Why is it necessary to burn the hydrogen fuel coming out of the engine for the lift of the rockets?
If it is done to create a greater reaction force on the rocket then why can't we do the same lift with just adjusting the speed of the hydrogen gas going out of the engine like we can release them at a great pressure (and also by adjusting the size of the nozzle opening) and thus at a greater speed?
Is it possible for rockets to fly without burning the fuel and just releasing the fuel with a great force? (I know the rockets are too massive . )
How does the ISP of the ordinary rocket engines compare with the one in my question? Most of the answers have done the comparison (and a great thanks for that), but help me with the numerical difference in the ISP's.
(Compare it using any desired values of the amount of fuel and other required things for taking off.)