Why are rocket engines so complex?

[RCgothic]

But they don't have to be complicated.

I personally fly these occasionally. Solid propellant, no moving parts:

 

 

This is also a complete complete liquid rocket thruster. Not the most efficient or the largest. It's a pressure-fed monopropellant thruster that flew on the Mercury capsules:

 

 

This next one is slightly more complicated because it's bipropellant and a little larger, but it's still basically two valves, an injector and a nozzle. R-4D used on Apollo.

 

 

The complexity comes in when you:

-Want to use fuels that don't self-ignite (add starters).

-Want the chamber pressure to be higher than the tank (add pumps).

-Want to use a more advanced engine cycle (add tap-offs, turbos, etc).

-Want to operate continuously (add cooling channels for temperature management).

-Want to Gimbal (add hydraulics and joints).

-Want the engine to pressurised the tanks instead of using bottled helium (add heat exchangers and pipes)

-Want a variable throttle (add more complex injector assemblies).

-Want better combustion stability in a larger engine (add baffles).

-Want a comprehensive engine-management system for all of the above for maximum efficiency, diagnose faults, and have non-catastrophic emergency shut-downs (add a butt-load of sensors and an onboard computer).

-Want additional information for engine development purposes (add another butt-load of sensors).

It all stacks up quickly.

Edited November 5, 2021 by RCgothic

[wumpus]

19 hours ago, RCgothic said:

The complexity comes in when you:

-Want to use fuels that don't self-ignite (add starters).

-Want the chamber pressure to be higher than the tank (add pumps).

-Want to use a more advanced engine cycle (add tap-offs, turbos, etc).

-Want to operate continuously (add cooling channels for temperature management).

-Want to Gimbal (add hydraulics and joints).

-Want the engine to pressurised the tanks instead of using bottled helium (add heat exchangers and pipes)

-Want a variable throttle (add more complex injector assemblies).

-Want better combustion stability (add baffles).

-Want a comprehensive engine-management system for all of the above for maximum efficiency, diagnose faults, and have non-catastrophic emergency shut-downs (add a butt-load of sensors and an onboard computer).

-Want additional information for engine development purposes (add another butt-load of sensors).

It all stacks up quickly.

And you more or less need all the above to get the thrust and Isp high enough to get into orbit, at least with some reasonable amount of cargo.  Even more for escape velocity (although I think there are some pretty simple designs that create "ion thrusters" by blasting PVC with electrical current, so you could possibly go from orbit to beyond Mars with a solar panel and such a motor).

[AHHans]

I think the original question as to why many rocked engines are incredible complex has already been answered. (They don't have to, but when optimizing performance and efficiency is more important than cost then that's what you get.) But for anyone who just wants to revel in the glory of rocket engines - or just likes to watch top-grade rocket porn - I can recommend the latest video from Scott Manley:

 

[magnemoe]

On 11/5/2021 at 6:24 PM, wumpus said:

And you more or less need all the above to get the thrust and Isp high enough to get into orbit, at least with some reasonable amount of cargo.  Even more for escape velocity (although I think there are some pretty simple designs that create "ion thrusters" by blasting PVC with electrical current, so you could possibly go from orbit to beyond Mars with a solar panel and such a motor).

This, its not really that strange, compare a modern car engines with an simple engine like on an chainsaw or lawnmower. 
You want that much more complex engine on an car as its very fuel efficient, clean burning and provides lots of power. 
Yes you could get as much power out of an 75 year old engine who would be pretty much an scaled up lawnmower engine but it would use much more fuel and pollute much more.