Rocket engine cycles: How do you power a rocket engine?

Hi, it’s me, Tim Dodd. The Everyday Astronaut. Rocket engines are incredibly complex machines pushing the boundaries of material science and human ingenuity. And there’s a wide range of ways. You can actually power a rocket engine and make it work. They can be super simple, like just opening the valves of a tank. That’s under high pressure to having complex pumps arranged in a fashion so confusing, it’s a miracle anyone ever figured out how to build them in the first place. This video should really just be called how rocket engines work or how you power rocket engines because an engine’s cycle type or its power cycle is really what defines the engine. And there’s so many different cycle types. It’s really hard to keep track of them all. It’s like knowing the difference between a piston engine that’s naturally aspirated, hopped up on nitrous, turbocharged or supercharged. Now they all operate under the same basic principles, but they employ different techniques to reach power and or efficiency goals. For those of you who have seen my Raptor engine video, some of this might be a review and familiar. We’re still going to be drawing out those same, super simple and easy to understand diagrams, just like before, only this time we’re going deeper and we’re going over all the major engine cycles. Because maybe you were like me and after that video, you still couldn’t quite grasp why full flow was advantageous over just closed cycle, or maybe you don’t know what tap-off is or expander cycle. Well, here’s your chance to really dive into this topic. So today we’re going to talk about cold gas, pressure fed, electric pump fed, open cycle, closed cycle, full flow stage combustion cycle, tap-off and expander cycles. We’ll go over their pros and cons and lots of examples of each one. And if you’re more of a reader and want some links and sources, we’ve got an article version of this video up at There is a link to that in the description as well. Rocket engine cycles, master this topic, and you’ll have a really good grasp on rocket engines period. So let’s get started. 3, 2, 1. Right up top, I wanted to remind you that we do have this awesome full flow stage combustion cycle hoodie, as well as shirts and lots of other cool schematic shirts up at And another quick note, a great video to watch before for this one would be my why don’t rocket engines melt video, because we do talk about a few of the different cooling techniques and concepts in this video. So if you haven’t watched that video consider watching it before you watch this one. And after having watched both of these videos, I think you’re going to have a great start on understanding rocket engines better than ever before. And I’ll have even more to add to this series, like how to start a rocket engine and how injectors work. So I’m getting back down to the roots of bringing space down to earth for everyday people. It’s gonna be fun. But today we’re talking about rocket engine cycles. So first let’s talk about how they work in general, a rocket engine functions under a simple concept, throw stuff out the back as fast as possible period. The more stuff you can throw and the quicker you can shoot it out, the more thrust you can produce, the more thrust you can produce, the more stuff like fuel and payload you can lift. And the further you can potentially go in space. The speed of your exhaust is known as exhaust gas velocity, and it’s not only directly related to the thrust that pushes back on the vehicle, but it’s also directly proportional to the efficiency of the engine. So the faster we throw propellant out the back, the better. Just like the recoil from a gun or a cannon. That’s the basic concept at play here. Newton’s third law for each action there’s an equal and opposite reaction only instead of bullets or cannon balls. We’re shooting tiny molecules of gas at ridiculously high velocities. The way to do that is by converting pressure and though non intuitively much more important, heat inside the combustion chamber of a rocket engine into kinetic energy through what’s known as a de lavel nozzle or a converging diverging nozzle, which converts hot subsonic, high pressure gas into cooler, supersonic, lower pressure gas. The challenge is getting the pressure and temperature as high as possible in the combustion chamber while managing the heat. But in general, the higher the temperature inside the combustion chamber, the better. We can perform a lot of work with that heat because heat is energy. All the energy contained in a system is known as Enthalpy. Enthalpy is volume times pressure plus energy, or in this case, heat energy. So really the higher the enthalpy is in the system. The more potential it has to perform work and heat energy has a huge effect on Enthalpy. And there’s one more important rule. We need to remember pressure always flows in a system from high to low. So if you have pressure anywhere, that’s higher pressure than the pipe or the tube or tank or whatever it’s connected to, it will always flow to, to the low pressure region. We’ll label our diagrams with very rough pressure and temperature numbers so you can get an idea of how they’re changing throughout the system. This is a thing engineers actually do. They’ll design a system and have a pressure budget based on how much pressure they need to create in order to keep everything flowing in the right direction. So keep all of this in mind throughout this video, and you’ll have an understanding of why engineers go to such great lengths and design incredibly confusing labryths, all in the name of rocket science. The simplest form of a rocket engine is just to simply store propellant in a tank at very high pressure, open a valve and let that pressure flow into the engine done. This is the basis of the cold gas rocket engine. As the name suggests the engine runs cold, meaning there is no chemical reaction or combustion that occurs just simply the expansion of a stored gas through the nozzle to produce thrust. And the name is literal because when expanding a gas, the temperature drops too, and that’s called the Joule Thompson effect. So it actually is physically a cold gas thruster. So the biggest limitation with a cold gas thrust is the lack of heat and the available pressure in the system. Remember our rule pressure flows from high to low. So in order to get high amounts of pressure and therefore thrust in the engine, it means the pressure in the tank always needs to be higher than it is in the engine. As you can imagine, this means you’ll want to store the propellant inside your tank as high of pressure as possible shove, just as much of it in there as you can, but the higher, the pressure you store your propellant, the thicker and heavier your tanks will need