Are Aerospikes Better Than Bell Nozzles?

– Hi, it’s me, Tim Dodd,
the Everyday Astronaut. I’m in Mississippi, at the
Infinity Science Center, just outside of NASA’s
Stennis Space Center, and I’m here to check out a rocket engine, but not just any rocket engine; there’s a very special rocket in here that’s kind of inside
out and promises to be as efficient at sea level
as a sea-level engine and as efficient in space as
a vacuum-optimized engine. Welcome to the aerospike. And with engineers relentlessly
trying to optimize rockets in a world where even a 1%
improvement is a massive leap, the aerospike engine seems
like a dream come true. Throughout history, there’s actually been a number of aerospikes that have made it really far into development. But to date, none have ever really flown, let alone been used on
an orbital-class rocket. If they’re so good, why
isn’t everyone be using them, or for that fact, why
isn’t anyone using them? Why aren’t these brand-new engines, like SpaceX’s Raptor
engine, that are built from the ground up, why
aren’t they aerospikes? So today we’re going
to look at the history of the aerospike engine,
and then we’re going to look at how nozzles work, including
things like overexpansion, underexpansion and expansion ratios. Then we’ll look at the pros and the cons, the physical limitations and problems of the aerospike engine. Then we’re gonna compare the aerospike to other traditional rocket engines. But that’s not all. I actually got some
never-before-seen photos and videos of some historical aerospike engines. Then I went out and
talked with some people who have actually worked on
and engineered aerospikes. Then we’re gonna look at
some compelling concepts and promising prospects for
some future aerospike engines. And hopefully by the end of the video we’ll know whether or not
the holy grail of rockets is just waiting to be utilized or if they’re just simply not worth it. So put on your favorite
aerospike engine T-shirt, grab some popcorn a drink and a notepad, ’cause this is a really long video and we have a lot to cover. So let’s get started. (intriguing music)
– Three, two, one, liftoff! – That’s one small step for man. – So those of you that
follow me on Twitter have probably been watching
this video come to life and have likely noticed
my tone change from “aerospikes suck” to a little more neutral stance on the subject. Does this mean I’m maturing? And wow, after literally
months of research, this turned out to be a monster. I mean, we ended up going
through dozens and dozens of hard-to-find documents
to be able to track down the actual numbers from their sources from some of these original
papers back in the ’60s. And although it took me a while, sorry, The more I learned, the more I realized just how much more I needed to learn, and the more context I knew
would need to be in this video in order to make it more understandable. So in order to make this more digestible, here’s the timestamps of the major topics. And I’ve got the links to those timestamps in the description and,
like all my videos, I actually have a full article
version up on my website,, if
you want to be able to search for a certain sections or
see some numbers later on. Again, the link’s in the description. After I posted my “Is
SpaceX’s Raptor engine “the king of rocket engines”
and “Why SSTOs suck” videos, I got so many people claiming
that the real holy grail of rockets is actually
the aerospike engine. And you’re not wrong. I mean, an engine that
magically, thanks to physics, is inherently more efficient
at almost any altitude? An engine that works as well at sea level as a perfectly-tuned sea level engine bell and still performs just
as well in a vacuum as a large vacuum bell? Wow! Yes, sign me up! Why isn’t there more love
for aerospikes engine in the aerospace industry? I mean, after all,
SpaceX engineers tackled the ultimate challenge by
developing a methane-powered full flow staged combustion cycle engine. If you could solve that crazy engine, why didn’t they try for an aerospike, something that promises some
pretty substantial gains and they’ve already been fully developed? After all, aerospikes
definitely aren’t new. They actually go all the
way back to the ’60s, when engineers were looking for ways to improve upon some early rocket engines. Now, keep in mind, the
engines at this point in time were relatively primitive, so the promise of an aerospike engine
was extra appealing. Perhaps the most notable
and promising aerospike was an aerospike version of the J-2 engine that powered the second and
third stage of the Saturn V. This was called the J-2T, and on paper it seemed to be a nice and
compact version of the J-2 while offering even
greater vacuum efficiency than the standard J-2. Although it hit the test stand 34 times and had some promising potential, it was shelved alongside the Saturn V and any potential upgrade path thereof once the Space Shuttle program began. But it was actually considered for use as the space shuttle’s main engine. But as we know, NASA
went with a closed cycle bell nozzled engine, the RS-25. Rocketdyne also took
spare parts from the J-2 and the simplified J-2S rocket engines and developed a linear aerospike engine known as the L-1 linear
test bed from 1970 to 1972, and it had 44 tests with
3,113 seconds of operation. But it would be almost 30 years before the concept would
be dusted off again and looked at with any
serious consideration, and this time it was for a
space shuttle replacement known as Venture Star. The Venture Star was a rocket
nerd’s ultimate dream rocket, a single stage to orbit, or
SSTO, fully reusable space plane that was to utilize a
linear aerospike engine called the RS-2200, that
not only made it look like the Millennium Falcon,
but it also promised nearly the same payload
capability to low earth orbit as the space shuttle it
was intended to replace. In order to minimize risk, Lockheed Martin began development of a
suborbital demonstration version of the Venturestar called the X-33 which was to use a smaller testbed version of the RS-2200 called the XRS-2200. It was fully operational
and had accumulated 17 tests and about 1,600 seconds
of test stand operations. But because of the overly ambitious use of super advanced carbon composite tanks, a few other technologies
that had yet to be perfected, and some interesting
politics, Lockheed Martin may have bit off more
than they could chew. The Venturestar program and the X-33, along with the RS-2200 and xRS-2200 linear aerospike engines,
were put on the shelf in 2001. (upbeat rock music) Okay, so what the heck? Is the aerospike just a bad luck engine? Being assigned only to programs which were destined to be canceled? Why aren’t any of these
plans being resurrected now with more modern technologies? Well let’s first get into how they work, and in order to do so, we
should do a quick overview of how and why a traditional
bell nozzle works so we know how to compare
them against aerospikes. I mean, after all, the
physics are the same between the aerospike and a bell nozzle. So, let’s dive in. Rockets work by taking a high pressure gas with molecules traveling in all directions and turning that pressure
into a high velocity flow in a very specific direction, and if all things are going smoothly, the direction of the
flow is exactly opposite of the direction that
the rocket is pointing. As we’ve talked about in my
video about the Raptor engine, the chamber pressure in rocket engines can be unbelievably high. SpaceX’s raptor engine is currently the king of chamber pressure at 270 bar and they’re aiming for 300 bar. Just to put into perspective
how much pressure that is, that’s like putting a BMW 740i sedan, which weighs nearly two tons, and placing it on
something not much bigger than an average sized US Postage stamp. That’s how much pressure is pushing down on every single square inch
of the combustion chamber. To convert that pressure,
engineers developed something called a
converging-diverging nozzle, or a de Laval nozzle, that
converts those high-pressure gas molecules traveling in all directions to a high-velocity gas
moving in one direction. Now we’re gonna go to fluid
dynamic city here real quick, and not just any old fluid dynamics but some super nutty stuff
where everything gets backwards and upside down as we’re
going to transition from a subsonic high pressure hot gas into a supersonic lower
pressure and cooler gas. So if our combustion
chamber was just a tube coming off the injector,
we’d end up with some hot gas moving out pretty fast,
but not super sonic. And a fun reminder here
about rocket engines and rocket engine efficiency: the faster the exhaust gas
is ejected, the better. So in order to accelerate a
gas moving through a tube, we can actually shrink
the size of the tube. The same amount of mass will
pass through this smaller area, and in order to do so,
it needs to speed up. I think the best example of this is a standard old garden hose. Turn it on and put your thumb
over the end of the hose. Now, ignoring a few potential loses, the same amount of water
will flow through the hose regardless if your thumb
is covering the end or not. The only thing that changes
is the water has to speed up to move the same volume of
water through a smaller hole. Now remove your thumb and
the water slows back down, but both methods would
actually fill a bucket in the same amount of time. One will just be a lot more
splashy than the other. So the same thing applies with
gas passing through a tube. We can keep shrinking the tube until we get to the point
where the gas is traveling at sonic speeds, or at
the local speed of sound. Now, I say local speed of sound
because the speed of sound goes up with the square
root of temperature, and because temperature in
rocket engines is really high, the speed of sound at the
throat can be five to 10 times faster than that of room
temperature at sea level. Once the exhaust gas is
traveling at the speed of sound, we can’t actually shrink the tube anymore, because here’s where things
get quite literally backwards. Once a gas is traveling
at the speed of sound, if you shrink the tube anymore, you simply end up choking flow. So from this point on,
engineers design the nozzle to be the perfect shape which will convert thermal energy into kinetic energy. So as the walls of the nozzle gets wider, it actually accelerates the gas, and the pressure and
temperature will lower. If you keep going with this
and you make the nozzle big enough, you’ll get to
