full-flow staged combustion rocket engines work by Scott Manley.

With that out of the way, let’s get on with the awesomeness and dive into the nitty-gritty details.

The SpaceX Raptor engine overview

The SpaceX Raptor engine is one of few full-flow staged combustion cycle engines in the world. This includes the ENTIRE history of rocket engineering too. And more impressive, SpaceX is pushing the envelope of what we believed was even possible.

SpaceX impressively develops and manufactures these rocket engines in-house. This gives SpaceX immense flexibility to quickly test, fail, tweak, and repeat. This is the lean start-up mantra applied to rocket engineering. I know, crazy right, who would’ve imagined we’d apply agile development to rockets!

Supply chain awesomeness aside, the Raptor engines have a huge responsibility. They’ll one day power the SpaceX Starship to Mars. So yeah, these engines need to be powerful and reliable, to say the least.

Now, if you’re wondering about the evolution of the SpaceX Raptor engine, check out the below table. It’s a timetable of all the incremental achievements on the engine. And take special notice of how the work didn’t happen overnight. This despite the engineering work taking place in the premier aerospace company led by Elon Musk.

DateDescription
October 2012SpaceX publicly announces concept work on rocket engine that'll be "several times as powerful as the Merlin 1 series of engines, and won't use Merlin's RP-1 fuel."
November 2012Elon announces a new direction for the propulsion division of SpaceX, with the development of methane-fueled rocket engines.
Early 2014SpaceX confirms Raptor engine will be used for the main Mars Colonial Transporter.
May 2014Raptor engine component testing begins (e.g. testing on Raptor injector elements).
April to August 2015SpaceX does 76 hot fire tests of a full-scale oxygen preburner.
January 2016US Air Force awards SpaceX $33.6 million to develop a prototype version of its methane-fueled reusable Raptor engine.
Early 2016SpaceX constructs new engine test stand at their McGregor test site in Texas for Raptor engine testing.
August 2016First integrated Raptor rocket engine completed and shipped to McGregor test site for development testing.
September 2017SpaceX completes testing with 200 bars (2,901 psi) combustion chamber pressure using new SpaceX created SX500 alloy.
February 2019First test flight (lasted 2 seconds) of Raptor engine with combustion chamber pressure of 170 bars (2,466 psi).
June 2020Raptor engine achieves a combustion chamber pressure of 300 bars (4,351 psi) on a ground test.
August 2020Raptor engine achieves a combustion chamber pressure of 330 bars (4,786 psi) on a ground test.
February 2021Starship prototype SN9 launched to an altitude of 10km and is destroyed on landing.
May 2021Starship prototype SN915 launched to an altitude of 10km and successfully lands back on pad.

A LOT of work still remains with both the engine and engine integration with the Starship. Undoubtedly though, the work accomplished to date is utterly incredible!

And this makes a great segway to discuss what the fuss is all about with these engines. To get the ball rolling, we’ll play the comparison game. We’ll compare the Raptor engine to the gas-generator rocket cycle engine.

The gas generator cycle engine overview

The gas-generator cycle engine, or called an open cycle, is very common. It may be one of the most common liquid-fueled rocket engines used on orbital rockets. The following are example use cases of the engine, which you’ve probably heard of:

  • F-1 rocket engines used on the SpaceX Falcon 9 second stage
  • F-1 rocket engines used on the Saturn V booster stage

The below schematic is an overly simplified workflow of how the engine operates. It’ll help you connect the dots over the core concepts of this engine.

gas generator rocket cycle engine
Photo Credit: Duk

At the most basic level, we want to drive loads of oxidizer and fuel into the combustion chamber. These are the blue and red lines pointing to the top of the combustion chamber in the schematic.

At the same time, we want to maximize the pressure in this combustion chamber to maximize thrust.  So engineers need to design around this requirement. And what’s important to know is, with any engine system, pressure flows from high to low.

So how is this rocket design achieved without a highly pressurized propellant tank? A well-operating turbopump system, of course!

Important Note: the turbopump system consists of two pumps and a turbine. Then there’s a mini rocket engine, labeled a preburner in the schematic. It’s in fact a burner, or better yet, called a gas generator. Finally, there’s a single shaft that connects the turbine to the two pumps. 

Now, the burner drives the turbine, and the pumps are loads on the turbine. The pumps then deliver a high volume of fuel and oxidizer to the combustion chamber. In short, the rocket turns the propellant’s chemical energy into kinetic energy. 

You can now see why the turbopumps are at a higher pressure than the combustion chamber. Where the inlet of the burner is the highest pressure point in the entire engine. And the farther you go downstream, the lower the pressure drops. A schematic in this article lists various pressure points in an engine’s cycle you can check out. 

