Hi, it's me, Tim Dodd,
the Everyday Astronaut. Liquid fueled rocket engines operate
by flowing fuel and oxidizer into a combustion chamber at ridiculously high
pressures in order to throw as much mass out the flaming end of the rocket as
quickly and thereby as efficiently as possible. But how does it get that way?
How on earth, and even more difficultly, how off earth do you get an engine up
to operating pressures and temperatures? Engines often utilize pumps
which themselves rely on
the engine to be running in order to get power and
having a rocket engine run and getting a rocket engine
running are two wildly different things. There is often an extremely
delicate dance to get the palms, valves, temperatures, and pressures all up
to operating conditions perfectly. Get it wrong by just a
few milliseconds even, and you can cause a detonation that
can completely destroy the engine. So today we're going to do a deep
dive on how exactly you start a rocket engine. We'll cover pretty much everything from
a simple solid rocket motor to all of the intricacies of liquid
fueled rocket engines, including thermally conditioning
the engine, the spin up process, the scary transient regions, and the
actual ignition of the propellants. Then we'll talk about some of the extra
challenges like starting a rocket engine in space, and then we're going to actually go step
by step inside a rocket engine during its startup. Now this
is a very long video, so here's the time
stamps for each section, and we've got the YouTube timeline
broken up into those sections too. And if you're more of a reader, we've got you covered with an article
version up at everydayastronaut.com with links and sources. That being
said, let's get started. 3, 2, 1, liftoff. Right up front, I need to suggest that you watch my
videos "Why don't rocket engines melt?" and "How to power a rocket engine" as
those videos will help with a lot of concepts that we talk about
in this video. Trust me, don't even try to watch this video without
having watched those because they lay a lot of vital groundwork. And
lastly, before we get started, I've got some great news. Our highly detailed one 100 scale metal
Falcon 9 rocket models are finally back in stock. Get them while you can
everydayastronaut.com/shop. Okay, assuming you have all the basic knowledge
of how a liquid fueled rocket engine works, let's actually start off with the most
simple rocket engine to start a solid rocket motor. By far, the easiest rocket to start
has to be a solid rocket motor. In fact, there's actually a good
chance that you've done this yourself. If you've ever launched a
model rocket or even fireworks, you've already pretty much experienced
the ignition sequence of a solid rocket motor. Solid rocket propellant
is exactly what it sounds like. It's a solid that is rocket propellant. It's usually like a sludge of
pre-mixed fuel and oxidizer, and all it takes is energy to get
the combustion process started. In the case of a small model rocket motor, that energy usually comes from the
heat from a set of small wires. Bigger solid rocket motors will usually
require more energy to get started, so sometimes they'll
have some black powder, which is basically gun powder that
will initially ignite from the wires, and then a chain reaction will occur
that will then light the solid propellant where it will continue to self sustain.
But once you start talking about large orbital class, solid rocket boosters like those on
the Space Shuttle or maybe those on the Space Launch System, otherwise
known as the SLS rocket, which is at the heart
of the Artemis program, there's basically a bomb sitting on top
of the booster that starts the ignition process. There's a device called a
NASA Standard Detonator or an NSD, and this is a little device used since
the Gemini program for things like separation events on booster separation
and fairings and frangible bolts, or in this case, they're the device that they actually
use to start the chain of reactions that'll get these massive solid rocket
boosters going. When it's go time, A signal is sent to not just one NSD, but a completely redundant NSD to
ensure the boosters light. After all, if one doesn't light, it would be very, very bad. And don't forget, once
you light a solid rocket booster, there is no shutting it down. The NSDs
burst through a thin barrier seal, which then lights a
pyrotechnic booster charge. Then that booster charge is what
ignites propellant inside and igniter initiator, which then lights
the main booster igniter, and that lights the entire surface
of the core of the booster virtually simultaneously. So for
a little overview here, it's basically a waterfall of explosives
that cascade down from the top of the booster until eventually the entire
booster is lit pretty much all at once. Basically stepping up from a
small charge, to a medium charge, to a large charge, to booster
ignition. Like I said, it's basically more or less a bomb
that's lit at the top of the booster or otherwise known as the forward segment
that starts the ignition process. It's really just about that easy.
That's kind of their whole point. They're super simple and because
of their easy ignition process, they're very reliable, but they definitely have limitations
such as not being able to be shut down, and they aren't nearly as high performing
as a liquid fueled rocket engine. But starting a liquid fueled rocket engine
is just not anywhere near that easy. There's a ton of steps you need to take, but perhaps the first and most important
thing to do is to purge the engine and thermally condition it for the extreme
environments it's about to encounter. Before you get a liquid
fueled rocket engine running, it has to be prepared for the ludicrous
temperatures it's about to face. And believe it or not, I'm not
talking about the extreme heat. I'm actually talking
about the extreme cold. Not only do most orbital liquid fueled
rocket engines run propellant through the walls of the engine to keep the combustion
chamber and nozzle from melting, but the pumps themselves will flow upwards
of thousands of liters per second of cryogenic propellants,
which can make the metals, valves and bearings brittle and
failure prone. This of course, is specifically true of any engine
that uses cryogenic propellants, so any liquid oxygen, also known
as LOx powered rocket engine, and especially those that run on
hydrogen or hydrolox and methane or methylox for their fuels, which
are also cryogenic propellants. These propellants all make up the
majority of rocket engines that we see on orbital rockets. But before you even
begin to chill down the engine with the propellants, you need to purge the
engine from moisture and air pockets, and this is usually done with nitrogen. If there's any water vapor in the
lines before cryogens are introduced, it'll freeze and cause damage to the
engine with the potential for clogged orifices, damaged seals, clogged
injectors, et cetera, et cetera. But if it's a hydrolox engine;
liquid hydrogen is so cold, it can even turn any residual nitrogen
into a solid block of ice and do all of the bad things we just mentioned. And an ice blockage can be really hard
to investigate because the evidence of the root cause of the failure melts away
by the time a human can even get close enough to investigate. Because of this hydrogen powered rocket
engines are usually purged with another inert fluid, helium. Helium is the only element
that has a lower melting point than hydrogen. So in the
presence of liquid hydrogen, it won't freeze solid. Purging and chill down of the engines
can be anywhere from a few hours before launch to a few minutes, but this
varies very much from engine to engine. So listen for something like engine
cool down or engine chill down, called out on comms... Stage one engine chill has started. All right, there's that call that
stage one engine chill has started. Okay, so once the engine
has been purged of moisture, we can start to actually thermally
condition the engine with the cryogenic propellants. This process involves
opening what's known as a pre-valve, which is the big valve that connects