the point where the exhaust is actually at the same pressure
as the ambient air outside, or in the case of sea
level, one bar of pressure. It’s actually kind of
amazing to me that we can go all the way from hundreds
of bar of pressure down to less than one bar in such
a relatively short distance. And it’s also weird to me that,
even though the exhaust gas is moving at unbelievable
speeds, the exhaust pressure can actually get really, really low. And the lower pressure we
can get the exhaust gas, the more we’ll have
converted high pressure into high gas velocity,
which is desirable. But as a rocket climbs in altitude, the ambient air pressure
outside of the rocket lowers, so the pressure at the end
of the nozzle becomes higher and higher than the ambient
pressure outside of it. You can see a rocket’s
exhaust plume expanding out in all directions as it ascends
into the vacuum of space. This is why vacuum optimized
nozzles are so freaking big. The perfect example of this
is SpaceX’s Merlin engine, which has the same combustion chamber between the sea level version
and the vacuum versions. The sea level version
with its sea level nozzle is small enough that
SpaceX can fit nine of them inside the 3.7 meter wide Falcon 9. But the upper stage vacuum
engine has only one engine because the vacuum nozzle’s
bell is about three meters wide. Just look at how snuggly it fits inside the interstage of the Falcon 9. You can tell whether or not a nozzle is optimal for the
altitude it’s at by looking at the exhaust coming out of the nozzle. A nozzle that’s perfectly
matching the atmospheric pressure will have exhaust shooting out straight in a perfectly expanded plume
or a perfect column of flame. Now if a nozzle is too small
for the altitude it’s at, it’s called being
underexpanded and you can see the exhaust immediately
getting wider and wider as soon as it exits the nozzle. If the exhaust is traveling out
away from the rocket at all, that’s energy literally being
spent thrown out the sides. You want the exhaust traveling
as straight as possible. At sea level, nozzles are
virtually always too big and that’s called being overexpanded. You can tell when a rocket’s
nozzle is overexpanded because the exhaust gets squeezed
tighter by the ambient air as it exits the nozzle and
it will even form shock waves as pressure grows and
expands and compresses again. These are called mock
diamonds or shock diamonds, and although they look
super cool and super pretty, they’re actually a sign of inefficiency from an overexpanded nozzle. So now you might be thinking, why don’t they just use
vacuum-optimized nozzles at sea level, since rockets only spend their first few seconds at sea level, ambient air pressure drops in half by only about 5,000 meters
or so, and they spend most of their time in the vacuum of space? Well there’s a few reasons,
but first remember, the further away the
nozzle exit pressure is from the ambient pressure,
the less efficient it is. So a sea level engine in
a vacuum is inefficient and a vacuum engine at
sea level is inefficient, but there’s actually
another, even bigger reason why you can’t actually use a
vacuum engine at sea level. It’s because the nozzle would likely break due to something called flow separation. This is where the ambient air squeezes in against the flow of the exhaust and it actually climbs up into the nozzle, and it does that so
much that it can create these sudden shock waves
and sudden pressure spikes called flow separations
which will most likely damage the nozzle. A nozzle can really only
go down to about 40% overexpanded before you
get flow separation. Well, that is unless you look at the Space Shuttle’s
main engine, the RS-25, which used a unique trick
to curl the nozzle walls in ever so slightly, which allowed them to get down to about 14% overexpansion without flow separation at sea level. This allowed them to be
very, very overexpanded, which made them more efficient throughout the entire
eight-and-a-half-minute ascent since they fired from sea level and they stayed running
all the way into orbit. And considering how little
amount of time is spent at sea level, maximizing nozzle design for the vacuum of space was
definitely a good thing. But for some reason
it’s still really weird to think that just regular
old air at sea level can be higher pressure
and actually squeeze in on rocket exhaust. But the pressure at the tip of the nozzle is actually pretty low; it’s just moving very, very, very, very, very fast. And even if you have a huge vacuum nozzle like the vacuum Merlin 1D, it’s still too small
for the vacuum of space. If it were to match the
ambient vacuum pressure, the nozzle would need
to be infinitely long, and since that doesn’t sound
like it’d be very light weight or fit inside of our universe, engineers find the happy medium between packaging, mass and performance. The term for how big the throat
of the combustion chamber is compared to the exit of the bell is known as the expansion ratio. This is the ratio between
the area of the throat vs the area of the nozzle exit. Again, for ease of reference, let’s look at SpaceX’s Merlin engine which has the same chamber
and throat diameter, but with different bells. A sea level Merlin has a
nozzle that’s 0.91 meters wide whereas the Merlin Vacuum engine has a 2.89 meter wide nozzle. Since both engines have
the same throat of 226mm, this means the sea level
Merlin has an expansion ratio of 16:1 while the Merlin Vacuum engine has an expansion ratio of 164:1. I think we tend to look at
a rocket engine’s nozzle and go “Hey, look at that huge
rocket engine!” when really, we’re seeing the most
insignificant piece of the engine. As a matter of fact, many of
the rockets we see on display are missing the actual
engines or the turbo pumps and they just have dummy mock up nozzles. So the next time you hear someone say, “Check out these huge engines,” you can push your glasses up on your nose and say, “The final size of
the nozzle has only to do “with the expansion
ratio of the engine bell, “and it does not necessarily correspond “to the output of the engine.” It’s kind of like pointing
to the wheels of a car and saying “Look at that engine!” Because it’s just like
how the wheels of a car transmits the energy of
the engine to the road, the nozzle converts the
energy of the rocket engine into workable thrust. (upbeat rock music) Okay, so you can’t really
fire a vacuum-optimized engine at sea level. and this, really,
is where aerospikes come in. The idea behind aerospikes is you allow the ambient air pressure to actually form the walls containing
the flow of the exhaust so it’s always in nearly ideal
conditions at any altitude. And not only will you not destroy
your engine by lighting it at sea level; it’s actually
gonna perform pretty darn well. And again, I know it
doesn’t really seem like it, ’cause it’s just what we’re
used to, but at sea level, there’s actually an awful
lot of air around us. And you don’t really realize
how much force it exerts until you remove air from
something and you see how much force the atmosphere
can actually apply. So with an aerospike, the throat
of the combustion chamber, or chambers, aims the exhaust gas at what is essentially just a
slice of a traditional bell. When the engine is at sea level, the higher pressure
ambient air outside of it pushes the lower pressure
exhaust against the wall. So now instead of the ambient air pressure squeezing in on the
exhaust and pushing it away from the nozzle wall, the ambient air pushes the exhaust further
into the nozzle wall, which makes flow separation
physically impossible. As the vehicle climbs in
altitude, the ambient air pressure decreases and so does the
pressure on the exhaust flow. This essentially makes the wall
that holds the exhaust grow, changing its expansion
ratio with altitude. So really, the air, or the lack of air, forms the outer wall of aerospikes and, due to the atmosphere
getting thinner and thinner as the rocket’s altitude increases, the virtual nozzle actually
grows with altitude. But there’s still an expansion
ratio for aerospike engines, and this is based on how
long the spike itself is. The exhaust needs to
form against the spike, and once the spike’s gone,
the exhaust will expand out beyond it in a vacuum just
like a traditional bell. Most aerospike concepts have
a flat base at the bottom, or a truncated spike, which
takes the turbine exhaust and puts it through a heat exchanger, and that creates a pressure
zone in the wake of the rocket that actually adds a little
bit of additional thrust in higher altitudes,
further aiding the traits that are advantageous. In order to make an aerospike engine, you need a differently
shaped combustion chamber than a traditional one,
or multiple chambers that form the unique shape
needed to make an aerospike. There’s two main types of aerospikes: there’s the toroidal aerospike
and the linear aerospike. With the toroidal aerospike,
the combustion chamber is like a donut kind of, and the throat is an opening pointing
inwards-ish towards a spike. A linear aerospike has
rows of combustion chambers that all point onto a
flatter wedge-shaped ramp and has at least two sides, and the exhaust ends up
meeting each other at the tip. Most rocket concepts that
utilize an aerospike engine are single stage to orbit concepts, and that is a rocket
that is only one stage and goes straight from sea level to orbit with no separation events. Now I’ve already done a
video on why SSTOs suck, and trust me, they’re super
cool, like yes I want that and maybe someday we’ll have
it, but physics just simply dictates that there’s a
lot better uses of rockets. After all, if the key is truly reuse, not putting the entire vehicle through punishing reentry
forces is a good place to start. That’s why we’re seeing
fully reusable concepts like SpaceX’s StarShip be be
a huge multistage vehicle. But an aerospike is a
pretty good way to do a single stage to orbit rocket, since you can essentially
utilize a vacuum-optimized engine at sea level and not
only will it not explode, it’ll operate about as well at sea level as a sea level optimized rocket engine. By the math, here’s
where a working aerospike can nearly double the
payload capacity of an SSTO compared to a good bell nozzle engine, but more on that later when
we’ll look at whether or not an aerospike truly is the
best option even for SSTOs. (upbeat rock music) Okay, so other than SSTOs,
in general, if aerospikes are so great, why isn’t it
being used on every rocket? I mean, especially on first
stages where the vehicle transitions from sea level