to be. And this is rocket science after all so thick and heavy tanks, aren’t really a great option for most applications. At some point, a tradeoff occurs where thicker and heavier tanks with higher pressure don’t actually pay off and it’s not worth it anymore to pursue a higher pressure in the system. So great lengths go into making tanks hold as high of pressure as possible while being as lightweight as they can. A common method for making lightweight high pressure tanks is by using sea COPVs or composite over-wrapped pressure vessels, COPVs are metal tanks wrapped really tightly with composites, such as carbon fiber or kevlar in order to basically keep them from bursting under extremely high pressures, COPVs typically operate at pressures around 300 or 400 bar, but some can get up to around 800 bar even, and as the name implies, a cold gas thruster are often gaseous propellants, which are less dense than a liquid propellant filled tank. However, nitrous or butane can be used, which are stored in liquid form under pressure. And of course, a less dense propellant means it requires again, even bigger and heavier tanks to hold the same relative mass of propellant, which has a bad runaway effect and ultimately takes away from your payload capacity in a hurry. Cold gas thrusters typically use helium or nitrogen for their high compressibility and relatively low molecular weight, which means they can be thrown out at greater velocities. Hydrogen or other propels could also be used, but I haven’t really seen any good examples of those. Now because the pressure and the temperature are low in the system, the specific impulse or the efficiency of a cold gas thruster is low. It’s usually only around 60 seconds of specific impulse, which is three or four times lower than even the most basic pump fed engine, because they’re going from high pressure inside a tank, to low pressure as it expands out the nozzle, you can actually run into a limit on how much you can expand the nozzle before you start turning your gas into a liquid while still in the nozzle. And that’s not good. So again, that’s actually another huge limitation of cold gas thrusters is just in the overall enthalpy or really the lack of heat in the system, which prevents them from really being that efficient. However, they’re very simple and extremely reliable and really only have one moving part, a simple valve, that’s about it. And despite their low efficiency, because of their simplicity and lack of parts and weight, sometimes they’re actually the best choice for really small spacecraft like small sats or cubesats, because think about it. There’s some things you just can’t really shrink down that much more like a valve can only really get so small. So if you have to have two, those or four of those or whatever that adds up in a hurry, another example of cold gas thrusters are the little tiny maneuvering thrusters on the interstage of the Falcon 9 rocket that reorient the booster and help guide it to its landing point using nitrogen. Another example was that MMU or the Manned Maneuvering Unit that was used on three Space Shuttle missions. You know what I’m talking about! The rocket jet pack. That thing had 24 cold gas thrusters stored in just two propellant tanks with about 18 kilograms of gaseous nitrogen. But now what if, instead of just having that high pressure gas flow straight to the engine, we instead had it pressurized and pushed out another propellant, the, that could react and expand to offer higher performance. Well, that’s definitely an option. The next most simple engine design is called a pressure fed engine. This is similar to a cold gas thruster in that it has almost no moving parts, just some valves, but it can all offer much higher performance by tapping into the chemical reaction of the propellants. There’s two common types of pressure fed engines, mono-propellant, otherwise known as monoprop or dual or bipropellant engines, otherwise known as biprop. So let’s start off with monoprop engines. A monoprop engine is a lot like a cold gas thruster, but instead of just a single tank, there’s a high pressure tank with an inert gas, usually helium or nitrogen, and a lower pressure tank with a propellant often hydrazine. When you want to run your engine, you just simply open the main valve to the engine and maintain pressure in your propellant tank by modulating another valve also known as a regulator between the pressure and tank and the fuel tank. It really is about that simple. Then the secret to monoprop’s success is converting some chemical energy in the fuel into pressure and heat through some kind of energetic reaction. That’s right. You can actually have an energetic reaction with just your fuel running over a catalyst. So if you use a fuel that reactive and a strong reducing agent, such as hydrogen peroxide or hydrazine, and you run it against a catalyst bed like potassium permanganate, or iridium infused allumina, you can harness that chemical reaction to create pressure and heat. But wait, if we’re creating pressure and heat, how does that not just go backwards up the system? Well, don’t forget there’s high pressure pushing in on one side of the catalyst bed and basically an open hole through the nozzle on the other side. So that high pressure will want to flow out the nozzle to low are easier than the high pressure at the injector, but we also have to have even higher pressure pushing that fuel into the catalyst bed. And here’s where we still have to have a very high pressure COPV storing a lot of high pressure gas in our pressurant tank. Now a lot of thought also goes into just how big to make the throat of a nozzle as the size of the throat basically lets us know how much pressure will build up inside the combustion chamber. So if the throat is too small, there actually is a risk of pressure going back up the system. And of course that is bad. But another good thing is now we can use a denser liquid propellant in our fuel tank and keep that fuel tank at relatively low pressure. It just has to be higher pressure than the engine. So as liquid fuel is drained out, we backfill the voids and keep that fuel tank pressurized with the pressurant tank. So we gain efficiency in a few ways here. First we gain the density increase of being able to use liquid propellants, which means much smaller tanks for the same mass of fuel. We don’t need all of our fuel to be stored at crazy high pressure, which means they can also be stored in lighter tanks. And we can unleash the chemical energy inside the fuel to create heat. This leads to monoprop engines being typically about two to three times higher specific of a impulse or more efficient than a simple cold gas thruster. Which generally makes them a great choice for reaction control thrusters or other engines where