Original subtitles

hi it's me tim dodd the everyday astronaut

I'm here

at spacex's brand new launch facility in poker chica

Texas to check out

the holy grail of rocket engines and that

spacex's upcoming raptor engine in engine like this

has never actually been used on a rocket before now

this is a methane powered

full flow stage 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 going to 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 f1 engine that powered the saturn v

the rd 180 and blue order b e4

that also runs on methane and as if the full flow

stage combustion cycle wasn't enough

space is also doing something else unique

they're probably that thing with liquid methane

and that's something that's actually never

been done on an orbital class rocket

so

we're going to 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 away 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

and 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 time stamps

if you want to jump to a certain section

also links in the description to each section

as well as an article version of this entire

video at my website

everydayness 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 so let's start off with this

rockets are basically just

pellant 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 weigh over simplified even more

the faster you can throw that propellant

the better now

the easiest way to do this is by storing all the

repellent 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 letter rip

this is called a pressure fed rocket engine

and there's a few main types cold gas

mono and by propellant pressure fed engines

you'll often find these used in reaction control

systems because there's 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 overlapped pressure vessels or cops

they're capable of storing gases at almost 10

000 people gsi 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 mash

shove 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

hmm the second option sounds like a pretty good idea

but pumps

moving hundreds of leaders of fuel per 2nd

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 at 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 stage combustion cycle

but you've still got some limiting factors here

like

how high pressure always wants to go to low pressure

and how he has that habit of melting stuff

so you've got to keep all these things and 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 gonna 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 stage combustion cycle

and lastly

we'll look at the full flow stage combustion cycle

and perhaps in a future video

I'll try and do a full rundown of all

liquid fuel rocket engines including fun

new alternatives like

the electric pump fed engines seen on rocket

lab's electron rocket

so 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 engines 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 close cycle counterparts

now I'm in a way

way oversimplify this

so it's as easy to grasp as humanly possible

in real life there's literally dozens of valves

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's an ignition source for the pre burner

and the combustion chamber

but again for the purpose of

making this as simple and as digestible as possible

just no

so there's a lot of stuff missing from these diagrams

but

for now we're just gonna focus on the flow of these

engines so we can

grasp that concept 1st

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 pre burner

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 power is the engine in the open cycle system

the

spent propellant from the ore burner 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 in

egg syndrome situation

that makes it pretty difficult to start up

since the pre burner

that powers the turbo pump

needs high pressure fuel and oxidizer to operate

so the ore burner requires the turbo pumps to spin

before it can get up to full

operational pressure itself

but the turbo pumps need the pre

burner to fire in order to spin the turbo pumps

but the pre burner 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 pre burner

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 rp1 or rocket propellant

1 and liquid oxygen

notice how black

the smoke is is coming out of the pre burner 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 ore burner at a less than

optimal ratio

so either too much fuel known as fuel rich

or too much oxidizer or

oxygen rich running rp1 engine

fuel rich

means you'll see some

unburnt fuel appearing as dark clouds of soot

the highly pressurized unburnt carbon molecules

bond and form polymers

which is a process known as coking

this set 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

huh welcome to the closed cycle

the close cycle or stage combustion cycle increases

engine efficiency by using what would normally be lost

exhaust and connects it to the combustion chamber

to help increase pressure and

also increase efficiency

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 unclogged 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 soviet solved it

the first operational

closed cycle engine they made was the nk 15

designed for their n1 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 rp1 like the merlin

you can't run your ore burners

fuel rich because of the coking problem

so if you want to create a closed cycle engine with rp1

the answer is running the ore burner oxygen rich

easy as that right

well now you're blasting super heated

highly pressurized gaseous oxygen

which will turn just about anything into soup

right at your precision machine crazy load 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

