- Hi, it's me, Tim Dodd,
the Everyday Astronaut. This is SpaceX's Raptor engine that powers is their Starship
and SuperHeavy booster. When this thing is running, there is unbelievably hot combustion, continually burning at
the heart of this beast. In fact, the gases inside
the combustion chamber can reach about 3,500 K, about half as hot as
the surface of the sun, which is above the melting
point of pretty much any material out there. But this heat is kind of what makes a rocket engine function. I mean, after all their only real purpose is to convert the chemical
energy inside the propellant into pressure and often therefore heat so they can produce thrust. And that brings up an obvious question. How in the heck do
engines survive this heat? How did rocket scientists
figure out how to keep an engine running continuously while
harboring combustion inside that's hot enough to not just melt the metal walls containing it, but actually vaporize the metal or more pedantically sublimate the metal from solid to a gas in an instant, but we'll just say melt for
simplicity in this video. So today we're going to talk
about the tricks engineers employ to keep rocket
engines from melting. Here's the timestamps to the sections we're going to be going
over such as heat sinks, fuel to oxidizer ratios, ablative cooling, regenerative cooling, film
cooling and radiative cooling. And we're also going to be
showing you some awesome examples of each of these as well. And if you're someone who
would rather read about this, instead of hear me talk
about it, don't forget we do have an article version of this up at our website at
everydayastronaut.com, and there's a link in the
description to that article. Rocket engines, too cool to melt, but how? Let's get started. - [Announcer] Three, two, one, (mumbles) (upbeat music) - This is a combustion chamber. At the top of it is the injector face. This is where the fuel
and oxidizer get pumped into the chamber at very high pressures. The fuel and the oxidizer
will mix somewhere downstream of the injector face inside
of the combustion chamber and they'll ignite and then combust, assuming there's adequate
and continuous flow that repellents will burn. But with ridiculously hot
gasses squeezed together in between the metal chamber walls, we certainly don't want the metal to melt. So perhaps the easiest
thing we can do is just make the metal walls so thick,
even the scorching hot gases can't heat up all the metal
enough to actually melt it. The walls act, as what's
known as a heat sink or a giant thermal conductor that can handle a high heat
load for a period of time before it warms up all of the
metal to its melting point. The use of exotic materials
might be a fitting choice here. Something that can handle
extreme heat and still be strong, such as inconel or other alloys, but of course, price or
weight or other factors certainly come into play. Obviously, first off, if you're using thick
metal walls as a heat sink, they're going to be quite heavy. And in a world where rocket
engineers would eat a hat for even a 5% weight reduction,
making your engine's walls so incredibly thick they don't melt is not only far from optimal, it's downright primitive. The other problem is you
can realistically only run an engine for so long
before you eventually heat it all up to its melting point. So it has inherent
limitations and viability. This option isn't a very good choice for main propulsion engines, where they might need to run for several minutes continuously. But this option is actually
valid for smaller engines, such as reaction control thrusters. Reaction control
thrusters are often pulsed and run in shorter duration burns. This allows them an opportunity
to cool down between pulses. This keeps them simple and reliable, but with limitations on continuous runtime to keep them from melting. But what if you could just lower the temperature of the combustion? A less hot gas might make it
so you could use thinner walls and might make long duration
burns more survivable. Well, that actually is an option. (rocket engine roaring) The next relatively
simple thing you can do to keep your engine intact
is to simply run the engine with more fuel or more oxidizer otherwise known as fuel
rich or oxidizer rich, which can lower the
temperature of the exhaust. This ratio is the fuel
to oxidizer mass ratio. Now, if you want it to burn
100% of your propellant and have absolutely as much of it react with each other as possible, you'd need to burn it at what's known as its stoichiometric ratio. This is where there's the exact
amount of fuel and oxidizer to perfectly react as
completely as possible with each other, which leaves
no propellant left unburnt. But burning out your stoichiometric ratio also means you're releasing as much heat from the chemical bonds as possible too. While that might actually
be ideal in some situations, that is not ideal in a rocket
engine for a few reasons, the more heat your engine is producing, the more you need to cool
your engine via other methods, so it doesn't melt and you're
going to need to employ more of the tactics that we're going to be discussing in this video. So rocket engines need to run
their fuel to oxidizer ratios a bit off from stoichiometric. Main combustion chambers
