Original subtitles

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

cite rich cycles

like having to have a really

really strong metal alloy

they also have 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

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

space 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 measure and grams 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 liter 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

space chills their rp1 and 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 leader 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