to vacuum-ish conditions. Well there’s two main issues
that plague aerospikes: weight and heat. Let’s start off with heat as it’s actually perhaps the biggest
issue and it contributes to less than ideal performance
and additional weight. That’s right. Now, despite looking like
there’s a lot less to cool, an aerospike engine actually has a horrendous time keeping itself cool. I mean, after all, a
rocket engine is basically just getting exhaust gas as hot
and high pressure as you can and eventually make it
all move in one direction without it melting the engine trying to contain and direct it. And an engine melting, well, means you no longer have an engine. And as soon as your rocket no
longer has a working engine, it tends to fall right
back to Earth in a hurry. (rumbling) The primary way most
liquid-fueled rocket engines cool themselves is through a process known as regenerative cooling. This is where liquid
fuel is actually piped through the walls of
the combustion chamber and actually most of the nozzle as well in order to cool them down. There’s also other tricks
that rocket engines can do to keep themselves from melting. There’s film cooling with fuel or exhaust, ablative cooling and
even radiative cooling. Ablative cooling, like how a
heat shield typically works, is where you allow parts
to sublimate or ablate, and in doing so, they
take heat away with them. A good example of this is the RS-68 engine that powers ULA’s Delta IV Heavy and powered the now-retired Delta IV. Although it runs on liquid hydrogen just like the space shuttle,
it’s exhaust isn’t clear like the space shuttle’s main exhaust. Instead, it has this orange glow to it. That’s actually from the composite nozzle that uses graphite to ablate away, which takes the heat away with it. This is a little less common in modern liquid-fueled rocket engines, and it’s of course not a good choice if you plan to reuse your engine since you likely would have burnt through the majority of it during the flight. Film cooling can take a few forms either by injecting additional fuel inside the combustion
chamber along the walls to keep those areas cooler,
by pumping cooler exhaust down along the nozzle, or both. We’ve touched on the gas
generator exhaust before in my video about raptor engines, but the exhaust from a
gas generator or preburner is relatively cool since it needs to be a low enough temperature for the turbine to be able to survive being subject to it. It’s just super weird to
think that you can take hot exhaust, like 1,000 celsius, mix it with hotter exhaust gas,
like around 2,500 celsisus, and it’ll actually end up being somewhere between the two temperatures. My dumb brain tends to think that if you add those two together, it’s the sum of those two
numbers and not the average. I guess it’s a good thing that
I’m not a rocket engineer. But hey, I guess I’m proof
that it’s never too late to start learning thermodynamics. This cooler exhaust gas helps the nozzle, or usually the nozzle
extension, be in contact with a cooler gas than
if it were in contact with the main combustion chamber exhaust. You can see this utilized on
the F-1 engine that powered the Saturn V as well as the
Merlin 1D Vacuum engine. And looking at the Merlin 1D vacuum, you can see streaks in
its niobium alloy nozzle which is radiatively cooled but also takes the gas generator exhaust and
uses it to cool the nozzle. The streaks are from little fins inside the exhaust manifold
that maintain the structure. Side note, I used to
think these streaks were from the pintle injector,
but come to find out, it’s from the exhaust
manifold’s support braces interacting with the film cooling. The other way to keep the
walls of the combustion chamber and nozzle cool is by having
additional fuel injectors along the perimeter of
your injector plate. The injector plate is
generally the entire top of the combustion chamber
and is where the fuel and oxidizer mix, hopefully really well, and where the primary combustion occurs. I’ll do an in depth video
on injectors here soon, but for now, just know in general they mix to a predetermined amount
of fuel and oxidizer which balances performance and heat. The further away from
their ideal ratio the fuel and oxidizer are, the cooler
the combustion will be. Since oxygen is a heavier
molecule than most fuels and can’t be accelerated as quickly as a lighter fuel molecule
can, engines tend to run fuel-rich, and they change the ratio in order tot control the
temperature and performance. And in order to locally cool,
say, the outer perimeter of a combustion chamber
where the exhaust comes in contact with the chamber walls, you can add a few more fuel injectors so there’ll be a layer
of more fuel rich exhaust that’s in contact with the walls. The most problematic part of
a rocket engine is the throat. This is where the heat
absorbed by the wall of the nozzle is the highest. Luckily, this high temperature exhaust is only coming in contact
with a small amount of surface area, and with a
finite amount of cool fuel being pumped through the
walls, having only a small area in contact with the hot
exhaust gas is a good thing. This is backwards from what we normally tend to deal with in life. If you want to cool something off, you normally want a big surface area to radiate a lot of heat away
like a radiator does in a car, but that’s not what’s happening here; we have a heat source
and it’s going to melt whatever it touches. Think about it like if you
had a red hot fire going and you have a garden hose running to try and keep stuff you put
in that fire from melting. Stick a metal rod in there a little, the hose will probably do a
fine job in keeping the rod from melting, but as you put more and more of that rod in the fire,
eventually the hose will not be able to keep up with the task of keeping the rod from melting. This is where a larger engine actually has a big advantage
over a small engine. The volume of an engine, and therefore the amount of propellant
per second in the engine, goes up by the cube of
the engine’s radius, but the surface area
needed to cool the area only goes up by the radius squared, so larger rocket engines
have a lot more propellant to cool only a little more surface area. Now let’s make up some numbers here. Say the proportions of
the engine are the same, but one engine has a
throat diameter of 50mm and one has a throat of 100mm. The engine with the 50mm
diameter throat will have an area of 1,963 square-millimenter
and a circumference of 157mm. The engine with a 100mm diameter
throat will have an area of 7,854 square-millimeters and a circumference of only 314mm. In other words, the 100mm
throat engine could flow four times more exhaust
while only needing to cool twice as much surface area. But wait, it’s still twice
as much to cool, right? But guess what, if you’re flowing four times as much exhaust
through the throat, it also means you’re flowing
four times as much fuel as coolant through those areas as well. So really, in a sense
there’s twice as much fuel as coolant to cool a given area when you double the
diameter of your throat. That’s a good thing. So a larger engine actually
gets easier and easier to cool. But of course, they start to
bring on their own problems, such as combustion instability, but in general a bigger
engine is easier to cool. And right here is the biggest
problem with aerospikes. By design, the throat of
their combustion chamber has substantially more
surface area to cool. For a toroidal aerospike,
the combustion chamber basically has a giant
plug in the middle of it which greatly increases the surface area of what needs to be cooled. Let’s do some simple math
based on those last chambers. Let’s pretend we’re trying to keep the same throat area of
7,854 square-millimeters that we had with that
100mm diameter throat. Well in order to get
that same throat area, we’d have to have say a
141mm outer diameter throat and then a 100mm inner diameter plug. This means we now have more than doubled the amount of surface
area we need to cool. We went from having a
circumference of 314mm to a circumference of both 314mm plus an additional 444mm
for the outer diameter for a total of 758mm. These numbers are a bit arbitrary here ’cause we really could stretch this just about any way we want, but one thing that you can’t overcome is the fact that you’re inherently at least doubling the surface area at the exact place where it’s
the hardest to keep cool. But truth be told, engineers
don’t often suspend a plug in the middle of a normal
combustion chamber; it’s more common to make a
ring for a combustion chamber. But even so, a ring still
has an inner diameter and an outer diameter
that you now have to cool. The J-2T was actually a
perfect example of this. It took the turbomachinery
from a J-2 engine and was put inside a
toroidal aerospike engine. But just look at this picture
of the J-2 vs the J-2T. You can tell just how
much substantially bigger the throat is, and now we have two throats of that larger diameter just to make up the same throat area as the standard J-2. The math works out to being
almost 15 times harder to cool the J-2T than it
is to cool a standard J-2. That’s certainly not trivial. And don’t forget you have
to remove that heat somehow. If regenerative cooling
through the walls isn’t enough, you have to add more film cooling, change your overall burn ratio to cool all of the combustion
chamber, or maybe use heavier or more expensive metals
with higher melting points. Any and all of these come at
the expense of performance. Although we’re still doing research on the combustion dynamics
of aerospike engines, but so far they seem to be
a few percent less efficient at reacting the propellant
in the combustion chambers, which wastes some other highly cherishable chemical energy that’s in the propellant. And of course, it’s possible
this all could be fixed with more work, but it could just be due to some of the many compromises required to make aerospikes
actually function, period. So really, the other thing
that is a limiting factor for aerospikes is weight. In general, it seems like
aerospikes have a much harder time achieving a high thrust-to-weight ratio when compared to their
bell nozzle counterparts. This is mostly due to the
larger amount of surface area for the combustion chamber or chambers, the additional plumbing
to feed all the chambers, and the additional support and
structures between the spike. Take a look at the xRS-2200 and look at how much stuff there
is between the ramps. Although it seems like
