Rocket Pumps

For low Pressure it might be better to stick with pressurised tanks. Pistions and such come into their own vs. Turbines at low flow.

Four designs worth looking into are:

Soviet RD-1 (Gear Pump)
The Agena Upper Stage secondary Propulsion system (Gear Pump)
Astrid Launch vehicle (1994) (hydrazine powered reciprocating pistion pumps)
The Chevaline Submarine launched Missile had an apsolute mess of interconnected pistion pumps.

I'm not a fan of pressure feds. Once you run the numbers, its does not look good, even ullage mass can be pretty high because of the high pressure. The relatively low chamber pressure means very poor engine performance at least in the atmosphear. Far better to have a pump with plenty of margin and don't get crazy with chamber pressure.

The SSME have crazy pressures (the LH2 HPTP has a exit pressure of ~50MPa IIRC) which creates the huge pump problems.

If you can live with say 5MPa chamber pressure the numbers start to look pretty good, and the pump R&D look sane too.

...unless you use the ullage mass and the heavier tanks to form a passive recovery system.

Pump feed engines can have uneven feeds that screw with your mixture and melt your engine. They are moving parts, many many hot moving parts. The Shuttle has *twelve* seperate turbo pumps in its main engine cluster and every one of them has to have every blade inspected.

If flotsam falls into you engine it will clog an injector on a pressure feed engine. It has the potential to destroy a pump. The majority of development costs and problems goes into dealing with making pumps reliable.

Mind you if you know what you are doing then it can be easy. The Russians use turbo pumps on pretty much everything, even some manouvering thrusters.