simplicity and reliability matter most. Some good examples of monoprop engines are the reaction control thrusters on many satellites or the reaction control thrusters on the Soyuz spacecraft as well as the Mercury capsule. But what if we still need higher performance? Well, there’s another type of pressure fed system that is still relatively simple, but can offer even higher efficiency. The bipropellant pressure fed engine Bipropellant pressure fed engines are basically the same as a monoprop engine, but instead of just one propellant and pressurant tank, there’s a pair of them. One set stores the fuel and the other set stores the oxidizer, but they work together in a similar fashion. They still are very simple and rely only on a few valves opening to actually operate a pair of valves will open the fuel and the oxidizer tanks to the engine. And then another set of valves maintain adequate pressure from the pressurant tanks to each of the propellant tanks. The advantage of a biprop system is you can utilize a more energetic and efficient propellant. In fact, you could even run a fairly conventional propellant such as RP-1 or methalox if that’s what you wanted to do. But many bipropellant systems will utilize hypergolic propels or those that will spontaneously combust when they come in contact with each other. This makes it so the bipropellant system can be the ultimate in simplicity and reliability while still offering decent performance. At the end of the day, you’re still limited in your performance by the total amount of pressure in the system, you can only get so much pressure in the engine as you have upstream. So the pressurant tanks are still your limiting factor with a limited quantity and just like cold gas thrusters, or any of the pressure fed systems. At some point, the rade offs of getting a higher pressure in the system increases the weight so much that is just not worth it. In fact, a fully and only pressure fed rocket has never made it to orbit. So in other words, all stages, if you had your booster and your upper stages all pressure fed, none of them have ever made it to orbit. By most conventions, even with lightweight carbon composite tanks and other new age tricks, it’s considered basically impossible due to its limited overall performance. But one company did have a solution that was Firefly. Their original Alpha design was going to utilize an aerospike engine to overcome some of the limitations of a pressure fed system. And they thought that thing could make it to orbit. In Firefly space systems we were developing a, a carbon fiber pressure fed rocket. So on a pressure fed rocket to get that chamber pressure, you have to pay for it in the pressure of the tanks. Which means thicker tanks. Thicker thicker tanks. So in the pressure fed rocket. You’re really trying to lower the pressure in the propellant tanks. But that being said, many rockets have utilized pressure fed upper stages, such as SpaceX’s Falcon 1 upper stage with the Kestrel engine or Astra’s Aether engine on their second stage. But the Space Shuttle’s OMS pods or the Orbital Maneuvering System is maybe the best and most simplified look at a bipropellant pressure fed system. We can clearly see the two pressurant tanks, a fuel tank and an oxidizer tank. All right, there laid out perfectly. Now, despite not being as commonly used on launch vehicles, it’s extremely common for reaction control thrusters, and you’ll see it used on almost every United States spacecraft from the Space Shuttle to SpaceX’s Crew Dragon capsule to the Apollo Command and Service Module, or even the Gemini capsule. Pressure fed engines have a big limiting factor. And that’s the weight of the tanks increases the higher the pressure in the system is. So what if instead of having high pressure, heavy tanks, you used low pressure, lightweight tanks and used a pump to shove propellant into the engine. Well, although that can get really complicated, there’s one system that’s extremely simple. Okay, so we want to pump our propellants into the engine. Obviously, a pump can increase the pressure a lot and as we know, pressure is a good thing. But running a pump requires energy, a lot of energy. So perhaps the easiest way to power a pump is with electricity. That’s right. That is now a reality in spaceflight. You can run a rocket engine with pressure from a pump run by an electric motor powered by a lithium battery. So now we’re adding a little bit of complexity, but our biggest advantage is now we can lower the pressure inside the tanks. We can go from something like 30 bar down to only about three bar of pressure. And as you can imagine, that makes the tanks much, much lighter. So as long as the weight saved on tanks is lighter than the weight of the pumps, motors and batteries, it’s a win, but even with super lightweight composite tanks and the most energy dense and efficient batteries, there’s a limit to how big you can scale up the system. Pumps can require thousands of horsepower. In fact, the RD-170’s pumps require 170 megawats of power or 230,000 horsepower. Imagine having an electric motor that big! This is lucid heir’s motor unit. It weighs 74 kilograms and puts out about 500 kilowats and it’s one of the most advanced motors you can find today. It would require roughly 340 of these motors to power the pumps of the RD-170 and at 74 kilograms, we’re looking at about 25,160 kilograms just for the motors to spin the pumps. So yeah, just the motors alone to power the pumps would be about two and a half times heavier than an entire RD-170. That’s not including the weight of the batteries, which don’t get lighter as they drain their charge. So we’d need several additional tons of batteries in order to power this. And that’s not even taking into account the power density of batteries, which is 50 to 100 times less power dense than RP-1. So even if you optimized a motor and batteries to be substantially lighter, maybe even half the weight of those on a high performance electric vehicle, we’re still looking at something a lot heavier and less power dense than a turbine and propellant. Because of these limitations, electric pump fed rockets were actually considered impossible until just recently as battery technology is finally good enough to even make this a viable solution for an orbital launcher, but just barely. Rocket Lab pushed through that barrier and it was successful in making the first electric pump fed orbital rocket in history, the Electron. Since they paved the way, Astra has followed in their footsteps with the Delphin engines on the first stage of their rockets as well. And it’s become a popular choice for new rocket companies because of its relative ease of development and higher performance than a pressure fed system. That being said, it’s still quite limited on