tend to run fuel rich because this usually results
in lower thermal loads and higher efficiency, not to mention scorching hot gaseous
oxygen is typically avoided as much as possible, since it sure loves to turn metal into soup
and the rocket engine will end up running engine rich and that's definitely not good. (rocket exploding) But beyond just the main
combustion chamber's fuel to oxidizer ratio, you can also run your
preburner or your gas generator very fuel rich or oxidizer rich in order to keep it cool too. In fact, you can burn it at a
different, more conservative, cooler temperature than you would your main combustion chamber. And quick side note, if you don't know what a
preburner is or a gas generator, don't worry, I've got you covered. Check out my Soviet rocket engine video. In the beginning in the glossary, I go over a lot of terms
that will be really helpful for this video and frankly, all
videos about rocket engines. Okay, back to keeping your
preburner or gas generator cool with your fuel to oxidizer ratio. This is extremely important
because it's hard to cool a spinning turbine. You're really stuck with
a set amount of heat that it can actually take
based on the materials. And you just kind of simply have to change your fuel to oxidizer ratio
until the heat load is bearable. And unlike the fuel rich
main combustion chambers, turbines can be designed to
run fuel rich or oxidizer rich. For example, the space shuttle's RS-25 main engines ran fuel rich through their preburner while the Soviet designed NK-33
ran oxidizer rich propellant through their close cycle preburners. But all of this is rocket
science we're talking about and performance and
optimization is everything. So we don't want a big heavy engine or one that's sandbagging its
performance to run cooler, but we also don't want our
engine to melt, or do we? (rocket engine roaring) Welcome to perhaps one of the most simple and effective ways to cool an engine, and that is, well to allow it to melt, but just a little bit. This is called ablative cooling. Ablative cooling is
where you use a material that will vaporize and
gets thrown into the wake, which takes the heat along with it. It's most commonly made
out of carbon composites, which have a very high melting point. This is also the same principle many spacecraft use on their heat shields. When a spacecraft is coming back into the atmosphere for
re-entry, it gets crazy hot and the heat shield
just accepts that heat. It takes it right in the face. And then when the surface gets too hot, it basically melts a layer
away, taking the heat with it. This prevents heat from penetrating
deeper into the vehicle. And this is the same idea you can use to cool a rocket engine. Inside the metal walls
of the combustion chamber and the nozzle will be a
layer of carbon composites. As the propellant burns inside the engine, the carbon liner will
slowly get chewed away. Now, if you're anything like me, you might think this sounds
like kind of a terrible idea, but it's actually an incredibly efficient and reliable method of cooling since it's self-regulating,
has no moving parts and is incredibly effective, but it does have its limitations. Obviously, if you're looking
to reuse your engine, you should probably look elsewhere. In fact, some engines may not be able to go through their
full testing before use because it will wear down
the ablative chamber walls. And because the chamber and
throat walls get thinner, their performance can actually
decrease throughout the burn as your throat's area will increase a lot compared to the nozzle exit, which will decrease the
overall expansion ratio of the engine, which is the opposite of
what you want as you ascend into thinner parts of the atmosphere. And it's also quite heavy. The ablative material is just extra stuff that weighs a decent amount. Perhaps one of the most
famous examples of this is the Apollo Lunar Ascent engine, which could not be test
fired as a complete unit before being lit on
the surface of the moon to get astronauts home. Yes, that's a terrifying thought. Some other examples of
ablatively cooled engines are SpaceX's first Merlin
engine, the Merlin 1A, which flew on the first
two Falcon 1 flights and United Launch Alliance's
Delta IV engine, the RS-68A. On the RS-68A, you can easily
tell this engine is ablative because the engine runs on
hydrolocks, which usually has a nearly completely transparent (mumbles), just water vapor, like we
saw on the space shuttle. But the RS-68A's exhaust is bright orange because the carbon that
is ablated and ejected from the engine continues to react with the oxygen in the atmosphere, which makes it burn bright orange. Sometimes reaction control thrusters use ablative chambers too, since they are often used in
short durations and have a set amount of propellant they
can use before they run out. So engineers can design the
wall thickness to match the maximum use case. Okay, but what if ablative cooling is just not cool enough for you? Well, then we've got a few
more options to check out. (rocket engine roaring) Perhaps the most common way to keep a liquid fueled rocket engine from melting is by using regenerative cooling. This is where they flow some
