although it's Scott Manley here with the
50th anniversary of Apollo 11 already
being celebrated you're no doubt seeing
pictures like this all the time plenty
of articles about these amazing f1
engines and the raw power they produced
to get the Saturn 5 moving and so I
think it's appropriate do we actually
learn how the engines themselves started
you see a rocket engine is a complex
piece of machinery and it doesn't simply
go from zero to a hundred percent thrust
in a fraction of a second it's not just
turned on simply there are multiple
moving parts that all need to
synchronize their activities so that we
can see this gorgeous sequence here as
their engines light and they suck the
gas around them and of course the whole
vehicle eventually begins to move under
thrust and so this is what the mighty f1
engine looks like it is a simple pump
fed engine burning kerosene and liquid
oxygen
it's pump is powered by a gas generator
which means that it dumps its exhaust
over the site actually it dumps its
exhaust around the inside of the engine
Bell now because this is a NASA program
we have lots of documentation including
this page in the manual that has the
exact startup sequence so I took these
resources and drew my own diagram with
nice color-coded lines so you can see
what's what at the top we have the
low-pressure fuel and oxidizer lines
going into the turbo pump out of those
come the red and the blue high-pressure
oxygen and fuel lines getting tapped off
of those there's a small pair that gets
fed into the gas generator to power the
turbine and then min output goes
straight into the main rocket engine and
that's where the thrust comes from no
the question is how do you get to this
state this is what it's like winners
operating and the turbine is spinning
but obviously it doesn't start like this
for most of the time the rocket is
sitting in the pad there is no fuel or
anything in any of these lines the flow
of the propellant into the engine is
blocked by these three prevails up the
very top now about 10 minutes before
launch that's when these valves open in
the
let fuel and oxidiser a flaw into the
engine but it doesn't flow out of the
engine Bale because we have the mean
fuel and the mean LOX valves holding the
these back the next critical moment is
about t-minus nine seconds when you hear
the announcer say ignition sequence
start
the first thing happens is four
pyrotechnic devices get fired they're
there in two redundant pairs two are in
the gas generator and two are in the
engine Bell to catch the gas generator
exhaust these devices will burn for
about six seconds and while they burn
they will burn through a little metal
wire which is connected to a circuit so
when all four of those wires are broken
the sequencer knows to trigger the next
step in the ignition process the
sequencer will then direct the mean
oxygen valves to open and that allows
liquid oxygen to flow into the engine
bell into the combustion chamber but
there is no combustion happening at this
point we do get a cloud of liquid oxygen
coming out and you can see this in some
of the videos this flaw is largely
driven by the fact that the oxygen tank
is pretty high up so this is gravity
just pooling the oxygen down and it's
developed several atmospheres of
pressure because the first stage of the
Saturn 5 is over a hundred feet tall and
that corresponds to several atmospheres
of pressure when you account for the the
mass of the Lak oxygen that oxygen also
flows through the turbo pump and it
actually starts driving the turbo pump
causing it to turn which also then
applies pressure to the fuel side of the
system the next step is the fuel and
oxidizer valves leading to the gas
generator are opened this lets the
propellants flow in there they start
combusting and they start generating
exhaust gases which drive the turbine
which drives the turbo pump and as the
pump picks up speed the pressure
downstream from it starts to get higher
also the exhaust gases from the turbo
pump finally start arriving in the
nozzle and those are getting burned off
but they're not like a raw high-energy
rocket flame they're more like a casual
combustion they're not generating any
thrust but it's important that it gets
burned up so that it doesn't
accidentally flow back through the
system and cause problems when they
really ignite early on in the ignition
you can obviously see these billowing
flames that aren't
generating any thrust when sensors
detect the pressure in the main fuel
line is about 26 atmospheres it then
opens a valve to the hyper gall
cartridge this is a small part of the
fuel line with a burst disks at either
end so when the fuel is allowed to flow
through it it breaks these disks and the
contents of that pipe get pushed through
the engine and into the combustion
chamber so this contains the starter
fluid this is the triethyl aluminium and
the trifle boring that ends up going
into the engine and when it contacts the
oxygen that's when the main combustion
starts with a combustion chamber being
fed a small amount of fuel the pressure
inside the chamber rises and that pushes
against sensors in the injector plate
which when they reach a certain
threshold they then send a signal to
trigger the final operation opening the
main fuel valve and as both the fuel and
the oxidizer valves are open the engine
then ramps up to full power and about a
second before liftoff they should be
generating over 700 tons of thrust per
engine if all the engines are generating
that thrust the main computer gets the
signal and liftoff occurs there's a few
other wrinkles are important first of
all the valves are actuated by a high
pressure hydraulic fluid as is the
engine gimbal on the ground they have a
high-pressure hydraulic line that comes
in but once the fuel loop gets to high
enough pressure this thing switches over
and it uses the high pressure fuel
coming out of the turbo pump to open the
valves and of course to move the engine
once it's in flight secondly when you
are bringing 700 tons of thrust per
engine you don't light all the engines
at once because that would create too
big a shock to the spacecraft
instead they light the middle engine
then another pair 200 milliseconds later
and the third pair 200 milliseconds
after that that all gives them enough
time to ramp up to a hundred percent
thrust before launch while not hitting
the structure with too much thrust at
once another step the engineers took to
smooth the engine thrust buildup is that
the combustion chamber is regenerative
Li cooled that means the floor
the fuel through the walls to keep it
cool but before they launch they
actually pre fill all these pipes that
run through the combustion chamber walls
with ethyl glycol so that that has to
get pushed out of the way first by the
fuel flowing through these so so
initially the fuel flowing into the
combustion chamber is diluted by this
ethyl glycol which reduces its energy
and as that gets flushed out then the
fuel gets to its fuel concentration and
the engine reaches fuel thrust now of
course we have all the telemetry so you
can actually see this thrust build up in
graphs this is Apollo 11 this is the
inner the thrust being built up over
time and you see that roughly one second
before launch they're all generating
pretty much 100% thrust you can see that
the engine five in the center goes first
and then engine three and four one go
second and then you get engine tuned for
those go last but around two and a half
to two seconds you'll notice there's a
little kind of blip in the thrust
buildup on four of those engines but not
in engine five in the center and that is
a result of helium from the Pogo
suppression system getting ingested into
the oxygen two pipes now engine 5 in the
middle doesn't have any of the Pogo
suppression hardware so this only
affects the four outer engines but once
the flow settles down the engines reach
stable levels of thrust and of course
everything works just fine and so there
you have it that's how you turn on a 700
ton thrust engine that's consuming more
than two tons of propellant per second
I'm Scott Manley fly safe
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