Kerbal Space Program Doesn’t Teach… Nozzles

Kerbal Space Program Doesn’t Teach… Nozzles


Intro music Hello! It’s Scott Manley here. Now, I like to talk about the educational merits of Kerbal Space Program. But I am acutely aware there are many things that it doesn’t in fact cover in the field of rocket science… such as nozzles. Now nozzles are what many people think of as “rockets”. They are the bell shaped business end of the rocket… which expel firey awesomeness during the launch process. But in reality it is just one part of a whole rocket engine. In the rocket engine what you have is fuel and oxidizer being mixed inside a combustion chamber. And then these exhaust gases escape out through a nozzle. Which is carefully designed by rocket scientists… to harness the most thrust and efficiency.. from those chemical reactions. The classic nozzle design actually predates rocket science. The “De Laval” nozzle was originally designed for a steam turbine in the nineteenth century. And what happened in the “De Laval” nozzle is: you have high pressure exhaust gases being forced through a small throat. And this is narrow enough that the gases end up getting choked at the speed of sound. Beyond this the nozzle expands outwards and because of the magic of fluid dynamics as the pressure drops and they travel down the gasses accelerate and they end up going at supersonic speeds. and of course in rockets, the faster those exhaust gasses go the more efficient your rocket is. Getting the right nozzle shape and dimension can make a huge difference to the performance of a rocket engine. and uh, the atmosphere complicates these things you see, it turns out that the ideal exhaust gas pressure is the same as the external atmospheric pressure Now this means that for booster rockets designed to be used at sea level, their engine bells will be smaller, whereas engines designed to be used in the vacuum of space will have much longer and wider nozzles to try and get the gas pressure, or the exhaust gas pressure down close to that of the vacuum. the Merlin engines on the Falcon 9 are a perfect example of this. Since the first and second stage engines are almost identical, but on the first stage there is room for 9 engines and their associated nozzles but on the second stage there is a single engine designed for vacuum and its engine bell is almost as wide as the rocket itself. As large as it is, this second stage engine bell is a compromise. To actually get the exhaust gas pressure down to match the vacuum of space would require an infinitely large nozzle. So engineers pick a compromise size which gets the gas pressure down low enough while still fitting inside the second stage fairing. incidentally there is funny story about that second stage engine nozzle. During an early launch they discovered cracks on the engine bell extension and it was decided to fly out an engineer who would enter into the interstage
and with a pair of metal snips he essentially snipped off the bottom
four feet of the rocket nozzle and the rocket flew and they didn’t need the
extra performance that the engine bell gave. This is of course in our actual
bona-fide real life example of rocket surgery! The designers of the RL 10 rocket which
is used in the center have a neat trick for actually fitting
their engine nozzle into an interstage some versions of it have a nozzle
extension which comes down and and extends the nozzle after the staging happens
so this way they can make the interstage smaller and save on size if not complexity back inside the atmosphere
things are a little more complicated The ideal nozzle matches the
exhaust pressure to the atmospheric pressure but Rockets are generally known for going up and as they go upwards they will pass
through different air pressure so the perfect design for sea level is not as
good once you reach higher altitudes and ultimately space typically the range and nozzles for a
launch vehicle booster are slightly over-expanded at launch and they’ll reach some
perfect performance at altitude and then as they blaze skywards it will become
under expanded by the time the engine cuts out but there’s actually a limit to how big you
can make a simple rocket nozzle and still have the engine operate stably.
If the exhaust pressure gets too low then the atmosphere
can push back inside the rocket nozzle and this causes the exhaust to detach from the rocket and the engine nozzle walls and this can
cause all sorts of instability that can lead to control problems and
other undesirable phenomena typically this occurs at about forty
percent of atmospheric pressure for a simple nozzle. The Space Shuttle main
engine is an interesting example in this regard, if you recall the space shuttle
would light its main engines on the ground and use them all the way into
space so if they need to they needed to design
an ancient which would work well at all altitudes The designed exhaust pressure is about
thirteen percent of atmospheric pressure that’s below the forty percent that of course
would lead to flow separation and instability So the designers redesigned their nozzle
very slightly that towards the end as it curves outwards they suddenly change
the curvature in a little and that surprises the exhaust gases
just enough the pressure rises near the edge and keeps its pinned to that surface
and keeps the combustion stable this provides decent performance on the
ground and all the way up into space although they weren’t as good as they could
have been because they did originally have a proposal for a longer nozzle
extension and another thing over expanded nozzles are the source of those
beautiful chains of shock diamonds in exhausts Because the exhaust gases for an
over expanded nozzle are below that of atmospheric pressure as they come out
they get squeezed in and then the pressure gets too high and the bounce
back so you get a hot spot as it bounces in and out and that’s why you’ll get the
chain of shock diamonds on certain types of engines, of course as the engines go
higher up in the atmosphere the pressure drops and the nozzle becomes no longer
over expanded so it’s a phenomena that is related to exhaust gas pressure
versus atmospheric pressure you don’t see it in space incidentally now I
focused on conventional nozzles but because of the altitude dependence
there is an incentive for scientists come up with the nozzle designs are
efficient and across a wider range of atmospheric pressures and there’s been a
number of different altitude compensating designs the Kerbal space
program players are probably most familiar with the toroidal aerospike
engine which has a ring-shaped combustion chamber that pushes the gases
inwards towards a spike shape structure and this provides a performance which is
more consistent across different atmospheric pressures but it’s not the
only altitude compensating design there are other things like plug nozzles
linear aerospikes and of course engine bells that drop down like on the RL 10
Anyway, nozzle geometry is a critically important part of rocket
design and oddly enough it’s one of those places that kerbal space program
frequently gets it wrong since the engine designs were built by
modelers who weren’t really rocket scientists for example the ‘Poodle’ engine-bell is a
lot smaller than say the ‘Mainsail’ and yet gets much better efficiency
(better specific impulse) I hear one day this may get fixed,
regardless I hope to be back with more rocket science that kerbal space program
doesn’t teach you I’m Scott Manley, fly safe.

