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Basically all rockets that I know of have a pump feeding fuel and oxidiser into the combustion chamber. The turbopump is one of the most complicated and expensive components of the entire rocket. If a pumpless gravity-fed engine were feasible, it would've been done by now.

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But it's not immediately obvious to me why the rocket engine needs a pump when fuel and oxidiser can be provided through gravity (and acceleration). I have a couple of ideas why the pump might be necessary, but would like to have a definitive explanation.

Ingolifs
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    Most smaller engines are pressure fed; the tankage is pressurized usually with helium or nitrogen and propellants are forced into the chamber by the tank pressure, with no turbopump involved. – Russell Borogove Jul 14 '20 at 00:06
  • Related:https://space.stackexchange.com/q/19647/6944 – Organic Marble Jul 14 '20 at 02:05
  • Fuel pressure needs to be higher than chamber pressure for it to run to the right direction. Chamber pressure needs to be very high for the engine to be efficient. Theoretically you can build a rocket a few miles tall to build the fuel pressure using nothing but gravity but in practice that has some many flaws. – user3528438 Jul 14 '20 at 06:10
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    "Gravity" <-- doesn't exist above a certain altitude, for all practical purposes. So once you shut down your engine, you can never restart it. Might as well be a solid-fuel system. – Carl Witthoft Jul 14 '20 at 12:56
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    @CarlWitthoft gravity exist everywhere. You meant acceleration, but that is not altitude dépendant neither. – Antzi Jul 14 '20 at 13:45
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    @CarlWitthoft, That's lazy. The scare quotes you put around "gravity" make it clear to anybody who knows what you're talking about that you know what you're talking about—that when you said "gravity" you actually were talking about something that is not gravity—but any newbie who reads your comment will only remember you saying, "gravity does not exist above a certain altitude," and that will only reinforce their ignorance. – Solomon Slow Jul 14 '20 at 13:57
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    @Ingolifs, What Carl is saying is that once a spacecraft enters orbit, any remaining fuel (and everything else aboard) will be weightless. There's plenty of gravity up there, but you can't feel it when you're in orbit because "orbit" effectively means that you are always falling. (The reason you don't hit the Earth is, you're always missing it because of your horizontal speed.) Anyway, If the fuel can't feel its own weight, then gravity is of no help in getting it to enter the combustion chamber. – Solomon Slow Jul 14 '20 at 14:07
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    @SolomonSlow Those are not "scare quotes" ( neither are those!). I was quoting the word from the OP's text. Perhaps I should have written "force due to gravity." Further, the OP says nothing about orbit ; the force due to earth's gravity once you are even half-way to the Moon is essentially zero. – Carl Witthoft Jul 14 '20 at 14:50
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    @Antzi I meant "Force due to Earth's gravity" – Carl Witthoft Jul 14 '20 at 14:51
  • fyi I've just asked Would rotating detonation rocket engines always require high pressure fuel pumps? – uhoh Jul 16 '20 at 08:48

4 Answers4

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The performance of a rocket engine - its specific impulse - is directly proportional to the velocity of exhaust gas (and nothing else!). That velocity is achieved by releasing the combustion products from pressurized combustion chamber (pressurized by continuous production of exhaust gas by burning the fuels) and the higher the pressure the more you can accelerate the exhaust gas - obtain better performance.

To inject fuels into the combustion chamber you need to push them in at pressure higher than present in the chamber. That necessitates plumbing and infrastructure capable of withstanding these pressures - thick-walled, bulky and heavy. If you pressurize the entire tank, the entire tank must be pressure-proofed - made robust enough to withstand the high pressures. That will result either in exceptionally thick, heavy tank, or - practically - a tank that is moderately heavy but only holds a very moderate pressure. That converts to low combustion chamber pressure and poor performance.

The turbopumps are a way around this - the tank must only withstand very modest pressure needed to get the fuel to the pump, and then only the small segment of the infrastructure past the pump must be reinforced, and it can be reinforced by quite a bit (it's small!) providing very high chamber pressure - great engine performance.

Still, pressure-fed rockets aren't all that uncommon; most of early-days rocket engines were pressure fed. It's often more economical to go with a simpler, bigger, low-performance stage of a rocket than to develop something of excellent performance that just costs a lot.

As for pressurizing through gravity and acceleration - 10m of water produces 1 bar of pressure differential in 1g. Liquid oxygen, RP-1, hydrogen, methane etc are all less dense, but let's use water for ballpark numbers and upper bound using some extremes. Saturn V was 111 meters tall. Let's give it a pretty oppressive 6g of acceleration and run the fuel from the very tip to the engine. You're still getting only the very modest 66 bars. You could improve it by pressurization but you're already carrying all this mass on top of a 111m tall tower, the structural overhead will be massive! Meanwhile, SpaceX's Merlin, their workhorse, goes at nearly 100 bar and is rather mediocre performance-wise.

