One of the most amazing endeavors man has ever undertaken is the exploration of space. A big part of the amazement is the complexity. Space exploration is complicated because there are so many interesting problems to solve and obstacles to overcome. There are things like: the vacuum of space, heat management problems, the difficulty of re-entry, orbital mechanics, micrometeorites and space debris, cosmic and solar radiation, restroom facilities in a weightless environment, and so on...

          However, the biggest problem of all is harnessing enough energy simply to get a spaceship off the ground. That is where rocket engines come in. Rocket engines are on the one hand so simple that you can build and fly your own model rockets very inexpensively. On the other hand, rocket engines (and their fuel systems) are so complicated that only two countries have actually ever put people in orbit. Here, we will look at rocket engines to understand how they work, as well as to understand some of the complexity. 

          Rocket engines are reaction engines. The basic principle driving a rocket engine is the famous (2nd law) Newtonian principle that to every action there is an equal and opposite reaction. A rocket engine is throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result. 

          You can view the specific structure of each class of rocket here.