the engines to the propellant tanks. The propellant will fill the pumps and
fill all the way down to the main valves, which is usually one or a number
of valves for fuel and oxidizer. Then the pumps will just soak in a bath
of cryogenic propellant until the pumps eventually get down to
those frigid temperatures. In comparison to cryogenic propellants, the engine is actually
scorching hot under normal, everyday human comfortable
ambient temperatures. Even if an engine was hanging out in
Antarctica during a deadly blizzard, it would still pretty much immediately
boil off any cryogenic propellant that touches it. So there needs to be a vent line to
remove the boiled off propellant. Oxygen can usually just
be dumped overboard, which poses little to no risk when
being vented into the atmosphere. But in general, a cryogenic fuel that now has boiled
off into a gas like methane or hydrogen needs to be recaptured
and either chilled and pumped back into the rocket, or it can be vented away from the rocket
in a controlled manner where a flare can prevent a buildup from accidentally
detonating because if you just vent gaseous methane or hydrogen out into the
atmosphere in an uncontrolled manner, don't expect very good things to happen. This is why you see those giant
sparkler lit before Space Shuttle. SLS or Delta IV Heavy launches. It's not to light the actual
engines and start combustion, it's to burn off the gassiest hydrogen
that will exit the engine during the startup process before main
combustion chamber ignition. These are called radial outward
firing initiators or roofies. They're just preventing a large cloud of
hydrogen from gathering and potentially detonating, but as you know,
could be very, very bad. But the engine isn't chilled down just
to protect the engine from the cold temperatures. It's also to protect the propellant
from the relatively warm engine even when it's just resting at ambient
temperatures before the engine is running. If the propellant is beginning to boil
before it reaches the impellers and the pumps, it can cause cavitation or
basically bubbles in the liquid, and those bubbles can actually chip
away at the pumps and completely ruin them. Elon Musk talked about this a little bit
when we were talking about the Raptor engines pumps. So what you're trying to avoid is
cavitation or bubble generation. If you start generating
bubbles, the bubbles will actually eat away at your blades. Like it's weird like bubbles would
chip away metal, but they will. And, and if you cavitate too much then
you're gonna just be a bubble generator and you'll lose
pressure and starve the engine. And not only can those bubbles
chip away at the pumps, the bubbles can also cause the pumps to
over speed and or starve the engine of the propellant and potentially
create stoichiometric conditions, which is the most energetic and highest
temperature conditions and of course, those can be catastrophic. Getting an engine down to operating
temperatures is absolutely vital and it's monitored very closely by the engine's
computers that run the engine. In fact, it was a fear that one of the RS-25
wasn't cooling down prior to ignition that caused the first scrub of Artemis 1 on
its very first launch attempt in August of 2022. It's a scrub. It is officially a scrub? Officially. All right. Launch director called it. Launch director called a scrub. Engine number three's temperature sensor
wasn't showing the engine had cooled down enough to the required temperatures, so the launch attempt
was scrubbed for the day. It turns out the engine was likely
indeed at the right temperatures, but the sensor itself was erroneous. But it just goes to show how
important engine chill down is. If an engine's not
cooling down as expected, no diligent launch conductor
would proceed towards liftoff. Some liquid fueled rocket engines don't
use any cryogenic propellants such as hypergolic propellants. Hypergolic propellants are those that
will spontaneously combust when they come in contact with each other. But hypergolics are happy as a clam at
room temperature, and because of that, the engines don't really need
to be chilled down. In fact, many intercontinental ballistic missiles
can sit for years fueled up and ready to go, and can be fired within a moment's notice.
There've been a handful of US-based hypergolic orbital rocket engines such
as the LR-87 used on the Titan 2 or the AJ-10 on the Delta 2's upper stage. But the Soviet Union developed a
ton of hypergolic rocket engines. For those of you who have watched my
entire history of Soviet rocket engines, video may know the Soviets
well, specifically propulsion
engineering legend, Valentine Glushko, loved
hypergolic rockets. Just take a quick glance at this
family tree that we put together. Any orange or yellow engine near the
bottom of the chart is hypergolic. Yeah, that's a lot of
hypergolic rocket engines. Some of the more noteworthy ones are
the RD-275s on the first stage of the Proton rocket or the mighty RD-270. That was a full flow stage combustion
cycle engine designed for a massive lunar rocket that sadly never left
the drawing board. Okay, so we've chilled our engine
down or we're using hypergolic. So we're already at
operating temperatures, but now we need to get the pumps spinning
and get them up to operating pressures of potentially hundreds of
Bar and tens of thousands of RPM. Perhaps the number one
rule with rocket engines, and I guess the universe in general is
pressure always wants to flow from high pressure to low pressure. So engines need to have extremely high
pressure upstream in order to not send a flame backwards through the system, which would inevitably lead to a
catastrophic failure. As you likely know, having a simple pressure
fed engine is easy. You just open some valves and
ta da, you're running an engine. The rep propellant is already
stored at high pressure, so it'll naturally flow
into the combustion chamber
at the necessary operating pressures. But what about an orbital class rocket
engine that has turbo pumps that spin at absurd speeds? We're
talking high power, high chamber pressure engines. Take a look at an open cycle turbo
pump powered rocket engine while it's running. Notice the fuel pump and the oxidizer
pump are connected by a shaft to a turbine that powers them. Turbines
can be insanely powerful with some engines making hundreds of thousands
of horsepower from just a simple turbine. So what powers the turbine? Where does that power
actually come from? Well, there's usually a preburner or a gas
generator of some kind that's kind of like a rocket engine on its own right
and it's sole purpose is to produce energy to power the turbine. But take
a closer look at the gas generator. It's fed by the pumps. The pumps
are powered by the gas generator, but the gas generator is fed by the pumps, but the pumps are powered by the gas-
Okay, yeah, you, you get the point! Okay, here's the second step of starting a
liquid pump fed rocket engine: spin up. Now, there's a handful of ways you can
actually get the pumps up to speed, but perhaps the most common
is by utilizing a separate
high pressure gas to get the pumps spinning. This can either be
supplied with an onboard system like helium stored in a high pressure COPV
or Composite Overwrap Pressure Vessel, or it could be supplied by ground systems
where weight doesn't matter since it doesn't contribute to the overall
mass of the rocket. In either case, high pressure helium or nitrogen is
pumped into the gas generator to get the turbine spinning up to operating
speeds. For a short period of time, the engine's pumps are being powered
by basically a cold gas thruster, which isn't very efficient. A cold gas thruster has a low
specific impulse or a low efficiency, and you would want to continue to run
the engine off the system any longer than absolutely necessary. But it's simple and reliable since it's
just opening up a valve and letting high pressure gas flow into the engine, into the preburner or gas generator
through the turbines until the pumps are spinning fast enough to begin combustion.
