Rocket Design is all about optimising mass, within cost constraints while accepting a certain amount of risk. Both pressure fed and pump fed systems have their pros and cons, but how do they compare at different scales(Micro(<100 kg),small(<1000kg),medium(<5000 kg),heavy for LEO). I've mostly seen pump fed systems but I would like to know when pressure fed systems are used and how they perform in comparison.
$\begingroup$ As far as I know most if not all orbital rocket main thrusters are pump fed. $\endgroup$– Everyday AstronautMar 26, 2019 at 12:40
1$\begingroup$ @EverydayAstronaut: Currently, probably. I recall pressure fed system were a big thing early on, and quite popular in military (ballistic, short, medium range) systems. $\endgroup$– SF.Mar 26, 2019 at 13:24
This supposed to go in comments because it's not a complete answer but I got to put it here. I'm considering rocket scales based upon LEO payload. Micro - payload to LEO in few kgs, Small - payload to LEO in few hundreds of kgs. medium - payload to LEO in few tons. heavy - payload to LEO in few tens of tons. ultra heavy - payload to LEO in few hundreds of tons.
Here I'm analysing rocket performance in terms of empty rocket and payload mass. As rocket scales up, empty rocket weight decreases so payload can be mass increased . For pressure fed rocket system, walls of propellant container should be thick, increasing empty rocket weight. For micro, small and medium pressure fed rockets, payload mass may be less than 4%. So it's only feasible for heavy and ultra heavy rockets.
But if turbo fed is implemented for heavy rockets, payload can be increased significantly. Same can't be done for ultra heavy rockets, because there is a limit on how big a turbos can be made because of their high RPM. So it makes sense to do pressure fed for Ultra heavy rockets. This is one of the reason/factor in designing Sea Dragon Ultra heavy rocket.
$\begingroup$ Also, I believe it is more suitable for lower stages as upper stages mass fraction with payload increases. It's good to know that ultra heavy rockets find usage of pressure fed. $\endgroup$ Apr 19, 2019 at 10:52