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hey hey marcus house with you here and once again i've put together another midweek video
for you all to enjoy today we are going to take a close look into how spacex manages
starship pressurization and engine startups we will also peer into the past to see where those
solutions were used in previous generations of launches are you ready for this because we
are about to go straight into the deep end of the pool join me here and let's dive right in
so we will start off here with what many of you would find the most familiar that being spacex's
starship this vehicle once in later stages in development will be kept pressed in flight
by using what's called autogenous pressurization but before we get into that we need to go over
some necessary background starships raptor engines run using two propellants the fuel liquid methane
and the oxidizer liquid oxygen remember these for later on now both of these propellants can
be viably produced on mars through a chemical process known as the sabatia reaction which
we've covered before a link to that is in the description if you want to check out that after
this video now elon in his 2016 international aeronautical congress meeting spoke about this
type of pressurization here with both the fuel and oxidizer potentially available to be produced on
mars the only thing left is to figure out how to repressurize the main tanks during flight this
is where autogenous pressurization comes into play this is the process which allows a rocket
to bring its fuel tanks back up to pressure by utilizing the resources already on board
nitrogen and helium are the two most common inert gases used for tank pressurization however neither
is available in abundance on mars therefore leaving only two options left the methane and the
oxygen by heating up the propellants already in the main tanks spacex can generate gaseous methane
and oxygen which can then be pumped back into their respective tanks to repressurize for flight
autogenous pressurization is especially helpful for long duration missions to other celestial
bodies in the solar system and may actually prove absolutely essential in certain instances such as
a mars mission looking deeper into the autogenous pressurization cycle specifically for starship
we can understand how exactly the liquid propellants can be turned into their gaseous
form to repressurize the tanks but first let's go through why we need to repressurize the tanks in
the first place starship is made up of stainless steel which requires extra reinforcement to
support itself but reinforcement adds weight right which is really not what rocket engineers like
without structural support the weight of the heavy ship can cause it to buckle and rupture
as famously demonstrated here by the atlas agena so what other options are there to provide this
support well by pressurizing starships tanks two things are accomplished firstly by keeping the
main tank structure as rigid as possible starship gains the structural support that comes along with
that but secondly and most relevant to this video the pressure is also available to push fuel down
into the bottom of the tanks and feed propellant into the engines themselves these factors help
show why tank testing is such an important focus of spacex's starship development program as we
watch it unfold the tank section of starship does not have any extra supporting ribs or stringers
running through it therefore requiring it to be pressurized as long as extensive loads such
as fuels cargo and crew sit above the process of autogenously pressurizing the vehicle begins with
running the liquid propellants through a heat exchanger sitting directly on the pre-burners
of the raptor engine now corey here aka seabass productions has produced some lovely renders to
help illustrate this exact process for us so be sure to check out his channel for more of these
amazing videos keep in mind that these visuals here are by our understanding of the idea and by
no means reflect the real plumbing for those heat exchangers now back to those pre-burners there are
two of them and they are like mini rocket engines that both run at much higher pressures than the
main combustion chamber one of them runs fuel rich combustion while the other runs oxygen rich
which allows for a higher efficiency of combustion in the main chamber when the two mixtures meet
together now these visuals here are from tim dodd the everyday astronaut so go and check out his
channel later as well to learn about topics in a much deeper dive these propellants that are going
into the heat exchanger come from a small feeder pipe running off the main propellant manifold
that supplies the pre-burners the cryogenic liquid propellants are now converted into their gaseous
form after being run through a heat exchanger and then can be sent back into their respective tanks
now although the propellant is now gaseous it isn't exactly hot but rather warmer than the super
chilled liquid that it just was as you can see the gaseous methane runs up a long re-pressure and
pipe into the top of the methane tank that methane will then allow the main tank to pressurize
in the exact same way the gaseous oxygen runs up a slightly shorter pipe into the head of the
