The simplest rocket engine doesn't really have moving parts. It's a chamber where the fuel burns and the nozzle which shapes the exhaust. The moving part is the valve somewhere which turns it on.
The more complex rocket engine includes a pump. But today it's feasible not to make a turbopump, but instead use electric pump - batteries get better, and Electron rocket from Rocket Lab uses this approach for years already.
With jet engines you necessarily have to accept incoming air, compress it and burn with fuel - otherwise it's not a jet engine. Batteries are unfortunately still a bit heavy, so electrical aviation is just getting off the ground slowly.
You can't build a liquid-fueled rocket engine without a pump, can you? The liquid will just stay in the tank unless there's something pressurizing it to a higher pressure than you achieve inside the rocket's combustion chamber, won't it?
An electric pump still sounds much more complex than a jet engine, which has, I believe, one moving part to both compress that incoming air and harness the exhaust. Admittedly, it's a moving part subject to high stresses, high temperatures, stringent balancing requirements, and demanding aerodynamics, so the larger number of parts in the electric pump might still be easier to make.
Ultimately I think long-distance aviation will probably get electrified by way of abundant renewable energy powering electrical synthesis of synfuel on the ground which its engines burn.
> You can't build a liquid-fueled rocket engine without a pump, can you?
You most certainly can, and it was done. Why do you think otherwise?
> The liquid will just stay in the tank unless there's something pressurizing it to a higher pressure than you achieve inside the rocket's combustion chamber, won't it?
True, but you can have the pressure in the tank bigger than in the combustion chamber, right?
> An electric pump still sounds much more complex than a jet engine, which has, I believe, one moving part to both compress that incoming air and harness the exhaust. Admittedly, it's a moving part subject to high stresses, high temperatures, stringent balancing requirements, and demanding aerodynamics, so the larger number of parts in the electric pump might still be easier to make.
Yes, the additional materials requirements and others can make single rotating part harder to get right than electrical parts, which can be developed independently from the rest of the system.
> True, but you can have the pressure in the tank bigger than in the combustion chamber, right?
I guess you can if people have done it. I've never built a rocket myself, so I don't know, but I thought the combustion-chamber pressure had to be crazily high to get the high exhaust velocity you need for propulsion.
Surprisingly you don't need to have that large of the pressure in the chamber to get to the sonic speed in the throat - and more than that in the diverging nozzle. E.g. 3 bar pressure in the chamber could be enough for that.
French Diamant rocket, the one used to launch their first satellite, had a pressure-fed first stage. Lunar Expedition Module from Apollo program had a pressure-fed ascent stage.