Some engines will use a tiny little rocket motor called a starting cartridge
to do pretty much the same thing. It's quite literally kind of a small
rocket motor of either solid propellant or hypergolic propellants that
act as a gas generator, almost like the cartridge in an airbag
whose sole purpose is to generate a lot of gas to quickly inflate an airbag. So for engines that
require multiple restarts, such as upper stage engines that might
have multiple burns to get to their desired destinations or
even SpaceX's Merlin engine, which has to light up three to do that
boost back re-entry and landing burn on a Falcon 9, you have to have enough nitrogen or helium
or starting cartridges to get through every single one of those startups. We should probably take a quick little
second here and mention the Rutherford on Rocket Lab's Electron rocket, that thing's electric pump fed. And of
course we talked about that in my "How do you power rocket engine" video,
but now in context of starting, just think about how much easier it
would be to start an electrically pumped edge engine. You literally just send electricity
to pumps and work on the timing, and it's pretty much done from there on. Like it'd be so easy to tune and figure
out the timing when it's all just direct correlated to the speed of a motor. Now, it might be tempting to think you could
just use a small electric motor to get these spinning similar to
how a car engine starts. Just get the pump spinning for a
brief moment and then let the engine run itself. The problem is the power
requirements for these pumps is insane. We're not just talking about peak rpm, we're talking about
horsepower, so RPM and torque. Or more specifically, RPM
multiplied by torque. In fact, the RS-25s fuel preburner
delivers about 200 horsepower per kilogram. Think about that. How big is a 200
horsepower engine or motor? I'm fairly certain that you won't
find one that's only one kilogram, but there's another way to spin up an
engine that doesn't require a separate source to get the pumps up to speed. It's called bootstrapping or tank head
or sometimes called deadhead starting, and it's where you carefully allow the
engine to initially light up using only the tank pressure and the energy in
the thermal difference between the propellant and the engine. The RS-25 on the Space Shuttle
and SLS do this. At the beginning, the turbine will begin to spin up
because the hydrogen flowing through the engine and the walls and the combustion
chamber and the preburner boils off like we mentioned before. But instead of just having it bleed
out and go to the flare stack, during the startup process, we can actually run that high pressure
gas through the preburner into the turbine to begin spinning it. Then
there's a very delicate and precise dance to let in some oxygen and light
the preburners under low pressures. That weak combustion will slowly gain
pressure as the pumps begin to spin up over the next few seconds. As the pumps
increase in speed, the pressure rises, which increases the
power of the preburner, which then increases the
speed of the turbine, which increases the speed of the pumps,
which increases the pressures. Yeah, it just keeps going that way and keeps
rising until the pumps are at adequate speed for main combustion. Fireflie's Tom Markusic mentioned they're
looking into trying to bootstrap their Alpha rocket's second stage
engine, the Lightning, if at all possible to remove the
nitrogen spin start system from the upper stage. We do a nitrogen spin
start from the ground. Although we've recently
did a series of tests where we're trying to figure
out how low we can go on that, that spin start when before the bootstrap itself. So we've definitely been playing with
the level of spin start that's required, but we do spin start them. And,
yeah, what we found that we is, we can probably just tickle this thing
with a little nitrogen at altitude and get it to run much lower... very low spin start requirements. Yeah,
the dream would be to deadhead start it, but we haven't gotten that brave yet. But bootstrapping is definitely a feedback
loop that takes very precise control to get up to speed. In fact, bootstrapping is extra difficult because
the engine spends a lot of time in what's called a transient, which is the time between
being off and up to full power. And this is perhaps one of the most
complicated and challenging pieces of this puzzle. Transients are the in between moments. So in between the engine being stationary
and at full power and vice versa, or even between throttle settings, the
engine is actually in a transient state, but once the engine is
running and at a steady state, it's relatively easy to keep running.
