SpaceX’s new Raptor engine is a methane fueled full flow staged combustion cycle engine and it’s so hard to develop, no engine like this has ever flown before!
Now this topic can be really intimidating so in order to bring the Raptor engine into context, we’re going to do an overview of a few common types of rocket engine cycles then compare the Raptor to a few other common rocket engines, like SpaceX’s current work horse, the Merlin, The Space Shuttle’s RS-25, the RD-180, Blue Origin’s BE-4 and the F-1 engine.
And if that’s not enough, not only is SpaceX using a crazy engine cycle, they’re also going to be using Liquid Methane as their fuel, again something that no orbital rocket has ever used! So we’ll also go over the unique characteristics of liquid methane as a rocket fuel and see if we can figure out why SpaceX went with Methane for the Raptor engine.
We’ll also break down and explain all the different engine cycle types so you know what the full flow staged combustion cycle is, how it works, and how it compares to the other cycles.
So by the end of this article hopefully we’ll have the context to know why the raptor engine is special, how it compares to other rocket engines, why it’s using methane and hopefully find out if the Raptor engine will be the new king of rocket engines…
In case you didn’t notice when you clicked on this article… this is a VERY VERY long article… sorry, not sorry. BUT, if you’re anything like me, you keep hearing a lot of hype about the Raptor engine and you want to appreciate it… but you don’t even know where to start.
Well, I’ve spent quite a while really studying up on this subject so I can lay down a good foundation in order to help us fully appreciate the Raptor engine, well and quite frankly, all rocket engines!
And if you’re anything like me, maybe you’ve stared at diagrams like this for hours and your head explodes every single time. So to avoid that, I’ve personally whipped up some very simplified versions of the rocket engine cycles for us all to enjoy, which will hopefully help us grasp these concepts.
Now we’re going to start with a super quick physics lesson, but bear with me, we’ll dive in and get plenty of nitty gritty details… we’ll leave no stone unturned and by the end of this article hopefully you’ll have a strong grasp of how rocket engines work, the different versions of liquid fueled rocket engines, why methane is a valid choice for fuel, and you’ll know exactly how the Raptor engine stacks up against some other popular rocket engines.
Rockets are basically just propellant with some skin around it to keep it in place and they have a thing on the back that can throw said propellant really really fast. And to way over simplify it even more, the faster you can throw that propellant, the better.
The easiest way to do this is by storing all the propellant in your tanks under really high pressure, then put a valve on one end of the tank and a propelling nozzle that accelerates the propellant into workable thrust. DONE! No crazy pumps, or complicated systems, just open a valve and let er rip.
This is called a pressure fed rocket engine and there’s a few main types; cold gas, monoprop and bipropellant pressure fed engines. You’ll often find these used for reaction control systems because they’re simple, reliable and quick.
But pressure fed engines have one big limiting factor. Pressure always flows from high to low, so the engine can never be higher pressure than the propellant tanks.
In order to store propellant under high pressure, your tanks will need to be strong and therefore thicker and heavier. Look at composite overwrapped pressure vessels or COPVs. They’re capable of storing gasses at up to almost 10,000 psi or about 700 bar.
Despite this, there’s still a limited amount of propellant and pressure they can store. This does not scale up very well when you’re trying to deliver a payload to orbit.
So smart rocket scientists quickly realized in order to make the rocket as lightweight as possible, there’s really only ONE thing they could do. Increase the enthalpy. That would be a great 90’s metal band name. You’re welcome internet.
Enthalpy is basically the relationship between volume, pressure and temperature. A higher pressure and temperature inside the combustion chamber equals higher efficiency and more mass shoved through the rocket engine equals more thrust.
So in order to shove more propellant into the engine, you could either increase the pressure in the tanks, or just shoot the propellant into the combustion chamber with a high powered PUMP. Huh, the second option sounds like a good idea!
But PUMPS moving hundreds of liters of fuel per second require a lot and boy do I mean a lot of energy to power them. So what if you took a tiny rocket engine and aimed it right at a turbine to spin it up really fast? You could exchange some of the rocket propellant’s chemical energy for kinetic energy which then could be used to spin these powerful pumps.
Welcome to turbopumps and the staged combustion cycle! But you’ve still got some limiting factors, like how high pressure always wants to go to low pressure and how heat has that habit of melting things… So you’ve got to keep all these factors in check while trying to squeeze every bit of power out of your engine.
Now there’s actually a lot of cycles here we could talk about, but I’m going to stick with the three most common, or at least the three that matter the most when putting the Raptor into context.
We have the gas generator cycle, the partial flow staged combustion cycle and lastly we’ll look at the full flow staged combustion cycle… and perhaps in a future article I’ll try and do a full rundown of all liquid fueled rocket engines including fun new alternatives like the electric pump fed engines seen on Rocket Lab’s Electron rocket.
