Original subtitles

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 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 close cycle counterparts

now I'm gonna

way way over to simplify 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

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 know

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 pushed 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 and the open cycle system

the spent propellant from the pre

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 pump 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 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 eye 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

an 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 sit 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

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

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 soviets solved it

the 1st

operational closed cycle engine they made was the nk

15 designer 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 lower 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

to stand the crazy conditions of an

oxygen rich pre 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