Aerospike Engines – Why Aren’t We Using them Now?

Ever since the earliest rockets we’ve
seen them working with a bell-shaped engine nozzle from the first German v2
rockets right up until the spacex falcon heavy but the ubiquity of the bell
shaped rocket nozzle doesn’t mean to say that it’s the best way to do things in
fact they have a major drawback which is one of the reasons why we use multistage
rockets, however since the early 1970s there has been an alternative rocket
engine which has much greater efficiency and was to power the next generation of
single-stage-to-orbit spacecraft to replace a space shuttle but if it’s so
good why hasn’t even been flown yet. The engine in question is the aerospike, a
design that goes back to the 1950s when it was first developed by Rocketdyne. Now
whilst this might sound like an exotic new type of propulsion the aerospike is
not actually a whole new engine it’s actually just a different way to contain
and control the thrust but any rocket be that liquid or solid fuel produces and
it replaces the Bell type combustion chamber. Now if we’ve been using the Bell
type rocket nozzle combustion chamber since the very first Rockets right up
until now and what’s wrong with them. For a rocket to work correctly and produce
enough thrust to lift it into orbit then you have to control the burning rocket
fuel. If there was no rocket nozzle then the burning rocket fuel would just
expand uncontrollably in all directions and very little of it will be converted
into useful thrust. What the rocket nozzle does is convert the high-pressure
combustion of a rocket fuel into an ultra-high speed flow of gas exiting the
nozzle in one direction and at atmospheric pressure. The ratio of the
size of a narrow end to the wide end of the nozzle must also
allow for the atmospheric pressure where the rocket will operate. Now this sounds
complicated but it’s actually quite simple. Everything including you, me
the ground and rocket engines has atmospheric pressure pressing on us in
all directions due to the weight of the atmosphere as the air is pulled down by
the Earth’s gravity. If you imagine a column of air measuring
one square inch or 2.54 square centimeters from sea level to the top of
the atmosphere, that air would have a mass of 6.67 kilograms or
14.7 pounds.we don’t feel it because we have evolved to live
at that pressure but if you were to go up to 12 kilometers that pressure
would be just one-tenth of sea level and at 50 kilometers it will be one
hundredth of sea level simply because the column of air is now much shorter
and thus weighs much less. That air pressure also pushes on the gases
exiting the rocket nozzle. For a rocket to work at its most efficient and
produce the most amount of thrust the gases exiting the rocket nozzle must be
at atmospheric pressure so where they exit out as straight as possible. This is
fine if your rocket is traveling sideways through the atmosphere like a
air to air missile then the air pressure doesn’t change much. A rocket
going into orbit however starts at sea level with a lot of air pressure and
ends up in a vacuum with no air pressure. Because a bell nozzle has a fixed shape
and size its maximum efficiency and thrust will only be achieved at one
altitude. Design it to have maximum thrust at sea level and takeoff and the
exhaust gases will over expand and lose thrust at high altitudes because of a
lack of air pressure, design it to have maximum thrust at high altitudes and at
takeoff the air pressure is so much but it constricts the gas flow inwards which
leads it to separating from the nozzle wall and becoming unstable
usually resulting in the engine blowing up.
In an ideal rocket nozzle it would change its shape as the altitude and air
pressure changes this is what an aerospike
does. It uses the pressure of the air surrounding it
air surrounding it to have the same to have the same effect as the bell of a traditional rocket engine. The two main types of aerospikes which have been developed
are the annular or round ones and the linear or straight ones. The round ones
are where we get the name from, if you imagine an inverted bell shape it
becomes a round spike, this spike becomes one side of a virtual Bell whilst the
air pressure surrounding it compresses the exhaust gas against the inner spike,
although many now use a truncated spike which has a much shorter design than the
equivalent engine bell. As the rocket goes from takeoff at high air pressure
to the low air pressure at high altitudes the shape of the exhaust flow
changes due to the changing air pressure to keep it at its optimum shape and
optimum thrust. This is what’s called an altitude compensating rocket nozzle.
Although it might not be as efficient as a bell any given altitude it outperforms
them at all others. The Space Shuttle main engines which were used from
takeoff to space were much less efficient at sea level than they were in
space. With a rocket engine like the aerospike you could use just one
engine that will work efficiently from takeoff to space and avoid the need for
multiple stages with different rocket engines optimized for different
altitudes effectively becoming an SST o or single-stage-to-orbit.
So if these are so good why aren’t they used to date no major rocket launches
have used an aerospike despite much research and development being done
during the 1960s and 70s and then in the 1990s. As a follow-on to the successful
J-2 engine which was used on the Saturn third stage Rocketdyne set about
developing and building both toroidal and linear aerospikes using the turbo pumps and engine infrastructure of the J-2. one of the
biggest problems with a aerospike engine is the cooling of the spike. in
the toroidal or round design the spike is long and heavy which makes it
