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Understanding liquid fueled rocket engines

  1. Jan 24, 2010 #1
    I'm troubled by the problem of pumping fuel to a combustion chamber. The big problem is that pressure cannot be directed in some particular direction. So when you see propellant leaving the chamber with a great pressure, it should be kept in mind that the propellant is attempting to leak, with the same pressure, back to the pipes through which the fuel is being injected. I have understood that most of the time the solution to this problem is that the liquid fueled rocket engines simply use extremely powerful pumps. I've found a following joke about this:


    My first question is that where do these pumps get their energy from, like in Saturn V and in Space Shuttle? If you don't think about it much, you might think that perhaps some battery is enough to run the pumps, because it's the combustion chamber from where the real energy to the propulsion comes from, but that wouldn't make sense to me, because the pumps need to fight against the incredible pressure created in the chamber.

    Is part of the energy that is generated in the chamber, somehow directed back to the pumps? How could one accomplish that? Or do the pumps have completely own combustion engines, like cylinder engines burning their own fuel?

    Another fact which I have found surprising is that the LEM did not use pumps at all. I have the https://www.amazon.com/NASA-Apollo-...=sr_1_1?ie=UTF8&s=books&qid=1264369699&sr=1-1 and it says that

    I am left puzzled by this, because according to my intuition the helium injected to the tanks would need to be in equally high pressure as the pressure created in the combustion, meaning that equal thrust might as well be achieved by simply shooting out the pressurized helium.

    Earlier I had also considered a question that perhaps burning a fuel in the rocket engines is useless, because one might achieve the same propulsion by simply using the pumps to shoot some passive propellant, like water. I have not understood the thermodynamics of this, but I have been left under impression, that if there are two engines, which both create the same pressure at the mouth of the chamber, but other one shoots lighter particles, and other one heavier particles, then the one with lighter particles would create stronger thrust. Although I don't feel like understanding why that would be the case, I believe that that is the way it is, because otherwise, according to my reasoning, the rockets would work just as well by merely shooting passive propellant with pumps.

    But I'm still left unable to come up with an answer to how one could avoid the need of the LEM's helium to be in an unreasonably high pressure when released to the fuel tank.
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Jan 31, 2010 #2
    One possibility is that the LEM had valves which allowed propellant to only enter the chamber, but not leak back to the tanks. I guess this would mean that the burn was not continuous, but consisted of very rapid sequence of discrete explosions, the valves always closing when propellant explodes, and then opening again for new propellant to enter the chamber. I've never heard of such thing, but one cannot debunk such idea by watching videos only.
  4. Jan 31, 2010 #3


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    Staff: Mentor

    The pressure of the pump might be the same as the pressure of the gas in the combustion chamber, but the volume of the gas in the combustion chamber is much, much higher than the volume of the liquid being injected.
  5. Jan 31, 2010 #4
    I see. That remark solves the biggest trouble.
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