What is a reaction mass and how is this different in a pyro rocket?
The reaction mass in a water rocket is the water. It is the force caused by accelerating this downwards that makes the rocket go upwards. In a water rocket, the energy that forces the water out is stored in the compressed air (you can prove this by filling up a water rocket to the top and then seeing if there is any significant flight). In a pyro rocket, the energy that forces the reaction mass out of the rocket is stored as chemical energy within the reaction mass. Water rocketry is therefore special in that the reaction mass and the energy storage can be separated.
