>>12643372Liquid fuel rockets are only about one hundred years old.
Solid fuel rockets are cheap as far as expendable rockets go, and keeping the companies that produce them in business is of strategic importance due to the fact that solid fuel rockets are useful for ICBMs.
Nuclear thermal rockets are an option, but the risk of contamination in the event of a vehicle failure is a problem.
Ion and plasma rockets require a lot of electricity and/or produce too little thrust to be used for launch vehicles or even for human spaceflight at all.
Nuclear pulse works, but it's insane.
Beyond those options you have really exotic engines or megastructures that are still in the realm of scifi.