or all of the propellant through the walls of the
combustion chamber and the nozzle before it goes through the
injectors and into the chamber. Yeah, that's right. Even though the walls and the nozzles of rocket engines look super thin, there's often a very
small channel where fuel can run up and down the walls
in order to keep them cool. This was a major breakthrough
that allowed rocket engines to run more or less indefinitely. Early versions of regeneratively
cooled engines would have the main chamber and
then basically a liner on the outside of it to run
the coolant or fuel through. Later it became common to actually just use pipes to form the
walls of the combustion chamber. In fact, the RL10 engine still uses braze tube construction today, although they are working
on modernizing that process. A more modern practice is
to mill a cooling channel in the walls of the combustion
chamber and the nozzle, and then use a copper or a
nickel alloy to close it off, and then that will be the
inner wall of the chamber. Copper and nickel alloys are
used because of their ability to transfer heat from the
walls into the coolant, thanks to their high thermal conductivity. Now you might be asking, doesn't that boil the fuel
before it reaches the chamber? Yeah, it sure can. And this process of boiling
the fuel can actually be used to spin the turbines to run
the pumps of the engines. This is what the expander cycle is. It harnesses the energy of the
thermal expansion of the fuel going from liquid to gas. It is extremely efficient, but it is limited by the
amount of available heat. We'll talk more about the expander cycle in an upcoming video, where we go over pretty much
all of the cycle types and talk about their pros and cons. Most engines use the fuel as coolant, but oxidizer could be used as well. But a fun little thing to know is when cryogenic propellants are used, the outside walls of the rocket nozzles can be unbelievably cold, while the inside of the
walls are scorching hot. Talk about a temperature gradient. Check out this video of an RS-25 running. If you went up and
touch the outside of it, your hand would freeze solid, touch the inside and it
burned off instantly. It's honestly crazy to me that
these two extremes can exist in such close proximity. One of the challenges with
regenerative cooling is that the pressure inside the walls
needs to actually be higher than the pressure of the
combustion inside the chamber. That's right. The tiny little channels
flowing cryogenic fluids inside the thin little walls of a
rocket nozzle are actually under higher pressure than the main combustion chamber itself. This is because of the fact
that the walls are essentially just tubes that feed the injectors
and pressure always flows from high to low. So the injectors always
have to have higher pressure than the combustion chamber. But it's just crazy to me that
those tiny little passages inside the walls need to handle that really high pressure too. Seems like with such thin walls, it'd be so easy to spring a leak. Well, if a leak does spring,
there's some good news. Let's pretend a leak
springs on the inner wall of a regenerative cool chamber, because the pressure
inside the walls are higher than the pressure inside the chamber, all that will happen is
fuel'll leak in a little and actually provide
some additional cooling. Firefly CEO, Tom Markusic,
explained this to me when I interviewed him at their factory outside of Austin, Texas. - Yeah, engines are amazing (inaudible). I haven't seen this here as much, but I've seen places where the engine will actually erode itself down to the point where it starts leaking and then it's happy, and it doesn't go any further. It just like self regulates. - Yeah, interesting. But wait, if fuel leaks into the chamber, how would that not just make
it explode more and make an even bigger problem? How on earth can that help cool an engine? Well, that brings us to our next method of cooling, film cooling. - [Announcer] Lift off, we have lift off. - Another common method of
cooling is film cooling. This is where you inject a fluid between the combustion chamber and nozzle surface and the hot combustion gases, because don't forget, fluids
are either gas or liquid. So this can be done in either form with liquid or gaseous propellants. The basic mechanism at play
is creating a boundary between the wall and the hot
combustion gas that creates a thermal insulation with a
cooler fluid in between them. So how exactly do they do this? Well, let's start off
with liquid film cooling. One easy way to do liquid film
cooling is to literally just have a higher concentration
of fuel or oxidizer injectors on the outer perimeter
of the injector face. Engineers will almost always use fuel since the main combustion
chamber usually runs fuel rich, so there'll be a whole ring
of extra fuel flowing around the outer perimeter that
won't have the right amount of oxidizer to react with. This will produce a ring of
even more fuel rich combustion, which prevents heat from transferring from the main combustion
gases and the walls, but not only that, most of
the fuel closer to the walls won't react at all because it won't have any oxidizer to react with. So it actually just travels