performance and it doesn’t scale up well for larger launch vehicles. So what if you need more performance, you’re trying to make a larger launch vehicle? Well, I think it’s time we tap into the energy of that rocket fuel to power our pumps. As we’ve mentioned, it can take a lot of energy to spin a big pump, fast enough to flow ridiculous amounts of propellant into a combustion chamber at very high pressures. Although 230,000 horsepower is at the upper end of the power requirements. Tens of thousands of horsepower is still in the realm of most orbital rocket engines. So what if instead of having an electric motor spin the pumps, we basically take a small rocket engine and point it at a turbine and allow that high velocity, high pressure, hot exhaust gas to perform mechanical work well. That’s the idea behind the open cycle, otherwise known as the gas generator cycle. You basically create a small rocket engine whose sole purpose is to generate high pressure hot gas that can be used to spin the pumps that feed the engine. One of the earliest examples of this was the German designed V2 rocket with the A-4 engine. Instead of utilizing the main propellant, they basically used a monoprop rocket to spin the pumps. They chose high concentration hydrogen peroxide as the fuel, and then they ran it over a potassium permanganate catalyst that can create enough energy to spin the turbine up to the right speeds. I like this solution because it’s using a simple pressure fed monoprop rocket engine to spin a larger, more powerful pump to increase the overall output of the engine. It’s a pretty clever idea. And it’s still in use today on the Soyuz rocket on the RD-107A and the RD-108A. But that’s a little less efficient to have another set of tanks for your gas generator fuel. And then you’re also only getting the energy from a monoprop engine instead of just tapping into the high energy rocket fuel that’s already right there inside the rocket. So a more sophisticated and higher performing option is to use the main propellants themselves inside the gas generator. So they’ll take some of the fuel and just a dash of oxidizer and send it through the gas generator to create high pressure hot gas to power the turbine. They will run this combustion extremely rich because, as those of you that watch my, “Why don’t rocket engines melt” video might know, engineers keep the temperatures of the gas generator low enough to not melt and destroy the turbine. The gas generator is fed from the pumps themselves. So the pressure inside the gas generator can be really high, whatever it takes to get the turbines up to right speed to power the pumps. But of course, this brings up a conundrum. If the pumps are powered from the gas generator and the gas generator is powered by the pumps, how in the heck do you get this process started? Well, starting a rocket engine is a whole different topic altogether. So we’ll save that topic for another video. But the most common thing to do is just basically shoot a separate cold gas thruster at the turbine to get it spinning first, before the engine actually starts to run, and this is called a helium spin start. With a gas generator, all the exhaust gases that were used to spin the turbine are simply dumped overboard, or perhaps maybe used to cool portions of the nozzle. But the gases are not added into the main combustion chamber. That’s why this is called open cycle. The exhaust gas is used and then it’s expelled to the open air or space. It’s not used in any part of the main combustion process. The downside with the open cycle is that there’s a ton of unburnt fuel in the exhaust. See how dark and sooty the exhaust from the gas generator can be? That’s unburnt fuel just being wasted and thrown away. So the open cycle wastes some of the propellant in order to power the pumps. But generally it’s considered worth it because it is a relatively small amount of fuel compared to the amount of fuel used inside the combustion chamber. Some good examples of gas generator, open cycle engines are the Merlin 1D engines on SpaceX’s Falcon 9, the F-1 and J-2 engines on the Saturn V, the RD-107A and RD-108A as we mentioned on the Soyuz, the RS-68 on the Delta-IV Heavy and many, many others. It’s easily. One of the most common cycle types for orbital rockets being extremely effective, highly capable, and relatively easy to develop compared to other more exotic cycle types. But what if it’s not enough? What if you’re still not hitting your target performance and you’re crying at the thought of wasting fuel and dumping it overboard? Well, what if we could somehow pump that exhaust right back into the engine and not waste any fuel? The closed cycle or staged combustion cycle, something rocket engineers quickly sought to try to master after seeing the potential for incredibly high performance. In the closed cycle, they don’t just attach the gas generator exhaust pipe to the main combustion chamber and cross their fingers. That would be very bad for a few reasons. The pressure run through the turbine is usually kept to a minimum and we lose most of that pressure to spin the turbine. So the main combustion chamber gases would want to go backwards through the system. And of course that’s not good. But also if you’re using RP-1 or any other carbon based fuel, that sooty exhaust is terrible for injectors and regen channels and all sorts of things. So you’d likely clog your engine up and it would cease to function in a hurry. First off, instead of just using a little fuel and oxidizer to run the gas generator, we’ll actually put all of the fuel or oxidizer through the gas generator and the turbine. And now we won’t call it a gas generator. It’ll actually be considered a preburner because that’s exactly what we’ll be doing. Pre-burning the propellant, a little preburning it before it gets into the main combustion chamber for the real full burn. Now, remember, again, we’re going to be flowing either all of our fuel or all of our oxidizer through the preburner and the turbine, and depending on which one it is, that’s which type of closed cycle. So either fuel rich or oxidizer are rich. Let’s start off with the oxidizer rich closed cycle, since it was the first closed cycle engine developed. As those of you who watched my “Complete guide to Soviet rocket engine history” would know, Soviet engineers were able to overcome the challenges of oxidizer rich stage combustion in the late 1950s, already with the S1.5400. This Soviets chose to go with the oxygen-rich stage combustion cycle because when running on kerosene based RG-1 or RP-1 you’d run into the coking and sooting issues. So this means they decided to run all of the oxidizer through the turbine, and then pipe that into the main combustion chamber. Then they inject just exactly as much fuel as they need