Practice English Speaking&Listening with: Is SpaceX's Raptor engine the king of rocket engines?

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- 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,

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.


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.


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 ...


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)



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.


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.


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.


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.


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?


Is it the most powerful engine?


Is it the cheapest engine?

Probably not.

Is it the most reusable engine?


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.

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I'm Tim Dodd, the Everyday Astronaut

bringing space down to earth for everyday people.

(exit music)

The Description of Is SpaceX's Raptor engine the king of rocket engines?