there’d be less nozzle, which may be true, there’s honestly more of literally about everything else. Now imagine you had, say, a hydrogen leak in one of these additional
tubes, pipes or valves, which is really common, because hydrogen’s really hard to contain. There’s a substantially
more points of failure in an engine like this. And of course weight is
extremely important with rockets, and the thrust-to-weight ratio
is a really important metric in rocket engines. After all, extra weight directly affects your payload capacity and overall
performance of the rocket, so a lightweight and powerful
engine is always a good thing. Another thing that can be difficult to do with aerospikes is how
to gimbal or steer them. Because of the large attachment
points to the vehicle, it can be impractical to try and move the entire spike with
thrust vector control. Although, one good thing
is a gimbaled spike will never exceed its own perimeter, unlike a nozzle which can
run into nearby engines. A common solution for thrust vectoring is to throttle individual chambers to perform differential thrust,
by off-putting the thrust, it will induce pitch
or yaw to the vehicle. Although it’s not as effective as steering the entire engine, it is a viable option. The RS-2200 linear aerospike was to use both actuated thrust
vectoring and throttling. On the pitch and roll axis
it had actuated control and for yaw control it’d
throttle individual chambers all the way down to 18%. And the last big deal with aerospikes is actually the lack of real flight data. Now although this seems
like a simple solution, or a perfect submission for
the “Why Don’t They Just” segment on “Our Ludicrous Future Podcast” that I’m the cohost of. I mean, why don’t they
just fly an aerospike? Which by the way if you don’t listen to Our Ludicrous Future
podcast here on YouTube or on your favorite podcast player, definitely get on that if
you want your weekly fill of space news, Evs, futurism,
et cetera, et cetera. Sorry, shameless plug. But the thing that hasn’t
really been proven out is what happens with aerospikes
between supersonic speeds, or above mach one, up to
hypersonic speeds of mach five. And this is a non-trivial thing. But one really really cool experiment was with the LASRE experiment, which was taking a scale
mockup of the X-33 tail and aerospike engine,
putting it on an SR-71, and flying above supersonic speeds. There were actually seven research flights with the LASRE experiment on an SR-71, and although they never
actually fired the engine, they did perform cold
firing tests at Mach 1.58 and ground firings which gave them all the information necessary to move on. But regardless of how cool it looks, the aerospike engine still
lacks any real runtime data in these supersonic and hypersonic regimes or almost any real full
scale flight data, period. (upbeat rock music) Okay, I think it’s time we actually put up some aerospike engines against
more traditional engines and see how they compare. So let’s start off with our line up of traditional bell engines. For this we’re going be
looking at original J-2, the J-2S that was intended
to replace the J-2 engine, then we’ll look at the
Space Shuttle’s main engine, the RS-25, as well as
SpaceX’s Raptor engine. The reason I chose these engines is because they’re substantial engines that have a lot of data,
they’re either alternatives to the aerospike, the basis of aerospikes, and/or considered the
best engines ever made. This way we get a variety of engines to compare different aerospikes to, and each of these engines
performs in the vacuum of space or, in the case of the
RS-25 and the Raptor engine, they operate in space and at sea level like an aerospike does. Now, for aerospike engines,
we really only have three engines that really made it through a legitimate certification
and testing program, but only two that I have
good hard numbers on, and then there’s one more engine that would have perhaps
been the king of aerospikes. So the engines with the
most data that I could find were the J-2T 250K toroidal
aerospike and the xRS-2200. By comparing these engines to the J-2S which it was based off of, we should get a good idea
of their performance. The other aerospike we’re going to look at is the RS-2200, which
was the full scale engine that would’ve actually
powered the Venturestar. Now all of these numbers will have a pretty big asterisk next
to them, because this engine never actually hit the stand
and it relied on a lot of new, unproven-ish technologies
to get these numbers, such as a lightweight carbon
ramps and ceramic tubes. So the numbers quoted here
are what they would’ve needed to reach in order to make
the Venturestar work, and truth be told, getting an engine to hit your targets can
be very, very difficult. But the numbers on Raptor
are just the current numbers and, just like the Merlin engine, we do expect to see SpaceX
continue to push this engine and develop it beyond these
numbers in the very near future. And one more note, I should also point out that all of these engines are
drawn to scale of each other. So let’s start off with
each engine’s fuel. Every engine here runs on liquid
hydrogen and liquid oxygen otherwise known as hydrolox
except the Raptor engine which runs on liquid methane
and liquid oxygen, or methalox. I wanted most of the
engines to be hydrolox so we’re comparing the most
apples to apples engines since hydrolox engines
have the most potential for a high specific impulse. But it’s still fun to compare them here to the methalox Raptor. Next let’s look at their cycles. And again, if you need a
rundown on engine cycles, I go into most of these in my
video about the Raptor engine and its full flow staged combustion cycle. But the one engine cycle
type that I didn’t talk about is the tap-off method
which actually just uses the primary combustion
chamber to power the turbine. The engine here that
uses the tap-off method is the J-2S which was intended
to help simplify the engine and bring the cost down
since it eliminated the preburner or gas generator. The original J-2, the
J-2T 250K, the XRS-2200, and the RS-2200 all use a gas generator. The RS-25 is closed cycle, fuel
rich, and the Raptor engine is of course full flow
staged combustion cycle. Now let’s look at their thrust output. We’ll do both sea-level and
vacuum thrust in kilonewtons. The J-2 produced 486 kN at sea level and 1,033 kN in a vacuum, the
J-2S couldn’t actually fire at sea level and in
space produced 1,138 kN, the J-2T-250K produced 731 kN at sea level and it would do 1,112 kN in space, the XRS-2200 was 909 kN at sea
level and 1,184 kN in space, the RS-2200 would have
hopefully produced 1,917 kN at sea level and 2,201 kN in space, the RS-25 produces 1,859 kN at sea level and 2,278 kN in space
and the Raptor engine produces 1,960 kN at sea
level and 2,150 kN in space. Now let’s look at their
efficiency measured in seconds or the engine’s specific impulse. You can think of this
like the fuel economy of a gas-powered car. So a high specific
impulse would be similar to a high mpg or km/l. The best way to think of specific impulse is imagine you had one kg of propellant. For how many seconds can the engine push with 9.81 newtons of force? The longer it can sip on the fuel while still pushing that hard, the higher its specific
impulse, and therefore the more work it can do with
the same amount of fuel, in other words it’s fuel economy. So the higher the specific
impulse, the less fuel it takes to do the same amount of
work, which is a good thing. A fuel efficient engine
is extremely important. The J-2 was 200 seconds at sea level and 424 seconds in space, the J-2S couldn’t fire at sea level and was 436 seconds in space, the J-2T was 290 seconds at sea level and 441 seconds in space, the XRS-2200 was 339 seconds
at sea level and 436 in space, the RS-2200 was planned to
have 347 seconds at sea level and 455 seconds in space, the RS-25 is 363 at sea level
and 453 seconds in space, and the Raptor engine is
330 seconds at sea level and 355 seconds in space. Now as we mentioned
previously in the video, a huge factor in the specific impulse of an engine is the expansion ratio. So let’s see if there’s
a strong correlation between expansion ratios
and their specific impulse. The J-2 was 27:1, the J-2S was 40:1, the J-2T was 80:1, the XRS-2200 was 58:1, the RS-2200 would’ve had a crazy 173:1, the RS-25 is 69:1 and the Raptor is 35:1. It should be noted that although the J-2S has a higher expansion ratio, it’s dimensions look almost
identical to the J-2. That’s on purpose; it’s
because the throat diameter is actually smaller on the
J-2S compared to the J-2 in order to increase the expansion ratio but also to keep the
same general dimensions which would make it a drop-in
replacement of the J-2. And lastly, let’s look at
their thrust-to-weight ratio. Now this one we definitely
need to remind you that these numbers could
be off by a little bit as each company might quote the engine with varying amounts of hardware, or have different metrics
for this, but in general, we can use these numbers
as a decent rule of thumb. The J-2 was 73:1, and
although the J-2S was simpler with no preburner, it
was a little heavier, and had a 69:1 thrust-to-weight ratio, the J-2T was 63:1, the XRS-2200 was 35:1, the RS-2200 was planning to hit 83:1, but would’ve needed to hit 75:1 in order to make its SSTO
capabilities possible, the RS-25 is 73:1 and the Raptor is 107:1. So now that we have all of
these numbers, we can really see how aerospikes compare to
more traditional engines. And in general, the J-2T and XRS-2200 should be compared to the J-2S since that was really what
was meant to replace the J-2. And when we do so, we can definitely see why both of these engines
were advantageous. They offer great vacuum performance and they can still be used at sea level, something the J-2S physically couldn’t do. And in this case, the J-2T even seems to have been a better
choice for an upgrade to the J-2 than the J-2S
in literally every metric except for thrust-to-weight ratio. It was really a compelling option. And now when we compare all
of these to the Raptor engine, we can clearly see that the Raptor engine with a sea level nozzle and methalox isn’t nearly as efficient in a vacuum, but with a mighty impressive
thrust-to-weight ratio and solid sea level specific impulse, it’s sort of out there on its own. Now had the RS-2200 actually
made it through development and truly hit all of these numbers, it would’ve been a very impressive engine. Not only does it have the
