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In a tap-off cycle, combusted gases are extracted from the combustion chamber and routed to the turbines, which then drive fuel/oxidizer pumps.

In a pressure-feed cycle, fuel/oxidizer is fed into the combustion chamber by high tank pressures. A pressurant gas (e.g. helium) is also fed into the fuel/oxidizer tanks to keep the tank pressure constant.

Would it be possible to combine the concepts of these cycles, by using the tap-off gas directly as pressurant gas? On the face of it, this would lead to a very simple cycle, but I can't find any material on it. Why is this not done/possible?

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Besides the safety concerns mentioned by Hobbes, there are other things to consider.

The gases at the combustion chamber are at a very high pressure and temperature and as such are excellent at driving turbopumps. They are exhausted at a much lower temperature and pressure, having given up a considerable amount of useful energy in an efficient expansion cycle. More importantly, the turbopump is able to achieve a higher pressure (albeit at lower volume flow) than the combustion chamber pressure, as it is effectively 2 independent machines connected by a shaft: a turbine and a pump.

In a pressure fed cycle, gas from the combustion chamber, besides being very hot, is necessarily at a slightly lower pressure than tank pressure. It is difficult to make a lower pressure gas pressurise a higher pressure tank. Surprisingly, it is possible if the low pressure gas has greater volume and here are some tricks employed in steam engines to make this happen, but it requires a considerable waste of fluid. https://en.wikipedia.org/wiki/Injector (see section on feedwater injectors.)

The other issue is the nature of the combustion gases. They are mainly H2O and CO2. They need to be cooled before entering the tank, or, if not, they will reach the temperature of the tank contents as soon as they enter it.

In Oxygen or Hydrogen, these substances will instantly freeze and be useless at maintaining pressure, other than by the heating effect they have on the tank medium. In a kerosene tank, the H2O would condense and sink to the bottom, which might not cause problems if the percentage was small. Only the CO2 would go to pressurizing the tank.

All in all, it's more complicated than you make it sound. A separate helium tank is a better solution.

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  • $\begingroup$ Pretty much covers everything. Maybe worth mentioning, that you can use fuel for pressurization in an expander cycle. You still need a pump, though. $\endgroup$
    – Rikki-Tikki-Tavi
    Commented Apr 4, 2016 at 7:40
  • $\begingroup$ I've posted a follow-up question $\endgroup$
    – spookysys
    Commented Apr 13, 2016 at 18:28
  • $\begingroup$ For pressurization of a kerosene tank, cheep nitrogen will do, expensive helium is not necessary. For the oxygen tank, nitrogen may be used too, gaseous nitrogen will not be liquified by the temperature of LOX. But for a liquid hydrogen tank, only helium or hydrogen may be used, nitrogen will be solidified by the temperature of LH2. $\endgroup$
    – Uwe
    Commented Dec 22, 2016 at 8:48
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    $\begingroup$ I think the point about "higher pressure tank" could do with a bit more clarification. The key point being that you only get flow where there is a pressure gradient to drive it. Without a pump, the tanks must be at a higher pressure than the combustion chamber or there will be no flow to deliver propellant to the engine. No matter where you tap off, the pressure must be lower than the tank pressure because it is "downstream". That tap off will then not be at sufficient pressure to put anything back into the tanks. $\endgroup$
    – Anthony X
    Commented Dec 22, 2016 at 22:50
  • $\begingroup$ @AnthonyX 1. since the question is about pressure-fed rockets, we are talking about tanks with higher pressure than combustion chamber by definition (not a good design choice for large rockets, but anyway...) 2. As you can see in the wikipedia leak it is actually possible to use steam from a boiler to fill the boiler with water using a venturi injector, no matter how counterintuitive that may be. It's very wasteful of fluid, though, so while it is done with water / steam you wouldn 't want to do it with fuel. $\endgroup$
    – Level River St
    Commented Dec 23, 2016 at 0:06
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Most engines are run with an imperfect (non-stoichiometric) mix of oxygen and propellant, so the exhaust will have either oxygen or propellant in it.
Running propellant-rich, hot exhaust into a LOX tank can lead to ignition of the remaining propellant, so you'd have a fire inside your tank. The same goes for oxygen-rich exhaust being run into a propellant tank.

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    $\begingroup$ That certainly isn't the only reason... $\endgroup$
    – Level River St
    Commented Apr 3, 2016 at 21:22
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    $\begingroup$ You might call the result of that running "tank-rich"... $\endgroup$
    – Tristan
    Commented Dec 22, 2016 at 17:24
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    $\begingroup$ @LevelRiverSt it's not the only reason, but it is quite sufficient to make it a bad idea all on its own $\endgroup$
    – Leliel
    Commented Sep 6, 2017 at 21:50
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The book Rocket Propulsion Elements, Sutton, has a sketch of a missile that use solid grain to generate gas that pressurize a tank of liquid propellant separated from the hot gas with a piston

Propulsion system pressurized by solid grain

The advantage is that a missile can be stored for much longer time, it is not necessary to worry about the wear suffered by the turbo pumps or possible leaks of a pressurized system to helium. I believe that the drawback is the weight of the pistons, and a eventuali leak betwen the hot gases and the liquid propelant could cause an explosion.

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    $\begingroup$ Welcome to Space Exploration! It would be good if you could edit this answer to clarify what the results and tradeoffs of this design are. $\endgroup$
    – Nathan Tuggy
    Commented Dec 22, 2016 at 2:34

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