- Hi it's me, Tim Dodd,
the Everyday Astronaut. I'm here at SpaceX's
brand new launch facility in Boca Chica, Texas to check out the holy
grail of rocket engines and that SpaceX's upcoming Raptor engine. An engine like this has
never actually been used on a rocket before. Now this is a methane powered full flow staged combustion cycle engine. Talking about a rocket
engine that's this complex can be really intimidating. And in order to put it into
context against other engines and other engine cycles we're gonna do a full
comparison of the Raptor engine versus a bunch of other engines including SpaceX's current workhorse the Merlin engine against the RS-25, the space shuttle main engine. the F-1 engine that powered the Saturn 5. The RD-180 and Blue Origin's BE-4
that also runs on methane. And as if the full flow
of staged combustion cycle wasn't enough, SpaceX is also
doing something else unique. They're powering that
thing with liquid methane and that's something that's
actually never been done on an orbital class rocket. So we're gonna take a look at
the characteristics of methane and see if we can figure out why SpaceX chose methane instead of any other common propellant. Now this engine isn't
really the best at anything. It's not the most powerful. It's not the highest thrust
to weight ratio of any engine. It's not even the most efficient but it does a lot of
things really really well. So by the end of this video hopefully we have all
the context understand why the Raptor engine is special how it compares to other rockets why it's using liquid methane and then hopefully we'll
know if it really is the king of rocket engines. Let's get started. (electronic music) - [Voice over] Three, two,
one, blastoff. (mumbles) - In case you didn't notice
when you clicked on this video, this is a very, very long video. Sorry, not sorry, but if you're anything like me you keep hearing a lot of
hype about the Raptor engine and you want to appreciate it but you don't even know where to start. Well, I've spent quite a
while really studying up on the subject so I can
lay down a good foundation in order to help us really
truly fully appreciate the Raptor engine. Well, and quite frankly
all rocket engines. And if you're anything like me maybe you've stared at
diagrams like this or like this or like this one for hours until you feel like your
head's going to explode. So in order to avoid that I've actually whipped up
some really simple versions of rocket engine cycles
for all of us to enjoy which will hopefully help us
grasp these crazy concepts. But in case this isn't your first rodeo here's the timestamps if you want to jump to a certain section. There's also links in the
description to each section as well as an article
version of this entire video at my website, Everydayastronaut.com in case you want to
study some of the numbers a little more in depth or see sources of some of the material. Now we're gonna start off with
a super quick physics lesson but bear with me. We're gonna dive in and get
plenty of nitty gritty details. Okay. Let's start off with this. Rockets are basically just propellant with some skin around
it to keep it in place and they have a thing on the back that can throw said
propellant really, really fast and to way oversimplify it even more, the faster you can throw
that propellant the better. Now the easiest way to do this is by storing all the
propellant in your tanks under really high
pressure then put a valve on one end of the tank and a propelling nozzle that
accelerates the propellant into workable thrust. Done. No crazy pumps or complicated systems just open a valve and let her rip. This is called a pressure
fed rocket engine and there's a few main types: cold gas, monoprop and
bipropellant pressure fed engines. You'll often find these used
in reaction control systems because they're simple,
reliable, and they react quickly. But pressure fed engines
have one big limiting factor. Pressure always flows from high to low so the engine can never be higher pressure than the propellant tanks. In order to store propellant
under high pressure, your tanks will need to be strong and therefore thicker, and thicker, and heavier, and heavier. Look at composite overwrapped
pressure vessels or COPDs. They're capable of storing
gases at almost 10000 PSI or 700 bar. And despite this there's still a limited amount of propellant and pressure they can store. And this does not scale up very well when you're trying to
deliver a payload to orbit. So smart rocket scientists
quickly realized in order to make the rocket
as lightweight as possible there's really only one
thing they could do: increase the enthalpy. That would be a great metal band name. You're welcome Internet. Enthalpy is basically the relationship between volume pressure and temperature. A higher pressure and temperature inside the combustion chamber
equals higher efficiency and more mass shoved
through the rocket engine equals more thrust. So in order to shove more
propellant into the engine you could either increase
the pressure in the tanks or just shoot the propellant
into the combustion chamber with a really high powered pump. The second option sounds
like a pretty good idea. But pumps moving hundreds
of liters of fuel per second require a lot, and boy do I mean, a lot of energy to power them. So what if you took a tiny rocket engine, and aimed it right a turbine to spin it up really, really fast? You can exchange some of
the rocket propellant's chemical energy for kinetic energy which could then be used to
spin these powerful pumps. Welcome to turbo pumps and the staged combustion cycle. But you've still got some
limiting factors here like how high pressure always
wants to go to low pressure and how heat has that
habit of melting stuff. So you've got to keep
all these things in check while trying to squeeze every bit of power out of your engine. There's actually a lot
of different variations of the cycles that we could talk about but I'm going to stick
with the three most common or at least the three that matter the most when putting the Raptor into context. We have the gas generator cycle the partial flow staged combustion cycle and lastly we'll look at the full flow staged combustion cycle and perhaps in a future video I'll try and do a full rundown of all liquid fueled rocket engines including fun new alternatives like the electric pump fed engine seen on Rocket Lab's Electron rocket. (slow music) Let's start with the gas generator cycle known as the open cycle. This is probably one of
the most common types of liquid fueled rocket engine
used on orbital rockets. It's definitely more complicated than a pressure fed system
but it's fairly simple, well at least compared to their
closed cycle counterparts. Now I'm gonna way, way oversimplify this so it's as easy to grasp
as humanly possible. In real life, there's
literally dozens of valves, a hive of wires, and extra tiny little pipes everywhere, helium to back pressure the tanks fuel flowing through the nozzle and the combustion chamber to cool it and there is an ignition
source for the preburner and the combustion chamber. But again for the purpose
of making this as simple and as digestible as possible, just know there's a lot of stuff missing from these diagrams. But for now we're going to focus on the flow of these engines so we can grasp that concept first. The gas generator cycle works by pumping the fuel and oxidizer into
the combustion chamber using a turbo pump. The turbo pump has a few main parts a mini rocket engine called the preburner, a turbine connected to a shaft and then a pump or two