best vacuum efficiency, it also would’ve managed to beat the RS-25’s thrust-to-weight ratio. That being said, I’ve actually
heard from multiple sources that, had Lockheed just
simply stuck with the RS-25 for the Venturestar, it
would’ve been just as good of an option and was
already a complete engine compared to the RS-2200
which would’ve still had a full risky and costly development ahead. But now, with that in
mind, I think it’s time we hear from real experts
who have either worked on aerospikes or who have
opted against using them. (upbeat rock music) So I actually reach out to
Tory Bruno, the CEO of ULA who actually worked on the X-33 and Venturestar program to
hear what he has to say. “The toughest part about
the design and operation “of an aerospike engine
is thermal management. “A traditional aerospike
engine is conical in shape “and can really struggle with heating “as the spike tapers down. “The linear aerospike design,
together with the strategy “of truncating the taper, goes a long way “to simplifying this problem,
but it is still there.” I also asked him if he
thought the RS-2200 engine could have met the metrics
it was intended to hit. He said: “I’m sure the engine would
have worked and been practical. “The complexity of so many
separate engines, however, “would have presented a
weight growth challenge “as well as significant propellant “flow management complexity. And I just had to ask
him if, after working with aerospikes, if he
loved them or hated them. “I love them. “Aerospikes are just plain cool, “and linear aerospikes
are the coolest type.” I agree Tory. I also asked Elon Musk why he hasn’t opted to use an aerospike engine
for anything at SpaceX, and here’s what he said. – You know, I’ve internally
asked this question so many times, like, “Guys, “shouldn’t we maybe do an aerospike?” You’ve got to get your
combustion efficiency. There’s really two parts to … When you have a rocket engine,
what are you trying to do? You’re trying to shoot things out as fast as possible in a straight line. – Yes. Converting as much thermal and
pressure into kinetic energy. – Yes, exactly. So you have your combustion efficiency. So, what percentage of max theoretical combustion efficiency are you? And then, what’s your nozzle
efficiency, which is really … Are you straightening the flow and shooting the molecules
out in a straight line so that you’re going the other direction; Newton’s Third Law. – Yup. – So … With a traditional combustion chamber, you can get to a very high
combustion efficiency, ’cause the molecules are
all sort of bouncing around in there, they’ve got a time
to combine and do their thing. And then when you sort of
choke it through the throat, that gives them sort of
more opportunity to combine. We think we can probably
get to 90, certainly 98.5, hopefully 99% of theoretical
combustion efficiency. This is so, if God himself
came and knitted together the molecules, you’re 1%
better, okay, maybe 1.5% better. That’s very high efficiency. – Because of full flow stage combustion. – Full flow stage combustion, exactly. You’ve got a gas-gas interaction. So you’ve got two hot gases combining. – Yeah. – And with a relatively simple reaction. Only thing that would be
simpler would be hydrogen. You actually want to say, what is the actual achievable
combustion efficiency times the theoretical chemical energy? That’s the real number. – Right. – This is where methane
starts to look really good. – And so you don’t think– You’re just telling me now, spoiler alert, you’re probably never gonna see a full flow stage combustion cycle aerospike engine produced by SpaceX? – You know … If somebody can show that we’re
wrong, that would be great. – Yeah. – If somebody can explain,
like, “Wow, there is a way “to make your design
better,” this is a gift. – Right. – I would be like, “Thank
you for this great gift. “Wow, this is awesome.” It’s definitely … The worst thing would be like
we was to do this dumb design and stick with our dumb design. That would be insane. – Right. Obviously. – I would love it if somebody could show how an aerospike is the smart move, in which was we will just do an aerospike. – Yeah, you’ll just do an aerospike. Aerospikes, with Elon Musk. (chuckling) So to summarize what Elon was saying, clearly him and his team are
focusing first and foremost on combustion efficiency over other things like, say, an altitude
compensating nozzle. And the other person that I
think has some cool expertise on the subject is Peter Beck, the CEO and co-founder of Rocket Lab. ‘Cause not only has Peter
built aerospikes himself, but he has a great view of why a company like Rocket Lab hasn’t pursued them. Why not aerospikes? Why haven’t aerospikes been flown? Did you view them, or
did you look at them? Is it something you would love
to do at some point, or what? Tell me the actual rocket builders– – The curse of the aerospike. – Yes!
– Yeah, yeah, yeah. – Where’s our aerospike? – I’ve got an engine down
there that’s an aerospike. I mean, I’ve done my time
on aerospikes myself. – Really?
– Yeah. I mean … I guess they’re attractive for all of the right physics reasons. But a pain in (bleep) for all
of the engineering reasons. And it kinda cancels itself out. I mean, if physics says it’s better, but engineering and
trying to engineer them, it’s far more complicated. And the mess and complexity
you end up driving into them, and this is my personal
experience, driving into them, versus just a typical conical bell, it’s just not worth it. – So you physically are adding mass to potentially gain a little bit of– – You end up at the same point. In my experience, you end up
exactly in the same point, except with a much more
complicated system that’s unproven. – And I think cooling has
always been a big issue because you’re just … It’s really hard to do
that in the middle of a … (chuckles) in the middle of it. – Then it’s other things like TVC. It’s very easy to gimbal a chamber. Piece of cake. But a whole aerospike, do you
gimbal the whole aerospike, or do you have a multiple pore aerospike where you just throttle the aerospike? And then what about all the altitude compensation effects that you get? I mean, you have the
same kind of complexities with multiple engines, the
plume-plume interactions. As your ascending through atmosphere, you’ve got all those
plume-plume interactions and the plumes are changing. And then your control
system’s changing as well, because the TVC works … Functions differently at
different plume-plume interactions and recirculation zones. It’s all a pain in the (beep). But it’s still less of
a pain in the (beep) than something like an
aerospike, in my opinion. And the other thing to think about is that if we were a lab, maybe that’s … a pure research lab,
it’s a difference story. We’re a commercial company. R&D is … It’s not fun anymore. R&D is just expensive and time-consuming. So, you know, pick your battles. – And while I was sitting
there chatting with Peter, it made me realize that the
Electron uses nine engines, and I wondered if they could do anything like gimbaling inwards
to utilize some sort of aerospike-like effects. Here’s his response. Even if all your engines are
able to thrust vector control, is it advantageous to actually
aim them into the center at near mico or anything as they get closer to the vacuum of space, or does that not really … – Oh, there’s stuff you can do. (laughing) – I may have just figured some stuff out! Interesting. So there might actually
be something there. I mean, I’ve heard that the Falcon 9 does something like that too. It’s pretty cool that those companies are able to use multiple engines to do kind of an aerospike-like effect. But perhaps the best summary
of the aerospike engine comes from Vector Aerospace who worked on several aerospike engines, including one of their
first engines in April 2002 which was test fired in front of Elon Musk and Tom Mueller of SpaceX. The engine unfortunately
only lasted 200 milliseconds before it blew the graphite plug right off the injector face. But this wasn’t their last attempt. They continued to pursue
different aerospieks including a 10-chamber,
1,300-pound thrust aerospike engine which also unfortunately failed
on it’s 2009 flight test. In 2016, Vector released this statement, which I honestly think
summarizes aerospikes perfectly. “While aerospike engines can
provide performance advantages, “the larger number of parts and components “means that they are usually heavier “than their regular
bell-nozzle counterparts “in terms of thrust-to-weight
and, more importantly, “require very high component reliability.” So the general consensus
I’ve gotten from people who have worked on aerospikes
is it’s just not worth it. Not only is the development
and research costly and risky, but the net gain might not be any better than a more conventional
tried-and-true bell engine. And this is echoed when you
learn about Firefly aerospace who had a really
promising aerospike engine known as the FRE-2 which
was to be a 12-nozzle methane-powered toroidal aerospike that was to power their Firefly Alpha. After a company reset,
the aerospike plans died and now the company is planning to utilize a traditional bell nozzle as well. What’s going on? Where are our aerospike engines? So how many companies
are currently working on aerospikes as we speak? So far as I can tell, one-ish, kind of. Well, maybe two. ARCA has a pretty intriguing video series called “Flight of the Aerospike”
and they are promising to make a low-cost and
simple SSTO aerospike rocket. But oddly enough, the latest video in the “flight of the Aerospike” series shows the company testing
their brand-new systems using a traditional bell nozzle and simplifying their
rocket to running on steam. Now I want this company to succeed and I really want them to fire
off their linear aerospike, but from what I can tell,
they’ve got a very, very long way to go before they would
ever be close to flying one. And although the company
has been around for 20 years and has yet to fire an aerospike, perhaps they’ll make some progress and get anything flying someday. But I’m starting to think
this too will fall into the “well, it turns out it
wasn’t worth it” category. The other company that’s
working on an aerospike is RocketStar, who is
pursuing an aerospike, but so far their engines seem to be only in the high-powered model rocket category, although they do have
plans for a Starlord rocket which would use an aerospike, but this is very much a paper
rocket so far as I can tell. But aerospikes still