rp1 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

stand the crazy conditions of an oxygen rich are burner

with a closed cycle engine

you don't just use some fuel and some oxidizer

and burn that in the pre burner to spin the turbine

you actually shoot all all of the rich propellant

through the turbine

so with an oxygen rich cycle

all of the oxygen actually goes through the ore burner

and just

the right amount of fuel goes to the pre burner

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 pre burner

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 are burner

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 cannot 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 closest glory

you'd be wrong

it took the United States a little bit longer

but they eventually figured out a close 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 ore burner

but wait we just learned that fuel rich are burners

exhaust is so city that it pretty much ruins anything

right well sure

if you're using rp1 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 have avoided the problem of blasting crazy

high pressure oxygen at anything

deer in precious

but now we've opened up a new can of worms

hydrogen is significantly less dense than rp1 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 rp1 and locks are relatively similar in density

and in ratios

they can be run

on a single shaft using a single ore burner

because of this

the engineers at rocketry pursued an

engine known as the rs 25

which would go on to power this space shuttle

they realized that

because of the large difference between the pumps

they might as well have two different ore burners

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 this hydrogen

on the same shaft

right next door to the liquid oxygen pump

if some of that

hydrogen would leak out of the pre burner

it would start a fire in the locks pump

which is catastrophically bad

hydrogen is also very hard to contain because it's so

not dense or undersea

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 in hurt helium

genius but take a look at how different the locks

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 ore burner

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 are burners 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 close 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 stage combination Russian cycle

which basically just combines the two cycle methods

we just talked about

with the full flow stage combustion cycle

you take two ore burners

one that runs fuel rich and one that runs oxygen rich

the fuel rich ore burners powers the fuel pump

and the oxygen rich are burner powers

the locks pump

this means the full flow stage

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 ridge 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 rp1

due to the coking problems with a fuel rich ore burner

but other fuels are still valid to use this design

but more on that in a minute

the advantage of this 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 pre burner

the turbines can run cooler

and at lower pressures

because the ratio of fuel and 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 pre burner since it's all wasted

and you want it to be as hot as withstand

to make it more efficient but with the full flow cycle

all of the fuel

and all of the oxidizer goes through the ore burners

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 ridge

that the temperatures at the turbines will be

much lower

and this means longer life

spins for the turbo pump assembly

it also means more

combustion happens in the combustion chamber

and less than the pre burner now here's the crazy part

only three engines have demonstrated the full

flow stage 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 rocketry worked on an integrated

power demonstrator

called wait for it

the integrated overhead demonstrator which

again never made it past the test stand

and the third attempted developing a full flows

stage 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 stage combustion cycle engine to reach orbit

well actually just about anything

this engine does will be a 1st

this means spacex had to tackle some

crazy crazy problems

I mean not only that same problem that plugs

oxidize or rich cycles

like having to have a really really strong metal alloy

they also had to learn how to control

you know two different

pres burners and two different cycles

to create the highest pressures of

any chamber pressure ever

they just beat the rd 180s record of about 265 bar

when they hit 270 bar

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

engine 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

and for myself

it helps a lot to see them all

together on the same screen

at the same time

since the raptor engine can't run a fuel

rich pre burner using rp1

you'd think the next most logical

choice would be hydrogen

well spacex didn't opt for either rp1 or hydrogen

they went with liquid methane

so now we finally have another topic to touch on

why did spacex 2's liquid methane for the raptor engine

what are the qualities that make it advantageous over

hydrogen or rp1

today no liquid methane or

otherwise known as methylock's engine

has gone to orbit

so what qualities does it have that make it desirable

let's take a look at methane compared to

rp1 and hydrogen

let's put methane in between rp1 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 the tanks are

smaller and lighter for a given massive fuel

a smaller tank

equals a lighter rocket

so here's the density of these

three fuels measured in graham's per liter

in other words

how much does one leader of this stuffed way

or really what's its mass starting off with rp1

one leader is around 813 grams

rp1 is 11 times more dense than hydrogen which is only

70 grams per liter

and methyls 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 than 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 rp1

but spacex chills there are rp1 in their falcon

9 and falcon heavy

for about a two to 4% increase in density

but historically rp1'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 bounces