SF.
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    Very comprehensive answer, thank you. – Ingolifs Jul 14 '20 at 09:17
  • Great explanation! – Organic Marble Jul 14 '20 at 11:38
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    Also, it'd only be 66 bars at the very beginning, with tanks full. As you consume propellant the "gravity fed" propellant pressure will drop. The last dregs in the tanks would be at effectively zero pressure, if you could keep them flowing into the engine that long. – Christopher James Huff Jul 14 '20 at 18:13
  • Maybe a ramjet sort of rocket engine, that doesn't need pump? – zephyr0110 Jul 15 '20 at 16:27
  • @Prakhar: the problem is a relatively short segment where it's viable - requiring both super/hypersonic speed and atmosphere. Breaching 5-6 mach in atmosphere becomes very tricky (and we need 21!) so the rocket spends a relatively short time in the atmosphere at hypersonic speeds, most of acceleration taking place above Karman line. – SF. Jul 15 '20 at 18:36
  • THen there is the "pistonless pump" concept [I "invented" it, but so did others in parallel, and it had a number of prior then unknown inventors] as pursued by Flometrics on NASA contract a few years ago. This replaces a turbopump in selected systems at vastly lower cost. Two or more gas pressurised pump chambers per-propellant cyclically provide pressurised fluid. In basic form one controlled valve and some non return ... – Russell McMahon Jul 15 '20 at 20:36
  • ... valves are required per pump. || 2018 DARPA LOX Methan firing || Images| Tanks are essentially unpressurised. – Russell McMahon Jul 15 '20 at 20:39
  • Your starting comment: "The performance of a rocket engine - its specific impulse - is directly proportional to the velocity of exhaust gas (and nothing else!)." is distinctly misleading - the real-world performance of a rocket is heavily (no pun intended) dependant on MASS. – MikeB Jul 16 '20 at 11:12
  • @MikeBrockington: Mass of what? The engine? The entire launch stack? The dry mass of the last stage? Mass of the engine alone is almost non-issue. Mass of propellant is, and an engine with higher specific impulse will deliver more delta-V out of lower propellant mass. – SF. Jul 16 '20 at 11:32
  • All of that is indeed relevant, so why did you initially say "and nothing else" ? – MikeB Jul 16 '20 at 16:18
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    @MikeBrockington: Performance of the engine is understood as its specific impulse and it depends on exhaust velocity, period. Performance of the spacecraft is a broad, nebulous term with no firm definition. – SF. Jul 16 '20 at 16:20
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In high performance engines the chamber pressure is much too high to be gravity- (or even pressure-) fed.

The Space Shuttle Main Engine had a chamber pressure in the ballpark of 3000 and 3500 psi (~200 to ~240 bar). Pumps are required to inject the propellants into a chamber containing such high pressure; head pressure is not a practical means.

If your question is really "Why does the chamber pressure need to be so high" I will delete this.

Organic Marble
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In addition to the great answers already given, I would note that all major liquid rockets are “gravity-fed” in a way. They do in fact rely on either gravity or acceleration to push the fuel and oxidisers to the bottom. Where this is most challenging is at stage separation, where the rocket is briefly almost in free fall (– well, free ballistic rise) and the upper stage may need small e.g. solid thrusters to both get away from the lower stage, and to slosh the fuel down towards the engines.

Just, as already said, it would be highly problematic to get that down-pressure high enough to actually press the fuel into the combustion chamber. That's where a pump is pretty much needed, because a tank that's able to withstand the pressure is only practical on a small scale. The designers of the Sea Dragon actually thought otherwise, but I'm pretty sure they were just wrong; this rocket would never have worked as intended.

leftaroundabout
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  • I don't think your claim about gravity-feed is valid even at the moment of launch. – Carl Witthoft Jul 14 '20 at 14:52
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    You may think that, but then you are wrong. Good luck feeding the turbopumps with vapours, in absence of some acceleration pushing down the liquid phase. – leftaroundabout Jul 14 '20 at 15:14
  • and yet is that not the point of compressors? Else how would you restart a cold engine in orbit, where there is no net external force to keep liquid fuel at the exit port of each tank? – Carl Witthoft Jul 14 '20 at 15:31
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    You don't, generally. In orbit, there's no need for much Δv, so mostly pressure-fed monopropellant thrusters are used. Of course these can also provide the necessary acceleration to get a pump-fed engine to start, if necessary. – leftaroundabout Jul 14 '20 at 16:03
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To add one more reason for turbopump pressurization - combustion stability. It is of paramount importance that the combustion of fuel and oxydizer is performed in a stedy, controllable and safe manner. Taken for granted and needs no explanation, right? But the physical reality of reaching this desirable condition made generations of engineers loose their sleep over it. Long story short, the high pressure feed from turbopumps, among other measures, helps tremendously in solving this problem.

On the gravity feed part the original question, it really does not matter what is the origin of the force that the fuel is experiencing; be it gravity itself, acceleration of the rocket or hydrostatic pressure of the column of fluid itself - in any conceivable scenario that pressure if far too low for the engine to operate in a useful way, as explained earlier by other posters. Just wanted to note that if you were onboard an accelerating rocket and blindfolded, there is no way to tell how much of the force is felt from gravity, and how much from acceleration. This is how gravity and inertial forces are equal, and the fuel feels it this way, too. Given that the acceleration is commonly in the range of 3 to 6g, and that the rocket starts its slow pitch tilt towards horizontal almost immediately after liftoff, the effects of gravity rapidly fade in comparison to other forces, either by increse in the ratio of acceleration forces vs gravity, or by constantly decreasing the component of gravity along the longitudinal axis of the rocket during the pitch program execution.

Hope this helps.

Mitar
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