to make enough heat and energy to spin the pumps fast enough to create high enough pressure and heat, because remember it will drop as it runs through the turbines as it’s converted into mechanical work, and it then will go into the combustion chamber. And what’s our rule, pressure always flows from high to low. So the pressure inside the preburner needs to much higher than the combustion chamber. That way when the pressure drops across the turbine and then drops again in the injector, it’s still coming into the main combustion chamber with some margin of safety. A good rule of thumb is two times higher pressure from the preburner through the turbine and about 20% higher pressure between the back of the injector and the chamber, but engineers tend to chew away at these as they gain confidence with an engine and they try to pursue higher and higher performance. But now we have kind of the ultimate question, how do you get the preburner to be higher pressure than the main combustion chamber in the oxidizer rich stage combustion cycle? All of the oxidizer will get compressed up to very, very high pressure since it all will flow through the turbine, but that same thing isn’t true for the fuel. The majority of the fuel will just flow into the combustion chamber. So it only needs to be pressurized to about that 20% higher pressure than the combustion chamber, but we still need some of the fuel to be high enough pressure to power the preburner. So in this case, there’s actually stages to the pump. Most of the fuel will go through the first stage, which gets it to be high enough pressure for the main combustion chamber, but then some of it will actually go into another stage of the pump that increases the pressure to be high enough pressure for the preburner. Now you might be thinking, wait, the oxidizer has already been burnt in the preburner. How can it be burnt up again in the main combustion chamber? Well, since there was only a dash of fuel to react with, the vast majority of oxidizer is unreacted, but it has warmed up from a liquid to a hot gas, but it still has most of its chemical energy. When it enters the main combustion chamber where it will then react with fuel, then the main combustion process happens, unleashing the remaining energy from the propellants. The oxidizer rich stage combustion cycle is very, very hard since you’re creating high pressure, hot gas oxygen, which sure loves to react with absolutely everything. It requires fancy metal alloys that can handle those extreme environments. The Soviets mastered this as the majority of their engines were oxidizer rich staged combustion cycle, including the NK-15 and NK-33’s for the N1, the RD-170 on the Energia and the RD-180 on the Atlas V. This is actually extremely difficult and something the United States still hasn’t achieved for an orbital rocket. Although Blue Origin’s BE-4 oxygen-rich closed cycle methalox engine will fly on United Launch Alliance’s upcoming Vulcan rocket, and Blue Origin’s New Glenn rocket. There’s also Launcher’s kerolox E-2 engine, which is oxygen-rich closed cycle with impressive performance. But neither engine has currently flown as of the making of this video, but hopefully they will soon and will take that title of the first US built oxygen-rich closed cycle engine. But the United States didn’t completely give up on staged combustion. They just went the other way. They pursued fuel rich stage combustion for an engine at the heart of an icon, the RS 25 on the mighty Space Shuttle. What if we basically swapped the process of the oxidizer rich stage combustion cycle and just put all of the fuel through the turbine and just used a little oxidizer kind of like most gas generators wouldn’t that avoid the issues of having hot gaseous oxygen? Well, yes, yes it does. But as we mentioned, it brings up another problem, coking and soot. But wait, what if we didn’t use RP-1 or any other carbon rich fuel? When engineers were designing the main propulsion system for the Space Shuttle, they went with liquid hydrogen and liquid oxygen because they can run hydrogen fuel rich through the preburners and not have it create soot. And the engine will happily run with hot gaseous hydrogen. This seems like a pretty obvious solution, but wait, fuel rich staged combustion cycle comes with its own challenges besides the potential for soot, especially if you’re using hydrogen. Hydrogen is extremely undsense. I don’t think that’s a word… Sparse or lightweight, which of course means it takes up a lot of space in tanks, but it also takes a large pumps with a lot of stages to get it to the right pressures. It’s common and perhaps the most simple to have a single shaft for your pumps and your turbines, assuming everything on that shaft can operate at similar speeds and although single shaft hydrolox fuel rich close cycle engines have been made, like the Soviet union’s RD-0120 at the heart of their Energia booster, the US went with a different solution that raised its own set of problems. Their design for the RS-25 called for dual preburners on two different shafts, both fuel rich, one preburner would power the fuel pumps and the other would power the oxygen pumps. But having high pressure hot gaseous hydrogen in your preburner on the same shaft as high pressure liquid oxygen is a recipe for disaster. If any of that hot gaseous fuel would make its way up the shaft and meet oxygen, it would be game over. So US engineers had to develop an extremely elaborate purge seal, that prevented propellant traveling up or down the shaft by having even higher pressure inert helium in the middle of it. So if anything, leaks, it would leak out from that higher pressure helium side and into the preburner or the oxygen pump. So looking at our diagram here, you can see the two separate turbines and preburners of a dual shaft fuel rich cycle engine. Of course, one preburner powers, the oxidizer pumps and the other powers the fuel pumps. Since these preburners run fuel rich, it means all of the fuel will go through one of the preburners and turbines before it goes into the main combustion chamber. So we see that approximately half of the fuel flows through each preburner and turbine. And exactly opposite of the oxidizer rich closed cycle engine, in this case only a dash of oxidizer goes into the preburners. Just enough to create enough energy to spin the pumps up to the right speeds to create the high pressure needed to get the propellant through the preburner and turbine and into the combustion chamber. And again, opposite the oxidizer rich closed cycle engines, in this case, most of the oxidizer will only go through a single stage pump that gets it to be high enough pressure to go into the main combustion chamber. But the bit of oxidizer that goes into the preburners will flow through a