oxygen tank therefore pressurizing its main tank now due to each propellant now being in a gaseous
state and being much warmer than the rest of the cryogenic propellant they will both cool down in
their respective tanks and eventually condense back into their liquid forms now as it does this
it also very slightly warms up the rest of the propellant in the tanks and they further expand
to continue pressurizing the tanks as well with heating up cryogenic propellant using the engines
and the indirect warming up of propellant in the tanks through thermal equilibrium a consistent
level of pressurization can be maintained throughout the flight but one major problem does
exist which is that that engine must be running for autogenous pressurization to work as heat is
needed to heat up the propellant to circumvent this one idea could be that starship in orbit
would orient its heat shield site to face the sun therefore allowing the propellant to expand
at a very slow rate under indirect heat as we saw with the starship flight of sn8 and sn9 spacex
is continuously getting better at transferring propellant into the header tanks and keeping it
nice and chilled during that flight spacex is also likely in the process of producing an actively
cooled system to ensure that the header tanks are kept chilly during the long duration missions
so yes let's shift gears and look at some of the past vehicles that demonstrated autogenous
pressurization the most well-known vehicle to use this method was actually the space shuttle
the orbiter vehicle that had the three rs-25 space shuttle main engines which used liquid oxygen and
liquid hydrogen to fuel them now these propellants were stored in the shuttle's orange external tank
and by running the liquid hydrogen in flight through the rs-25 main combustion chamber via
custom grooved pipes and channels autogenous pressurization can take place as the propellant is
heated up by the combustion gases the now gaseous hydrogen would then be sent back into the external
tank to effectively repressurize the vehicle while it was in flight just as starship hopes
to complete as well now as you might be thinking by now what about the liquid oxygen well it's a
similar process and it would instead run through a heat exchanger built into the oxygen pre-burner
which would then be re-piped into the external tanks through the external pressure and lines
these 5-inch external lines can be seen next to the main 17-inch liquid oxygen feed pipe together
with both the liquid hydrogen and the liquid oxygen being heated up by the engines during the
flight the external tank could maintain proper fuel flow without any helium or nitrogen for
pressurizing the tanks in flight which of course saved valuable payload mass now saying all of that
while on the ground the shuttle's external tank would be kept under pressure by helium which was
supplied via the ground support equipment the space shuttle's main engines would also
utilize helium from the ground equipment to ignite and then would continue in flight via
the autogenous pressurization system now this next example isn't exactly a past vehicle but i
did think it was worth mentioning here as it uses nearly an identical system as the space shuttle's
external tank the space launch system or the sls will also conduct tank pressurization in flight
by autogenously pressurizing the tanks once again the process is nearly the exact same as the space
shuttle the space launch system however won't have as much of the complexity with its vehicle
to tank interfaces and should be much easier to handle while on the ground and especially in
flight as well the sls core stage just like the shuttle will have that massive oxygen feed line
running down the leeward or the non-wind facing side the same two repression lines will also be
located there next to that main feed line as with the shuttle helium via the ground equipment would
pressurize the vehicle before launch sls with this configuration is slightly easier to work with as
the engine pod is directly connected to the main tanks and no longer separated by a secondary
interface putting these two together we'll see shuttle's legacy hardware from the 80s supporting
this super heavy lift launch capability in the space launch system of today a truly amazing
feat of engineering now the final vehicle that we're going to cover from the past is the titan ii
intercontinental ballistic missile and later the titan ii gemini launch vehicle this vehicle was
the successor to the american mercury program and the precursor to the apollo moon landing program
now that means that this vehicle was operational back in the 60s that is amazing right but
the titan ii differed from the other launches as it used hypergolic propellants
these types of propellants are normally stored at room temperature and upon contact of both the fuel
and the oxidizer both will spontaneously combust together this allows for reliable combustion
within the main chamber and the turbo pump cycles without the high risk concern over engine ignition
and engine failure similar to the other systems that we've just covered a pressure line would run
up the side of the vehicle and would constantly be adding high pressure gas from the engine's turbo
