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There is an apparent need to have pressurants in both pressure fed and pump fed systems. However, since pump fed systems can create the necessary pressure, they can work with less pressurants and lesser storage pressures making tanks thinner. What other differences exist between these two systems within the context of pressurants and would someone care to elaborate on it?

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  • $\begingroup$ This could be a related question to " Why do pump and pressure fed liquid engines need to operate at high pressures? " $\endgroup$ – Rajath Pai Mar 25 at 12:33
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While you're correct in your assessment of "since pump fed systems can create the necessary pressure, they can work with less pressurants and lesser storage pressures making tanks thinner" you may be missing the degree to which these two differ. Some other concerns exist, but they are secondary - the main issue is the vast difference in pressure needed.

Of top importance in rocketry are the engine thrust and specific impulse - both depending on speed at which the gases are ejected from the nozzle, thrust additionally accounting for mass flow ratio. Directly related to energy of the propellant obtained through combustion, which is derived from combustion temperature and combustion chamber pressure. In short, the higher the chamber pressure the more efficient and more powerful the rocket. There are some more factors to that, but this one is one of most important.

And the fuel and oxidizer must be delivered into the chamber - pushed into it at pressure higher than the combustion chamber pressure plus whatever resistance the plumbing, injectors, possibly regenerative cooling system too, create.

For pump-fed systems, the pressurant needs only to overcome the (minor) resistance of plumbing between the tank and the pump - even less so if the pump provides suction, just assuring so there are no vacuum bubbles which would cause serious trouble, assuring uninterrupted flow with no resistance, and even some help from the pump.

In pressure-fed system the pressurant must overcome the combustion chamber pressure, plus all resistances along the way (injectors etc).

This will result both in the tank being significantly more rugged, holding against pressure equivalent to that of the combustion chamber instead of token increase over ambient, and also it will severely limit the combustion chamber pressure - and as result, the engine efficiency. The short ruggedized piping between the pump and the chamber is much, much smaller than the entire big tank, so one can afford to keep it much thicker and more durable, able to withstand much higher pressures.

Obviously, the trade-off is the cost, complexity and failure modes of the pump. It's a very complex and expensive device that also can fail in many ways pressure fed system wouldn't. (admittedly, it also protects against some failure modes characteristic to pressure-fed systems though.)

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  • $\begingroup$ Does scale of the vehicle warrant the usage of one over the other? Pressurant will always be a mass of little value but has a better reliability and less chances of failure due to usage of a simpler system. Do pressurants also help against sloshing and its effects within tanks as the fuel drains . How does pressurant volume and ullage compare in a pump fed system $\endgroup$ – Rajath Pai Mar 26 at 11:22
  • $\begingroup$ @RajathPai: All very good questions which I believe deserve to be asked on the site and get full answers on their own instead of whatever may fit in the comments. $\endgroup$ – SF. Mar 26 at 11:34
  • $\begingroup$ Thanks for the answer, and I will put them up as questions. $\endgroup$ – Rajath Pai Mar 26 at 12:12

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