Original subtitles

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

Three, two,one blastoff

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, Everydayastronautcom

in case you want to study some of the numbers

a little more in depth

or see sources of some of the material

Now we're gonna start off with a super quick physics lesson

but bear with me

We're gonna dive in and get plenty of nitty gritty details

Okay

Let's start off with this

Rockets are basically just propellant

with some skin around it to keep it in place

and they have a thing on the back

that can throw said propellant really, really fast

and to way oversimplify it even more,

the faster you can throw that propellant the better

Now the easiest way to do this

is by storing all the propellant in your tanks

under really high pressure then put a valve

on one end of the tank

and a propelling nozzle that accelerates the propellant

into workable thrust

Done

No crazy pumps or complicated systems

just open a valve and let her rip

This is called a pressure fed rocket engine

and there's a few main types:

cold gas, monoprop and bipropellant pressure fed engines

You'll often find these used in reaction control systems

because they're simple, reliable, and they react quickly

But pressure fed engines have one big limiting factor

Pressure always flows from high to low

so the engine can never be higher pressure

than the propellant tanks

In order to store propellant under high pressure,

your tanks will need to be strong

and therefore thicker, and thicker,

and heavier, and heavier

Look at composite overwrapped pressure vessels or COPDs

They're capable of storing gases at almost 10000 PSI

or 700 bar

And despite this there's still

a limited amount of propellant

and pressure they can store

And this does not scale up very well

when you're trying to deliver a payload to orbit

So smart rocket scientists quickly realized

in order to make the rocket as lightweight as possible

there's really only one thing they could do:

increase the enthalpy

That would be a great metal band name

You're welcome Internet

Enthalpy is basically the relationship

between volume pressure and temperature

A higher pressure and temperature

inside the combustion chamber equals higher efficiency

and more mass shoved through the rocket engine

equals more thrust

So in order to shove more propellant into the engine

you could either increase the pressure in the tanks

or just shoot the propellant into the combustion chamber

with a really high powered pump

The second option sounds like a pretty good idea

But pumps moving hundreds of liters of fuel per second

require a lot, and boy do I mean,

a lot of energy to power them

So what if you took a tiny rocket engine,

and aimed it right a turbine

to spin it up really, really fast?

You can exchange some of the rocket propellant's

chemical energy for kinetic energy

which could then be used to spin these powerful pumps

Welcome to turbo pumps

and the staged combustion cycle

But you've still got some limiting factors here

like how high pressure always wants to go to low pressure

and how heat has that habit of melting stuff

So you've got to keep all these things in check

while trying to squeeze every bit of power

out of your engine

There's actually a lot of different variations

of the cycles that we could talk about

but I'm going to stick with the three most common

or at least the three that matter the most

when putting the Raptor into context

We have the gas generator cycle

the partial flow staged combustion cycle

and lastly we'll look at

the full flow staged combustion cycle

and perhaps in a future video

I'll try and do a full rundown

of all liquid fueled rocket engines

including fun new alternatives

like the electric pump fed engine

seen on Rocket Lab's Electron rocket

(slow music)

Let's start with the gas generator cycle

known as the open cycle

This is probably one of the most common types

of liquid fueled rocket engine used on orbital rockets

It's definitely more complicated

than a pressure fed system but it's fairly simple,

well at least compared to their closed cycle counterparts

Now I'm gonna way, way oversimplify this

so it's as easy to grasp as humanly possible

In real life, there's literally dozens of valves,

a hive of wires,

and extra tiny little pipes everywhere,

helium to back pressure the tanks

fuel flowing through the nozzle

and the combustion chamber to cool it

and there is an ignition source for the preburner

and the combustion chamber

But again for the purpose of making this as simple

and as digestible as possible,

just know there's a lot of stuff missing

from these diagrams

But for now we're going to focus

on the flow of these engines

so we can grasp that concept first

The gas generator cycle works by pumping

the fuel and oxidizer into the combustion chamber

using a turbo pump

The turbo pump has a few main parts

a mini rocket engine called the preburner,

a turbine connected to a shaft

and then a pump or two that push propellant

into the combustion chamber

Now you might hear the turbo pump assembly

called the power pack because it really is

what powers the engine

In the open cycle system,

the spent propellant from the preburner

is simply dumped overboard

and does not contribute any significant thrust

This makes it less efficient since the fuel

and oxidizer used to spin the pumps is basically wasted

Now the funny thing about a turbo pump

is that it kind of has a chicken

and egg syndrome situation

that makes it pretty difficult to start up

since the preburner that powers the turbo pump

needs high pressure fuel and oxidizer to operate

So the preburner requires the turbo pumps to spin

before it can get up to full operational pressure itself

but the turbo pumps need the preburner to fire