What is a rocket, exactly?
Take a great big bloody bomb.
Make it explode in one direction, at a slower than immediate rate, make it continue to do so until one is at one's destination point or needs to replace that one direction bomb.
As a practical example, I know of one intermediate range tactical missile that uses thermate as its propellent, with some modifying binders mixed in.
A second example would be all manned NASA rockets, which use hydrogen and oxygen.
Now, take that bomb, assemble it with loads of moving parts and electrical circuits, well, it gets complicated.
Rocket science isn't as much science as it's partially an art.
An example from early and even current rocketry, liquid fueled engines can suffer from pogo oscillation, due to structural components moving due to acceleration. That results in slowing fuel/oxidizer rates, then normal rates. That has destroyed quite a few rockets.
Now, who wants to design a rocket to personally fly up into space?
Not I!