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

rp1 is burned at 2.7 grams of oxygen

to one gram of rp1

hydrogen burns at

6 grams of oxygen to one 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 1 141 grams per liter

it's a little more dense than rp1

so burning locks and rp1 at a 2.7 to 1 ratio

for every leader of locks

you'd need a little over half a liter of rp1

next up let's do hydrogen

now with hydrogen being 11 times less dense than rp1

you'd think it need a tank that's 11 times bigger

but luckily

engineers have found that it pays to burn locks and

hydrogen at a

6 to 1 ratio for a good compromise

this means for each

leader of locks you'd need 2.7 liters of hydrogen

so your fuel tank needs to be approximately

five times larger compared to rp1

so yeah that helps

that's why when we look at a hydrogen powered delta 4

versus an rp1 powered falcon 9

you can see the fuel tank is much

smaller than the locks tank on the falcon 9

but the delta 4 is about the opposite

the locks tank is much smaller than its fuel tank

so now let's take a look at methane

and this one gets kind of interesting

locks 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'd need 0.73 liters of methane

for every liter of locks

in other words your fuel tank need to be about 40

bigger for methyls than it would need to be for rp1

despite rp1 actually being almost twice as dense

and compared to hydrogen

its fuel tank would be about 3.7 times smaller

so the fuel to oxidize

a ratio helps make a methane fuel tank

a lot closer to an rp1 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

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 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 weight 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

and an ideal in perfect world

an rp1 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 rp1 engines

seeing around 350 seconds like the marlin 1d vacuum

around 380 seconds from methane powered engine

like the raptor vacuum might be someday

and about 465 seconds for a hydrogen powered engine

like the arl 10b2

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

rp1 can burn up to 3

670 kelvin hydrogen 3070 kelvin

and if you haven't guessed it by now

methane is again between the two at 3

550 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

may rp1 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

and at least boils off at a temperature

similar to locks

so there's that

and because it's so close to the temperature of locks

the tanks can share a common dome

which makes the vehicle lighter

locks and hydrogen's temperatures very so wildly

that locks will boil off

hydrogen and the hydrogen will freeze locks solid

now onto the exhaust

what are the byproducts of combustion with these

engines rp1 is really the only one of these three

that really pollutes with

any unburnt 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

so 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 as a consideration

so stand by

because I think that video is gonna be awesome now

one metric that we're just

kind of going to gloss over really quick

but and talk about it generally is the cost

and these tend to very considerably

and it's actually really

hard to pin down the exact prices reliably

so for the considerations

rp1 is basically just a highly refined jet fuel

which jet fuel is 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 cost 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 spaces 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 saba a 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 resources

this is called in situ resource utilization or is

now you might be thinking well if there's 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

and 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 produced on the surface of mars

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

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 line up each

engine by their fuel type and their cycles

so let's start off with spacex's

open cycle merlin age and that powers their falcon

9 and falcon heavy rockets

no entergomesh's

oxygen rich closed michael rd 180

that we see power the atlas 5 rocket

and rocketdyne's open cycle f1

that powers the saturn v

which all three of these engines run on rp1

then we have spacex

this is full flow stage combustion cycle raptor engine

that will power the starship and super heavy booster

and then we have blue origins

close cycle oxygen rich methane powered be 4 engine

that will power their new glen rocket and

uola's upcoming vulcan rocket

and then we have aether rocket dine's close 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

and the b 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 b e4

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 subjective 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 web site 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 mega newtons of thrust

the rs 25 produces 1.86 mega newtons

the raptor currently is at two megatons

the b e4 is hoping to hit 2.4 megatons

the rd 180 3.83 megatons

and the f1 is still the king out of these at

6.77 megatons

now there was an engine called the rd 170

which actually produced more thrust than the f1

but since it barely flew

I figured it wasn't as relevant in this lineup

I thought it'd probably be 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 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's the rd180 which is 78 to 1

then we have the b e4 at a round

80 to one button

keep in mind

we don't actually have a really good number on this

so there might be some wiggle room there

then the f1 is 94 to one

then we have the rafter which is at about 171 for now

and lastly the merlin is

actually the leader here

with an astonishing 198 to one 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 f1 engine

at 263 to 304 seconds

then the merlin engine at 282 to 311 seconds

then we get the rd180 at 3011 seconds to 338 seconds

and somewhere in that same ballpark is the bee 4

which is around

310 to 342nds

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 f1 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