But why is the transient so hard? Well, remember this whole
chicken and egg scenario we had. We're just getting the pumps up to speed. How do the pumps run if they're
powered by a gas generator that itself runs on the pumps? While the biggest challenge with the
whole startup process is the fact that everything is intertwined and a change
anywhere in the system will immediately have an effect upstream and
downstream from the change. So let's say you're trying to
bootstrap an engine such as the RS-25, like we touched on earlier. The first thing you're going to do after
you purge and thermally condition the engine is you're going to introduce some
liquid hydrogen into the pumps and the preburner on its way in it will
flash boil into a gas that gas expands and that expanding gas is what's
going to start to blow through our turbine and get our pumps spinning
up. Stoke Space CEO, Andy Lapsa, explained this to me when I was standing
with him on their test stand with their incredible engine that bootstraps. The start transient and to some extent
also the shutdown transient are some of the highest risk moments in a rocket
engine, right? And if you think about it, like our little engine, a thousand horsepower in the pumps
bigger engines have tens of thousands of horsepower going through the pumps, and they go from zero
to full power in maybe a couple seconds, right? So you've gotta control
all those horses and make sure they're running in the
right direction during that time. So in early development is
usually one of the higher risk activities for rocket engine. Quick little side note, wow, this is
honestly one of my favorite conversations, and Andy Lapsa and his
company, Stoke Aerospace, is working on an absolutely
incredible rocket. Okay, but that boil off is spinning up
the turbine, which spins the pumps, so now the propellant is
starting to flow a little faster. As long as the pressure in the system
continues flowing in this direction during this transient region, you're golden. The problem is there's often slight
delays between action and reaction. So it's almost like there's waves of
pressure changes moving throughout the system, and if those get too extreme, the pressure gradient might actually
start to flow backwards and you can stall out your startup sequence, or even
worse, potentially blow up your engine. Take a valve opening, for example, as a valve opens and introduces
say the oxygen into the preburners, that too will have an effect
on the pressure and the flow, and then the preburner ignition will
affect the pressure and the flow. It's all just a constant
back and forth feedback loop. But now we've touched upon another
piece of the puzzle: ignition. We mentioned how the preburner will
light under relatively low pressure, but how does it actually
light what starts the ignition process? Just because you combine your fuel and
your oxidizer doesn't mean you're going to get combustion. In
fact, most repellants, to the dismay of rocket engineers, will kind of happily coexist in the
same space and may even exit the system without unleashing their
fiery goodness at all. Or even worse, they may accidentally ignite
at the wrong time or place, which is often catastrophic. Perhaps
you've heard the term hard start. This is basically when the propellant
combust at the wrong ratios or perhaps at the wrong time or place. The worst hard starts can severely
over pressure the engine or maybe overspin the turbines or cause an
energetic detonation that can completely destroy the engine and sometimes even
the test stand and or the rocket that they're actually attached to. So the timing and precision
of ignition is vital. Like most engines, rocket engines typically require an
external source of ignition to begin the combustion process. But what if we
just use propellants that don't need an ignition source at all?
Yes. You know, hypergolics, we've already touched on them earlier
and, and almost every video I've made, but being able to spontaneously combust
is one of their biggest advantages. And the fact that they don't need any
additional considerations to keep liquid at cryogenic temperature is just
really a pure bonus. I mean, don't get me wrong, they're a total pain in the butt in
other ways like being super toxic and carcinogenic, but in the case of
ignition and sheer reliability, they're hard to beat. Which is why they're so common for
maneuvering thrusters and on orbit engines where reliability and long duration
missions matter since they won't boil off. But for LOx based propellants, if you put your fuel and oxidizer
in contact with each other, it likely won't lead to combustion. If
you took liquid methane and you poured it into a container of liquid oxygen,
don't really expect anything to happen. Even if you swirled it around
without a source of ignition, the two will coexist surprisingly
peacefully. Even in gaseous form, this is true for the
most part, but as a gas, it's much more likely to ignite
with just a small ignition source. So non-hypergolic fuels and oxidizers
not only need to be properly mixed for stable combustion, they also need an ignition source to
initiate the combustion process in the first place. But once stable
combustion has been achieved, and assuming there's adequate and
stable flow of your propellants, the initial ignition process is no
longer necessary and it can actually be self-sustaining. The combustion in the chamber acts as
a continual source of ignition. Okay, so we've actually got to
light up our propellants. Perhaps the simplest option outside of
hypergolic propellants is what the Soviet Union decided to do with the R7 and
what Russia still uses today on the Soyuz, which is basically just
some giant matchsticks. Yep, literally just some large braces with
a pair of pyrotechnics that are put inside each combustion chamber and are
all lit prior to the propellants arriving in the combustion chamber. Now, this is also possible because the gas
generator on the RD-107 and RD-108 is basically just a monoprop thruster, so it doesn't need a separate
source of ignition. So you could, the main thing you really have to worry
about on those engines is actually igniting the main combustion chamber. ut it's not very elegant and it's
probably not a great option once you're in space either. So for that, we'll have to look into some other options
that are capable of space starts and potentially restarts. Perhaps one of the most common methods
is to basically use a spark plug. Okay, maybe not your off the shelf
spark plug from a Ford Fiesta, but the concept is the same, take a high electrical voltage
and have it jump a gap, which provides the perfect opportunity
to ignite the propellant with ionized electrical energy.
As you can probably imagine, it does require a good amount
of electrical energy to
power a rocket engine size spark plug. So rockets using this approach may need
to carry heavy batteries or utilize ground support equipment to run
current through the igniter at startup. Another method is similar to
a spark igniter, actually, it's again similar to the automobile
industry with diesel engines, and that's using a glow plug. Glow plugs also require a lot of
electricity to convert to heat, similar to an incandescent light bulb,
an electrical heater coil, or a toaster. There needs to be enough heat energy on
the surface of the glow plug to initiate combustion of the propellants. And again, once stable combustion has been achieved,
the glow plug can just be turned off. Another method is to use a laser. Lasers can very precisely focus the
beam of energy and tune it to excite the propellant molecules, so it has the potential to be more
efficient than other electrically driven devices.
Now, it's not very common yet, although Airbus does have some variance
of optical ignition for some of their boosters. These methods are
all great and commonplace, but they all require massive
amounts of electrical power, which is likely stored in big and
not so energy dense batteries. So what if you actually could just use a
chemical reaction to start the ignition process? This is actually pretty common
and extremely effective. In fact, if you've ever seen a
Falcon 9 or Heavy launch, you may have noticed a green
flash right at the beginning of ignition. That, my friends, is the injection of a pyrophoric ignition
fluid coming in contact with oxygen to begin stable combustion.