So let’s start with the gas generator cycle otherwise known as the open cycle. This is probably one of the most common types of liquid fueled rocket engines used on orbital rockets. It’s definitely more complicated than a pressure fed system, but it’s fairly simple, well at least compared to their closed cycle counter parts.
Now I’m going to way way over simplify this so it’s as easy to grasp as possible. In real life there’s literally dozens of valves, a hive of wires and extra small pipes, helium to back pressure the tanks, fuel flowing through the nozzle and the combustion chamber to cool it, and an ignition source for the preburner and the combustion chamber….
But again, for the purposes of making this simple and as digestible as possible, just know there’s a lot of stuff missing, but we’re focusing on the flow of these engines to get that concept down. Then once you grasp these simple versions, it’ll be much easier to look at one like this and not have your head explode.
The gas-generator cycle works by pumping the fuel and oxidizer into the combustion chamber using a turbopump. The turbopump has a few main parts, a mini rocket engine called the preburner, a turbine connected to a shaft and then a pump or two that push propellant into the combustion chamber.
You might hear the turbopump assembly called the powerpack because it really is what powers the engine. In the open cycle system, the spent propellant from the preburner is simply dumped overboard and does not contribute any significant thrust. This makes it less efficient since the fuel and oxidizer used to spin the pumps is basically wasted.
Now the funny thing about a turbopump is that it’s kind of has a chicken and egg syndrome situation that makes it pretty difficult to start up since the preburner that powers the turbopump needs high pressure fuel and oxidizer to operate… So the preburners requires the turbopumps to spin before it can get up to full operational pressure itself, but the turbopumps need the preburner to fire in order to spin the turbopumps… but the preburner needs the turbopumps etc etc
This makes starting a gas generator tricky. There’s a few ways to do this, but we don’t need to get into all of that in this article, that sounds like a fun topic for a future article though.
So back to the turbopumps, remember, pressure always flows from high to low, so the turbopumps need to be a higher pressure than the chamber pressure. This means the inlets leading into the preburner is actually the highest pressure point in the entire rocket engine, everything downstream is lower pressure.
But notice something here. Take a look at SpaceX’s Merlin engine which runs on RP-1 or Rocket Propellant 1 and Liquid Oxygen. Notice how black the smoke is coming out of the preburner exhaust…
Why would it be so sooty compared to the main combustion chamber which leaves almost no visible exhaust? Well that’s because rocket propellant can get super hot…. Like thousands and thousands of degrees Celsius. So to make sure the temperature isn’t so hot it melts the turbine and the entire turbopump assembly, they need to make sure it’s cool enough to continually operate. Running at the perfect fuel and oxygen ratio is the most efficient and releases the most energy, but it also produces a crazy amount of heat.
In order to keep the temperatures low, you can run the preburner at a less than optimal ratio, so either too much fuel known as fuel rich or too much oxidizer or oxygen rich. Running an RP-1 engine fuel rich means you’ll see some unburnt fuel appearing as dark clouds of soot. The highly pressurized unburnt carbon molecules bond and form polymers which is a process known as coking. This soot starts to stick to everything it touches and can block injectors or even do damage to the turbine itself!
So what if you didn’t want to waste that highly pressurized propellant… I mean afterall, since it’s running cooler by being fuel rich, doesn’t that mean there’s a bunch of unburnt fuel literally being wasted? What if you could just pipe that hot exhaust gas and put it into the combustion chamber? Welcome to the closed cycle!
The closed cycle or staged combustion cycle increases engine efficiency by using what would normally be lost exhaust and connects it to the combustion chamber to help increase pressure and therefore increase efficiency.
So let’s take the Merlin engine and try closing the loop. Let’s take that exhaust and just pipe it into the combustion chamber! UH OH!!!! OH NOOOOOOO we just put a bunch of soot and clogged all your injectors. You do not go to space today.
But there’s a few solutions to this problem, so let’s see how the Soviets solved this problem. The first operational closed cycle engine they made was the NK-15 designed for their N-1 moon rocket, they later upgraded it to the NK-33 and then many versions from there stemmed out including the RD-180 which is what is used on the Atlas V today.
Since the NK-15 and NK-33 runs on RP-1 like the Merlin, you can’t run your preburners fuel rich because of the coking problem… so if you want to create a closed cycle engine with RP-1 the answer is running the preburner oxygen rich. Easy as that right? Well now you’re blasting SUPER HEATED highly pressurized gaseous oxygen which will turn anything into soup, right at a precision machined, crazy high tolerance turbine blade.