have some promise, right? I mean, most of the concepts we looked at utilized the open cycle
or gas generator engines. Couldn’t there be ways to
improve upon aerospikes that would make them more worth it? (upbeat rock music) There’s two pretty promising
ideas or technologies that might actually help
aerospikes find their place on the bottom end of an orbital rocket, the first being 3D printing. 3D printing allows for advanced designs that can help make cooling channels and combustion chamber
shapes that would normally be physically impossible to manufacture super easier to manufacture
at the push of a button. There’s companies like Amaero, I think that’s how you say it, who have built additively
manufactured aerospikes out of Hasteloy X which
is a high-strength, nickel-based superalloy. It’s really cool to see how they can print these crazy shapes that are
required for aerospikes. The other concept that
very well might work hand-in-hand with 3D printing is a dual expander cycle aerospike engine, and one in particular is known as the Dual Expander
Aerospike Nozzle or DEAN. What’s cool about DEAN is it takes the biggest problem of
aerospikes, which is heat, and makes use of it in the expander cycle. The expander cycle is
where a rocket engine takes the liquid fuel
that cools the chamber, heats it up to the point of phase change, turning it into hot gaseous
fuel and then using that to spin the turbine which
then will power the engine. The problem with the expander cycle is actually the opposite of aerospikes. They have a thrust output limited based on how much liquid fuel an engine can actually heat up and
turn into gaseous fuel. So, due to the square-cube
law, you eventually run out of surface area
to heat up the fuel. The theoretical limit is
approximately 300 kN of thrust before the surface area
to volume of fuel ratio gets to the point where you can no longer power the pumps adequately. But by having more surface
area to heat up fuel, like an aerospike engine inherently has, you can actually get away
with a more powerful engine, which might be a cool trick
for future aerospike engines. But to date, the DEAN
concept has only existed in theoretical papers
which targets an engine with 111 kN, 383 seconds
of impulse in a vacuum and a thrust-to-weight ratio
of 108, which I should note, these specific impulse and
thrust-to-weight ratios put it on par with the Raptor engine. But again, research papers
and actual working hardware are very, very different things, which is why many of
these concepts are studied only in research labs and often
don’t make it much further nor do they end up being
commercially viable, for now. (upbeat rock music) Okay, wow. You’re still here with me. This ended up being a lot longer video than I would’ve ever imagined. So let’s do a quick summary. Aerospikes. Super cool on paper, a
super tantalizing concept that seems to beckon even the
most astute rocket engineers, perfect for your SSTO
concept that might also need to be reconsidered, and likely
to drain your bank account and make you age a few
decades trying to develop it. But .. But … they are super cool and as manufacturing and
material sciences advance, perhaps we’ll see companies able to utilize and
exploit their advantages and close the gap on the bell nozzle which has dominated
the aerospace industry. But perhaps the biggest
reason we generally don’t see too many aerospikes popping
up in modern times is simple. If they truly offered a
clear performance advantage, I would assume everyone
would be developing them, but the fact of the matter
is, they kind of wind up right back in the same place
as a normal rocket engine. Now you could probably
say the exact same thing about SpaceX’s Raptor
engine which utilizes a full flow staged combustion cycle. That was considered far too difficult to really ever be worth it previously. And that rocket engine
is extremely complex and unbelievably hard
and expensive to develop. But the thing is, the full
flow staged combustion cycle is just physically the best way to extract all your energy from your propellant. I mean, they actually
perform better in virtually all metrics than other
rocket engines, period. Whereas the aerospike
might gain some advantages in, say, altitude compensation, but it lacks advantages in other areas which puts it right back
on par with other engines. So as far as where to spend
your time and research, that’s why most engineers
have pursued things like closed cycle
engines, full flow staged combustion cycle, or the
path of easiest manufacturing and development like Rocket
Lab’s electric pump fed engines. And just another note here
while wrap up about aerospikes, and this is kind of similar
to how I ended my SSTO video. It’s not really the aerospikes that suck, but it’s their application
here on Earth that sucks, ’cause Earth is kinda just the
wrong planet to utilize them. Ironically, if Earth’s
atmosphere was any thicker, the aerospike would actually become significantly more advantageous, even for a multistage rocket. But with Earth’s atmosphere as it is, the bell nozzle is just too
close to the same performance, making it a much easier
and less risky choice in the aerospace industry. If it wasn’t for their
thermally backwards nature, their advantages may actually
outweigh their disadvantages, but perhaps their tendency
to melt is the biggest reason why we just simply haven’t really seen one leave Earth’s atmosphere. But all in all, I think the best way that I can summarize the
Aerospike is to compare it to like the rotary engine seen on cars like the RX-7 and RX-8
and their predecessors. For those of you familiar
with car engines, you may know about the rotary engine. They’re super simple; there’s
only three moving parts, They’re small, lightweight
and very powerful. On paper, the rotary engine
really should have been a more popular choice
in the automotive world, but in practice, they’re a
nightmare to cool and lubricate and have awful fuel economy and poor lifespan and reliability. That being said, they’ve
maintained a major cult following, and in some aspects, they
still are a good choice. People have always
argued that if there was as much engineering put
into the rotary engine as there has been for the piston engine, maybe the rotary would
be about as efficient and reliable as a piston engine. And in the same way,
sure, maybe the aerospike could improve and be a more viable choice with an increase in
research and development. But much like the rotary engine, would it really be worth it? Would it really ever truly be
better than the alternative? The art of rocket engineering is balancing all of your variables,
not only in performance but also reliability,
cost, development risk, manufacturing and tying them all together to make the right choice for your vehicle. So at the end of the day, an
aerospike’s biggest advantage is being able to fire a
vacuum-optimized engine at sea level, but in all reality, why do you want to do that? Unless you are for whatever
reason required to do an SSTO, you really never would need to do this, because it’s much easier
and much more common to just fire a sea-level
engine on your first stage and a vacuum-optimized
engine on your upper stage. So what do you think? Do you think aerospikes are
just too cool to ignore? Think we’ll ever see one
propel a vehicle into orbit? Or do you just think the bell
nozzle is the right choice? Let me know your thoughts
in the comments below. And be sure and let me know
what other questions you have about aerospike engines, rocket engines, or rocket science in general. And be sure and stick around because I have a perpetually
growing list of awesome videos, and I’m trying my hardest
to crank them all out. Special thanks to Charlie Garcia for chatting with me for hours
and hours about aerospikes and helping me grasp a lot
of these technical points. He’s my go-to personal,
actual rocket scientist, and he has an awesome
YouTube channel as well, so definitely check it
out and start learning even more in depth with Charlie Garcia. And I owe a really big thank
you to Martian Days on Twitter. He did those beautiful engine renders, and he worked really,
really hard to make sure they’re perfectly in
scale with each other. And of course, owe a huge thank you to my Patreon supporters. If you can’t tell, I’m constantly tweaking and trying to upgrade my stuff, so better video, better audio. I flew myself down to Mississippi to make sure I had the best
footage of an aerospike engine. I just think I’m trying to
make the best product possible, and especially with my Patreons
in our Discord channel, who literally help me piece
all the stuff together. And I owe a special thanks to Ghost Rider, who helped me dig through literally dozens of research papers. There’s a couple others too, but you guys just keep me sane through all these times, so I owe the biggest
thank you to you guys. If you want to help contribute
or add your own facts or opinions or just fact-check
our stuff as I research it, go ahead and consider
becoming a Patreon supporter, where you’ll gain access
to our exclusive Discord, our exclusive subreddit, and
even exclusive live streams as well but going to Thank you, guys. And of course, while you’re
online, be sure and check out my web store, where you
can find things like this. Limited edition aerospike shirt. We do limited runs of shirts
and merchandise these days, so if you see something in
that store that you like, you better get it right now, because there’s a good chance
we’ll run out of stock, and re might not restock it. So literally like the full
flow stage combustion cycle shirt, every time it’s in
stock we sell out right away. So if you see something in
stock, you better get it today, because it might not be there tomorrow. And don’t forget, if you work
in the aerospace industry, I actually give you 25% off all apparel as my thank you to you
for helping inspire me and also for helping get
humans off of this planet. And definitely be sure and subscribe to Our Ludicrous Future podcast. That’s the podcast that I’m a cohost of, where each week I give you
your rundown on space news. So if you want more of me and you’re like, “Tim, your videos take months to make,” trust me, I know. If you want to hear more
from me, every single week you can listen to Our Ludicrous
Future right here on YouTube or on you favorite podcast player as well. Thanks, everybody. That’s gonna do it for me. I’m Tim Dodd, the Everyday Astronaut, bringing space down to
Earth for every day people. (upbeat rock music)