that push propellant into the combustion chamber. Now you might hear the turbo pump assembly called the power pack because it really is what powers the engine. In the open cycle system, the spent propellant from the preburner is simply dumped overboard and does not contribute
any significant thrust. This makes it less
efficient since the fuel and oxidizer used to spin the
pumps is basically wasted. Now the funny thing about a turbo pump is that it kind of has a chicken and egg syndrome situation that makes it pretty difficult to start up since the preburner that
powers the turbo pump needs high pressure fuel
and oxidizer to operate. So the preburner requires
the turbo pumps to spin before it can get up to full
operational pressure itself but the turbo pumps need
the preburner to fire in order to spin the turbo pumps. But the preburner needs
the turbo pumps to ... Yeah. You can see where this is going. This makes starting a gas
generator pretty tricky. There's a few ways to do this but we don't need to get
into all that in this video. That sounds like a fun topic
for future videos though. So back to the turbo pumps. Remember pressure always
flows from high to low so the turbo pumps need
to be a higher pressure than the chamber pressure. And this means the inlets
leading to the preburner is actually the highest pressure point in the entire rocket engine. Everything else downstream
is lower pressure but notice something here. Take a look at SpaceX's Merlin engine which runs on RP-1 or rocket propellant 1 and liquid oxygen. Notice how black the smoke is coming out of the preburner exhaust. Why would it be so sooty compared to the main combustion chamber which leaves almost no visible exhaust? Well that's because rocket propellant can get super hot like thousands and thousands of degrees Celsius. So to make sure the
temperature isn't so hot that it melts the turbine and the entire turbo pump assembly, they need to make sure it's cool enough to continually operate. Running at the perfect fuel and oxidizer ratio is the most efficient and releases the most energy but it also produces a
crazy amount of heat. So in order to keep the temperatures low you can run the preburner at
a less than optimal ratio. So either too much fuel known as fuel rich or too much oxidizer or oxygen rich. Running an RP-1 engine fuel rich means you'll see some unburned fuel appearing as dark clouds of soot. the highly pressurized
unburned carbon molecules bond and form polymers which is
a process known as coking. This soot starts to stick
to everything it touches and can block injectors or even do damage to the turbine itself. So what if you didn't want to waste all that highly pressurized propellant? I mean after all since it's running cooler by being fuel rich doesn't that mean there's a bunch of unburned
fuel literally being wasted? What if you could just
pipe that hot exhaust gas and put it into the combustion chamber? Welcome to the closed cycle. The closed cycle or
staged combustion cycle increases engine efficiency by using what would normally be lost exhausts and connects it to the combustion chamber to help increase pressure and also increase efficiency. So let's take the Merlin engine and try closing the loop. Let's take the exhaust and just pipe it straight
into the combustion chamber. Uh-oh, oh no! We just put a bunch of soot
and clogged all the injectors. You do not go to space today my friend. But there's a few
solutions to this problem so let's see how the Soviets solved it. The first operational closed
cycle engine they made was the NK-15 design for
their N-1 moon rocket. They later upgraded it to the NK-33 and then many versions
from there stemmed out including the RD-180 which is what is used
on the Atlas 5 today. Since the NK-15 and NK-33
runs on RP-1 like the Merlin you can't run your preburners fuel rich because of the coking problem. So if you want to create a
closed cycle engine with RP-1, the answer is running the
preburner oxygen rich. Easy as that, right? Well now you're blasting superheated highly pressurized gaseous oxygen which will turn just
about anything into soup right at your precision machine crazy low tolerance turbine blade. Doing so is actually considered impossible by the United States, and they basically gave up on trying. They didn't think a metal alloy existed that could withstand these
crazy crazy conditions, and they didn't believe the Soviets had made such an efficient and powerful RP-1-powered engine until after the collapse
of the Soviet Union and the US engineers got to see them and test them out firsthand. But the Soviets had indeed
worked their butts off, and they had made a special
alloy that can magically with science withstand
the crazy conditions of an oxygen rich preburner. With a closed cycle engine, you don't just use some
fuel and some oxidizer and burn that in the
preburner to spin the turbine. You actually shoot all
of the rich propellant through the turbine. So with an oxygen rich cycle all of the oxygen actually
goes through the preburner and just the right amount of
fuel goes to the preburner. You only need enough to give the turbine the right amount of energy
to spin the pumps fast enough to get the right pressures
for the preburner and the combustion chamber to make the right amount of power to shoot the thing into space. Just crazy. So back to this oxygen rich preburner. That now hot gaseous oxygen is forced into the combustion chamber
where it meets liquid fuel. They meet and go boom and we get a nice clean and efficient burn without really wasting any propellant. But still like all engines
the chamber pressure can not be higher than the pump pressure so the pumps actually have a lot of weight on their tiny little metal shoulders. Now if you're sitting there thinking that the United States just sat back and let the Soviets have
all the closed cycle glory, you'd be wrong. It took the United States
a little bit longer but they eventually figured
out a closed cycle engine. But it was very different
from the oxygen rich cycle. The United States pursued
a closed loop cycle but they went with a fuel rich preburner. But wait, we just learned
that fuel rich preburners' exhaust is so sooty that it pretty much ruins anything, right? Well sure if you're using RP-1 or any other carbon heavy fuel that's definitely going to be the outcome. So the United States went with
a different fuel: hydrogen. Okay, so now we've avoided the problem of blasting crazy hot high pressure oxygen at anything dear and precious but now we've opened
up a new can of worms. Hydrogen is significantly
less dense than RP-1 or liquid oxygen. It's so much less dense, it takes a huge and
really complex turbo pump to flow the right amount of hydrogen into the combustion chamber. Since RP-1 and LOX are
relatively similar in density and in the ratios they can
be run on a single shaft using a single preburner. Because of this the
engineers at Rocketdyne pursued an engine known as the RS-25 which would go on to
power the space shuttle. They realized that because
of the large difference between the pumps they might as well have two different preburners, one for the hydrogen pump and one for the oxygen pump. So that's what they did. But having two separate shafts
created another new problem. Now engineers were putting high pressure hot gaseous hydrogen on the same shaft right next door to the liquid oxygen pump. If some of that hydrogen would
leak out of the preburner it would start a fire in the LOX pump which is catastrophically bad. Hydrogen is also very hard to contain because it's so not dense,