Doing this was actually considered impossible by the United States, so they basically gave up on trying. They didn’t think a metal alloy existed that could withstand these crazy crazy conditions, and they didn’t believe the Soviets made such an efficient and powerful RP-1 powered engine until after the collapse of the Soviet Union and US engineers got to see them and test them out first hand! But the Soviets had indeed worked their butts off and made a special alloy that can magically, with science, withstand the crazy conditions of an oxygen rich preburner.
With a closed cycle engine, you don’t just use some fuel and some oxidizer and burn that in the preburner to spin the turbine, you actually shoot ALL of the rich propellant through the turbine. So with an oxygen rich cycle, all of the oxygen actually goes through the preburner and just the right amount of fuel goes to the preburner. You only need enough to give the turbine the right amount of energy to spin the pumps fast enough to get the right pressures for the preburner and the combustion chamber to make the right power to shoot a thing into space AH
Sorry, so back to this oxygen rich preburner, that now hot gas oxygen is forced into the combustion chamber where it meets liquid fuel. They meet and go boom and we get a nice clean and efficient burn without really wasting any propellant! HUZZAH!
BUT STILL like all engines, the chamber pressure cannot be higher than the pump pressure, so the pumps actually have a lot of weight on their metal little shoulders.
Now if you’re sitting there wondering if the United States just sat back and let the Soviets have all the closed cycle glory, you’d be wrong. It took the US a bit longer, but they eventually figured out a closed cycle engine, but it was very different from the oxygen rich cycle… The United States pursued a closed loop cycle but they went with a fuel rich preburner. But wait…. We just learned that a fuel rich preburner’s exhaust is so sooty, it will ruin just about anything… right?
Well sure, if you’re using RP-1 or any other carbon heavy fuel, that’s definitely going to be the outcome… so the United States went with a different fuel, Hydrogen! Ok, so now we’ve avoided one problem of blasting crazy hot high pressure oxygen at everything dear and precious, but now we’ve opened up a new can of worms… Hydrogen is significantly less dense than RP-1 or liquid oxygen. As a matter of fact, it’s so much less dense it takes a HUGE turbopump to flow the right amount of hydrogen into the combustion chamber.
Since RP-1 and LOX are relatively similar density and ratios, they can be run on a single shaft using a single preburner…. But with Hydrogen you need to run even more Hydrogen per mass of oxygen, so between being less dense and needing a higher fuel ratio with Hydrogen, the pumps are drastically different between the Hydrogen and LOX.
Because of this, the engineers at Rocketdyne pursued an engine known as the RS-25 which would go on to power the space shuttle. They realized that because of the large difference between pumps, they might as well just have two preburners, one for the hydrogen pump and one for the oxygen pump. So that’s what they did!
But having two separate shafts created ANOTHER new problem. Now engineers were putting high pressure hot gaseous hydrogen on the same shaft and right next door to the liquid oxygen pump. If some of the hydrogen would leak out of the preburner, it would start a fire in the lox pump, which is catastrophically bad. Hydrogen is also very hard to contain, because it’s so not dense, un dense? Lightweight? It likes to sneak through cracks and get out anywhere it can. So engineers had to make an elaborate seal to keep the hot hydrogen from sneaking out.
The seal required for this is called a purge seal and it’s actually pressurized by helium so it’s the highest point of pressure, so if the seal leaks, it just leaks inert helium! Genius. But take a look at how different the LOX turbopump and the hydrogen turbo pump seals look. You can tell how much more engineering time and effort had to go into those lox seals. THE PEOPLE THAT THINK OF THIS STUFF ARE NUTS!!!!!
Ok now that we’ve talked all about the dual burner, fuel rich RS-25, here’s a simplified diagram of that. I didn’t bother making the fuel pumps different sizes as I just want to focus on the flow here and help make that simple.
But do note, both preburners of the RS-25 BOTH run fuel rich, they just power different pumps. The RS-25 is still considered to be about the best engine ever made with a fairly high thrust to weight ratio and unmatched efficiency.
So the closed cycle improves the overall performance of the engine and is highly advantageous, so how could it get any better than this?
We’re finally ready to talk about the full flow staged combustion cycle which basically just combines the two closed cycle methods we just talked about. With the full flow staged combustion cycle, you take two preburners, one that runs fuel rich and one that runs oxygen rich. The fuel rich preburner powers the fuel pump and the oxygen rich preburner powers the LOX. This means the full flow cycle needs to tackle the oxidizer rich problems, which again is solved by developing very strong metal alloys.
So, SpaceX developed their own superalloy in house that they named SX500. According to Elon, it’s capable of over 800 bar of hot oxygen-rich gas. That may have been one of the biggest hurdles in developing the Raptor engine.