See the taps from the main fuel and oxidizer lines to the burner in the schematic? Only a small amount of fuel and oxidizer enters the burner to generate gas for the turbine. The turbine then drives the insanely powerful pumps. The combustion chamber then becomes flooded with highly pressurized propellant. It’s almost like a looped domino effect.

This now leads us to where it reads “Exhaust” on the schematic. This is our catch-22 situation. At this node, the highly pressurized unburnt propellant from the turbine goes overboard.

Not surprisingly, the engine efficiency isn’t optimal due to this exhaust. Because again, you’re wasting some highly pressurized propellant. Instead, this exhaust waste could have entered the combustion chamber. This would have helped with maximizing the produced rocket thrust.

As we’ll learn, increasing propellant flow through the combustion chamber increases thrust. And the more pressurized and hot the propellant is to an extent, the better. BUT, let’s hold our horses. The open-cycle design has the following limiting conditions:

  • A cap on the amount of propellant flow
  • Limit on how pressurized and hot the burner can become
  • Sooting resulting from unburnt fuel exiting the burner exhaust

These limitations make the engine not good for heavy load applications. And this will set the stage for our full-flow staged combustion engine discussion. Before we get started though, let’s mathematically show these limitations.

The mechanical power to drive the pumps 

The following equation is the mechanical power required to drive the pumps:

P_{p} = \dot m_{ox} \dfrac{\Delta P_{op}}{\eta_{op}\rho_{ox}}+\dot m_{f} \dfrac{\Delta P_{fp}}{\eta_{fp}\rho_{f}}

Where,

P_{p} = mechanical power to drive the pumps
\dot m_{ox} = oxidizer flow rate
\dot m_{fp} = fuel flow rate
\Delta P_{op} = pressure rise in oxidizer pump
\Delta P_{fp} = pressure rise in fuel pump
\eta_{op} = oxidizer pump efficiency
\eta_{fp} = fuel pump effficiency
\rho_{ox} = oxidizer density
\rho_{f} = fuel density

So to increase P_{p} we need to increase the propellant flow rate or increase the pressure. But as we learned, there’s a limit to how much we can increase these variables.

If we increase \dot m_{fp} too much, we’ll waste fuel and have sooting issues. If we increase \rho_{ox} and/or \rho_{f} too much, we can damage the turbopump from scorching temperatures. And again, this is why these engines aren’t designed for heavy payload applications.

We’ll refer back to these relationships, so remember them. They’re important when we discuss the advancements of the full-flow staged combustion engine.

Important Note: to maintain low temperatures, you adjust the fuel to oxidizer ratio. This is important since extreme temperatures will destroy your turbine and pumps. So, you either use more fuel (fuel-rich), or more oxidizer (oxidizer-rich).

What’s more, if you run an engine fuel-rich using RP-1 fuel, you’ll quickly see unburnt fuel. This takes form as dark clouds of soot exiting the burner exhaust.  The soot will stick to everything around it like glue too. And with the many bells and whistles of rocket engines, you don’t want to block any passages.

In short, engineers control the turbopump temperature with fuel-rich oil. But since you can’t increase the oxidizer, your only option is to increase RP-1. Then though, sooting increases and you waste a lot of fuel. In the end, you need to down-regulate engine operations to play it safe. 

Finally and not to forget, the sharing of a common shaft by the two pumps has risks. If the oxidizer leaks through the seal on the shared shaft into the fuel pump, things can go BOOM!

The full-flow staged combustion cycle engine overview

Now, we can go over the awesomeness of the full-flow staged combustion cycle engine. Also, commonly called the full-flow closed cycle.

As a side note, other engine designs existed between the open cycle and the full-flow closed cycle. But for the sake of brevity, I’m going to skip over these other design iterations.

full flow staged rocket cycle engine
Photo Credit: Duk

With this engine, the propellants burn in stages. And unlike the gas generator cycle, there are two burners, appropriately called preburners. One is fuel-rich and the other is oxidizer-rich. The fuel-rich preburner not surprisingly powers the fuel pump. While the oxidizer-rich preburner powers the oxidizer pump. This sounds like the so obvious design that should’ve always existed, right?…

In the schematic, you see the fuel-rich preburner taps off the oxidizer flow. So a lot of fuel passes through this preburner but very little oxidizer. And vice versa for the oxidizer-rich preburner. This in return produces mostly unburned vaporized propellant. This is what we want!

The hot gases spin their respective turbines and drive their respective pumps. Where the pump speed is just fast enough to optimally pressurize the preburners. Then in return, best pressurize the combustion chamber.