difficult to cool the tip of a spike to stop it from melting. This was mostly
overcome with the development of the new copper alloy called NARloy-Z in the
1970s which allowed longer use of high temperatures. The design were also
changed with a truncated spike with some of the exhaust gases being passed
through the center to achieve a similar result of a long spike but with much
less mass. However the linear version was even more flexible. In this the round
spike is straightened out into a v-shape with the combustion chambers on either
side, the beauty of this design is that can be made modular so that it can have
more combustion chambers added to make a longer a more powerful engine. Back in
the 70s they use combustion nozzles made into small banks which were stacked
side-by-side on both sides of the “V” center. Although aerospikes were
proposed for the space shuttle, as the Apollo program was wound up in the early
1970s development work on the aerospikes also stopped and the space
shuttle went on to use conventional Bell engine nozzle designs. Things stayed
pretty much like this for the next 20 years until NASA was looking to develop
the next generation of space shuttle using an all-new single-stage-to-orbit
design. The design brief was to be able to come up with a completely new launch
vehicle that would be fully reusable and would greatly reduce the cost of getting
into space from $10,000 per pound to $1,000 per pound.
Lockheed Martin won the contract to build the revolutionary design
designated the X-33 and one of the key features was the use
of the linear aerospike engines. Development work continued on the XRS-
2200 linear aerospike and by now with the use of electricmagnetic nozzles it meant that the fuel system could be controlled on a nozzle
by nozzle basis a bit like the fuel injection on a car. This allowed for much
greater throttling control and allowed for the thrust vectoring by turning off
different sections of the engine. This also removed the need to have heavy
complex engine gimbals. Work progressed well up until 2001 when due to issues
with the X-33’s composite fuel tank and cost overruns
the project was cancelled again taking the aerospike engines with it. Progress
has been made with NASA testing small scale solid rockets with a toroidal
full-length aerospike in 2004 but since then there have been no large aerospike
engine developments. So why don’t new companies like SpaceX and Blue Origin
use the aerospike engines with all the efficiencies they bring wouldn’t they be
the perfect match for a low-cost route to space. As far as we know SpaceX has
looked at using aerospikes but given the fact that no large-scale aerospike
has ever been flight tested it would be a very big risk when you’re looking to
set up a commercial orbital space company. One of the driving principles of
the space race was “To do the job good enough and no more”, basically mainly that
once you have developed your spacecraft or your rocket engine to do what it was
designed to do then that’s it you stop there. The technology SpaceX and others
are using is well known and tested. The way they use it might be different like
relighting the main angels descend back to earth but the engines themselves are
a known quantity. Commercial companies have to make a profit in the end and
taking on a major task like developing a new untested engine design is something
but could quickly sap those profits away. Although the X-33 project almost got
them to flight testing, it’s still new technology and needs more money and more development. It would also mean completely redesigning the Rockets away
from the tried and trusted but yet limited traditional
bell nozzle engines. All this takes time time which could be used launching with
traditional engines and getting money into the companies. Although there would
be a saving of up to 40%, the fuel is actually one of the cheapest parts of a
rocket launch, Elon Musk said himself that the Falcon 9 costs $60 million
to build but only $200,000 to fuel. The bringing back of the boosters
and the central core with the engines brings far greater savings than could be
achieved with a change to aerospike engines. It seems as though until the
price of launches has been driven down to as low as conventional Bell nozzle
engines will allow that arrow spikes will remain on the drawing board.
The single-stage-to-orbit vehicles which perfectly compliment the aerospikes
capabilities seem a long way off since the cancellation of the X-33 and the
renewed interest in returning to the moon using variations of conventional
engines that date back to the early 1960s. There are smaller companies like
Arco space which are developing small-scale aerospike engines for
single-stage-to-orbit satellite launches but unless there is a radical change in
the space market only time will tell if arrow spikes will ever get used for
future space vehicles. So what does it take to design a rocket engine how much
thrust does it take to lift a spacecraft and how fast must we launch an object
from the surface of the earth to get it to leave. You can look up the answer if
you want but if you like me then you want to create things by yourself our
sponsor for this video, is dedicated to doing just that turning you
into a living breathing and most importantly calculating scientist head
on over there and prove for yourself just what it takes to get a rocket into
orbit. Having a strong math and science skills set is crucial because it opens
up so many ways to explore the universe. To support curious droid and learn more
about brilliant go to and sign up
for free. so if you’re ready to launch off a planet the first 200 people will
get 20% discount off of the annual premium subscription