Eugene Islam

100 thoughts on “Kerbal Space Program Doesn’t Teach… Nozzles

  1. You didn't mention that even in the vacuum of space and engine can be overexpanded and the exhaust can freeze, which will reduce the efficiency of an engine.

  2. Great stories… but you forgot to mention that rockets in space is total and 100% make believe BS. A rocket engine might work to some degree but the efficiency would drop to almost zero. That is of course pure theoretically and assuming that space would exist.

  3. nozzles.. nozzles are still being studied.. 70 years later we are still in their infancy..
    theres not much to teach ahahahha

  4. How about having an extention for the nozzle that you slide down when you reach the right altitude? You can't get much more Kerbal than that.

  5. God, but that clip of the shuttle's main engines igniting is the sexiest twelve seconds of footage on earth. The way the combustion builds and suddenly condenses into an organized thrust diamond, and then all three bells gimbal inward as the shuttle starts pushing against the launch clamps like an excited greyhound — Gets me pumped for rocket science every time.

  6. But if in the vacuum of space you need an infinitely sized nozzle, couldn’t you use space itself as a nozzle, basically just get rid of the nozzle completely (of course I realise that this is wrong otherwise rocket engineers would’ve been doing this but I’m interested din the response)

  7. Hi. Are stages glued together? How does the first stage get attached? Are there small explosives to eject it? Thanks.

  8. "…these exhaust gases escape out through a nozzle, which is carefully designed by rocket scientists…" No, really?
    Missy: I'm always bragging to my friends about my brother the rocket scientist
    Sheldon: You tell people I'm a rocket scientist? I'm a theoretical physicist.
    Missy: What's the difference?

  9. One thought… What does gas pressure actually do in vacuum? In vacuum and 0 gravity every atom/molecule you "shoot" will by Newton have a counter reaction… and in vacuum and 0 gravity that will be the only thing that is "thrusting" you… no shape or form will actually make anything better (again 0 gravity, vacuum)… It's just depends on what you are shooting in one direction and that force will propel you in the opposite direction… Gas pressures and the likes is needs something else then vacuum…

  10. "The magic of fluid dynamics"… in this case also known as the Venturi effect.
    I wonder who came up with the nozzle design for the very earliest Chinese black powder rockets?
    Couldn't have been that hard. After all… it's not Rocket Surgery.

  11. @5:36 the shock diamonds that you get in jet engine exhaust are due to sound waves cancelling not due to bouncing in and out

  12. Why couldn't we make dynamic rocket nozzles that change relative to the atmospheric pressure? It would be smaller on the ground and it could expand as it went higher up until it reached a vacuum.