Pyroforks are a type of hypergolic. It's a fluid that will spontaneously
and virtually instantaneously react with oxygen. So just inject some pyrophoric fluids
into the combustion chamber along with oxygen and ta-da, you've
got yourself a flame. SpaceX uses an ignition fluid called
TEA-TEB or triethyl aluminum triethyl borine. It's stored in their own canisters
onboard the rocket and allows for multiple restarts of their Merlin engines. Now, not every engine is connected
to a TEA-TEB canister, only the engines that would do a relight, so three of the nine Merlin engines on
the booster and the vacuum Merlin engine on the upper stage. But on the ground, the Falcon 9 and Falcon Heavy are lit
using ground supplied TEA-TEB because why not keep those systems and their weight
on the ground if you don't need to carry that mass off the launch pad? Now, SpaceX has taken a little bit of a hybrid
approach to ignition with their new Raptor engine. The Raptor engine uses
what are called a torch igniters. Torch igniters are basically just a fancy
version of the lighters that you might use to light a candle. You may or
may not hear them called ASIs or Augmented Spark Igniters. The ASI has its own little small spark
ignition and then its own supply of methylox that allows
for the torch to burn. So it in itself is almost like a little
flame thrower that's initially lit with a spark ignition. Then that torch remains lit through
the rest of the startup sequence. So it's kind of like the cascading effect
of the solid rocket booster ignition, this uses a smaller electrical
ignition source to ignite a small baby flame thrower. But surprisingly, this is only done on the preburners
because believe it or not, the Raptor engine has no igniters
in the main combustion chamber. So you got torch igniters for
the main chamber but Raptor 2 has no torch igniters in the main chamber. So you can see it's much
cleaner around the chamber area. How do you... how does it light then? Well, that's secret sauce! Although Elon was tight-lipped about how
exactly they get away with ignition in the main combustion chamber, it's actually assumed that SpaceX is
getting away with homogenous combustion or basically spontaneous combustion when
the methane and oxygen come in contact with each other. But the main reason that they can get
away with that is because in the Raptors full flow stage combustion cycle, both the fuel and the oxidizer are
preburned in their own preburners, and then they arrive in the main
combustion chamber as hot gases already. So take the fact that it's a gas-gas
interaction along with ridiculously high pressures seen in the combustion
chamber, no ignition source is necessary, the propellant is already plenty
hot and bothered and ready to burn. Well, I think that pretty much covers all the
major parts of starting a rocket engine, but before we get into some examples, let's actually cover one more challenge: starting a rocket engine in space. Starting a rocket engine on the
ground is pretty challenging, but at least it has a few
things going its way. First off, the rocket can utilize a lot of
ground support equipment for startup. This means the rocket doesn't need to
provide crazy amounts of high pressure starting propellant,
electricity, ignition fluid, or whatever else you may
need. And best of all, gravity is pulling down on the rocket
and the rep propellant held inside. So the more dense liquid propellant will
sit at the bottom of the tanks while only at the very top of the tank. Opposite of the feed
tubes will have gases. But the same thing isn't true when
trying to light an engine in space. Well, not just specifically in space, but more when you are not accelerating
or not on the ground because when not accelerating, everything is in the same inertial
reference frame. In other words, when the engines are off and the
spacecraft is just floating in space, there is no acceleration, meaning the propellant is more or less
just a blob floating around in a tank. So in order to start a
rocket engine in space, we first need to make sure the liquid
propellant is at the bottom of the tank, so the pumps engine and or the
thrusters can suck up the liquid as intended and not get gas
bubbles, which can be really bad. A common method for doing this is by
using what's known as ullage thrusters. This is where you either use a small
solid rocket motor or cold gas thruster to settle the propellants. You can almost think of it like a
butterfly net trying to catch butterflies. By moving through space, it'll basically catch and collect
all of the liquid propellant. You'll see footage of ullage thrusters
at stage separation for many rockets. This incredibly iconic imagery of the
S-IVB ignition is actually from a Saturn 1B, but it's the same stage that was
used on the upper stage of the Saturn V. Not only did the ledge motors
help separate the stages, but it also did the propellant settling. This may also need to be done
for any restart in space. Say you have a circularization burn or a
geostationary transfer burn or a direct to geostationary orbital insertion
burn, or whatever you might have. If it's a liquid fueled rocket engine,
you'll likely have to do an ullage burn. And similar to a rocket needing
to do a spin start again. If you need to do multiple burns, you might need to either have multiple
solid rocket cartridges or use a cold gas thruster so you can fire
it up whenever you need to. Cold gas is pretty common for ullage
thrusters since it might also be used as maneuvering propellant for stages. Even
the Falcon 9 booster has to use cold gas thrusters to settle the propellant
before boost back and entry burns. You can use a cold gas thruster in
space because it uses gaseous propellant stored at high pressure, so there is
equal pressure throughout the entire tank, the gases won't blob about. But some spacecraft don't need
ullage thrusters at all. In fact, many maneuvering thrusters run on liquid
by propellants, usually hypergolics, and when they're in space in
order to do precise maneuvering, they can't do an ullage burn every
single time they need to do a little maneuver. Take SpaceX's
Dragon capsule, for example. As it approaches the
International Space Station, you'll see it firing its
Draco thrusters like crazy. These little tiny impulses help precisely
control and maneuver the vehicle with extreme accuracy. The Dracos run
on hypergolic liquid propellants. So how do they fire these engines
in space when the tanks have liquids in them?
