OK, I'm ducking for cover as I write this...but I figure that sometimes it's good to ask a really dumb question : the answers can be surprisingly enlightening...

Why do (most) traditional high-power liquid fuelled rocket engines have to inject the propellants into a combustion chamber at high pressure? That is, why do they need to use big turbopumps to get pressures of (I believe) 10 - 200 bar....why not just use smaller pumps to suck the fuel out of the tanks and inject it at lower pressure into the engine...if the rate of fuel use is the same, and if combustion efficiency is the same (itself debatable), why wouldn't this produce as high a thrust?

I say "most" engines because some, smaller engines are - I believe - just pressure fed, i.e. used Helium gas under pressure to force the propellants out of their tanks and into the engines. Were the LM engines like this? They seem to have respectable ISPs of ~300s.

I can think of these speculative reasons for the necessity of using high pressures :

-if you don't shoot the raw propellants in at high pressure, the force of the combustion happening in the chamber will prevent you from squirting any more in

-it lowers the volume of the combustion chamber (perhaps to an extent where you can get a lighter engine...the reduced chamber diameter could dominate over the need for thicker chamber walls in a higher pressure unit)

-with cryogenic propellants, if you don't keep them under high pressure they would have boiled off as they neared the hot engine. (Doesn't explain why you still use high pressure engines with non-cryogenic propellants)

-you get a contribution to the overall thrust from the fact that the propellants were injected under pressure (must be tiny though).

Any ideas which of the above - if any - dominate? Or are there other reasons?

Has anyone ever done calculations to see if you could make a large booster without turbopumps...using a pressure-fed system with lower ISP but without the weight, cost and complexity of the pumps and high-pressure combustion chamber? Do engines which just use (He) gas from a reservoir achieve input pressures close to those of turbopump-based units?