100 thoughts on “Are Aerospikes Better Than Bell Nozzles?

  1. Timestamps for mobile users:
    6:20 – How Nozzles Work
    16:00 – How Aerospikes Work
    19:55 – The Problems With Aerospikes
    32:50 – Comparing Aerospike Engines To Bell Engines
    41:30 – What The Experts Say
    51:35 – Future Aerospike Prospects
    54:00 – Summary

    Also sorry about including my ignorance on mixing temperatures ? when I realized that a few years ago, it kind of blew my mind but I hadn’t really thought about it too much I guess. And with no formal education or even ever having taken a single physics class, I definitely lack some of those basics. But I thought it was fun to include so people can remember that it’s ok to keep learning at all times!

  2. IMO, aerospikes would only likely be used in point-to-point transport on Earth where atmospheric oxygen is used. Future Concorde replacement.

  3. Writing a comment despite having no creative or constructive input because this video is amazing and deserves every YouTube algorithm point I can give it.

  4. Man I’ve been waiting for this teaser video for a bit and it was everything and more I had hoped it would be. I feel
    like I could hold my own with anyone in a conversation about them now! Well probably not. But I’d go into it cocky as hell! Thanks Tim!

  5. Stunning work. Such a wonderful and thorough explanation of the aerospike engines and sonic fluid dynamics. Thankyou Tim for your amazing efforts

  6. The aerospike engine is the perfect engine for any planet and atmosphere? Kind of just flying from our planet to another one and back. No separate engines for different atmospheres. So yes, please continue the work on aerospikes! ?

  7. Outstanding video Tim. You have outdone yourself. Thank you for putting this all together, it was very interesting and informative. Bravo.

  8. Aerospikes are just part of the equation, the ideal system being an actively dynamic adjustable nozzle similar to how the Pratt and Whitney J58 engines controlled thrust distribution on the SR-71. This would negate multi stage nozzle requirements and provide better efficiency. As with modern combustion vehicles, dynamic systems such as Variable Valve Timing, Electronically Adjustable Velocity Stacks, Adjustable Exhaust Valves and Active Aero clearly demonstrate dynamic systems are the future to producing more power and efficiency. Current rocket nozzle designs are still very primitive and have a long way to go however this requires massive aeronautical research, significant engineering costs and further complexity which is the last thing we need right now. I'm convinced dynamic thrust systems are in store for the future beyond even what an Aerospike can offer and it's just a matter of time before these engineering challenges are solved.

  9. This time you have really outdone yourself! Thank you for all of the time and effort you put into this, it was more than worth it!

  10. "Huh, it's going to be difficult to summarise the aerospikes vs nozzles in a 1 minute video. Wait a second, it's a whole hour!"

    Serious props, this must have taken a huge amount of effort! The animations are really slick too, I always found it hard to explain the concept of how the "virtual bell" works with aerospikes

  11. Ok, I haven't watched the whole thing yet but considering the sea-level and vacuum engines – couldn't they just make nozzles with variable exit diameter at least for the first stages? Or probably the optimization is too small to be worth increasing complexity…

  12. What if in a parallel world the aerospike was the more practicle engine and the bell was the elusive engine, simply because they focused only on developing areospike engines.

  13. I lived 30 miles from there in 1993/4… why we never went.. (We lived in D'Ibberville when my dad was stationed at Keesler AFB)

  14. Question: The Raptor uses Methane? Is this because they see the economic value of the Sabatier Reaction and the potential of re-using CO2 (atmosphere of mars) to be converted to a fuel source? (Just asking a nosy question, -James Walper, Libertarian Party of Canada)

  15. Tried to skip two times, but I'm glad i didn't. Please make sure to make future videos as short as possible or divide them into parts.
    Thumbs up for all the efforts you have made in order to make this video.