  1. Time to mock nas'a
    And drive them
    Round the bend
    Their CGI space race
    … Is coming to an end
    #WWJD #spaceXFAGS
    #nasaFAGS #usaFAGGOTS

  2. I believe we have propulsion systems better than rockets. What do you think all those ufo sightings are……….swap gas?

  3. This is just Bull Shit all this fighting. There's no reason why we should be fighting we should be pulling our resources and building cool stuff. I ride with Russians and we get along just fine so I don't know why all these politicians find so damn much to argue about. If politicians and generals wan to find things to quibble about then they should dress in battle gear and go at it and leave the rest of us to build things and have fun. Before they involve the rest of us in their arguments they should be thrown into a ring fight until only one comes out.

  4. I think the biggest reason is that the added efficiency of aerospike engines still isn't enough to make SSTO viable on Earth, and there's not much reason to use aerospikes on anything that isn't SSTO. If we lived on a planet that with a bit less gravity or if we had chemical reactions that produced a bit more energy then maybe SSTO and aerospikes would have a role. But given our current constraints, we need at least two stages in order to lift any significant payload fraction to orbit. Once you're stuck using two stages anyway, you can just use two different engine bells.

  5. There's lot of talk but no real new technology or space exploration. Manned missions are minimal and I doubt we will ever leave low Earth orbit for a long time to cone… In that case, everything said about Mars are pipedreams.

  6. You pointed out why quite well. The current technology is tried, tested, and _proven_. When it comes to developing new technologies, private companies actually don't usually have the resources to develop them on their own. If they do, they're still usually not going to without government funding to help defray the costs, as the R&D of new technology is one of the largest and riskiest money sinks a company involved in any technology engages in. They will work toward making the current tech as efficient as they can engineer until they either can't, or it would cost too much to do so.

  7. Aerospike can still be used for non-SSTO rockets.

    With SpaceX they can use it on the 1st stage part of the rocket and if the 2nd stage is needed to land then they can use the Aerospike on it as well.

    Some crazy reusable rocket that lands in different atmospheres as part of a mission to multiple bodies in one trip, Aerospike is the way to go.

  8. You described exactly why something like this is never achievable with capitalism. And we've buried socialism, which allowed existence of unprofitable, but potentially usefull R&D/production, for quite a while. Thus, no aerospike forquite some time for us.

  9. You clearly know too little about space and present your own hypothesis as facts. How about you go ask Musk or SpaceX directly of why SpaceX doesn't use Areospikes

  10. OK, so aerospikes are better than bell nozzles over the duration of the launch to orbit. However, the amount of fuel required is roughly the same. So the problem is getting the engine(s) to lift the massive fuel tank(s) all the way to space. Where the nozzle design beats the aerospike is its requirement to stage; i.e. so that you can switch to larger nozzle engines for the next phase of the launch. This presents the opportunity to also ditch the tanks that contained the expended fuel. If the aerospike was 2 or 3 times more efficient, that would reduce the fuel requirement and therefore the mass of the tanks; and that might tilt the balance. However, there is only so much energy you can squeeze out of a chemical rocket.

  11. So basically there was technology lacking second nobody wanted to take a risk . For sum reason the aero style had something that made government and scientists stray away.

  12. Isn't there a bigger reason why rocket engines are multi-stage? Specifically that when you drop a stage you're dropping the mass and getting a better change in momentum for the same amount of thrust?

  13. It's was actually the president of NASA who caused the problems, everyone knew the composite fuel tanks would be a problem and so tried to change to an aluminium alloy but the CEO wanted to test all of these new technologies together. And so the venture star was doomed. So sad

  14. ARCA is testing fullsize Aerospike engine and just got new funding

  15. This is very pertinent to a very recent interview with Musk. Because he wants to make reusable rockets, aerospike might be the answer.

  16. Having just eaten noodles with chilli, jalapenos AND sriracha sauce, there`s more than atmospheric pressure going on..
    Why? Because mouthjoy is worth risking a fiery prolapsed anus, you`re welcome 😉

  17. Sort of like the way a high performance jet engine changes it's exit orifice for different conditions of speed and thrust?

  18. once again capitalism screws humanity over by prioritizing immediate profit rather than investing in long-term benefits.

  19. Fine video…minus your enunciation folly and propensity to overuse the suffix "s," folly born of mouth-breathing.

  20. You forget the biggest disadvantage you can have two optimized bell nozzles and seperate stages and lose the weight of the dropped weight making the whole vehicle more efficient. The aerospike might be the most efficient whole flightprofile engine with stage separation and 2 optimized nozzles the aerosüike just falls behind.

  21. I'm sure I propped against you during a rugby game once. Its MathS anyway not math. Just like Physics isnt Physic.

  22. because they know the universe/space doesnt exist so they are putting it off as long as they can and in the mean time they show you 3-d graphics of what can happen… in your mind…

  23. Nice explanation. Everyday Astronaut mentioned this to Elon and I did not know what it was. I do now, it's a nice design but nobody uses it.

