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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.

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    $\begingroup$ 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. $\endgroup$ – Russell Borogove Jul 14 at 0:06
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    $\begingroup$ "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. $\endgroup$ – Carl Witthoft Jul 14 at 12:56
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    $\begingroup$ @CarlWitthoft gravity exist everywhere. You meant acceleration, but that is not altitude dépendant neither. $\endgroup$ – Antzi Jul 14 at 13:45
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    $\begingroup$ @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. $\endgroup$ – Solomon Slow Jul 14 at 13:57
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    $\begingroup$ @Antzi I meant "Force due to Earth's gravity" $\endgroup$ – Carl Witthoft Jul 14 at 14:51
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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.

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    $\begingroup$ Very comprehensive answer, thank you. $\endgroup$ – Ingolifs Jul 14 at 9:17
  • $\begingroup$ Great explanation! $\endgroup$ – Organic Marble Jul 14 at 11:38
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    $\begingroup$ 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. $\endgroup$ – Christopher James Huff Jul 14 at 18:13
  • $\begingroup$ Maybe a ramjet sort of rocket engine, that doesn't need pump? $\endgroup$ – Prakhar Jul 15 at 16:27
  • $\begingroup$ @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. $\endgroup$ – SF. Jul 15 at 18:36
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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.

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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.

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  • $\begingroup$ I don't think your claim about gravity-feed is valid even at the moment of launch. $\endgroup$ – Carl Witthoft Jul 14 at 14:52
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    $\begingroup$ 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. $\endgroup$ – leftaroundabout Jul 14 at 15:14
  • $\begingroup$ 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? $\endgroup$ – Carl Witthoft Jul 14 at 15:31
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    $\begingroup$ 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. $\endgroup$ – leftaroundabout Jul 14 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.

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