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This is something I just don't understand. The way I see it, propellant can be pressurized all on its own in the tank. Just pump a lot of the stuff in there and make sure the tank is strong enough. Any fluid can do this. There shouldn't be any need to pressurize it with something else, such as helium, which adds mass especially if you consider the helium tank.

I also don't understand how you can mix helium with the propellant like that. How do you ensure that when you open the valve, the propellant comes out instead of a propellant-helium mix?

Note: im talking about small rockets like thrusters in an orbital satellite, or maybe a little bigger like the Apollo CSM engine (which was also pressure-fed). I'm not talking about large rockets launched from the ground, which certainly can't be pressure-fed systems.

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  • $\begingroup$ You should think about a halve empty tank, there should be something in the empty halve to hold the pressure. $\endgroup$ – Uwe May 30 '17 at 8:58
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Interesting and non-trivial questions.

Most propellants are not self-pressurized because as soon as the engines turn on the pressure would drop precipitously as the tanks quickly emptied and the propellant was unable to vaporize fast enough to keep up. LOX, RP, and H2 are the most common launch vehicle liquid propellants and none vaporize fast enough to maintain pressure. (N.B. this is at launch vehicle scale. There are self-pressurized LOX/Methane thrusters for reaction control systems.)

So in practice pressurization is performed with another gas, typically nitrogen or helium (kind of air that makes your voice funny)stored at very high pressure in a separate tank and fed into the ever-increasing ullage in the main tanks as they are emptied. Nitrogen is used because it is cheap and dense (even more dense if liquified, although I’m not aware of any such systems); helium is light.

This is a bummer in practice as the pressurization system add significant complexity and mass to a rocket.

As for mixing of helium and the propellant, they tend not to mix too much because of the vastly different densities and the large accelerations during boost. That said, special care is given to design diffusers which keep the pressurization gas from being shot into the liquid propellants like the worlds most amazing soda stream.

As for ensuring the liquid is near the outlet valve instead of the ullage gas, this is easy when the rocket is on the ground and under acceleration. It is a real problem for upper stage ignition after the main booster has shut off when the rocket is in free-fall. There are lots of solutions here, which is probably a topic for another question, but the short answer is tiny rockets to settle the propellants just before ignition, ensuring some small amount of acceleration remains to keep propellant settled, using screens or other features in the tank that take advantage of capillary action or surface tension to keep liquid near the outlet, etc.

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    $\begingroup$ I edited my question to explicitly exclude large rocket launches, because those can't have pressure-fed systems in the first place (only turbo-pumps can supply that kind of mass-flow). Not sure if this changes your answer any. $\endgroup$ – DrZ214 Jun 3 '15 at 5:43
  • $\begingroup$ It doesn't really. You're right that pressure fed rockets would only be feasible at the very smallest end of launch vehicles, but the same problems apply to pump fed rockets. For example, the Saturn V was pump fed, but the Up-Goer Five shows all the massive helium tanks it had. $\endgroup$ – Adam Wuerl Jun 4 '15 at 6:04
  • $\begingroup$ Wasn't the Saturn V's helium supply merely for preventing negative pressure as the fuel and oxidiser were drawn out of the tanks, rather than for providing positive pressure to actively force them out? $\endgroup$ – Sean Jun 23 '18 at 16:36
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Given that you are using a liquid propellant you cannot rely on its own compressibility to achieve the required pressure. Liquids do not compress very well.

This is especially true as you need a high pressure for your thruster and to make it worse: usually you do not want this pressure to change through the time of engine usage. Therefore, the common solution is to achieve a constant, high pressure of the propellant by inserting an inert (i.e. non-reactive) gas to the propellant tank. In addition, this allows to modify the propellant pressure during use.

The orientation of the liquid propellant is a difficult topic. As said in other comments, the commonly used techniques include: - using a flexible bladder that separates the liquid from the gas and compresses around the liquid - feeding the liquid droplets to the exhaust port by dapilar force, e.g. by small fibers or plates or a special wall-coating - enforcing pre-acceleration of the tank (towards the exhaust port!) e.g. by rotation of the spacecraft or pre-acceleration into the desired direction through vernier engines (another use for the Nitrogene or Helium, by the way...) - in general by avoiding 'sloshing' of the liquid in the tank by adding almost separated compartments in the tank.

During propellant budget computation you will also have to take into account that there always is going to be a residual amount of propellant in the tank that cannot be used for combustion in a planned, controlled operation.

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  • $\begingroup$ Thanks I should have remembered that bit about incompressible liquids. Water, iirc, is imcompressible, but what about Hydrazine, UDMH, and N2O4? $\endgroup$ – DrZ214 Jun 3 '15 at 21:31
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    $\begingroup$ I’m not sure what compressibility has to do with it. Liquids can be stored at very high pressures despite being essentially incompressible. The issue is that when propellant is consumed the pressure collapses because the liquid can't vaporize fast enough. An example of a self-pressurizing fuel would be butane in a lighter: as it burns more vaporizes to keep the pressure relatively constant at butane's vapor pressure. $\endgroup$ – Adam Wuerl Jun 4 '15 at 6:07
  • $\begingroup$ DrZ214, liquids are by definition "incompressible". But of course what that means is that the change in volume when you apply pressure is so small as to normally be negligible. $\endgroup$ – c roald Dec 14 '15 at 16:26
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It's easy enough to have sufficient pressure when the tank is full. But with your method, you need to pump in enough gas to keep pressure up until the tank is empty.
Say you'll use the tank until 5% of the fuel is left. In that case, your pressurant gas has expanded by a factor of 20, and has dropped pressure by a factor of 20. If the pressure at that point is 1 bar, you need to start at 20 bar. That means you need a very heavy tank: 20 times heavier than a tank that's good for 1 bar.
So what you do instead, is build a small high-pressure tank, and connect it to the large tank via a pressure regulator. This way you always have enough pressure in the tank, without needing to build a heavy main tank.

The ideal scenario is when you can use the propellant itself to provide pressure. Now, propellant always boils a bit. In fact before launch, tanks are usually vented to prevent the boiloff increasing the tank pressure too much.
Bu when the stage is in use, as Adam said, propellant on its own doesn't boil off fast enough to keep the tank pressure up.

There is an interesting new development in this area: ULA is working on a rocket stage (ACES) that will use one or more small internal combustion engines to heat the propellant. They found they can generate enough boiloff this way to keep the second stage tanks pressurized.

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  • $\begingroup$ "So what you do instead, is build a small high-pressure tank, and connect it to the large tank via a pressure regulator." Are you talking about sump tanks? $\endgroup$ – DrZ214 Jun 3 '15 at 21:29
  • $\begingroup$ No. I'm talking about the way fuel tanks are usually pressurized: a high-pressure reservoir of helium or nitrogen that vents gas into the propellant tank as needed. $\endgroup$ – Hobbes Jun 4 '15 at 7:24
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Use small high pressure helium tank located inside the tank and well isolated with electric heater, its volume capable of expelling the LOX at a pressure that would be adequate for full thrust of combustion chamber size this saves room.

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    $\begingroup$ This does not answer the question. It offers an explanation of how to arrange tanks of helium & LOX, but does not state why helium or nitrogen are used in pressure fed systems. You should also be more careful with you spelling. $\endgroup$ – Fred May 30 '17 at 9:09

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