Luckily, the fuel rich side only pumps fuel, so if some hot fuel leaks through the seal on the shaft, it just comes in contact with more fuel, no big deal. Full flow likely wouldn’t work with RP-1 due to the coking problems with a fuel rich preburner, but other fuels are still valid to use this design, more on that in a minute…
The advantage of this system is that since both the fuel and oxidizer arrive in the combustion chamber as a hot gas, there’s better combustion and hotter temperatures can be achieved. There also is less need of that crazy sealing system as we mentioned earlier, which makes for less refurbishment. That’s a good thing when you plan to reuse your engine over and over with little to no maintenance between flights.
And lastly because there’s an inherent increase in mass flow, or how quickly all the propellant is shooting into the preburner, the turbines can run cooler and at lower pressures because the ratio of fuel and oxidizer needed to spin the turbopumps is much lower. And think of it this way, in an open cycle, you only want to use as little fuel and oxidizer as possible in the preburners since it’s all wasted, and you want it to be as hot as withstandable to make it more efficient.
But with a full flow cycle since ALL of the fuel and ALL of the oxidizer goes through the preburners, you can burn as much propellant as necessary to power the turbopumps…. BUT, your fuel to oxidizer ratio will be so crazy fuel rich and oxygen rich that the temperatures at the turbines will be much lower and this means longer lifespans for the turbopump assembly. It also means more combustion happens in the combustion chamber and less in the preburner.
Now here’s the crazy part. Only three engines have demonstrated the full flow staged combustion cycle… EVER.
In the 60’s the Soviet’s developed an engine called the RD-270 which never flew, and in the early 2000’s Aerojet and Rocketdyne worked on an integrated powerhead demonstrator called, wait for it, the integrated powerhead demonstrator which again, never made it past the test stand.
And the third attempt at developing a full flow staged combustion cycle engine is SpaceX’s Raptor engine!!!! TA DA!!! WE DID IT!!!
That’s right, the Raptor engine is only the THIRD attempt at making this crazy type of engine. It’s the first to ever do any work and leave the test stand! And fingers crossed, it’ll be the first full flow staged combustion cycle engine to reach orbit too. Well, actually, just about everything this engine does will be a first.
But all this means SpaceX had to tackle many very difficult problems. Not only did they need to solve the same problems with an oxygen rich cycle, they also had to precisely control fuel to create the highest chamber pressure of any rocket engine ever, at 270 bar, finally beating the RD-180’s record of around 265 bar. And they’re not done, they’re targeting 300 bar, which is just plain nutty, more on that in a second.
Since the Raptor can’t run a fuel rich preburner using RP-1, you’d think the next most logical choice would be Hydrogen… well SpaceX didn’t opt for either RP-1 or Hydrogen, they went with Liquid Methane! So NOW we finally have another topic to touch on… why did SpaceX chose liquid methane for the Raptor engine? What are the qualities that make it advantageous over hydrogen or RP-1?
To date, no liquid methane or methalox engine has gone to orbit, so what qualities does it have that make it desirable? Let’s take a look at methalox compared to keralox and hydrolox… let’s put methane in between RP-1 and Hydrogen… you’ll see why pretty quickly…
So let’s start off with perhaps the biggest factor when designing your first stage. The density of the propellant. Having a denser fuel means the tanks are smaller and lighter for a given mass of fuel. A smaller tank = a lighter rocket.
So here’s the density of these three fuels measured in grams per liter, in other words how much does one liter of this stuff weigh, or really what’s its mass?
Starting off with RP-1, one liter is around 813 grams, RP-1 is 11 times more dense than Hydrogen which is only 70 grams per liter, and Methalox is right in the middle at 422 grams per liter.
Remember how airships or zeppelins used to be filled with hydrogen to make them “lighter than air”… That’s because Hydrogen is so much less dense than our atmosphere, it makes for an excellent, albeit flammable gas for a balloon. I mean, we all remember the Hindenburg… right?
It should also be noted that 813 grams per liter is an average for RP-1, but SpaceX chills their RP-1 in their Falcon 9 and Falcon Heavy for about a 2%-4% increase in density. But historically, RP-1’s density is right around that 813 grams per liter.
So in the case of density, Methane is kind of right in the middle of the others. But there’s more to it than just density, we also need to take into consideration the ratio of how much fuel is burned compared to how much oxidizer is burned. This is the oxidizer to fuel ratio.
So here’s where things get a little more interesting and the tables turn a little. Rocket engineers have to take into account the mass of the fuel and the corresponding weight of the tanks, so they don’t actually burn propellant at the perfect stoichiometric combustion ratio, they find the perfect happy medium that balances tank size with thrust output and specific impulse.