down along the walls of the combustion chamber and a film traps between the combusted
gasses and the metal walls. And if it is a liquid, it will likely phase change
from a liquid to a gas, then that'll create a vapor
boundary layer as well as absorbed some heat energy in
the process of phase changing. It's common to actually
drill holes in the walls, assuming they're regeneratively cooled, to intentionally leak in some of the fuel, especially in particular hotspots, like the throat of the engine. Another weird benefit of using
fuel to cool is that you can create a carbon layer in the
form of coking along the walls when using a carbon-based fuel like RP-1. When you have a fuel rich combustion, there's a lot of carbon
that will be unburnt and can create soot. Just look at the exhaust
of the gas generator for the Merlin engine. SpaceX runs the combustion
inside the gas generator extremely fuel rich in order
to reduce this temperature, so the turbine doesn't melt,
like we mentioned before. As a by-product, you see
the exhaust coming out is very, very dark and sooty. This is the same type of
soot that can actually stick to the walls of the chamber and
often has a bad side effect, which is having that soot
attached to the injectors and potentially the cooling
holes or slots themselves, which can clog them up. But the soot that sticks to the walls is another thermal barrier that can help. Again, let's hear Tom
Markusic explaining that. We also saw great evidence of this soot when we were up close and personal with their (mumbles) engines as evidenced by the soot on Tom's face. - Also (mumbles) propellants, if some of the products that
condense on the surface of the chamber are beneficial for inhibiting, heat transferred to the wall. - [Tim] Right. - So once you get up about,
above about 1200 PSI, that sort of soot, carbon
layer starts to come off, so it kind of runs away, your cooling problems get worse, very kind of precipitously
as you go over that, (mumbles) just 100 PSI is
a little bit different, cause your soot away and
now you've got much more heat transferred to the wall. So there are other factors that play into what's going on in there. - [Tim] So the coking on
the combustion chamber can almost aid as like a, as an insulating barrier. - It absolutely does. - [Tim] Really! - It's really effective thermal
barrier coating for free. So all that soot you saw
on my face and (mumbles) that's great for thermal barrier coating. - By the way, if you haven't
seen this interview with Tom, I highly recommend it. Tom is literally a
rocket doctor after all, having a PhD in mechanical
and aerospace engineering. So it's packed full of great knowledge. But back to the Merlin engine with that really dark fuel rich exhaust
from the gas generator. There's another thing we
can do with that exhaust. We can use the gas itself to
film cool some of the nozzle, but there are a few caveats. You obviously noticed that
on the sea level of Merlins they are just dumping
that exhaust overboard. So it doesn't seem like they're cooling with that exhaust gas at all. But if we pop over to the
vacuum optimized Merlin, we can see they do something different with their gas generator exhaust. It isn't just simply dumped overboard, instead that turbine exhaust gas is pumped into the nozzle extension that
makes the engine optimized for use in a vacuum. In order to understand
why this makes sense, we do need to remember
something about a nozzle. As it expands after the throat, the exhaust gets cooler
and lower pressure, the further down the nozzle you go. So keep this in mind when we think about where we can pump in
that turbine exhaust gas. It has to be far enough down a nozzle where it's both higher pressure than the main combustion exhaust and it also needs to be at a
point where the amount of heat it needs to protect the
nozzle from can successfully be insulated by the film cooling
of the turbine exhaust gas as regenerative cooling is
often terminated at that point. Some other engines that did film cooling with their turbine exhaust are the engines on the Saturn V. Both of them, actually,
the F-1 and the J-2, utilized turbine exhaust film cooling to keep the lower portions
of their nozzles cool. In the case of the J-2, it still utilized re-gen
cooling below the manifold, but the F-1 stops doing
re-gen cooling at the manifold because the film cooling was enough to keep the rest of the
nozzle from melting. There's a really fun and distinct thing to notice when the F-1s were running. Notice the bright orange flame
front doesn't actually start at the end of the nozzle. There's this weird dark bit dancing round between the flame and the nozzle exit. That's the turbine
exhaust film cooling gas. Because it's so fuel rich, it takes a moment for it to find oxygen to burn with and ignite. This of course doesn't happen
until outside the engine when it eventually will react
with oxygen in our atmosphere. But what's confusing is we
can see a little bit of this in a closed cycle rocket engine that doesn't have a gas generator and therefore doesn't dump the
exhaust gas into the nozzle. I mean, check out the
RD-180 on the Atlas V. Notice there's little dark
spikes leaving the nozzle, it's kind of similar to the F-1. That's from the fuel rich film cooling and not gas generator exhaust soot. It's not as prominent, but