pump into the top of each respective propellant tank the beginning of the space race really
brought up some amazing technologies that have stuck with us right up until today now hopefully
you all found that interesting because before this video i didn't know that sls and space shuttle use
that same method of pressurization as starship is intending to now this level of engineering
never fails to surprise me and constantly reinvigorates my love for all things space
so as i stated earlier we're going to find out how raptor engine starts can happen on other celestial
bodies within our solar system in reality this is a fairly lengthy process to explain so i will
instead be cutting it down to make it a little more simplified now none of this is confirmed by
elon musk or spacex but it's generally what may happen nonetheless as we know the major places
starship plans to travel in the next decade mainly includes the moon and mars with possible deep
space missions venturing into the asteroid belt the process for starting a raptor engine can be
designated into three main sections priming the pre-burner turbines igniting the pre-burner and
lighting the main combustion chamber now just remember that most of this is unconfirmed and
going off our observations and our understanding of the engine and vehicle as they are right now
so we'll first start with the top of the engine and look at how the turbines will be started
as you can see in this render by quarry once again the liquid methane pre-burner has a small pressure
line coming from the skirt of the starship which houses the copvs with high pressure gases
a copv of course is a composite overwrapped pressure vessel which is capable of holding
upwards of 350 atmospheres on average those copvs store some of that autogenous pressurized gaseous
propellant in them for later use and engine starts so as that methane pressure line is opened super
pressurized gas is sent into them spinning up the turbine and then allowing for the methane to flow
into the pre-burner and begin we see the exact same thing happening here on the liquid oxygen
side as the pressure line sends in oxygen gas from the copvs now that super high pressure allows the
turbine blades to spin extremely fast and begin drawing in propellant to begin the pre-burner
combustion so you may be wondering now why it is called a pre-burner well it's called that because
that's exactly what it's named for it pre-burns the propellant so that when it enters the main
combustion chamber it can be mixed with a higher efficiency and control now that the turbines are
spinning fast the might of the raptor can begin being heard as the pre-burner on the methane side
gets going with a little bit of oxygen also going into it we can see the first stage of combustion
happening in order for proper combustion to happen a tad bit of oxygen is going into the methane
pre-burner and the opposite is happening with the methane going into the oxygen pre-burner now at
this point those copv's high pressure gases are no longer necessary as the pre-burners themselves
are now running through sheer main tank pressure so the copvs could just be shut off now both of
those pre-burners are started by a spark igniter that is recessed into the head of the pre-burner
chamber the final part to igniting the raptor engine is to send those pre-burned gases straight
into the main combustion chamber this is possibly the most simple part of the process although
spacex did have to design their own alloy in-house to handle those extremely hot oxygen-rich gases
at this point another set of spark igniters light the two pre-burned gases in the main
combustion chamber allowing them to combine and complete full flow stage combustion what spacex
has achieved with the raptor engine is borderline unimaginable they are the only people to make a
full flow stage combustion cycle engine that has actually flown i mean just listen to that roar
of the engine on the stand at their mcgregor texas test facility it is truly a remarkable achievement
so yes putting everything together what do you think autogenous pressurization systems
seem to be the way to go for those long duration missions moving forward but there are always a
few drawbacks the only alternative to this problem would be to revert to the traditional helium-based
systems like what's used for the falcon 9 but that is clearly what spacex hopes to avoid we saw sn8's
landing attempt to go awry due to the loss of header tank pressure as tweeted by elon musk
could it have been possible that an issue related to the autogenous pressurization system failed in
that vehicle it may be difficult for starship to incorporate this method of pressurization during
the incredible skydiving or belly flop phase of the flight but as we know if anyone can do it it
will be spacex with nasa's involvement in the starship program throughout the lunar starship
contract it's possible that spacex can leverage that partnership and pull ideas from the space
shuttle to enhance their own pressure and system now i want to hear your thoughts on this idea is
autogenous pressurization too difficult to achieve in the long term over long missions or is it the
best plan moving forward for spacex's long-term goals as always let me know in the comments and
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