un-dense, lightweight it likes to sneak through cracks and get out anywhere it can. So engineers had to make an elaborate seal to keep the hot hydrogen
from sneaking out. The seal required for this
is called a purge seal and it's actually pressurized by helium so that it's the highest
point of pressure. So if the seal leaks it
just leaks inert helium. It's genius but take a look at how different the LOX turbo pump and the hydrogen turbo pump seals look. You can tell how much
more engineering time and effort had to go
into the hydrogen seals. I mean the people that think
of this stuff are nuts. The RS-25 is still considered to be about the best engine ever made with a fairly high thrust to weight ratio and unmatched efficiency. Okay now that we've talked all about the dual preburner fuel rich RS-25, here's a simplified diagram of that. Now I didn't bother making
the fuel pumps different sizes and I just want to focus on the flow here and help make that as simple as possible. But do note both preburners
of the RS-25 run fuel rich so although they might look the same they power different pumps and I'll just let this
run here for a few seconds so you can study it for a bit but don't worry we'll also
put all these up on screen at the same time once we cover them all. So the closed cycle improves the overall performance of the engine and is highly advantageous. So how can it get any better than this? We're finally ready to talk about the full flow staged combustion cycle which basically just combines
the two cycle methods we just talked about. With the full flow
staged combustion cycle, you take two preburners
one that runs fuel rich and one that runs oxygen rich. The fuel rich preburner
powers the fuel pump and the oxygen rich preburner
powers the LOX pump. This means the full flow
staged combustion cycle needs to tackle the oxygen rich problems which again is solved by developing very strong metal alloys. So SpaceX developed their
own super alloys in house that they named SX500. According to Elon Musk it's
capable of over 800 bar of hot oxygen rich gas. That may have been one
of the biggest hurdles in developing the Raptor engine. Luckily the fuel rich side only pumps fuel so if some of that hot
fuel leaks through the seal on the shaft it just comes
in contact with more fuel which is kind of no big deal. So no need for one of those
really really elaborate seals. Full flow likely wouldn't work with RP-1 due to the coking problems
with a fuel rich preburner but other fuels are still
valid to use this design. But more on that in a minute. The advantage of the system
is that since both the fuel and the oxidizer arrive
in the combustion chamber as a hot gas there's better combustion and hotter temperatures can be achieved. There's also less of a need
for that crazy ceiling system as we mentioned earlier and that's definitely a good thing when you plan to reuse your engine over and over with little
to no refurbishment between flights. And lastly because there's an
inherent increase in mass flow or how quickly all the propellant is shooting into the preburner the turbines can run cooler and at lower pressures because the ratio of
fuel an oxidizer needed to spin the turbo pumps is much lower. And think of it this way in an open cycle you only want to use as little fuel and oxidizer as possible in the preburner since it's all wasted and you want it to be
as hot as withstandable to make it more efficient but with the full flow
cycle all of the fuel and all of the oxidizer
goes through the preburners so you can burn just exactly
as much propellant as necessary to power the turbo pumps. But the cool thing is your
fuel to oxidizer ratios will be so crazy fuel rich and crazy oxygen rich that the temperatures at the
turbines will be much lower and this means longer lifespans
for the turbo pump assembly. It also means more combustion happens in the combustion chamber and less in the preburner. Now here's the crazy part. Only three engines have demonstrated the full flow staged
combustion cycle ever. In the 60s the Soviets developed an engine called the RD-270 which never flew and in the early 2000s
Aerojet and Rocketdyne worked on an integrated
powerhead demonstrator called wait for it, the Integrated
Powerhead Demonstrator, which again never made
it past the test stand. And the third attempt to developing a full flow staged combustion cycle engine is SpaceX's Raptor engine. Ta-da, that's right. The Raptor engine is
only the third attempt at making this crazy type of engine. It's the first to ever do any type of work and leave a test stand and fingers crossed, it'll be the first full flow
staged combustion cycle engine to reach orbit. Well actually just about
anything this engine does will be a first. This means SpaceX had to tackle
some crazy crazy problems. I mean not only that
same problem that plagues oxidizer rich cycles like having to have a really
really strong metal alloy. They also had to learn how to control two different preburners and two different cycles to create the highest pressures of any chamber pressure ever. They just beat the RD-180's
record of about 265 bar when they hit 270 bar and they're not even done. They're hoping for 300 bar
inside the combustion chamber. That's nuts and we'll talk
more about that in a second but before we move on now
that we've done a rundown on all these engines cycle types let's put them all up on screen and let them run for a bit
so you can watch each one and compare them side by side. I know for myself it helps a lot to see them all together
on the same screen at the same time. (slow music) Since the Raptor engine can't run a fuel rich preburner using RP-1, you'd think the next most
logical choice would be hydrogen. Well SpaceX didn't opt for
either RP-1 or hydrogen. They went with liquid methane. So now we finally have
another topic to touch on. Why did SpaceX choose liquid methane for the Raptor engine? What are the qualities
that make it advantageous over hydrogen or RP-1? (dramatic music) Today no liquid methane or otherwise known as methylox engine has gone to orbit. So what qualities does it
have that make it desirable? Let's take a look at methane compared to RP-1 and hydrogen. Let's put methane in
between RP-1 and hydrogen. You'll see why here really quickly. So let's start off with
perhaps the biggest factor when designing your first stage, the density of the propellant. Having a denser fuel means
that tanks are smaller and lighter for a given mass of fuel. A smaller tank equals a lighter rocket. So here's the density of these three fuels measured in grams per liter. In other words how much does
one liter of this stuff weigh or really what's its mass. Starting off with RP-1, one
liter is around 813 grams. RP-1 is 11 times more dense than hydrogen which is only 70 grams per liter and methylox is right in the
middle at 422 grams per liter. Remember how airships or zeppelins used to be filled with hydrogen to make them lighter than air? Well, that's because hydrogen is so much less dense in our atmosphere it makes for an excellent albeit really flammable gas for a balloon. I mean we all remember
the Hindenburg, right? It should also be noted
that 813 grams per liter is an average for RP-1 but SpaceX chills their
RP-1 in their Falcon 9 and Falcon Heavy for about a
2 to 4% increase in density. But historically RP-1's density is right around that 813 grams per liter. So in the case of density methane is kind of right in