This all works out because the fuel and oxidizer arrive in the combustion chamber in the form of hot gas. As a result, you can reach hotter temperatures and improve combustion.

To point out, because of the two combustion stages, the propellant burns twice. First in the preburners and then in the combustion chamber. In the preburners, the propellant burns at a lower efficiency. Just enough to produce the required amount of energy to spin the turbines at the ideal speed. Because you don’t want to turn the turbines into a soup from high temperatures. Remember the problem with the gas generator cycle?…

In the combustion chamber, the propellant burns at max efficiency producing max thrust. And to top it off, no propellant goes to waste. So the engine maximizes the extraction of energy from all its propellant. This makes this engine ideal for high-power rocket applications. In short, a rocket built with SpaceX Raptor engines can more easily climb Earth’s gravity well.

Important Note: the SpaceX Raptor engine tech isn’t new. In fact, others have manufactured and tested similar engines before. But, they were never put into production with test flights.

It’s not hard to understand why. There are MANY engineering difficulties. For one, synching the start sequence of the two turbopumps. To make this happen, you need to design a complex feedback and control system. 

Then, engineers need to address the high-temperature piping for the flow of hot gases. This is especially a problem with the oxidizer-rich cycle. You need a material that won’t erode from the oxidization or melt when the engine runs.

The SpaceX metallurgy team created a super allow, SX500, to resist the oxidization. Also, they placed their oxidizer turbopump right on top of the combustion chamber. This location minimizes the amount of metal exposed to the flow of rich oxidizer.

Maximizing total power output with a Raptor engine 

Raptor engine unofficial combustion scheme (Photo Credit: livingjw)

Let’s now dig into the awesomeness of this engine with clear-cut examples.

For starters, you have near twice the mass flow entering the combustion chamber. This is because we have two turbines as shown in the full-flow schematic instead of one. So, there’s A LOT of propellant flowing through the preburners. This allows us to maximize the preburner outputs for optimal turbine speed.

Now get ready for this, there’s a pleasant surprise. The temperature and pressure drop across the Raptor turbines doesn’t spike. So as we increase the produced thrust, our invaluable turbines and pumps stay safe.

Again, for contrast, let’s go back to the operation of the open-cycle engine. In the open-cycle, you use as little oxidizer and fuel as possible in the burner. For one, some fuel will get dumped overboard and the price of every kilogram of cargo to space is HIGH. I’m talking about $2,720/kg kind of high, as advertised by SpaceX for the Falcon 9. Second, if you use RP-1, you’ll have sooting issues on your hand.

So, the burner runs at the highest allowed temperature set by the turbine specs. Because you have the entire turbine melting problem. But if that wasn’t enough, there’s the issue of thermal stress. The turbine blades go under large centrifugal stress and the material weakens.

Unfortunately, this is the only way to maximize efficiency at a reduced flow rate. To better explain with math, let’s look at NASA’s turbine thermodynamics:

TW = \eta_{t}c_{p}Tt_{4}(1-TPR^\dfrac{(\gamma-1)}{\gamma})

TPR = \dfrac{pt_{5}}{pt_{4}} = (\dfrac{Tt_{5}}{Tt_{4}})^\dfrac{\gamma}{\gamma-1}

Take note of the following variables,

TW = the turbine work per mass of airflow
TPR = the turbine pressure ratio
Tt_{4} = total temperature at turbine entrance
Tt_{5} = total temperature at turbine exit
pt_{4} = total pressure at turbine entrance
pt_{5} = total pressure at turbine exit
\gamma = specific heat ratio

As we increase the inlet temperature to the burner, the turbine efficiency increases. But as we just discussed, pushing the temperature of the turbine to its upper limits isn’t good.

A full-flow closed-cycle engine can bypass all these problems though. And yes, all without a spike in the temperature. It’s because ALL the fuel and ALL the oxidizer flow through the preburners. So you can burn just enough propellant to maximize turbine efficiency. Remember the equation for mechanical power that was dependent on the flow rate we discussed? That’s the key!

To summarize, the Raptor engine has the following going for it over open-cycle engines:

  • Increased mass flow rate
  • Separate fuel-rich and oxidizer-rich mixtures
  • Dedicated turbines for the fuel-rich and oxidizer-rich mixtures
  • Plumbing for all propellent to flow from the turbines to the combustion chamber
  • Use of liquid methane versus RP-1 or hydrogen for fuel, to limit sooting

Important Note: a tradeoff exists between efficiency and complexity. Using two gas generators allows for relaxed turbine requirements. Because the load to achieve max power distributes over two power sources versus one. In return, the turbines can operate at a low non-damaging temperature.