  24. Ref 2:28. 1 sq in is 2.54 sq cms? That is wrong. 1 sq in is 6.4516 sq cms. Wonder how many more "FALSE" information could be in this video. 🙁

  25. It seems aero spikes lack the drama of stages separation… As for human benefit, what has NASA achieved for the past 50 years ? About nothing. How muck does it cost ? Last year about 56 000 000 million dollars per day. Either the show is worth it – here comes in the drama – or the public may stop willing to fund an ever promising-it-for-tomorrow company. Last promise I heard is "back to the moon some time in the 20s"… Anybody wants to bet ?

  26. one logical reason why the arospike engine is not used. It is for a single stage rocket. We don't have a single stage rocket , that can reach orbit. So the first nosel is designed for a specific hight. The second for another hight and the 3th for outer space. So there is no use for arospike.

  27. @Curious Droid
    I think there is a common misconception of what underexpansion and overexpansion actually means. In your vid at 3:51 you state that a sea-level optimized nozzle, when lifted to area of low pressure, would have its gasses overexpand. While factually correct, the term of overexpansion is actually used in reference to nozzle itself and not gasses. So, an aforementioned nozzle would be underexpanded, ie – not expanded enough for optimal gas flow under low atmospheric pressure.

    Similarly, a vacuum-optimized nozzle, launched at sea level would be overexpanded – ie, expanded well beyond its optimal exhaust flow, allowing atmosphere to separate the flow from nozzle walls.

    This is kind of counter-intuitive and hard to remember, but still quite important to explain it correctly.

  28. Little announcement Everyday Astronaut is working on a video talking about these and he’s using his interview with Elon musk in the video

    After watching this I’m looking forward to the video with Musk now that I know more about aerospikes

  29. I guess there's a major "if it ain't broke don't fix it" mindset. I'd think that just fuel costs wouldn't be the motive of a more efficient engine, considering that most of the weight of a rocket at launch is fuel if they could reduce fuel requirements dramatically would allow a far greater payload fraction, thus allowing a smaller cheaper rocket to carry a given payload, or far greater payload on a similar sized rocket (more profit). I'd think commercial operators would be most motivated to do this, while NASA is more concerned with making something that works for fewer but more risky missions, for which reliability and confidence is a more important factor than cost. I'm sure SpaceX and the other commercial operators have thought about and researched this in great detail and determined that it doesn't make economic sense for them to try to develop these engines, the expected costs outweigh the performance potential.
    Another thought I have to ponder – what about variable-geometry bell nozzles? Afterburning jet engines, such as on fighter jets, basically always have a variable-geometry nozzle, typically with overlapping petals, which they need to operate efficiently in different conditions (different throttle settings, afterburner on or off, different altitudes). Could a similar approach, possibly even similar design, be used for rockets? Why isn't this done? My guess is that rocket engine exhaust is too hot and under too much pressure, far more than even afterburning jet engines, for such mechanisms to be possible or feasible.

  30. The main weakness of aerospike engine beside cooling of hot surfaces must be the poor efficiency and with two stage to orbit launcher, a bell shape of the engine can be easely optimized for low and high altitude and in fact on the latest Starship design, the second upper stage design carry both types which seems to be a must to combine efficiency at high altitude and safe landing with low altitude mandatory reignitions. This creates some additionnal weight penalty: the FalconX 16% reuse version load carrying penalty must reach about 30% on Starship. Since the cheap methane and oxygen combustion used on Starship is very efficient, the efficiency gap of an optimum engine with aerospike and wrong bell match got even wider, i would assume. Then a very importante factor is that the entire space system smooth landing rely on many engines critical reignitions. This and the atmospheric reentry at liberation speed of a light tin can structure are the major safety risks of such reusable system homologated for repeated crew carrying missions. Seems that only a single stage to orbit system project could save the current aerospike technology but this launcher requires much more progress on materials since a stainless steel single stage orbiter would be way too heavy. Weaved composites like the composite fan blades of the new Leaf jet engine shown how stiff complex composite assemblies could be robotized in the future, but we are still far from similar big structures.

  31. Да потому и не используем, что перевели нам только название, а остальное так и осталось… Лучше бы уже тогда и заголовок не трогали.

  32. If that engine was that great they didn't stop developing it, they just stopped taking about it's development, imo

  33. All they had to do was pick up where the X-33 left off. I call BS on all these so-called privateers like Musk and Bezos.

  34. Ideas for the future:
    Magnetics/magnetism, alternate fuels, hybrid alloys (titanium mixes and/or new ones), newer/better/lighter/more efficient ship and 'ship launcher' designs. And probably most importantly development of space engines (like FTL or the like) for use on shuttles (if calculated, may end up requiring to use for a shorter period, and if accurately measured, may require less fuel, lower costs, higher speeds obviously, lower heat?, lower weight?, and/or who knows what else?).

Leave a Reply

(*) Required, Your email will not be published