the middle of the two others but there's more to it than just density. We also need to take into consideration the ratio of how much fuel is burned compared to how much oxidizer is burned. This is the oxidizer to fuel ratio. So here's where things get
a little more interesting and the tables turn just a little bit. Rocket engineers have to take into account the mass of the fuel and the corresponding weight of the tanks so they don't actually burn propellant at the perfect stoichiometric
combustion ratio. They find the perfect happy medium that balances tank size with thrust output and specific impulse. Let's look at the mass ratios for fuel and oxidizer that the
engineers have come up with. So for these numbers RP-1 is
burned at 2.7 grams of oxygen to one gram of RP-one. Hydrogen burns at 6 grams of
oxygen to 1 gram of hydrogen and methane burns at 3.7 grams of oxygen to one gram of methane. These numbers can now help offset a little the massive difference in density. So let's visualize this to
help make it easier to digest. Liquid oxygen is 1141 grams per liter. It's a little more dense than RP-1. So burning LOX and RP-1
at a 2.7 to one ratio for every liter of LOX you'd need a little over half a liter of RP-1. Next up let's do hydrogen. Now with hydrogen being 11
times less dense than RP-1 you'd think it'd need a
tank that's 11 times bigger. But luckily engineers have
found that it pays to burn LOX and hydrogen at a 6 to 1
ratio for a good compromise. This means for each liter of LOX you'd need 2.7 liters of hydrogen so your fuel tank needs
to be approximately five times larger compared to RP-1. So yeah that helps. That's why when we look at
a hydrogen powered Delta IV versus an RP-1 powered Falcon 9 you can see the fuel tank is
much smaller than the LOX tank on the Falcon 9 but the Delta
IV is about the opposite. The LOX tank is much
smaller than its fuel tank. So now let's take a look at methane. And this one gets kind of interesting. LOX is 2.7 times more
dense than liquid methane but the burn ratio is 3.7 grams of oxygen to one gram of methane. So you need 0.73 liters of
methane for every liter of LOX. In other words your fuel tank would need to be about
40% bigger for methylox than it would need to be for RP-1 despite RP-1 actually
being almost twice as dense and compared to hydrogen its fuel tank would be
about 3.7 times smaller. So the fuel to oxidizer ratio helps make a methane fuel
tank a lot closer to an RP-1 tank than it is to a hydrogen tank. Another huge variable
with any rocket engine is how efficient it is. This is measured in
specific impulse or ISP but you can think of it kind of like a fuel economy
of a gas powered car. So a high specific
impulse would be similar to a high mile per gallon
or kilometer per liter. The best way to think of specific impulse is to imagine you had one
kilogram of propellant for how many seconds can the engine push with 9.8 newtons of force. The longer it can sip on that fuel while still pushing that hard the higher its specific impulse and therefore the more work it can do with the same amount of fuel. So again kind of like its fuel economy. So the higher the specific impulse the less fuel it takes to
do the same amount of work which is a good thing. A fuel efficient engine
is extremely important and now due to the
molecular way of each fuel and their energy released when burned there's a different potential for how quickly the
exhaust gas can be expelled out the nozzle. This means each fuel has a different theoretical specific impulse. In ideal and perfect world
an RP-1 powered engine could achieve about 370 seconds. An ideal hydrogen powered
engine could get 532 seconds and guess what? A methane powered engine
is right in the middle with 459 seconds. Real world examples of this are much lower with RP-1 engine seeing around 350 seconds like the Merlin 1D Vacuum run 380 seconds for a methane powered engine like the Raptor vacuum might be someday and about 465 seconds for
a hydrogen powered engine like the RL-10B-2. Next, let's talk about
how hot each fuel burns. A fuel that burns cooler
is easier on the engine and potentially makes
for a longer lifespan. RP-1 can burn up to 3670 Kelvin, hydrogen 3070 Kelvin, and if you haven't guessed it by now, methane is again between
the two at 3550 Kelvin. Speaking of thermal considerations let's look at the boiling
point for each of these fuels or at what point does
the liquid fuel boil off and turn into a gas. Since all of these fuels need to remain in their liquid state
in order to stay dense the higher the temperature the easier it is to store the fuel. A higher boiling point also means less or even no insulation on the tanks to keep the propellant from boiling off. And of course less insulation
means lighter tanks. RB1 has a very high boiling point even higher than water at 490 Kelvin. Hydrogen on the other
hand is near absolute zero at a crazy cold 20 Kelvin. That's insanely cold and it
takes serious consideration to keep anything at that temperature and like the Goldilocks it is methane is between the two at 111 Kelvin which although that's still very cold and requires thermal considerations it at least boils off at a
temperature similar to LOX so there is that and because it's so close
to the temperature of LOX the tanks can share a common dome which makes the vehicle lighter. LOX and hydrogen's
temperatures very so wildly that LOX will boil off hydrogen and the hydrogen will freeze LOX solid. Now, on to the exhausts. What are the byproducts of
combustion with these engines? RP-1 is really the only one of these three that really pollutes
with any unburned carbons being left in our atmosphere
alongside with some water vapor but hydrogen only produces water vapor and methane produces some carbon dioxide and water vapor as well. But an interesting note
now believe it or not as far as greenhouse gases go, water in the upper
atmosphere can be pretty bad but I'll be doing a video in the future all about how much rockets pollute talking about their air pollution, also their ocean pollution and even space debris is a consideration. So stand by because I think that video is going to be awesome. Now one metric that we're just kind of going to gloss over really quick and talk about it generally is the cost. And these tend to vary considerably and it's actually really hard to pin down the exact prices reliably. So for the considerations RP-1 is basically just a
highly refined jet fuel which jet fuel is a
highly refined kerosene which kerosene is a highly refined diesel. So it's safe to assume it's going to be more expensive than diesel. Hydrogen is also relatively expensive despite being abundant. Refining it storing it and transporting it can be hard but methane on the other hand
is basically the same thing as natural gas and can
be relatively cheap. Now when you're talking about
buying literally tons of fuel the fuel costs can add up quickly so although the cost of fuel
shouldn't factor in too much it certainly is a consideration but without hard data on this one I don't even want to put it on our chart. So instead let's talk about the more important aspect of the fuel that's manufacturing it. And here's where we get into specifically why SpaceX sees methane as an important or even a necessary part
of the company's future. SpaceX's ultimate goals
are to develop a system capable of taking humans
out to Mars and back over and over. The Martian atmosphere is CO2 rich. Now combine that with water
mining from the surface and subsurface water on Mars through electrolysis
and the Sabatier process the Martian atmosphere can
be made into methane fuel so you don't have to take
