What is the scientific principle is the basis for the rocket engine?

Answer 1

The rocket engine is based on the scientific principle expressed in the phrase:

"For every action, there is an equal and opposite reaction"

This is a rough approximation of Sir Isaac newton's third law of motion. His three laws of motion formed the foundation of the science and methodology that came to be known as classical mechanics.

The rocket fuel is ignited at the tail of the rocket. The somewhat uncontained explosion, or extremely rapid expansion of gases, pushes out in all directions. That rapid expansion pushes against the rocket, advancing the rocket's motion in the direction of the nose. In atmosphere, the rocket gets an advantage as the extremely rapid expansion pushes against the atmosphere, and transfers more energy to the rocket. In space, the action still works, as the gas is pushing still expanding in all directions, and so still expanding against the end of the rocket pushing it forward.

It is the EXTREMELY rapid expansion, due to the continuous "explosion" of the burning fuel that allows this gas to exert enough pressure to move the heavy rocket. The same amount of expansion stretched out over several days might not even be noticeable!