second stage of pumps to get it up to those higher pressures. So the RS-25 was the United States’ first closed cycle engine, but it wasn’t the only fuel rich engine developed. The Soviets also made the RD-56 and RD-57, which were both fuel rich staged combustion hydrolox engines developed for an upgraded N1 variant. They also made the RD-0120, which was at the heart of the Energia rocket. It’s the most powerful single chambered rocket engine the Soviet union ever flew. So the fuel rich stage combustion cycle trades one bit of complexity for the other, but even so there’s still one cycle type that combines many of the pros and cons of both cycle types to form one ultimate and confusing engine, but one pro makes it worth pursuing, but only for those daring enough to try. Full flow staged combustion engines is exactly what it sounds like. The full flow of propellant, so both the fuel and the oxidizer goes through a preburner and turbine. This means there’s both a fuel rich preburner and an oxidizer rich preburner. Let’s first just follow the propellants as they go through the pumps and turbines. Fuel and oxidizer arrive at the pump inlets at tank pressure. Then the pumps take them up to the full preburner pressures. Then we take almost all of the oxidizer and run it through the oxidizer preburner and turbine, but we do need to send just enough oxidizer over to the fuel rich preburner to give it enough power to power that turbine. And the same thing is true for the fuel rich side. Almost all of the fuel goes through the fuel rich preburner and turbine, but we do need to pipe over just enough fuel to the oxidizer rich preburner to have enough energy to spin those pumps. What’s left is extremely fuel rich gaseous fuel going from the fuel rich preburner and turbine to the combustion chamber. Then we have extremely oxidizer rich gaseous oxidizers going from the oxidizer rich preburner and turbine to the combustion chamber. This means both propellants wind up in the combustion chamber in hot gaseous form. And this is actually a huge advantage. A gas-gas interaction is extremely efficient and burns much quicker and more completely than liquid-liquid or liquid-gas interactions. Or as Elon Musk put it. Full flow station combustion. Exactly. Yeah. Uh, you’ve got a gas gas interaction, so you’ve got two hot gases combining we, we think we can probably get to ninty, certainly 98 and a half, hopefully 99% of theoretical combustion efficiency. This is so if God himself came and knitted together the molecules, he could do 1% better, okay. Maybe one and a half percent better. That’s how, that’s very high efficiency. Because of. Full flow stage combustion. Well, right off the bat, we can already see we’re having to deal with the problems of oxidizer rich stage combustion and fuel rich stage combustion cycle. So our oxidizer side has to be able to handle the problems of hot gaseous oxygen. The good news is at least we can couple the oxidizer rich turbine and shaft with the oxidizer pump. And we can also couple the fuel rich preburner turbine with the fuel rich pump. So at least we don’t have to deal with extremely elaborate seals that are cantankerous and difficult for reusability as they might require inspections and maintenance after each flight. But perhaps the biggest advantage of full flow staged combustion. Isn’t just the gas, gas interaction, or the relatively simple seals between the pumps and turbines, but it’s also the temperatures inside the preburners. I think a good way for this to click is by studying the oxidizer rich staged combustion engine. Look at all the at fuel going straight from the pump into the combustion chamber. What if we basically added just a dash of oxidizer to that stream of fuel and added another preburner to help power the pumps. It could share the load and decrease the amount of work the other preburner would have to do. The two preburners would basically be splitting the work needed to run the pumps. The less work the turbines need to do means less heat and pressure necessary inside the preburner to do the same amount of mechanical work, but let’s actually dive into this even more to help drill it in. Take a look at this equation for thermal energy. Now, for some perspective here, thermal energy and kinetic energy make up internal energy, which is that E in the Enthalpy equation. And we’re going to show you exactly how different the temperatures inside the preburners of different engine cycle types can be. Now bear with me, even seeing an equation on screen terrifies me, but we’ll make this as simple as possible. First we need to make up an engine so we can figure out how to fill in all the variables. So let’s say we’re designing a methalox engine that requires 25 megawats of shaft power in order to power the pumps for a 100 tonne thrust engine. We want to find the difference in temperature at the turbine between an oxygen-rich close cycle engine, a fuel rich close cycle engine and a full flow staged combustion cycle. What advantage would full flow actually give us assuming the engines thrust and chamber pressure were kept the same? That 25 megawats of required shaft power is pretty much entirely thermal energy in the preburner, which is equal to the mass flow of the propellants of the turbine times the specific heat of those propellants times deltaT or the change in temperature. Mass flow and specific heat are both known variables based on the engine and preopellant, so we can just plug those in by rearranging our equation to solve for Delta T or change in temperature. We’ll find the change in temperature is equal to the thermal energy divided by the mass flow rate times the specific heat. Now you may notice there’s a big difference in mass flow through the turbine between oxygen-rich closed cycle and fuel rich closed cycle, because we needed to take into account not only the density of each propellant, but also the engine’s fuel to oxidizer ratio. But these are good enough rough numbers for each of the system types. The biggest thing to notice is that full flow staged combustion cycle has the most mass of propellant flowing through the turbines because it has ALL of the propellant flowing through them, hence full flow. Not only that, full flow also gets to take advantage of the average specific heat of both propellants, and not to get too confusing here. It’s not just a 50 / 50 average, but we have to account for the propellant densities, each propellant’s respective heat flows, engine oxidizer fuel ratio, and even the exact power demands of each shaft. In our example, the change in temperature at the turbine is nearly twice as much in oxygen-rich clothes cycle compared to full flow and three times as much in fuel rich clothes cycle compared to full flow. That’s impressive. This is a dream come true for rocket