all the fuel you need to get home with you. You can make it right there
using Mars's resources. This is called in situ
resource utilization or ISRU. Now you might be thinking, "Well, if there is water
can't you just make hydrogen on the surface of Mars for your fuel?" Well, yes but one of the
biggest problems with hydrogen and long duration missions is
the boiling point of hydrogen. Remember, it takes serious considerations to maintain hydrogen in a liquid state and that's necessary
to be useful as a fuel so for SpaceX methane
makes a lot of sense. It's fairly dense meaning the rocket sizes are pretty reasonable. It's fairly efficient, it burns clean and it makes for a highly reusable engine. It burns relatively cool helping expand the lifespan of an engine which again is good for usability. It's cheap and easy to produce and can be easily reproduced
on the surface of Mars. (slow music) Okay. Yeah. We finally made it this far. and now that we have a strong grasp of how different engine cycles operate and the fuels they use we can finally line
them all up side by side and compare their metrics to help us appreciate
where each engine sits. So now we're going to lineup each engine by their fuel type and their cycles. So let's start off with SpaceX's open cycle Merlin engine
that powers their Falcon 9 and Falcon Heavy rockets. NPO Energomash's oxygen rich closed cycle RD-180 that we see power the Atlas 5 rocket and Rocketdyne's open cycle F-1 that powers a Saturn 5 which all three of these
engines run on RP-1. Then we have SpaceX's full flow staged combustion cycle Raptor engine that will power the Starship
and Super Heavy booster and then we have Blue
Origin's closed cycle oxygen rich methane powered BE-4 engine that will power their New Glenn rocket and ULA's upcoming Vulcan rocket and then we have Aerojet
Rocketdyne's closed cycle fuel rich RS-25 engine that
powered the space shuttle and will power the upcoming SLS rocket which runs on hydrogen. A few quick notes here. The Raptor and the BE-4 as
of the making of this video are still in development
so the numbers we have here are either their current state of progress like the Raptor which
is constantly improving literally every day and in the case of the BE-4, those are the target goals for the engine which Blue Origin has yet to hit. So just keep that in
mind that these numbers are definitely subject to change and now because of this
don't forget to check in with the article version attached in the description of this video. This video will likely date itself with some of these numbers and I can't update this video but I can update the website
when more info comes through. So if you're looking to use any
of these numbers as a source please, please, please
double check the website for any updates. Another fun note quick
is look at the RD-180. Now don't be confused. This is a single engine it just has two combustion chambers. There's only a single turbo pump that splits its power into
two combustion chambers. The Soviet Union was able to solve the crazy hot oxygen
rich closed cycle problem but they were unable to
solve combustion instability of large engines. So instead of one large combustion chamber they made multiple small ones. So first up let's take a look at their total
thrust output at sea level. Since all these engines run at sea level that's probably a fair
place to compare them. Let's go from the least amount of thrust to the most for fun. The Merlin produces 0.84
meganewtons of thrust. The RS-25 produces 1.86 meganewtons. The Raptor currently is at 2 meganewtons. The BE-4 is hoping to hit 2.4 meganewtons. The RD-180 3.83 meganewtons and the F-1 is still the king out of these at 6.77 meganewtons. Now there was an engine called the RD-170 which actually produced
more thrust than the F-1 but since it barely flew I figured it wasn't as
relevant in this lineup. I thought it'd probably a
good idea to go with engines that have actually been used a lot. Thrust is great but what's
maybe just as important when designing rocket is
the thrust to weight ratio or how heavy the engine is compared to how much thrust it produces. A higher thrust to weight ratio engine ultimately means less dead weight the rocket needs to lug around. Let's start from the
lowest to highest here. The lowest is actually the
space shuttles RS-25 at 73 to 1. Then there is the RD-180 which is 78 to 1. Then we have the BE-4
at around 80 to 1 but keep in mind we don't actually have a really good number on this. So there might be some wiggle room there. Then the F-1 is 94 to 1, then we have the Raptor which
is at about 107 to 1 for now. And lastly the Merlin is
actually the leader here with an astonishing 198 to
1 thrust to weight ratio. Yeah, that thing is a powerhouse. Okay. Thrust is great and all but who cares how powerful an engine is if it's terribly inefficient. So next up let's check
out their specific impulse which again is measured in seconds. So starting with the
least efficient engine which is the F-1 engine
at 263 to 304 seconds then the Merlin engine
at 282 to 311 seconds. Then we get the RD-180 at
311 seconds to 338 seconds and somewhere in that
same ballpark is the BE-4 which is around 310 to 340 seconds. Next up is the Raptor engine which is 330 seconds
to around 350 seconds, and lastly the king
here by far is the RS-25 which is 366 to 452 seconds. Wow. Now one of the factors
that affect both the thrust and specific impulse is chamber pressure. Now generally the higher the chamber
pressure the more thrust and potentially more
efficient the engine can be so higher chamber pressures let an engine be smaller
for a given thrust level also improving their
thrust to weight ratio. The baby here is actually the F-1 which only had 70 bar in
this chamber pressure. Now, I do need to pause here for a second and remind you that 70
bar is still 70 times the atmospheric pressure or
the same amount of pressure you'd experience at 700 meters underwater. Yikes. Okay so even the lowest chamber pressure is still mind-bogglingly high. So next up is the Merlin engine at 97 bar then the BE-4
will be around 135-ish bar then the RS-25 which is 206 bar then the RD-180 which has been considered the king of operational
engines at about 257 bar that is until the Raptor engine which is now kind of online which is considered the new
king of chamber pressure at 270 bars currently and
they hope to get that thing up to 300 bar. Again, 300 bar is like being three kilometers deep in the ocean. I can't even fathom. Okay, that's enough of the
specs of these engines. Now, let's look at their
operational considerations starting with their approximate cost. Now again this can be
kind of hard to nail down, so these are the best estimates that I could come up with. These numbers do factor in inflation to make them all in
today's dollars though. Let's go with the most expensive, and work our way down to
the least expensive engine. The most expensive engine
in the lineup is the RS-25 which has a sticker price of
over $50 million per engine. Yikes. Then we have the F-1 which was
about $30 million per engine then the RD-180 which is
$25 million per engine then the BE-4 which is
around $8 million per engine. and for the Raptor Elan has mentioned he thinks he can produce
the Raptor for cheaper than or close to the Merlin engine if they can remove a lot of the complexity that the current engine has. So for now we're gonna say $2 million is a pretty decent ballpark. Then we have the Merlin engine which is less than a million I think. Okay, well cost is one thing but another strong consideration