  13. the purpose of the nozzle must be to maximize thrust. thrust is the net force on the rocket in the forward direction caused by more rearward facing areas exposed to higher pressures than forward facing areas exposed to lower pressures. so the purpose of the rocket engine is to generate maximum pressure on all rearward facing areas and minimum pressure on all forward facing areas. pressure is minimized where velocity is maximized. however to generate higher velocities the flow must be accelerated. for the flow to be accelerated there must be an increasing pressure gradient in the upstream direction. these are major contributing factors to the internal shape of the rocket engine.

  14. Scott's got a vinyl collection!!! Just earned respect from Me. But I know I have more then you hehehehe. Would love to see a video on what you have.

  15. Can the shuttle detach from the fuel and srbs a few seconds or minutes after launch and divert to the runway like a glider if theirs a problem noticed just after launch that will cause a challenger style explosion. If so will the srbs and tank lose control as fast and crazy as they would in ksp when you lose it at full throttle or would they just fly a straight path into the ground or ocean?

  16. I've often joked that people who think rocket surgery is easy have never tried to re-weld a cracked engine bell at 50,000 feet. It's cold, everything shaking like crazy, the airspeed is about Mach 5 so the torch keeps blowing-out, etc. It's a pain in the ass, really.

  17. I quit rocketry when i was a kid, because , i wasn't allowed to run data and telemetry instrumentation on the rockets , but i have recently got back into it, because some rich ass hole put a car in to an orbital plane , i do work mostly on theory side , thank you for the inspiration , and yes im skeptical on the KSP,

  18. What’s the reason for the optimal exhaust pressure being the same as outside pressure? Intuitively it doesn’t exactly make sense to me. Thanks to anyone who replys

  19. Why can't they make the bottom of the rocket nozzle mechanically able to expand and contrast on it's own depending on the atmosphereric pressure?

  20. 2:16 There must be someone out there to thank for the "do not touch the rocket engine" warning. Then again, I'd have a hard time trying not to touch it as well, if it were right there.

  21. build an overly-large nozzle and near the bottom of it have 'blow-in doors' that stay open at low altitudes, but which close at higher altitudes. These doors would have to be made out of a material that is capable of withstanding the exhaust temperatures without any cooling, so probably would need to use something like tungsten in their construction.

  22. Hello smart persons of the comments!
    I'm building a valved pulse-jet engine (I don't need help with valves) and I'm wishing to get a bit more efficiency from it as that they are fuel guzzlers and make alot of heat. Adding a augmenter (a device that acts sorta like a bypass fan but only works well for pulse-jets) could give me a 40% boost but I'm looking for more power and efficiency.

    Would squirting some water mist/steam into the combustion chamber help efficiency by turning excess heat into usable reaction mass for more thrust? I might potentially turn part of the tailpipe into a small boiler if this does help.

  23. What advantage, if any, would be gained if the exhaust gases were given a slight spin as they travel through the constriction and nozzle. That could be achieved by "rifling" the inner surfaces.

    EDIT: I thought of a benefit for such spin: It would counter the instability that comes from a too-wide nozzle causing the atmosphere to detach the exhaust stream from the nozzle walls. The effect is analogous to the way a spinning bullet avoids tumbling during flight.

  24. A rocket scientist does not exist. There is no such animal. The correct terminology is
    Aeronautical Engineer. To understand rocket nozzle design, you must first understand
    The Method of Characteristics. This mathematical process was used in the design of
    shock free wind tunnels. Nozzles designed this way are very long and heavy.
    Modern nozzles are shorter and more efficient due to the work of J.V.R. Rao.
    The Method of Characteristics is coupled with Computational Fluid Dynamics to attain a superior nozzle.
    Nozzle gas recombination kinetic reactions and nozzle gas freeze point must be considered.

    G.V.R. Rao Exhaust Nozzle Contour for Optimum Thrust. Jet Propulsion Vol. 28 No. 6 1958
    Library of Congress TL780.A613
    G.V.R. Rao Contoured Rocket Nozzles. Ninth Annual Congress of The International Astronautical Federation. Amsterdam 1958 Vol. 2 Library of Congress TL787.i44

    K.N.C. Bray Journal Fluid Mechanics Vol. 6 1959 pages 1-32 Library of Congress QA901.J87

    K.N.C. Bray Simplified Sudden Freezing Analysis for Nonequilibrium Nozzle Flows. Journal American Rocket Society (ARS) Vol. 31 1961 pages 831-834 Library of Congress TL780.A613

  25. Scott – there's one important thing you didn't touch on in this video that I wish you would explain – though I think I figured it out (after several years of periodic wondering, finally) by extrapolating it from the info you included here. You should explain why a particular nozzle is optimal at a given altitude.

