With jet engines, it’s relatively easy to light it up and throttle. I know you can’t shut them down mid-flight and then turn them back on while still in the air, but you can easily throttle them. Now with rocket engines, you can only light most engines 1-3 times, and their throttle ability is minimal. Why is this so?
Thanks in advance.
In: Engineering
There are rocket engines you can start up hundreds of times. They are used to control the orientation of spacecraft or do small course correction maneuvers.
For the big rocket engines that launch things to space there is often no reason to ignite them more than once. You start them up, you run them until their stage runs out of propellant, you shut them down. They might throttle down a bit on the way, but running them at very low thrust would be inefficient.
For the first stage you can install some of the startup hardware in the launch pad instead of the rocket. That way it doesn’t add to the rocket mass.
Big rocket engines are incredibly optimized for their specific task – produce as much thrust with as little mass as possible. Let’s compare this to aircraft engines: The [Rolls-Royce Trent 900](https://en.wikipedia.org/wiki/Rolls-Royce_Trent_900) (powering e.g. the Airbus A380) produces ~350 kN of thrust with a mass of 6.2 tonnes. The [Raptor](https://en.wikipedia.org/wiki/SpaceX_Raptor) (the engine of Starship) produces 2600 kN of thrust with a mass of just 1.6 tonnes. That’s 7 times the thrust with 1/4 the mass. Rocket engines are always just a second away from destroying themselves. Everything needs to work as planned – that is easier if you can keep them running continously.
Latest Answers