for the cost of the engine is whether or not it's reusable. And here only the RD-180 and
the F-1 were not reusable or at least never reused which is different than
all these other engines which will all be reused multiple times. The RS-25 was reused over and over with the record being 19
flights out of a single engine. Well then again that's after
a few months of refurbishment. The Merlin is hoping
to see up to 10 flights without major refurbishment. We know a design goal for
the BE-4 is to be reused up to 25 times. And I think the Raptor engine hopes to see up to 50 flights but again
aspirations are one thing. We'll see how history tweets its claims. But one quick fun little story here is don't forget the Merlin engine which SpaceX currently uses on the Falcon 9 and Falcon Heavy rockets are already fired a bunch of times before they even make it to the pad. Each engine that is built goes
from Hawthorne, California to their test stand at McGregor, Texas, where it does a full duration burn then those engines go back to California where they're integrated onto the Octaweb which is at the base of the vehicle. Then they take the entire stage and they take it back out to McGregor for a full duration static fire. So it goes through the whole
mission basically again. Then they ship it to the launch pad where does a short static fire and then it flies the mission. So it's already done like three missions in duration of firing by the time it flies for the first time so I'm not entirely
sure what the most times a single engine has done
a full duration burn. We know that some of the
cores were set out on the pad and fired for a really
really really long time multiple times over and over so I think they've probably
done almost 10 flight full duration burns
out of a single engine. But you know I have no doubt they can probably do that if they say. I mean they have more experience in this than anybody already reusing engines without
really refurbishing them. So I'm gonna definitely
take their word for it. On the topic of price there's actually some things here that start to get really interesting when we start looking at these numbers. The first is an interesting metric that Elan talked about once
in a tweet in February of 2019 saying they hope to make
the Raptor get better at their thrust to dollar ratio. Now this is a really interesting concept when you think about it. Who cares how much an engine costs if one big engine is cheaper
than two smaller ones for the same thrust or vice versa. So let's actually take a look at the dollar to kilonewton
ratio of these engines. Starting with the most expensive dollar to kilonewton
engine which is the RS-25 at a crazy $26,881 to kilonewton of thrust then the RD-180 which is
$6527 to one kilonewton followed by the F-1 at
$4431 per kilonewton and then we get to the BE-4 which is $3333 to one kilonewton, the Merlin engine at $1170 per kilonewton and the Raptor at around
$1000 per kilonewton but now we can go even
another step further since we know their
dollar to kilonewton ratio but we also know their
reusability potential. Now we can predict their potential costs
per kilonewton per flight which changes based on how reusable these engines actually are. For start the RD-180 and the F-1 aren't reusable. Their price stays the same but for the rest of the engines, if we take into account how many flights they have/will have now we start to see the
RS-25 reusability pay off and kind of close the gap
bringing its potential cost down to just $1414 per
kilonewton per flight. But here's where things get crazy. Blue Origin's BE-4 has potential
to truly be game changing at around $133 per
kilonewton over 25 flights which could make it
about as cheap to operate as the Merlin at $117 per
kilonewton per flight. But if the Raptor engine
truly lives up to its hype it could bring this number all the way down to $20 dollars
per kilonewton per flight. Now that is absolutely game changing. Sure, money and reuseability
is a 21st focus for spaceflight but whatever happened to
good old proven reliability? For this let's first look at
how many operational flights each engine has had. Now at the moment of shooting
this video the Raptor and BE-4 haven't seen
any operational flights although the Raptor is starting
to leave the test stand and is being used on test
vehicles like the Starhopper. But for now, neither engine
has a real flight record. So let's look at the other engines. First we have the F-1 engine
which was used on 17 flights. Next up is the Merlin engine
which is at 71 flights and catching up quickly to the RD-180 which is at 79 flights but the king out of these was the RS-25 which saw 135 flights. Now lastly, how about
reliability and service? Between the number of
flights and this number we can get a pretty good sense of how truly reliable an engine is. This number is really
hard to just pin down since some of the engines
may have shut down early but the mission was still a
success on a few of these. So take a few of these
with a grain of salt. Again the BE-4 and Raptor
engine haven't flown yet. So those numbers are unavailable. Then we have the space shuttle main engine which is over 99.5% reliable but that gets hard to define when an engine doesn't fully shut down. And then we have the
Merlin at 99.9% reliable. It sure helps when you have 10 engines on each flight of the vehicle and with only one engine ever failing early on in its career, and despite that that
mission was still a success. The Merlin is a very reliable engine. Now to end this technically the RD-180 and the F-1 are 100% reliable but with the F-1 never
having shut down at all in any flight, it gets the bold here. And depending on how you
define success and reliability technically the RD-180 is
only kind of 100% reliable because it got really lucky ones. One time it shut down six seconds early on an Atlas 5 mission in 2016. This was due to a faulty valve but the mission went on to be a success because of some pure luck
with the center upper stage having enough spare Delta V to carry out the mission. Had that valve failed
even a second earlier that mission would have failed. (dramatic music) Seeing all these numbers
and considerations, it makes you realize
just how many variables go into designing a rocket. Change any one little thing and it can have this massive ripple effect on the entire design and the implementation of
the vehicle as a whole. So let's go back over all of this. Now that we know all
the cycles, the fuels, the aspirations of SpaceX
to see if we can figure out why the Raptor engine exists and figure out if it's
worth all the effort. Let's look at SpaceX's ultimate plan. Make a rapidly and fully reusable vehicle capable of sending humans to the Moon and Mars as inexpensively and routinely as possible. Not exactly your everyday
goal for a rocket, huh? In order to be rapidly and fully reusable the
engine needs to run clean and require low maintenance
with simple turbo pump seals and low preburner temperatures. A methane fueled full flow
staged combustion cycle engine sounds like a good fit. For reliability, redundancy, and scale of manufacturing considerations it makes sense to employ a lot of engines. In order to scale an engine down but maintain a high output chamber pressure needs to be high. Sounds like a methane fueled full flow staged combustion cycle engine is a good fit. For interplanetary trips methane makes the most sense because its boiling point makes it usable on long duration trips to Mars which guess what? You can produce methane on Mars. So for interplanetary trips