Let’s look at the mass ratios of fuel and oxidizer that the engineers have come up with… So for these numbers RP-1 is burned at 2.7 grams of oxygen to 1 gram of RP-1, Hydrogen burns at 6 grams of oxygen to 1 grams of hydrogen, and Methane burns at 3.7 grams oxygen to 1 grams Methane. These numbers can now help offset a little the massive difference in density.
So let’s visualize this to help make it easier to digest. Liquid oxygen is 1,141 grams per liter, it’s a little more dense than RP-1. So burning LOX and RP-1 at a 2.7 to 1 ratio, for every liter of LOX you’d need a little over half a liter of RP-1. Next up let’s do Hydrogen. Now with Hydrogen being 11 times less dense than RP-1, you’d think it’d need a tank that’s 11 times bigger… but luckily, engineers have found that it pays to burn LOX and Hydrogen at a 6:1 ratio for a good compromise.
This means for each liter of LOX, you’d need 2.7 liters of hydrogen! So your fuel tank needs to be approximately 5 times larger compared to RP-1… so yeah…
That’s why when we look at a Hydrogen powered Delta IV vs an RP-1 powered Falcon 9, you can see the fuel tank is much smaller than the LOX tank on the Falcon 9, but the Delta IV is about the opposite! The LOX tank is MUCH smaller than it’s fuel tank.
So now let’s look at that methane. Now this one is interesting. LOX is 2.7 times more dense than liquid methane, but the burn ratio is 3.7 grams of oxygen to 1 gram of methane. So you’d need .73 liters of methane for every liter of LOX. In other words, your fuel tank would need to be about 40% bigger for methalox than it would need to be for RP-1, despite RP-1 actually being almost twice as dense! And compared to Hydrogen, it’s fuel tank would be 3.7 times smaller.
So the fuel to oxidizer ratio helps make Methalox’s fuel tanks a lot closer to RP-1 than it is to Hydrolox.
Another huge variable with any rocket engine is how efficient it is. This is measured in specific impulse or ISP, but you can think of this like the fuel economy of a gas powered car. So a high specific impulse would be similar to a high mpg or kmpl. The best way to think of specific impulse is imagine you had one kg of propellant, for how many seconds can the engine push with 9.81 newtons of force? The longer it can sip on the fuel while still pushing that hard, the higher its specific impulse, and therefore the more work it can do with the same amount of fuel. IE, it’s fuel economy.
So the higher the specific impulse, the less fuel it takes to do the same amount of work, which is a good thing. A fuel efficient engine is extremely important! And now due to the molecular weight of each fuel and their energy released when burned, there’s a different potential for how quickly the exhaust gas can be expelled out the nozzle. This means each fuel has a different theoretical specific impulse.
In an ideal and perfect world an RP-1 powered engine could achieve about 370 seconds, an ideal Hydrogen powered engine could get 532 seconds and guess what, a methane powered engine is right in the middle with 459 seconds. Real world examples of this though are much lower with RP-1 engines seeing around 350 seconds (Merlin 1D Vacuum), around 380 seconds for a methane engine (raptor vacuum) and about 465 seconds for a hydrogen engine (RL-10B-2)
Next, let’s talk about how hot each fuel burns. A fuel that burns cooler is easier on the engine and potentially makes for a longer lifespan. RP1 can burn up to 3670 kelvin, Hydrogen 3,070 kelvin and if you haven’t guessed it by now, Methane is again between the two at 3550 kelvin.
Speaking of thermal considerations, let’s look at the boiling point of these fuels or at what point does the liquid fuel boil off and turn into a gas? Since all of these fuels need to remain in their liquid state in order to stay dense, the higher the temperature, the easier it is to store the fuel. A higher boiling also means less or even no insulation on the tanks to keep the propellant from boiling off. And of course, less insulation means lighter tanks. YAY.
RP-1 has a very high boiling point, even higher than water at 490 kelvin. Hydrogen on the other hand is near absolute zero at a crazy crazy cold 20 kelvin! That is insanely cold and it takes serious considerations to keep anything at that temperature. And like the goldilocks it is… methane is between the two at 111 kelvin, which although that’s still very cold, and requires thermal considerations, it at least boils off at a temperature similar to LOX, so there’s that!
And because it’s so close to the temperature of LOX, the tanks can share a common dome, which makes the vehicle lighter. LOX and Hydrogen’s temperatures vary so wildly that LOX will boil off hydrogen and the hydrogen will freeze the LOX solid!
Now onto exhaust, what are the byproducts of combustion with these engines? RP-1 is really the only one of these three that really pollutes with any unburnt carbon being left in the atmosphere along side water vapor, but Hydrogen only produces water vapor and Methane produces some carbon dioxide and water vapor. But an interesting note, believe it or not, as far as greenhouse gases go, water in the upper atmosphere can be pretty bad… But, I’ll be doing an article in the future about how much rockets pollute, talking about both air pollution, but also ocean pollution and even space debris as a consideration. So standby as that’s one that I think is very interesting!
