Rocket engines require the propellent(s) to be at higher pressure than the combustion chamber so that the fuel can make its way in.

A pressure fed rocket is the simplest kind of rocket. It simply has its tanks kept pressurised, usually by a compressed, inert gas, and the propellent(s) just force their way into the combustion chamber where they do their whoosh thing.

One downside to this design is that the fuel tank needs thick walls to keep the pressure in; which tends to be heavy. Another issue is that as the fuel level goes down, the pressure goes down.

Nevertherless, this is not a silly design, and is cheap to build, and has been used many times.

It is interesting to note that if Space Elevator suitable carbon nanotube cable becomes available then the strength/weight ratio of tanks may improve by more than an order of magnitude. This technique may then become the dominant way to build rockets; at significant improvement in cost (turbopumps are expensive).

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