a methane fueled full flow staged combustion cycle
engine sounds like a good fit. Methane is fairly dense meaning the tank size remains reasonable. Which again is good for
interplanetary trips not needing to lug around
a lot of dead weight making a methane fueled full
flow staged combustion cycle a pretty good fit. Okay so let's bring this
all back around now. Is the Raptor engine really
the king of rocket engines? Well rocket science like all things is a complex series of compromises. Is it the most efficient engine? No. Is it the most powerful engine? No. Is it the cheapest engine? Probably not. Is it the most reusable engine? Maybe. But does it do everything really well? Yeah it is truly a Goldilocks engine doing everything it needs
to do very very well. It is the perfect fit for
your interplanetary spaceship and despite its complexity SpaceX is developing this
engine at a rapid pace. I mean knowing how much
tweaking SpaceX did to their Merlin engine over a decade, we're just at the infancy
of the Raptor engine. It's only gonna get
better from here on out, which is crazy. So all in all the Raptor engine is the king of this application. It's a fantastic engine
to fulfill SpaceX's goals for their starship vehicle. Would it be the king
of other applications? Maybe, maybe not. And only that decision for the rocket scientists and
engineers who get to make all those crazy decisions
every single day. So what do you think? Is it worth all this hassle to develop such a crazy
and complex engine? Is this just the beginning
for the Raptor engine? And most importantly, is the Raptor engine really
the king of rocket engines? Let me know your thoughts
in the comments below. Okay I know I say this every video, but I honestly could
not have done this video without the help of my Patreon supporters. They not only kept me sane
for the past five months as I worked on this video but they also went over
all the data with me. They give me great feedback and suggestions in the
edits of this video. I you want to help support what I do or provide feedback in videos or help scripting research or if you just want to hang out and talk space consider joining our
exclusive Discord channel and our exclusive subreddit by becoming a Patreon member by going to Patreon.com/everydayastronaut. Thank you guys. Seriously, I couldn't have made this video without you. And while you're online, be sure and check out my web store. Seriously, I have really cool things like these F-1 T-shirts, tons of other shirts. There's lots of new merchandise popping up in there all the time so check back often. We have things like grid fin not-a-coasters and hats and shirts and mugs and prints, just literally tons of cool rocket stuff. So if that's your type of thing, be sure and check out my web store everydayastronaut.com/shop and then click on the music tab if you want to check out any of the songs used in this video. It's all music that I've
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Tweet from Elon about this video:
Tim outdid himself here, great video.
A fantastic starting point for anyone interested in rocket engines, the types of rocket engines, how they operate, and how they compare.
I think the only thing missing, if you can call it missing in such an information rich video, is how rocket designers decided how many engines to use on any given rocket stage. Which is dependent mostly on thrust to weight ratio of the whole propulsion stack (engine(s), pipes, and tanks) and the target delta v for that class of rocket. You see in the video Tim talks about tank size but didn't get into how that interacts with T/W ratio and Isp for the overall rocket thrust to weight of the rocket and total delta v.
That said, this was already a jam packed 50 min video so I completely understand why this complex idea was not included. Perhaps a sequel?
Regardless a fantastic video, well done Tim.
There were some questions in the comments, and I tried to answer them to the best of my limited knowledge. Since it all gets buried in Youtube comments, here is re-post, with added references:
Q: How do they cool the hydrogen down to such crazy low temperatures?
A: There are many ways in which low temperatures are obtained, and liquification of hydrogen uses a combination of several techniques. How exactly it is done, depends on the scale of the machine. Usually, hydrogen is compressed, pre-cooled, and then allowed to expand. When the gas expands, it cools, and some of it liquefies. Pre-coolong may use liquid nitrogen and/or refrigerators which also use expanding gas -- either helium or hydrogen itself. The details of how the real equipment works are quite intricate, for many reasons -- efficiency etc.
(A small hydrogen liquifier: https://aip.scitation.org/doi/pdf/10.1063/1.4860864?class=pdf
https://youtu.be/iKx8ip7ETFM?t=132)
Q: How do they make the metal alloys that can stand the very high temperatures - what are they made of? What are the nozzles made out of?
A: No alloys can withstand the temperature inside of the combustion chamber. The trick is to design the engine in such a way that the walls would not have to. Then, the walls can be made from metals that are actually not very heat resistant. Bell Aerosystems XLR81 (Model 8096) engine, which was used in hundreds of flights on Agena stage, launching American reconnaissance satellites, had the combustion chamber made out of *aluminum*! Russian RD-107 RD-108 used on Soyuz family of rockets and SpaceX Merlins use combustion chamber and nozzle with the inner liner made out of copper alloy. The important property of the material here is not heat resistance, but good heat conductivity. The cooling is so good, that the wall never gets too hot! There is a layer of relatively cool gas inside of the combustion chamber, immediately next to the wall -- which is specially generated by injection of more fuel next to the wall, and then there is the tremendous flow fuel inside the walls themselves. The liner is very, very thin -- a millimeter or less. The pressure in the cooling channels is higher than in the combustion chamber, so the liner is actually trying to bulge inwards, and it does not only because it is hard-brazed to the outer shell. See Wikipedia RD-107 article -- click of the beautiful photograph ot the engine and you will see how thin the walls are.
(XLR81:
https://qph.fs.quoracdn.net/main-qimg-18301d2c10f42f01efdbe490f8d5f546
https://www.quora.com/Is-it-common-for-a-rocket-engine-s-fuel-flow-rate-through-the-cooling-jacket-to-be-greater-than-through-the-injector-If-so-what-happens-to-the-extra-fuel-that-was-used-for-cooling-but-not-burned
RD-107:
https://upload.wikimedia.org/wikipedia/commons/f/ff/RD-107_Vostok.jpg
http://www.lpre.de/energomash/RD-107/index.htm
)
Q: How do they make the channels that the fuel flows through?
A: The channels can be made in several ways. Be best way, but also the slowest, is to cut the channels on the outer surface of the engine liner, and then braze the liner and the outer strong shell together. Assembling the engine is an amazing exercise -- a very tricky series of operations which have to be done very accurately without any defects. This is why the engines cost so much! if you look at the welds on the combustion chamber of the above RD-107 photograph, you might be able to guess how it is put together. It is quite a puzzle!
(Liner being machined: http://photos1.blogger.com/blogger/1613/496/400/Merlin1C_Chamber.jpg
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060005156.pdf )
Q: You explained that chicken/egg problem... so how are the engines ignited?