One metric we’re going to talk about, but only generally is the cost associated with each fuel. These tend to vary considerably and it’s actually really hard to pin down exact prices reliably. But for considerations, RP-1 is basically a highly refined jet fuel, which jet fuel is a highly refined kerosene, which kerosene is a highly refined diesel. So, it’s safe to assume it’s going to be more expensive than diesel fuel.
Hydrogen is also relatively expensive despite being abundant, refining it, storing it and transporting it can be expensive. Methane on the other hand is basically the same thing as natural gas and can be relatively cheap. When you’re talking about buying literal tons of fuel, the fuel cost can add up quickly, so although the cost of fuel shouldn’t factor in too much, it certainly is a consideration… but without hard data on this one, I don’t even want to put it on our chart, SO, instead… let’s talk about the more important aspect of the fuel. Manufacturing it…
Here’s where we get into specifically why SpaceX sees Methane as an important, or even a necessary part of their companies future.
SpaceX’s ultimate goals are to develop a system capable of taking humans out to Mars and back over and over. The Mars atmosphere is CO2 rich, combine that with water mining from the surface and subsurface water on Mars through electrolysis and the sabatier process, the Martian atmosphere can be made into Methane fuel! So you don’t have to take all the fuel you need to get home with you. You can make it right there using Mars’ resources.
This is called in-situ resource utilization or ISRU. Now you might be thinking, well if there’s water, can’t you just make Hydrogen on the surface of Mars for your fuel? Well, yes, but one of the biggest problems with Hydrogen and long duration missions is the boiling point of Hydrogen. It takes serious considerations to maintain Hydrogen in a liquid state necessary to be a useful fuel.
So for SpaceX, methane makes a lot of sense! It’s fairly dense meaning the rockets size remains reasonable, it’s fairly efficient, it burns clean and makes for a highly reusable engine, it burns relatively cool helping expand the lifespan of an engine, which again is good for reusability, it’s cheap and easy to produce and can be easily produced on the surface of Mars!
Ok yay! We’ve made it this far! Now that we have a strong grasp of how different engines operate and the fuels they might use, we can finally line them all up side by side and compare their metrics to help appreciate where each engine sits..
So now we’re going to line up engines by their fuel types and cycles. So let’s compare SpaceX’s open cycle Merlin engine that powers their Falcon 9 and Falcon Heavy rockets, NPO Energomesh’s oxygen-rich closed cycle RD-180 that we see power the Atlas V rocket and Rocketdyne’s open cycle F-1 engine that powered the Saturn V which all run on RP-1. Then we have SpaceX’s full flow staged combustion cycle Raptor engine that will power their Starship and Super Heavy booster and Blue Origin’s closed cycle oxygen rich methane powered BE-4 engine that will power their New Glenn rocket and ULA’s upcoming Vulcan rocket, and Aerojet Rocketdyne’s closed cycle fuel rich RS-25 engine that powered the space shuttle and will power the upcoming SLS rocket which runs on Hydrogen.
A few quick notes here. The Raptor and BE-4 as of the making of this article are still in development, so the numbers we have are either in their current state of progress like the Raptor which is constantly improving and in the case of the BE-4, those are the target goals for the engine which Blue Origin has yet to hit. So just keep these numbers in mind that they’re subject to change.
Another fun note… look at the RD-180… now don’t be confused, this is a single engine, it just has two combustion chambers! There’s only a single turbo pump that splits its power to two combustion chambers. The Soviet Union was able to solve the crazy hot oxygen rich closed cycle problem, but they were unable to solve combustion instability of large engines, so instead of one large combustion chamber, they made multiple small ones!
So first up, let’s take a look at their total thrust output at sea level since all these engines run at sea level, that’s a fair place to compare them. Let’s go from the least amount of thrust to the most, for fun 😉 The Merlin produces .84 MNs of thrust, the RS-25 produces 1.86 MNs, the Raptor currently is at 2 MNs, the BE-4 is hoping to hit 2.4 MNs, the RD-180 // 3.83 MNs ad the F-1 is still the king out of these at 6.77 MNs.
The RD-170 actually produced more thrust than the F-1, but since it barely ever flew, I figured it wasn’t as relevant in this line up, figured it’s better to go with engines that have actually been used, a lot!
Thrust is great, but what’s maybe just as important when designing a rocket is the thrust to weight ratio… or how heavy the engine is compared to how much thrust it produces. A higher thrust to weight engine ultimately means less dead weight the rocket needs to lug around.