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 moline

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 bars like being 3km 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 dollar 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 f1

which was about 30 $1000000 per engine

then the rd 180 which is 25 $1000000 per engine

then the be 4 which is around

8 $1000000 for engine

and for the raptor

elon 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 as a pretty

decent ballpark

then we have the merlin engine which is

less than 1 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

here only the rd180 and the f1 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 b4 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 treats these claims

but one quick fun little story here is

don't forget the merlin engine

which spacex currently uses on the falcon

9 falcon 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 in mcgregor Texas

where does a full duration burn then

those engines go back to California

where they're integrated onto the octagon

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 it 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 sat 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

elon talked about once in a tweet in February of 2019

saying they hoped 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 kila newton ratio of these engines

starting with the most

expensive dollar to killing newton engine

which is the rs 25 at a crazy 26

8181 dollars to kila newtons of thrust

then the rd 180 which is 6 527 dollars to 1 kila newton

by the f1

then we get to the be 4

which is 3 333 dollars to one kila newton

the borderland engine at 1

170 dollars per kilo noon and the raptor at around

1 000 dollars per kilo newton

but now we can go even

another step further

since we know they're dollar to kila newton ratio

well we also know their reusability potential

now

we can predict their potential cost per kiloton per

flight which

changes based on how reusable these engines

actually are

so for starters

since the rd180 and the f1 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

slash 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 1

414 dollars per kiloton per flight

but here's where things get crazy

blue origins b

e4 has potential to truly be game

changing and around 133

per kilohm newton over 25 flights

which could make it

about as cheap to operate as the merlin

at 117 dollars per kiloton per flight

but if the raptor engine truly lives up to its hype

it could bring this number all the way down to 20

per kaolinite per flight

now that is absolutely game changing

sure money

and reusability is a 21st

century focus for space flight

but whatever happened to good old proven reliability

for this

let's first look at how many operational flights each

engine has had

at the moment of shooting this video

the raptor and b

for 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 star hopper

but for now neither engine has a real flight record

so let's look at the other engines 1st

we have the f1 engine which was used on 17 flights

next up is the merlin engine

which is at

71 flights and catching up quickly to the rd180

which is at 79 flights

but the king out of these was the rs 25

which saw 135 flights now

lastly how about reliability in service

between the number of flights and this number

we can get a pretty good sense of how

truly reliable and 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 yes it take a few of these with a grain of salt

again the be4 and raptor engine haven't flown yet

so those numbers are unavailable

then we have this 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

that sure helps when you have 10

engines on each flight of the vehicle

and with only

one engine ever failing early on in his career

and despite that that mission was still a success

so the merlin is a very reliable engine now and this

technically the rd 180 and the f1 are 100 reliable

but with the f1 never

having shut down at all in any flight

it gets the bold here

and depending on how you define success in reliability

technically the rd180 is only kind of 100 reliable

because it got really lucky ones

one time it shut down

6 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 to sent our 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

man seeing all these numbers and considerations

it makes you realize just

how many variables go into designing rocket

and 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 ore burner temperatures

hmm

a methane fueled full

flow stage combustion cycle engine

sounds like a good fit for reliability

redundancy and scale of manufacturers

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

hmm sounds like a methane fueled full flow stage

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 stage 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 stage

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

space is developing this engine at a rapid pace

I mean knowing how much tweaking

space did to their merlin engine over a decade

were 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 I'll leave 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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