A: Different engines start in different ways. It's quite complicated due to the minutia that people seldom even think about -- like filling the channels in the walls of the nozzle with fuel before the engine starts, or not letting the fuel vapors into the combustion chamber so that they do not form explosive mixture with oxygen before the engine is started, etc -- that's why there are so many steps in the process and so many valves in the real engines. If we skip all that, Merlin 1D starts as follows: The liquid oxygen pump is cooled first with liquid oxygen -- otherwise the liquid would boil on the room temperature metal, and the pump will cavitate instead of pumping smoothly. Then the turbine is spun with compressed helium. This brings the pressure at the output of the pumps up. The oxygen injector opens first, and oxygen starts spraying into the combustion chamber. We see the white fog of evaporating pulverized liquid oxygen coming out of the nozzle. The pyrophoric igniter fluid is squirted into the chamber, and starts burning -- we see the green flame at this time. As the turbine continues to spin up, the fuel injector opens, and the combustion in the main chamber starts in earnest. We see the flame out of the nozzle. The gas generator is started in a similar manner and at about the same time as the main chamber ignites. As it starts producing the gas, this gas continues to spin the turbine up. The helium is turned off, and the engine quickly comes up to full power. Everything is closely monitored, and if something is out of spec, the engine is immediately shut down. This is the procedure in a nutshell -- the real process involves various purges etc, to make sure that the stuff goes where it should and does not go where it should not. You see these coming out of smaller drains on the second stage engine in flight, if you watch closely -- before it is started, while it is working, and after it is stopped. Start looking before the second engine burn in a launch to a GTO orbit.
(See references in this post: https://www.reddit.com/r/SpaceXLounge/comments/bo90oo/a_simplified_animation_of_an_open_gas_generator/enjs3ss?utm_source=share&utm_medium=web2x)
Q: The schematics were helpful, but what's all the other piping for, on the actual engines?
A: If you are really interested how real engines are constructed and work, the only material available would be for historical engines -- Apollo era engines. Russian RD-107 is also well described. Modern engines are all propitiatory, and also covered by ITAR, which makes it impossible to find the exact details. Everything that I have said above comes from various open sources -- books, comments from Tom Mueller, Elon Musk, other SpaceX engineers giving interviews in newspapers.
Here is a brain dump based on that engine lineup.
Jump in if you got corrections, updates (particularly fuel costs) or observations.
Merlin 1d (GG), Raptor(FFSC), BE-4(ORSC), RD-180(ORSC), RS-25D(FRSC), F-1(GG)
https://en.wikipedia.org/wiki/Comparison_of_orbital_rocket_engines
Merlin, RD-180 and F1 are kerolox.
Raptor and BE-4 are methalox.
RS-25 is hydrolox.
Liquid oxygen is cheap. Some reports of USD 100/ton.
Methane is the cheapest fuel (?500US/t). Then Kerosine (RP-1)(US1500/t). Then Hydrogen (?US3000/t). Then hypergolics (dont ask)
Methalox bulk density is roughly 90% of kerolox. Hydrolox bulk density is about 40% of kerolox.
Methane (and oxygen) is cryogenic. Hydrogen is deep cryo.
Merlin, raptor and BE-4 are restartable. RD-180s might be capable, but have never been flown that way.
There is a restartable RS-25 project : AR-22 that first testfired july 2018
All the engines are US, except the russian RD-180. Which powers the Atlas V rocket.
Merlin and Raptor optimised for cost. Typically light and cheap.
BE-4 optimised for cost/reuse. Not as high performance as raptor. But a decent methane engine.
RD-180, RS-25, F-1 optimised for mass : Typically expensive and heavy.
F-1 is handmade from the 60s for the saturn V. 700 ton thrust bell. And the least efficient and largest thrust of the bunch. RD-170 (not shown) is quad bell, more efficient and slightly higher thrust.
No-one makes combustion chamber/bells this large any more. Engine clusters and cc/bell clusters have taken over.
Rs-25 is early 80s tech, RD-180 is mid 80s.
Merlin 1d tech is roughly 2013.
Raptor and BE-4 is current, but not mission flown yet.
Raptor is the only (working) full-flow staged combustion engine on the planet at present.
Compare the ~200 ton cc/bells for a moment. Raptor, BE-4 (yeah 240), half a RD-180, and RS-25.
The raptor bell is the smallest and the most efficient footprint at SL. This is the result of FFSC and very high chamber pressures. Its footprint is tiny.
The rd-180 bell is slightly larger. But much heavier.
The RS-25 bell is gigantic (like its cost) because hydrogen (is low density).
Compare the methane bells.
Raptor(@30MPa) is essentially the same thrust as BE-4, but in a smaller, lighter package, possibly cheaper, along with a handy isp advantage. FFSC allows some extreme performance.
<rant>
Hydrogen at any level is maximising cost. Mass optimisation. Old school.
Methane at any level is minimising cost. Cost optimisation. New hotness.
Hydrogen may only make sense when ISRU methane in not available : icy moons, other than titan.
Even then, methane may win the cost optimization since methalox is ~78% oxygen by mass.
Any hydrogen rocket or spacecraft you can design, can be done in methane for a fraction of the cost.
And a fraction of the hydrogen problems (increased dry mass, boil-off, deep cryo).
</rant>
edits:700t f1, raptor footprint
Fantastic video, well done!
A small comment: Tim says "gas generator cycle" but then switches to calling the gas generator a "preburner". Though they are conceptually similar, a gas generator is a gas generator -- it is not a "pre-burner" because there is no "pos-burning" of the generated gas in the open cycle engine -- it just drives the turbine and is exhausted at low pressure. Actual preburners are usually much beefier, more complex units as they have to generate enormously higher pressure.
There are also famous rockets which do not even burn anything in the gas generator. German V2, many Russian rockets, including the famous R-7 / Soyuz type of rockets. They use a separate propellant, high concentration hydrogen peroxide, to power the GG. This is not entirely dissimilar to driving the pumps with electric motors using energy stored in the batteries, as done by the Rocket Lab's Electron rocket.
It finally dropped! Thanks for the heads-up!
He's missed one of the main advantages of the Full Flow Staged Combustion cycle, which is that it makes more power available to run the pumps, allowing for a higher chamber pressure.
They could also use FFSC to get the same chamber pressure from a lower temperature in the pre-burners (like he said in the video) but SpaceX has clearly gone for the highest possible pre-burner temperature, and the highest possible main combustion chamber pressure.
He also missed one of the disadvantages caused by hydrogen's low density. Lower density fuels require more power to pump. So the turbines and pumps need to be bigger, making the engine more expensive and giving it a poor Thrust to Weight ratio.
Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:
Decronym is a community product of r/SpaceX, implemented by request
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