Let’s start from lowest to highest here. The lowest is actually the Space Shuttle’s RS-25 at 73:1, then there’s the RD-180 which is 78:1, then we have the BE-4 at around 80:1 but keep in mind, we don’t actually have a really good number on this, so there might be some wiggle room there, then the F-1 is 94:1, then we have the Raptor at 107:1 (for now), and lastly the Merlin is actually the leader here with an astonishing 198:1 thrust to weight ratio…. Yeah… that thing is a power house.
Thrust is great and all, but who cares how powerful an engine is if it’s terribly inefficient? So next up let’s check out their specific impulses, which is measured in seconds. For this we’ll show both the sea level specific impulse as well as the specific impulse in a vacuum. So starting with the least efficient engine which is the F-1 engine at 263 to 304 seconds, then the Merlin engine at 282 to 311 seconds, then we get the RD-180 at 311 to 338 seconds and somewhere in that same ballpark is the BE-4 which is AROUND 310 to 340 seconds, next up is the Raptor engine which is 330 to around 350 seconds, and lastly the king here by far is the RS-25 which is 366 to 452 seconds! WOAH.
Now one of the factors that affect both thrust and specific impulse is chamber pressure. Generally, the higher the chamber pressure, the more thrust and potentially more efficiency the engine can gain. Higher chamber pressures let an engine be smaller for a given thrust level, also improving their thrust to weight ratio. The baby here is actually the F-1 which only had 70 bar in its chamber pressure. Now I do need to pause for a second and remind you 70 bar is 70 TIMES the atmospheric pressure… or the same amount of pressure you’d experience at 700 meters underwater… yikes… ok, so even the lowest chamber pressure is still mind boggling high.
So next up is the Merlin engine at 97 bar, then the BE-4 will be around 135 ish, then the RS-25 which is 206 bar, then the RD-180 which has been considered the king of operational engines at 257 bar, that is until the Raptor is now online which is the new king of chamber pressure at a whopping 270 bars currently and they hope to get it up to 300 bar!!! 300 bar is like being 3 km deep in the ocean. I can’t even fathom.
Ok, that’s enough of the specs of these engines, now lets look at their operational considerations. Starting with their approximate cost. Now, again, this can be kind of hard to nail down, so these are the best estimates I could come up with. These numbers do factor in inflation to make them all in today’s dollars.
Let’s go with the most expensive and work our way down to the least expensive. The most expensive engine in this lineup is the RS-25 which has a sticker price of over $50 million per engine… yikes. Then we’ve got the F-1 engine which was $30 million per engine, then the RD-180 which is $25 million per engine, then the BE-4 which is around $8 million per engine, then we have the Merlin engine which is less than $1 million and for the Raptor, Elon has mentioned he thinks they can produce the Raptor for cheaper than or close to the Merlin engine if they can remove a lot of the complexity of the current versions… so for now, we’ll say around $2 million as a pretty decent ball park.
Well, cost is one thing, but another strong consideration for the cost of the engine is whether or not it’s reusable. And here only the RD-180 and the F-1 were not reusable, or at least never reused… which is different than all of these other engines which will all be reused multiple times.
The RS-25 was reused over and over with the record being 19 flights out of a single engine, well after a few months of refurbishment… The Merlin is hoping to see up to 10 flights without major refurbishment. We know a design goal for the BE-4 is to be reused up to 25 times, and I think the Raptor hopes to see up to 50 flights, BUT, again, aspirations are one thing, we’ll see how history treats these claims.
On the topic of price, there’s actually some things that get really interesting when we start looking at these numbers. The first is a number Elon mentioned one interesting metric in a tweet in February of 2019 saying they hope to make the Raptor get better at their thrust to $ ratio.
Now this is a really interesting concept when you think about it. Who cares how much an engine costs if one big engine is cheaper than 2 smaller ones for the same thrust or vise versa. So let’s actually take a look at the $ to kN ratio of these engines. Starting with the most expensive $ to kN engine which is the RS-25 at a crazy $26,881 : kN of thrust, then the RD-180 at $6,527 : 1 kN, followed by the F-1 at $4,431 : 1 kN, and then we get to the BE-4 which is $3,333 : 1 kN, and then the Merlin at $1,170 and the Raptor at around $1,000 : kN.
BUT NOW we can go even another step further since we know their $ / kN ratio but we also know their reusability potential… Now we can predict their potential cost per kN per flight, which changes based on how reusable the engines are.
So for starters, since the RD-180 and the F-1 aren’t reusable, their price stays the same, but for the rest of the engines, if we take into account how many flights they have/will have, now we start to see the RS-25’s reusability pay off and close the gap, bringing it’s potential cost down to just $1,414 : kN, but here’s where things get crazy. Blue Origin’s BE-4 has potential to be truly game changing at around $133 : kN over 25 flights, which could make it about as cheap to operate than the Merlin at $117 per kN per flight. But if the Raptor engine truly lives up to its hype, it could bring that number all the way down to $20 per kN per flight. Now that is absolutely game changing.