    The fact that the ideal vacuum nozzle would be infinitely large was the clue that really clarified the situation for me. I've heard many times in many ways how rockets and jets try to lower the pressure to 1 atm, but this always seemed the opposite of what they were really doing – trying to create an output stream from the highest possible pressure…

    Ie, I think you should explain (if I understand it) a nozzle's optimal shape is one where (ideally) all propellant molecules have been theoretically repelled from the nozzle wall, out the exhaust opening, moving opposite the rocket's intended motion. In reality, molecules bounce around randomly, creating "pressure", and so the optimal nozzle is one where the exhaust pressure does not cause the pressure of the propellant molecules to push out of the nozzle's rim, since any such perpendicular-to-velocity pressure represents lost energy… right?

  26. Hey could you plese tell a software whick could be like ksp (large scale missions) and also explains rocket scice with all info like lenth pressure diameter and also user controlled thrust? ☺😊

  27. What would we do without Youtube and wonderful people who make great videos like these? OMG, we would be stuck with those horrible reality vomit inducing shows on mainstream media. Even the sciency ones on Discover and NatGeo get it wrong. They think we all have 2 second attention span and have to pan and zoom in every scene at super fast speed to make it seem like exciting action is happening otherwise we fall asleep.

  28. Scott, this video has bothered me for some time as it is an interesting problem. I have been playing with the idea of a solution that probably will not work, but it seems it is worth sharing.
    As the outside air density changes the optimum nozzle shape changes, I accept this. So I wonder if a pseudo nozzle can be made with several smaller nozzles? My thought is maybe you have 7 nozzles and at launch, you fire all 7. As you gain height you shut down 3 or 4. When in near orbit, you only use one. I know there is overhead in this, but if it is efficient enough, perhaps…

  29. Could you do a video going more into detail about the aerospike?
    Why is it always in testing limbo and not used and stuff.

  30. I'm confused. Why does velocity increase as pressure drops? I would think the more pressure, the more velocity. Thanks for a great video

  31. Awesome as usual, great work. I really wanted to say i purchased ignition as an audiobook and thank you for recommending it.

  32. Could you design a nozzle extension for a sea level Merlin 1D from carbon, much like the RL-10's for the second stage that could be jettisoned during reentry in order to allow it to be recovered much the way the first stages are if payloads are relatively small. Say for Space-x's cubesat array they are planning.

  33. Nice to know that we Swedes have made a major contribution to rocketry. Gustaf de Laval left this life poor but he had enriched science with a lot of fantastic stuff to his name.

  34. Soo… maybe im just stupid, but when the gases go outwards through the bell shape, yeah the pressure decreases, but why the hell does that make the velocity increase!?!? That goes against everything and anything ive ever known, my life is a lie.

  35. Uh, the "nozzle" is the ACTUAL "COMBUSTION CHAMBER" while the "combustion chamber" in your diagram is a "mixing chamber" and "combustion" cannot occur until there is "room" for EXPANSION.

  36. Actually, it has been fixed for a pretty long time now… the poodle engine has two larger engine bells. do you know why some rockets have two nozzles, like the poodle, the twin-boar and the bobcat?

  37. you can see the difference in the exhaust plume at different altitudes. aside from vectoring, bells create a thermal cushion yeah? like apollo 11 and the dark band of unburnt kerosene that causes the flame to stand off and i always wondered how that could be if combustion had already taken place, or was that just kerosene dumped around the bell (at turbine speeds) for cooling?

  38. If the boosters generally need smaller bell nozzles, why was the Saturn V's 1st stage F1 engines so massive, compared to the upper stages?

  39. 2:18 FYI, in aircraft, aerospace, and their related maintenance fields I can't tell you how many "warning signs" are 72pt bold font in a portrait landscape on 8.5×11 paper, taped to the item one is warned about 😂😂😂

  40. Glad I found your channel !
    My kind of topics ,intelligently explained from an informed and knowledgeable perspective !
    Man, that's totally awesome !

  41. Scott, do you have a video in which you explain exactly what the role of the heat exchanger is around rocket nozzles? Why on some rockets must fuel be run through this setup before introduced into the combustion chamber? Thanks!

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