There's a few ways to actually do this, but perhaps the most simple is with a
propellant management device or a PMD. This can be a simple device like a vein
screen or sponge to wick the propellant towards the tank outlets using surface
tension and some other really cool tricks. Another way is with tank
bladders. Yep, believe it or not, many in-space tanks will have a bladder
inside to separate the liquid from the pressure and gases. This way, the propellants can still be pressurized
to necessary pressures and the bladder will help keep gas
bubbles out of the liquid. These are most commonly
used on spherical tanks, but there's also some piston
tanks that do a similar thing, but these are more common on elongated
cylinders, as you can imagine. Although it is a piston, it's
usually just a moving seal. They're still actually pressurized by
helium. Most in space thrusters and engines will utilize one or a combination
of a few of these devices to best handle propellants in space. But what if I told you that there's an
even crazier way to light an engine in space without ullage thrusters
or bladder tanks or anything? Have you ever noticed the graded fence
looking section of the soy user proton rockets, or maybe you've seen these
cutouts on the inner stage of the Titan 2? Well, these are necessary to perform
the ultimate endearing space events, hot firing, otherwise
known as hot staging. This is where you light the engines on
the upper stage while the stage below it is still attached and running. Yep, you just light that engine while there's
still a rocket attached to the bottom of it. This is called "Fire in
the Hole" for obvious reasons. But by doing this, propellant is
already experiencing acceleration. So liquid will obviously be
at the bottom of the tanks, but when you light an engine, the
exhaust will have to go somewhere, and now you know why there's so much
of an open gap between the stages. All right, that pretty much sums
up how to start a rocket engine, but now let's actually go over a real
life example and see if we can follow along. Okay, my friends, I think in order to really put all this
together and dive really deep into this, we need to get inside a rocket engine
during startup and go step by step through the entire startup process of one of
the most complicated rocket engines, the RS-25. If we can walk
all the way through this one, I'm not gonna say that we've mastered it, but we should definitely have
a very good understanding. Oh, what's best of all with the RS-25 is
we have some incredible insights on how exactly this engine starts up because
the startup process is actually really, really well publicly
documented. So we can go really, really deep and actually explain what's
going on inside the engine for the entire startup process. And for this, I actually bought a book called
Space Shuttle Main Engine, the first 10 years by Robert E. Biggs,
and it's an awesome book. It goes really, really in depth into this process. But the coolest thing about this book
is it has these charts showing the pressure and valve openings
during the startup process. So we'll draw these out a little bit
more cleanly along with a diagram of the engine so we can go just step by step
really slowly and watch all the things change throughout the entire
process. Now, I must warn you again, if you haven't watched my how
to power a rocket engine video, you really will need to
have a good understanding of
the fuel rich closed cycle engine before you go through this portion
of the video because we get really, really in depth with the exact
components. But as a quick review, the RS-25 has two preburners
since it's fuel rich closed cycle, both pre burners are fuel rich, meaning all of the fuel will
flow through the preburners, one preburner powers the oxygen pumps
and the other powers the fuel pumps. Then there's a small boost pump on the
oxygen side that feeds the preburners, the higher pressure liquid
oxygen that they need to operate, and there's a handful of valves, the prevalve that shuts off
the tanks to the engines. Then there's the main fuel valve, which feeds the pre burners
and the regen cooling channels. But the regen system has a separate
valve called the chamber coolant valve, which can be throttled to redirect fuel
between the main combustion chamber and the regen system. Then
there's three oxygen valves, one that feeds each of the
three combustion chambers, so both of the pre burners and
the main combustion chamber. There's a few recirculation pipes that
feed boiling off gaseous propellant either back into the tanks or
vents them out into the atmosphere. There's also what's called ASIs
or Augmented Spark Igniters. There are three sets of ASIs, one in each preburner and one in the
main combustion chamber. These ASIs have their own fuel and oxygen supply lines
and will be the first items to receive each propellant in the system. Now there's obviously a million other
small tubes and pipes and sensors littering the real engine, but to
try to keep this understandable, I think we'll just leave it
at this for now. As we know, the first thing a cryogenic liquid
propellant engine needs to do is be purged and thermally conditioned for launch. So the RS-25 first goes into what
they call the Start Preparation Phase. During this period, the oxygen side of the engine is
purged of moisture using nitrogen, and the fuel side is purged with helium. Then the engine can have the
cryo propellants in its system, so they would open the main fuel pre
valve that allows liquid hydrogen to flow through the fuel pumps down to the
main fuel valve. There's a small recirculation flow that either dumps
some of the hydrogen or might even pump some of it back into the fuel inlet. Liquid oxygen fills the oxygen side of
the engine by opening the oxygen prevalve and then it flows through the
oxygen pumps up to three valves, which will all need to be very precisely
controlled during the startup process. The propellants are inside the engine for
an hour or more to fully condition the engine Throughout the stage, the main engine computer is constantly
monitoring the pressures and temperatures 50 times per second to make
sure everything is looking good. Then at about four minutes
before engine ignition, there's a final engine purge with
helium downstream of the main fuel valve. Now assuming everything is looking good
to go and it's all within the predefined parameters, the computer will go into what's called
the engine ready status At three seconds before engines start, the bleed valves for both the oxygen
and hydrogen lines are closed and the engine waits for the start command.
The engine is now fully thermally conditioned, purged of all
moisture from the atmosphere, and ready to take on the challenge of
what's to come because here's where things get super complicated. The moment
the start command is received, the first thing that will happen is
the main fuel valve is fully opened. This valve actually takes about two thirds
of a second to fully open even at its maximum rate. At the same time as
the main fuel valve is opening, the ASIs are powered up and ready to
ignite any propellant that they come in contact with. So ignition of the ASIs needs to
be before any mixed propellants are present in the system. As you can imagine, despite the engine being
cold by human standards, the actual components downstream of the
main fuel valve are still relatively hot, at least compared to the liquid fuel
since they haven't been soaking in that cold cryo liquid propellant, like the parts of the engine that
are upstream of the main fuel valve. This energy from the latent heat of the
engine is enough to begin spinning the turbine. The engine initially is starting up
and beginning to spin up as an expander cycle engine, otherwise known as bootstrapping
or deadhead starting
like we mentioned before. But it's facing some pretty
major thermodynamic instability. As the propellant flash boils, it'll create uncontrollable
but predictable oscillations, and that's one of the hardest things here. Since everything has a little
bit of a delayed reaction, you need to actually predict when the
peaks and dips and the pressure will occur during this process. These oscillations will occur for about
1.5 seconds until the main combustion chamber reaches what's known as prime. Now prime in this example is when the
pressure is stable on each side of the injector, well, specifically when the mass flow rate
is stable. So prime occurs in all three combustion chambers when there's stable
flow between the pumps and the chambers, each combustion chamber's targeted
prime time is very important so as to keep things moving
in the right direction. So we've introduced fuel into the system
and mostly flowing through the pumps and the turbines and beginning
to get the engine spinning. Fuel is flowing through the three
sparking igniters and they have electrical power. Now we've got to
start introducing oxygen. The first thing that's going to receive
oxygen is the igniter inside the fuel preburner. The system will start to flow liquid
oxygen basically as soon as that fuel preburner LOx valve starts to even open. At just 5% opening of the
fuel preburner oxygen valve, LOx is directed straight to the igniter. The timing of when the valve starts
to open and when the oxygen will start flowing into the igniter coincides
perfectly with the first dip in pressure during those pressure oscillations.