Sure, money and reusability is a 21st century focus for spaceflight, but whatever happened to good old proven reliability? For this, let’s first look at how many operational flights each engine has had. At the moment of writing this, the Raptor and BE-4 haven’t seen any operational flights, although the Raptor is starting to leave the test stand and being used on test vehicles like the StarHopper. But for now neither engine has a real flight record. Then we have the F-1 engine which was used on 17 flights, next up the Merlin engine which is at 71 flights and catching up quickly to the RD-180 which is at 79 flights, but the king out of these engines is the RS-25 which saw 135 flights.
Now lastly, how about reliability in service. Between the number of flights and this number, we can get a pretty good sense of how truly reliable an engine is. This number is really hard to just pin down since some engines may have shut down early but the mission was still successful on a few of these examples… so take these with a little grain of salt again. Again, the BE-4 and Raptor haven’t flown yet, so those numbers are unavailable, then we have the Space Shuttle main engine which is over 99.5% reliable, but that gets hard to define when an engine doesn’t fully shut down…
and then we have the Merlin at 99.9% reliable… it helps when you have 10 engines on each flight of the vehicle, and with only one engine ever failing in flight early on in its career, and despite that, the mission was still a success… so it’s a very reliable engine! Now to end this, technically the RD-180 and the F-1 are 100% reliable, but with the F-1 never having any shut down at all in flight, it gets the bold here… and depending on how you define success and reliability, technically, the RD-180 is only kind of 100% reliable, because it got really really lucky once.
One time it shut down 6 seconds early, on an Atlas V mission in 2016, due to a faulty valve, but the mission went on to be a success because of some pure luck with the centaur upperstage having enough spare Delta V to carry out the mission! Had that valve failed even a second earlier and that mission would have failed.
Man… seeing all of these numbers and considerations it makes you realize just how many variables go into designing a rocket. It’s so crazy to think about how if you tweak one little variable how many things that can affect. Change any one thing and it can have a massive ripple effect in the design and implementation of the vehicle as a whole.
So let’s go back over all of this now that we know the cycles, the fuels and the aspirations of SpaceX to see if we can figure out why the Raptor engines exists and figure out if it’s worth all the effort.
Let’s look at SpaceX’s ultimate plan. Make a rapidly and fully reusable vehicle capable of sending humans to the Moon and Mars as inexpensively and routinely as possible. Not your everyday design goal for a rocket huh? In order to be rapidly and fully reusable, the engine needs to run clean and require low maintenance with simple turbopump seals and low preburner temperatures. Hmm, a Methane fueled full flow staged combustion cycle engine sounds like a good fit….
For reliability, redundancy and scale of manufacturing considerations, it makes sense to employ a lot of engines. In order to scale an engine down but maintain a high output, chamber pressure needs to be high… Hmm sounds like a methane fueled full flow staged combustion cycle engine sounds like a good fit…
For interplanetary trips, methane makes the most sense because it’s boiling point makes it usable on long duration trips to say Mars, which guess what, you can produce Methane on Mars… so for interplanetary trips, a methane fueled full flow staged combustion cycle engine sounds like a good fit…
Methane is fairly dense, meaning the tank size remains reasonable. Which again is good for interplanetary trips, not needing to lug around dead weight… making a methane fueled full flow staged combustion cycle engine sounds like a good fit…
So… let’s bring it alllll back around now, is the Raptor engine really the king of rocket engines? Well, rocket science, like all things is a complex series of compromises. Is it the most efficient engine, no. Is it the most powerful engine, no. Is it the cheapest engine, probably not. Is it the most reusable engine, maybe. But does it do everything really well? YES. It truly is a goldilocks engine, doing everything it needs to do very very well. It is the perfect fit for your interplanetary spaceship.
And despite its complexity, SpaceX is developing this engine at a rapid pace. Knowing how much tweaking SpaceX did to their Merlin engine over a decade, we’re just at the infancy of the Raptor engine. It will only get better and better from here on out, which is crazy!
So all in all, the Raptor engine IS the king of THIS application. It’s a fantastic engine to fufill SpaceX’s goals for their Starship vehicle. Would it be the king of other applications? Maybe, maybe not, and I’ll leave that decision for the rocket scientists and engineers who get to make all of those crazy compromises!
So what do you think? Is it worth all the hassle to develop such a crazy and complex engine? Is this just the beginning for the Raptor engine? And most importantly, is the Raptor engine really the king of rocket engines? Let me know your thoughts in the comments below.
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