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 everydayastronaut.com. 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 everydayastronaut.com/shop.
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 <laugh>. 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
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the scenes content, head on over to patreon.com/everydayastronaut.
And while you're online, be sure and check out our awesome
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our schematics collection, our Future Martian collection
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over at everyday. astronaut.com/shop. Thanks everybody. That's gonna
do it for me. I'm Tim DOD, the everyday astronaut bringing space
down to earth for everyday people.