This helps make sure that you don't have backflow of hydrogen up
through the oxygen system, but also ensures that you're setting up
to have the right mixture ratio for the first bit of combustion. From here, the fuel preburner oxygen valve has to
do a lot of work to kind of ride the waves of those oscillations to flow
through the highs and the lows of the system, and again, they can't react to the
pressure oscillations because
of the delayed reaction between the valve opening and
the events happening downstream. So these oscillations had to be precisely
documented and predicted. In fact, every time you see the valve
moving during this period, you can pretty much assume that there was
an engine that blew up and changes had to be made from the lessons
learned. By this point, the pumps are spinning pretty quick and
the system is getting closer to reaching an equilibrium in all three
chambers, or again, hitting prime. At 1.25 seconds, it'll do a speed check of the fuel pump
turbine. It was found that the pump needed to be above 4,600 RPM in
order to continue moving forward into fuel preburner and main combustion
chamber ignition. Otherwise, there wouldn't be enough hydrogen
pressure to overcome the main combustion chamber pressure. At 1.4 seconds, the fuel preburner hits prime right when
there's that large dip in pressure and then that causes a rapid
rise in pressure. Now, this causes the fuel turbine to
spin up very quickly. In fact, there's virtually no back pressure after
the turbine from the main combustion chamber yet, since that hasn't
hit prime at this point. So the turbine spins up ridiculously
fast. If left unattended, it would actually overspeed the turbine
and would likely cause a catastrophic failure. So making sure the combustion chamber
hits prime at exactly the right moment is extremely important to provide the
necessary back pressure so the turbine doesn't spin up too fast. Now notice
we are spinning up the fuel turbine and pumps first. This ensures that the whole system has
higher fuel pressure and ratios that will ensure a cool fuel rich start. Obviously, it can't be so fuel rich that it
floods a system and can't be lit, but it's better to stay too rich rather
than to lean and be anywhere closer to stoichiometric. So let's actually walk backwards now a
bit to 0.2 seconds after engines start, that's when the main combustion chambers
oxygen valve will start to open to flow oxygen into the main
combustion chamber igniter. The main combustion chamber valve is
slowly open to just under 60% open. The delay and the slow rate of opening
makes it so the main combustion chamber igniter has oxygen at 0.85
seconds after engine start, and this will begin the main combustion
chamber ignition at a nice safe fuel rich mixture ratio. Main combustion
chamber hits prime at 1.5 seconds, which causes the pressure in the main
combustion chamber to rise rapidly and helps prevent over speeding the fuel
turbine with an increase in back pressure and therefore resistance
on the turbine. Okay, so let's actually go backwards again in
time and go through the oxygen preburner system. The oxygen preburner valve initial
opening is just 0.12 seconds after engines start, but it's designed in a way
that its initial opening is
all it takes to power the oxygen preburner igniter. The timing of this makes it so the
oxygen preburner igniter is lit at 0.95 seconds, just 1/10 of a second after the
main combustion chamber igniter. Now, the oxygen preburner valve is designed
to not really flow oxygen all the way through until about 46% open. Again, it's very important that the flow
of oxygen is generally conservative. It's a careful balance between giving
the system enough oxygen to begin combustion and help to provide the
power necessary to run the engine, but not giving it too much oxygen where
the engine can start to run lean and experience damaging temperatures.
As you can imagine, the oxygen preburner valve helps control
the power of the oxygen preburner, which controls the speed of
the oxygen pumps turbine, which is what controls the
speed of the oxygen pumps, which is what controls the
pressure in the oxygen system. So that one valve actually has a
huge effect on the entire engine. The oxygen preburner is the last of
the three chambers to hit prime at 1.6 seconds, and that's again done to ensure the oxygen
pressure doesn't get too high in the system. At 1.7 seconds
after engines start, the main engine computer verifies that
all three combustion chambers had proper ignition and are operating normally.
At the beginning of this phase, where the engine has all three
combustion chambers lit and primed, the main combustion chamber is at
roughly 25% of its rated power level, but it's far from stable. And if you
check out slowmo footage around this time, you'll actually notice the rocket engine's
nozzle just oscillating like crazy. You might also notice there's some big
spikes of flow separation going up into the nozzle, and that's because the pressure inside
the nozzle is actually still lower than atmospheric pressure at sea level. So the air's atmosphere is actually kind
of creeping back up into the nozzle and it has these big oscillations as
those shockwaves kind of form, and it has a bit of an
instability at this point. So in order to increase stability and
pressure and increase safety margin, the chamber coolant valve, which was fully open up until this
point is throttled down to 70%. This forces more fuel into
the main combustion chamber. It does this for 0.4 seconds to help
absorb variations in the pressures and temperatures. Now, up until this point, the main engine computer's actually
been operating in open loop control, meaning it's only receiving
pre-programmed commands, it's not like reacting to anything,
it's just a set of orders basically. But at 2.4 seconds after engines start, the computer goes into
a closed loop control, meaning that throughout the rest of
the ramp up to rated power level, the main engine computer is actually
reacting to the pressures and temperatures and making adjustments accordingly to
try and follow the path to ramp up. Most of this is done by controlling the
oxygen preburner oxygen valve because like we said, that really has a huge
effect on everything else. But remember, again, this is all pretty tricky since there's
a pretty big delay between reading the combustion chamber and the
temperatures and reacting to it. It's not actually like the reaction time, but just how long it physically takes to
say open a valve and for those changes to make a difference downstream. So this all has to be done just
incredibly carefully. At 3.8 seconds, the system goes into fully closed loop
mixture control. So not just closed loop has it head been operating, but now
it's fixed even with its mixture ratio, meaning only the fuel preburner
oxygen valve is used to ramp up the correct mixture ratio in
the main combustion chamber
trying to get to that 6:1 ratio, which will occur
right around 5 seconds. And this also means the engine has
fully reached operating power levels and during this ramp up period we're awarded
with those gorgeous shock diamonds or mach diamonds, and they're
just so perfect on the RS-25. It's honestly crazy to me that this all
happens in such a short period of time and that they could ever achieve the
reliability they did with a Space Shuttle and now with SLS. And now let's take a look back
at the chart again one more time, now that we hopefully understand a
little bit of what's actually going on. We can see how each of those dips of
pressure and the corresponding valve positions was obviously a learning lesson.