it's still quite noticeable, but you might actually
notice some dark spots in some other engines, such as the vacuum optimized Merlin engine or Rocket Lab's vacuum
optimized Rutherford engine. Rocket Lab, founder,
CEO and CTO Peter Beck was talking to me about
this in a recent interview. - You know, most engines,
will run some film cooling, so there's a small amount of film cooling. And in fact, if you look
at, we can actually, you can look at just about any rocket engine on an upper stage on a, like refracturing middle alloy nozzle, like an (mumbles) nozzle. You can see the dark spots. So you'll notice it's kind of on a nozzle, there's this kind of dark spots that run around the periphery
or those actually, you know, the locations of the injectors
up in the thrust chamber. And that's where you can, that's where the film cooling
injector kind of lies, is in line with one of those dark spots. - But notice the vacuum optimized Merlin and vacuum optimized Rutherford look a little different than the others. When they're running in space, their nozzle extensions glow bright red. What the heck is going on here? That is just the metal getting really hot and then radiating that
heat out into space. Well, this is another viable option, to keep your rocket engine from melting. (rocket engine roaring) Don't forget, there's a difference between radiative heat transfer and conductive or
convective heat transfer. Since there's no atmosphere in space, there isn't any air to pick up and conduct or convect the heat. So it's not like you can just
run a fan over your engine to keep it cool and expect
it to (mumbles) heat away. The only option here is
to radiate the heat away. Unlike convection or conduction, radiation doesn't need
matter to transfer heat, just like how the sun transfers heat through the vacuum of space via radiation. The nozzle extension in
the vacuum optimized Merlin and Rutherford are made
out of a very thin metal. Usually some alloys,
such as niobium alloy, or some other material that
can withstand high heat loads. Now, the problem with say
the niobium nozzle extension is it's very thin and relatively fragile, but even more problematic is how reactive niobium is to oxygen. This means engines with niobium can really only operate in a vacuum. And it also means their manufacturing is much more difficult. Another famous engine that
used a niobium nozzle extension was the AJ10 on the Apollo service module. That dark part is niobium. And it's a good thing that
engine worked well because otherwise it could have left
Apollo astronauts stranded orbiting the moon. Okay, I think that pretty much does it. I'm sure there's some other fun tricks, propulsion engineers employ, but that's really the main ones. And it's really fun to
see how engineers choose which systems work best for their engines. But I think to review this topic best, let's just look at the vacuum
optimized Merlin one more time because it employs almost
every type of cooling. At the gas generator, we
see both heat sink tactics and a very fuel rich exhaust employed. This makes sense because
the other forms of cooling can't be utilized on spinning turbines. You just have to use high
temperature metals and then lower the exhaust temperatures to
make sure it doesn't get hotter than those metals can handle. SpaceX uses regenerative cooling for the chamber walls,
throat, and the first bit of the nozzle. Then inside the engine, they employ some film
cooling with additional fuel in the chamber and near the throat. They also use film cooling
from the gas generator exhaust at the nozzle extension when the regenerative
cooling channels end. But in addition to the film cooling, the nozzle extension also
radiates additional heat away by using a thin, but high
temperature niobium alloy. The only form of cooling
the vacuum optimized Merlin doesn't appear to use is ablative cooling. But considering the upper
stages are only used once, this is one engine that
could probably get away with a little ablative cooling
in a spot if they needed to. I love all these little
tricks that engineers have figured out to keep
rocket engines from melting. It blows my mind that re-gen
cooling works for some reason. And also that you can use
exhaust gas from the turbine to film cool your nozzle
too, for some reason, both of those things
just seem kind of crazy and it's amazing that they work so well. Let me know if you have any
other questions or maybe let me know if you learned
something cool, literally, or if you have ideas for upcoming videos in the comments below. I owe a huge thank you to my
Patreon supporters for helping make videos like this possible. If you want to help me
continue to do what I do, head on over to
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patreon.com/everydayastronaut. And while you're online, be sure and head over to our web store for shirts like this, the
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or our schematics collection of our future martial shirt, lots of really cool stuff over at everydayastronaut.com/shop. Thanks everybody. That's gonna do it for me. I'm Tim Dodd, the everyday astronaut, bringing space down to
earth for everyday people. (upbeat music)