engineers. As the turbine heat load is often one of the biggest limitations in the system. Now, of course, this all varies drastically by propellant type, engine output and all sorts of other variables, but in general, our example helps illustrate full flow’s biggest advantage. But what this means is engineers can either reach their power levels at much lower temperatures in their turbines, or they could potentially increase the power of their engines assuming they’ve designed a system that can handle high heat loads, or of course some happy compromise of both. So you can either have more main chamber pressure or you can have the pumps, uh, put less stress on the pumps. Although looking at SpaceX’s Raptor engine, they like to push it all as much as possible for the ultimate performance. But as attractive as the positive attributes of full flow are to engineers, it’s often considered not worth it because of its extreme complexity. With everything intertwined and interconnected, one small change in one part of the engine can have this huge ripple effect on absolutely everything else. In the case or Raptor, you’ve got an oxygen power head and a fuel power head. And they’re different shafts, um, obviously, and you’ve got two turbines and two preburners. So,, and, they’re cross feeding one another. Yeah. So the start sequence for Raptor is insanely complicated, compared to the start sequence for Merlin. [Tim] It has. To be perfectly. Precise. [Elon] Everything’s it’s, it’s this basically, you’re doing this delicate dance between the fuel power head and the oxygen power head. And if they get outta sync, uh, then, then you can go stoichiometric in the preburner and melt or explode the preburners. Yeah. This means managing the timing of valves, startup, and even throttling is extremely hard to master and requires some very deep pockets to perfect, which is why we haven’t seen that many full flow engines developed. As usual, the Soviets were the first to develop a full flow staged combustion engine, the incredible RD-270. This engine ran on hypergolic propellants and was massive. It was only about 15%, less powerful than the F-1 that poweed, the Saturn V, yet it was far more efficient. Unfortunately it never saw flight as the massive UR-700 and UR-900 it was proposed to power were never given the green light. The United States also developed the turbo pumps of a full flow stage combustion cycle engine in the nineties called the “Itegrated Powerhead Demonstrator.” Aerojet and rocketdyne were successful in reaching full capacity of the power pack, but it would never make it onto a full engine. Today, SpaceX is utilizing the full flow staged combustion cycle on their Raptor engines that power their Starship and SuperHeavy booster. This all sounds so complicated. It needs two preburners and an insane amount of engineering to make it run. What if you could just get rid of the preburners altogether? Well, there’s actually two pump fed engine types that do exactly that. Okay. So hear me out. What if we just punched a hole in the side of the combustion chamber and just took that high pressure gas and used that to spin the turbines to run the pumps? Hmm. Now there’s an idea. This is essentially what the tap-off cycle or combustion tap-off cycle is. You can remove the complication and weight of having a preburner or gas generator and just use the main combustion pressure. You only lose a little bit of performance by having basically punched a hole in your engine, but you free up a lot of complexity for sure. The coolest thing is they can be fairly self-regulating because you can limit the amount of pressure the turbine sees with a choke or by how much you shrink down the throat leading to the turbine. The problem is the main combustion chamber gets really really hot because it doesn’t have any moving parts. And they’re usually regenerative cooled with fuel running through the walls. The main combustion chamber can see temperatures of around 3,500 kelvin, and that’s far too hot for a turbine. So engineers will sometimes need to dilute the gas before it reaches the turbine. Usually with some additional fuel that will help lower the temperature by making that exhaust more fuel rich. Then the exhaust can either just be dumped overboard or could be reintroduced into the nozzle at a point where it’s both higher pressure after its pressure losses and lower temperature than the main combustion to be used as film cooling to date. No tap-off engine has made it to orbit, but it has been used on multiple notable engines in the sixties. NASA developed a follow up to the J-2 engine on the Saturn V known as the J-2 simplified or just J-2S for short. As the name implies, it was meant to be simpler and higher performance by using the tap-off cycle. Now, although it was fully developed, it would never actually see flight. Blue Origin utilizes the tap-off cycle on their BE-3 engine that powers their New Shepherd rocket and Firefly could be the first rocket to reach orbit with their tap-off cycle Reaver and Lightning engine on their Alpha rocket. But there’s still one more system that has pumps, but doesn’t need a gas generator or a preburner. And that’s the expander cycle. Remember how heat is both our enemy and our friend in rocket engines? Well, there’s one really cool thing we can do with the heat of the engine. And that’s run the engine. Wait with the heat of the engine, we can run the engine? What the heck?! This is called the expander cycle and it harnesses the thermal energy of the fuel or the oxidizer, but most often the fuel that is used to cool the engine. Again, for those of you who watch my, “Why don’t rocket engines melt” video know, a very common and extremely effective way to cool a rocket engine is to actually pump the fuel through the walls of the combustion chamber and nozzle to help keep them cool. In the process of cooling the walls, the heat from the combustion chamber will be transferred into the fuel where it will absorb some of that heat energy. Some fuels can take on heat better than others. And in this sense, hydrogen is a very good fuel because of its heat capacity in all the other engine cycles. This fuel is usually just pumped into the combustion chamber as a hot gas, which usually has to interact then with a liquid oxidizer. But in the case of the expander cycle, we can actually take this heat energy and use that to spin the turbine. But there’s a few problems. This engine, again, like all the other engines has a big chicken and egg. If the engine isn’t hot, then how does it even power the pumps? Well, again, we definitely need to do a video on how to start a rocket engine because they often require a second source to get the pumps up to speed and get everything up to operational temperatures before they can run on their own. But