But, now you have two running turbines instead of one. This adds to the complexity of the design. As the saying goes, you can’t have your cake and eat it too. 

Maximizing specific impulse with a Raptor engine 

raptor engine test
Raptor engine test (Photo Credit: Elon Musk)

The increased total power output makes the SpaceX Raptor engine operate on steroids. To illustrate this, let’s revisit our turbines and pumps with a quick physics lesson.

With the turbines running at optimal levels, the high-powered pumps produce crazy pressure. The generated turbine power is near equal to the volume flow multiplied by the pressure drop across the turbine. The simple math relationship in calculating power in watts then becomes the following:

Volume flow x Pressure drop = \dfrac{\text{meter}^{3}}{\text{second}} \times \dfrac{\text{newton}}{\text{meter}^{2}} = \dfrac{\text{meter} \times \text{newton}}{\text{second}}

Where watt = \dfrac{\text{meter} \times \text{newton}}{\text{second}}

So, combustion chamber pressure increases as more mass pushes through the rocket engine. This then leads to greater exhaust velocity and thrust. Let’s now use another equation to paint this relationship.

C* = \frac{P_{c} \times A_{t}}{q}

C* is the characteristic exhaust velocity. It’s a measure of the amount of energy available from the combustion process. And P_{c} is the combustion chamber pressure.

Now, where does the specific impulse fit into all this, you ask?

Before answering this question, let’s talk a little about specific impulse. We measure specific impulse in seconds. I know, sounds confusing. But it’s actually straightforward if we tweak the framework of the definition.

Think of it as the time in seconds a given fuel weight onboard a rocket will produce a constant amount of thrust. So if a rocket has 100 pounds of thrust with a specific impulse of 200 seconds, it’ll burn 100 pounds of fuel every 200 seconds. In other words, it’s a measurement of how efficiently a rocket uses its fuel. Think of it like your car’s fuel economy, where you optimally burn fuel by driving X mph.

Let’s jump back to our discussion now. The following is the relationship between specific impulse (I_{sp}) and C*:

I_{sp} = \dfrac{C*C_{f}}{g}

C* can capture the impact of combustion, where C_{f} is the thrust coefficient for the nozzle impact. Thus, with C_{f} = 1, C* = I_{sp}.

So you’re asking, what do all these fancy equations mean in the end? It means the SpaceX Raptor engine can lift a heck of a lot of load out of Earth’s gravity well.

Physical space-savings with Raptor engines

spacex merlin and sea level raptor at hawthorne
SpaceX Merlin and sea-level Raptor at Hawthorne (Photo Credit: Brandon De Young)

With a high specific impulse, you gain much more physical flexibility. You can install more engines in the same area, previously allotted for a single-engine. So you get much more bang for your buck with a Raptor engine.

For example, you can fit four Raptor engines in the same area you fit one Space Shuttle Main Engine (SSRE). So you no longer need to use Solid Rocket Boosters (SRB) to supply extra thrust. In return, you save on costs and you improve safety. So you get the idea. These Raptor engines will become game-changers!

According to Elon Musk, the Super Heavy, the booster designed to push the Starship,

Will have 31 engines, not 37, no big fins and legs similar to ship. That thrust dome is the super hard part. Raptor SL thrust starts at 200 ton, but upgrades in the works for 250 ton.

You couldn’t design around the same thrust specs using any other existing engine. If you tried, the rocket would become insanely large and impractical.

What’s more, the high count of engines gives you huge redundancy. So if you lose an engine or two, you’re still in the safe zone. In fact, you can lose 6 Raptor engines and still theoretically make it into orbit. To point out, the specs of the Starship aren’t set in stone, thus, a lot of this is conjecture.

But let’s keep going. Most all SpaceX engines steer using gimbaling. In a gimbaled thrust system, you can swivel the exhaust nozzle of your rocket from side to side. This gives you control over the direction of the thrust, relative to the center of gravity of the rocket.

The significance is, if you lose an engine at the rim of the rocket, it’s not all lost hope. You adjust your thrust vector directions to compensate to maintain your flight course. That’s a backup to a backup you don’t want to do without when you’re traveling 17,600 mph to low Earth orbit. Talk about an insane nifty safety mechanism.

Reusability of a Raptor engine

We learned with the gas-generator cycle engine, there’s a potential for leaks. The oxidizer can leak into the fuel pump from the seal on the shaft. Because everything in the engine is almost connected in a big circular loop. Where all components depend on each other.