There were 19 tests through 23 weeks with eight turbo
pump replacements just to get through the first two seconds of startup. And
it took another 18 tests, 12 weeks, and five more turbo pump replacements
just to get up to full power. That's honestly actually fewer bits
of hardware than I would've guessed, but it still helps appreciate
how hard and how expensive engine development can be. Not to say there weren't
additional lessons to be
learned once they developed a reliable startup sequence, but operating at a steady state is
orders of magnitude easier than the dynamic startup process. Well, I think
that should do it for an example. I mean, that's about as deep as I can get, so I think it's time we wrap up by
touching on throttling and the opposite of startup shutting the engine down, and then we'll do some final
thoughts and a summary. Of course, startup isn't the only dynamic
situation a rocket engine faces. Sure, once an engine running, it's in a steady state and there
shouldn't really be any real surprises. But what happens when an engine throttles
down for maximum aerodynamic pressure or MaxQ maintaining desired peak
G loads or even when it's landing? As you can likely imagine, this varies
very much from engine to engine, and frankly, this is probably a topic
for another video. But in short, engines generally throttle by reducing
the flow to the preburner or the gas generator, so usually with one of the control
valves and often by reducing the flow of oxygen to maintain a fuel rich state. The same thing goes for shutting an
engine down. It's another dynamic event, and the general rule of thumb is you
never want to get close to stoichiometric conditions since engines
usually run fuel rich, you do this by first reducing the oxygen
flow and then the fuel flow. Generally, you want to shut an engine down
pretty much as quickly as possible, but you often have limits on how quickly
you can do that to avoid high G loads. For example, when the RS-25
was on the Space Shuttle, there was a limit to how quickly the
thrust could decay so as not to exceed the orbiter's structural limit. The initial oxygen preburner
oxygen valve motion was limited to 45% per second. The main oxygen valve also had to
be closed at a particular rate, but mostly to ensure there was sufficient
back pressure on the turbines so they wouldn't accidentally over speed
during the shutdown process. To me, it's just crazy how many considerations
there are to absolutely every single input and condition. Like I feel like there's a million lessons
to be learned that frankly can really only be learned the hard way by examining
scraps of an engine and trying to figure out what went wrong. I think it's
kind of this mindset that drives SpaceX to test their Raptor engines so quickly. They truly believe that by just
getting them out on the stand, not treating them as some
one-off golden pony or something, they can learn more quickly. There's countless lessons that they have
learned that have shaped safe operation during startup throttling and shut down. And I know that many of you are probably
wanting to know how exactly the Raptor engine starts. Its startup process isn't
as well publicly documented, of course, as say the RS-25, but it does have
some similarities. It of course, has two pre burners, like the RS-25, but the biggest difference is that one
preburner is fuel rich and the other is oxidizer rich. So this means the interaction between
the two preburners is even more intertwined. Changing the speed of one has a very
direct correlation and impact on the other one. In the case of Raptor, you've got an
oxygen power head and a fuel power head, and they're different shafts and you've
got two turbines and two preburners and, and they're cross
feeding one another. So the start sequence for Raptor is
insanely complicated compared to the slot sequence for Merlin. It has to be perfectly precise
cause each one relies-. Basically you're doing this delicate
dance between the fuel power head and the auction power head
and if they get out of sync then you can go stoichiometric
in the preburners and melt or explode the preburners.
Once it's running, it's a much easier situation. But if you get anything wrong with that
star sequence you're either gonna melt or explode the engine. Initial spin up of Raptor is done
with either helium or nitrogen. And as we mentioned before, there are torch igniters in the preburners
and likely some kind of homogenous ignition in the main combustion
chamber. And from there on, I honestly can't even begin to imagine
what goes on internally to balance the startup process. It's no wonder they're firing up raptors
about five times every day at this point. They've really gotta get all the kinks
worked out because there's going to be so many engines going through startups
simultaneously, it has to be perfect. So to summarize, starting a
rocket engine is very hard. Some are easier than others, but you can imagine why it's easy for a
company to come up with a rocket engine or concept and very hard for them to
get into production and operation. So when a company shares that they've
successfully started up an engine and made it all the way up into
operating power levels, it's very applause worthy as they've
likely made it through the biggest hurdles and development. And it's really fun
to see how different programs tackle starting an engine and what techniques
they might employ to achieve a reliable engine. But of course, starting a rocket
engine can be extremely complicated, like having valves changing their
position by just two degrees within a few milliseconds to avoid catastrophic
conditions from pressure oscillations like in the RS-25. Mastering this sequence can take
thousands of hours of development, which means millions of
dollars in time and hardware, which can mean years and years
of hard work and troubleshooting. It is pretty much a miracle to me that
people have figured out how to make rocket engines that are so complicated
and so reliable considering how close to catastrophe they are throughout
the entire startup sequence. But that pretty much does it. Hopefully this video helps give you
an appreciation of what engineers and scientists have to solve before your
favorite rocket has ever made it to the launchpad for the first time.
It's just absolutely incredible how much work has to go into this. Let me know if you have any
other questions or thoughts
in the comments below. I owe a huge thank you to my Patreon
supporters for helping make videos like this and everything we do here
at Everyday Astronaut possible. I also have to give a little extra
thank you to our Mission Directors and Commanders who were there for
some of the script read throughs. We did two of them for this script where
I'm sitting there reading through the entire thing in Discord and people are
giving me comments and feedback as we read. We kinda go like
paragraph by paragraph. We developed that script a lot
just from those read throughs. And then the rest of our
Mission Specialists and
Pilots ended up doing a script read through on their own and giving a
lot of good feedback and this video has really evolved a lot and I
learned a ton making this, which to me is always kind of the bookmark
on whether or not I think a video is going to be good. It's just based on how
much did I learn and I learned so much. So I really hope you guys do too. So if you want to help give your feedback
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