it’s also limited in thrust output based on the amount of heat energy available in the system. Think about it this way, as the engine grows, the amount of fuel flowing through the system increases too. That increase in fuel running through the walls also increases the cooling capacity, which normally is a good thing. But couple that with the fact that when you increase a chamber, this surface area goes up by the square of the radius while the volume goes up by the cube. This means it’s actually easier to cool a large rocket engine than it is to cool a small rocket engine. This is one of the main problems with Aerospike engines. Like we discussed in my video about Aerosspikes. This same situation is what limits the amount of a available energy that can be used to spin the pumps, a bigger and more powerful engine needs more energy to run the pumps and that same larger and more powerful engine doesn’t actually heat up the same percentage of fuel flowing through it. So there is a limit to how powerful it can actually be. And similar to closed cycle engines, the pressure needs to be quite high before it hits the turbine. It needs to be high enough that it can go through the turbine, which will cause a pressure drop, and it still will need to be a decent amount higher pressure than the main combustion chamber. This means the fuel pump needs to really do a lot of work to get the fuel up to those kinds of pressures. And if it’s liquid hydrogen, like it normally is, you’d better believe that’s going to be a huge pump with lots of stages in order to get the pressure high enough. In the case of a hydrogen fueled engine with a single turbine, engineers might even need to employ a gearbox inside the turbo pump so they can direct the required amount of speed and energy to the fuel pumps and send less to the oxygen pump, which doesn’t require nearly as much energy. So now you’ve again traded one form of simplicity and efficiency for another complication and more moving parts. But it is very efficient and it’s an effective way to use what is basically free energy in the system heat to power your pumps. It’s genius. Some examples are the upcoming Vinci engine that will power the upper stage of the Ariane 6, the RL-10 engine that powers the Centaur upper stage for the Atlas V and it will also power the SLS’s upper stage. These engines can reach ludicrous levels of efficiency. In fact, the RL-10B2, which utilizes the expander cycle with hydrolox propellants reaches 462 seconds of specific impulse. This is about the upper limits of what’s even possible for chemical rocket engines, but there is another version of the expander cycle called the expander bleed cycle. It makes the system a little bit simpler by not putting the fuel back into the combustion chamber after its spun the turbine. This means you can actually use more of the pressure in the heat to spin the pumps, and it doesn’t need to be higher pressure than the combustion chamber. So it’ll just use a little bit of the expanded hot gas and throw it overboard. Although it’s going to be a little bit of a waste of fuel, it’s still very efficient. This can help overcome the limitations of thrust since you can use more of the limited pressure and heat available to power the pumps. So it trades a little bit of efficiency for the potential of increased thrust and decreased complexity. There aren’t a lot of examples of the expander bleed cycle, other than the BE-3U that will power the upper stage of Blue Origin’s, upcoming New Glenn rocket and the LE-5A and LE-5B on Japan’s H1, H2 and their upcoming H3 rockets. There’s also another version of the expander cycle called the dual expander cycle that utilizes both the fuel and oxidizer assuming that both those propellants are used to regeneratively to cool the chamber. Okay. But really those are the major engine cycle types. So let’s wrap this up with a little overview and a few more thoughts. At the end of the day, there is no best cycle type each and every system has a unique way to power a rocket engine, but like all things rockets, there are trade offs and compromises to each and every system. Because who cares how high performance your engine is if it’s not reliable. There’s definitely the elegance and ease of pressure fed engines, but their performance is limited. Meanwhile, the electric pumped cycle is seeing a new rise in popularity with the increased density of lithium based batteries and more advanced material science. The gas generator cycle is still one of the most common cycle types and it blends a very healthy amount of performance and relative simplicity. It seems to be a great all around compromise. Closed cycle has always been much sought after and the Soviet’s made it look easy. Gains over open cycle engines should be expected, but with extra complexity and some new issues. Full flow is easily the most complex system, but has the potential to run the coolest turbines and the hottest chamber, which can definitely help it reach ridiculous levels of thrust. Tap-off, I’m actually surprised we haven’t seen developed more. It seems like it’s again, relatively simple and reliable, but it can actually still reach high levels of performance. Expander cycle is awesome and has proven to be a great choice on the RL-10, but it does have its limitations on output, which often leaves it off the table for a booster engine. There are also a few options of mixing and matching cycle types, like using a gas generator on just the fuel side and the expander cycle to power the oxidizer side or some unique combinations that engineers have come up with, but haven’t really seen the light of day yet. But yeah, that’s how you power a rocket engine. Did this video help you understand all the different cycle types? Let me know if you have any other questions or thoughts in the comments below and in the future, we’ll cover more exotic options like ion propulsion or nuclear rocket engines, because in my opinion, each of those deserves their own dedicated video as always. I owe a huge thank you to my Patreon supporters for helping make videos like this and everything else we do at Everyday Astronaut possible. If you want to gain access to our awesome Discord community or some extra live streams, or just some fun behind the scenes content, head on over to And while you’re online, be sure and check out our awesome web store for things like this, our full flow staged combustion hoodie, which also has a cool schematic on the back, along with the rest of our schematics collection, our Future Martian collection or our Soviet collection. There’s lots of really fun stuff over at everyday. Thanks everybody. That’s gonna do it for me. I’m Tim DOD, the everyday astronaut bringing space down to earth for everyday people.

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