But with the Raptor engine, the oxidizer-rich side only pumps oxidizer. While the fuel-rich side only pumps fuel. So, the fuel pump isn’t connected to the oxidizer. And if hot fuel leaks through the seal on the shaft, entering the fuel pump, it’ll be a non-issue.

Plus, sealings with gas-generator cycle engines take a beating in launches from leaks. Even when a launch goes smoothly. So it’s pretty obvious, we can better reuse engines if we remove sealings altogether. Because they’re a point of failure when it comes to reusability.

What’s more, without sealing requirements, your margin of error in design increases. This makes for a better and more reliable rocket. As we discussed earlier though, the points of engine failure increase with the Raptor. Because you now have twice as many turbines.

BUT, you lower the chance of catastrophic failures, from exploding pumps due to leaks. So this higher failure rate isn’t viewed negatively. Especially, when you build a rocket with many engines providing redundancy. And from another lens, each of the pumps becomes more reliable since the chance of leaks reduces.

The superiority of the Raptor engine over other rocket engines

By now, it’s clear as day why the Raptor engine is vastly superior to its predecessors. But we’re not done yet. Let’s notch it up one last level and compare SpaceX Raptor engine specs to other rocket engines.

One engine in specific catches my attention the most, the RD-170. It’s a Soviet-era liquid-fuel rocket engine that made its first flight in 1985.  It was one of the first rocket engines to use staged combustion, and it was often referred to as the,

“Most powerful rocket engine in the world.”

The Soviets designed the RD-170 to lift super-heavy launch vehicles into orbit. And for a long time, this engine held the record for the greatest produced chamber pressure. So in the comparison tables, take notice of this trailblazer one-of-a-kind engine.

Rocket enginePower cycleFuel typeChamber pressure
Raptor (developed by SpaceX)Closed (full flow)Methane33 MPa (4,786 psi)
RD-180 (developed by NPO Energomash)Closed (oxidizer-rich)RP-126.7 MPa (3,870 psi)
RD-170 (developed by NPO Energomash)Closed (oxidizer-rich)RP-124.52 MPa (3,556 psi)
RS-25 (developed by Aerojet Rocketdyne)Closed (fuel-rich) Hydrogen20.64 MPa (2,994 psi)
Merlin 1D (developed by SpaceX)Open (fuel-rich)RP-19.7 MPa (1,407 psi)

The produced chamber pressure of the SpaceX Raptor engine is simply incredible. Then when you throw in the fact the engine will have less wear and tear on it, it’s a no-brainer it’ll become a game-changer.

Now, let’s peek into the produced thrust of the engines. The important marker here is the thrust to weight ratio. The greater this value is, the heavier the payload a rocket can lift into space. So it’s no wonder Elon wants to use the Raptor engine for the Starship’s mission to Mars.

Rocket engineTotal thrust (sea level)Thrust to weight ratioSpecific impulse (sea level)
Raptor (developed by SpaceX)2.21 MN200:1330 seconds
Merlin 1D (developed by SpaceX)0.85 MN158:1311 seconds
RD-180 (developed by NPO Energomash)3.83 MN
78:1311 seconds
RD-170 (developed by NPO Energomash)7.25 MN75:1309 seconds
RS-25 (developed by Aerojet Rocketdyne)1.86 MN73:1366 seconds

Important Note: every small rocket advancement is important. Even a 1% boost in velocity, considered over the entire length of a launch, is HUGE. 

You need to consider escaping Earth’s gravity well, you need to travel 25,000 miles per hour. So, any boost in velocity will have a tangible impact on your fuel requirements. 

Conclusion

There you have it. The SpaceX Raptor engine in a nutshell.

It goes without saying, it’s an exciting time to be a space enthusiast. The advancements of the SpaceX Raptor engine are a testament to new frontiers that await us.

Now yes, the Raptor engine is still a baby in the pantheon of rocket engines. But it’s quickly maturing to become the premier engine that’ll one day take us to Mars.

Until then, the cost, reusability, and reliability all need a lot of work. In the meantime, we can appreciate how complex yet fascinating these engines are. Not to forget, every new engine advancement further showcases human ingenuity. So I need to give a shoutout to all the engineers who’ve worked on this amazing engine. They’re downright brilliant!

What’s more, I don’t think there’s a better company to lead this show than SpaceX. They iterate and test fast while making any necessary tweaks without any fuss. So I don’t doubt for a second, the SpaceX Raptor engine will become a staple in rocket engineering.

What are your thoughts on the SpaceX Raptor engine? How do you think the SpaceX Raptor engine will transform the rocket industry? What do you think is the next step in rocket technology?


Featured Image Photo Credit: Brandon De Young (image cropped)

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