  16. Super thorough video! You should do a video on altitude compensating nozzles designs hoping to combat the optimization problem like dual-bell or variable geometry. I believe they are far more likely to fly in the near future compared to an aerospike, although might fall in the same category as aerospike as 'theoretically great but practically not worth it'

  17. I don't know.. it's a nice video, but really it could be way shorter. Explain how it works, say it's not that advantagous and move on, really don't need to be an hour long

  18. Tap that like if you realised Tim's different coloured eyes before reading this comment, sub to Tim and BUY DAT SICK MERCH

  19. I've truly never seen a video both so entertaining and informative at the same time. This is terrific work, well-researched, and masterfully explained. I'm probably not the first to see the next Neil deGrasse Tyson here

  20. Seems the sweet spot for an aerospike would be something ssto ish that's large enough that your going to need lots of engines anyway. Like the old rhombus or nexus. Like once each of your 30odd sub rockets is the size of a j-2, you end up saving development costs. Which of course is ludicrously big, and risky, and drives people to the known solutions.
    Also, looking at the comparison again (saw that on twitter a couple of weeks back) it strikes me that the rs-25 killed the aerospike. It really is a damned good engine.

  21. Thank you so much for this, I love this video especially and all the research and test footage you dug up. BUT… didn't you just skip over the main advantage of aerospikes in your comparison to bell nozzle rockets?
    With a bell nozzle rocket, the thrust to weight ratio will always be half in a launch to orbit, since you will ALWAYS be carrying another set of bell nozzle rockets in your second stage that are just dead weight for half the trip. With an aerospike engine, you fly all the way to orbit with one engine, and then have the opposite issue, where in space you are lugging around a big empty fuel tank instead of jettisoning it… but does that even matter since you are floating around in space already?

    Shouldn't the need for two entire sets of bell nozzle rockets in two stages, carrying dead weight inside the hardest lift phase from Earth with twice the number of moving parts, be the main comparison with an aerospike engine? Even if an aerospike engine was exactly the same performance of a bell nozzle rocket– it would still be twice as good since you only need one of them.

  22. why not configure the raptor engines in ways that simulate the aerospike and do it by lots of little nozzles in a large aerospike nozzle?

  23. Tim. Congratulations on the completion of your Mammoth project.
    This is a high quality documentary without a doubt.

    When will you be taking over the Interwebz ??

  24. Any reason why you can't use purely ablatively cooled aerospikes? Entirely replace the ablative surface between flights or have some sort of in-orbit repair technique?

  25. Very very nice. Maybe your best one so far. Btw, your voice sounds a bit different when you quote the Raptor numbers. Anyways, about the Raptor numbers. Are they for a sea level Raptor? Meaning the vac Isp is for a sea level Raptor? If not I think Elon has mentioned slightly higher numbers.

    For your next project: Maybe an in depth documentary about Blue Origin if you get the access. One hears way too little about them. Want to know how the work on their engines, rockets, facilities and supporting utility systems progresses.

  26. At sea level, engines are less efficient. But I also suppose aerodynamic drag on the rocket body is a factor? Any idea about the orders of magnitude of these two effects on low altitude rocket flight?

  27. I've been following the development of the Skylon spaceplane. Obviously, they won't be using an aerospike engine but I think they are considering a different form of altitude compensation called an expansion deflection nozzle on their SABRE engine.

    It's basically similar to a traditional bell nozzle but with the addition of a centrebody or pintle which directs the exhaust gases along the outer wall of the nozzle.

    As an SSTO, Skylon will have to overcome the problem of engine efficiency as it gains altitude so it'll be interesting to see if the ED nozzle can work while avoiding the problems of an aerospike and if the SABRE does fly, how it compares to the bell nozzle.

    Have you come across this in your research and if so, what are your thoughts?

  28. Looks like we don't really need an aerospike engine if the bell nozzle was able to morph it's diametre according to level of altitude 🙂

  29. I think Aerospike engines may become more relevant once humans want to return to earth from other planets. It seems to me that bringing a multistage rocket engine to leave planet will be impractical so a single state rocket will have to be the solution… assuming we are still using chemical rockets at that point.

  30. 21:10 little nit: the coolant doesn't go down and back up as the image shows, instead it flows through the large pipe that's wrapped around the lower portion of the nozzle.

  31. Excellent documentary Tim. This is definitely very high quality content.
    One thing I would have added, just one, is that small aerospikes used in a vacuum would have lower profiles than massive engine bells for a similar performance. You wouldn't use the sea level efficiency, but could cram your engine in a smaller fairing, or not get in the way of landing legs, as I'm sure you've experienced in KSP!

  32. Awesome video, somewhere there are is an Alien civilization with a thicker atmosphere who are trying to get bell nozzles working cos they think they are cool but really aerospikes are the only logical way to go.

  33. Even for an actual would-be-an-SSTO Starship, they just have a sea level propulsion and separate vacuum propulsion systems. You don't need to get rid of a stage to utulize optimized nozzles instead of an aero spike.
    Awesome documentary video by the way.

  34. Hoping SpaceX improves the design in the future for later generations of their Starship platform. Curious Droid also covered this and of course reliability and duration of service has kept bell-designs going for so long, so it'll take a visionary like Elon Musk to bring them out.

    I recall Musk having mentioned (in your interview with him) that they considered AeSs but was more focused on propellents (which provided the cost of a pure-cryogenic fuel system in older engines isn't a small thing). He's gotten a massive cost benefit in using a LOX-RP1 configuration with great results.

  35. Superb review. Liked the look of Aerospike from DeBono books in 1970s for an SSTO solution. Asked Alan Bond (Reaction Engines) why he was not trying an Aerospike for SSTO. He gave most of the reasons you have identified plus another: the mass of the Aerospike in the tail of any aero-space plane would make the CG so far behind CP during re-nary to make the future X33/Venture Star untrimable, maintaining on day 1 it would fail. His reference was his early work on HOTOL which is why Skylon got wing tip engines. He also remarked that Earth was 5% too big for SSTO which is why Reaction Engines were going with Super Cooling Air for initial oxygen flight stage of SSTO.

  36. Wait. On the Thrust to Weight Ratio, do your figures include the weight required to haul a second/third stage engine into orbit that the SSTO wouldn’t require? Great video.

  37. A documentary with time stamps professional interviews, a summary, great explaining animations, a natural view, researched numbers and data and of course Tim dodd the everyday astronaut.
    Thanks for this awesome film

  38. 1 hour went by like nothing
    Great video.
    I see progress since your raptor video. (That was great too btw)
    This time I understood 99% from first time.
    (But I am going to rewatch it anyway)
    It's (truly) great work.
    I am proud to be your subscriber.

  39. Nop to spike engines unless a nuke engine can be used as a fuel. Maybe a spike engine could be used for larger ships/rockets like a car to a tank with racks.

  40. What, isn’t it obvious to all that mixing two gases with different temps will result around average temp between them and not the sum of them? It’s two volumes or masses actually. That’s the additional part the sum of both, but one cools the other and the other heats the first, translates to average temp.

  41. Ionize the flow, then charge up the aerospike "nozzle"' the same charge, should reduce a lot of the heat if the exhaust doesn't touch the flow itself. I think a sort of MHD plasma rocket aerospike could work extremely well?

  42. First you say, aerospike is too hot. Then you say, aerospike is super cool. Now I'm comphused. But I heard every word you said. It was a delight, 61:30 minutes, every one of them. BUT: How can rocket motors SUCK in any way as they have combustion chambers rather than vacuum chambers? That blows my mind. AND: Since aerospike engines get so hot that it will eventually warp the material, are we then on the way to invent WARP drive finally? Can't wait to see this happen.
    Now more seriously: If earth atmosphere was much denser, that would mean, earth gravitation would also be higher, so it would eat up aerospike's better performance anyway…

  43. Actually the government black boxed a highbred nozzle spike engine after complete development and it’s now in skunk works

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