rocket propulsion

Rocket propulsion is how a rocket pushes itself forward by throwing mass backward. It burns fuel and hurls the hot exhaust gas out the nozzle at tremendous speed; by momentum conservation, flinging that gas backward drives the rocket forward. Crucially, a rocket needs nothing to push against — it carries its own propellant, so it works just as well in the vacuum of space as in the air.

It is recoil, repeated continuously. Each little parcel of exhaust leaves carrying backward momentum, so the rocket gains an equal forward momentum. Because the rocket keeps losing mass as it burns fuel, the exact description needs a variable-mass version of Newton's second law, and it leads to the Tsiolkovsky rocket equation: delta-v = v_ex times ln(m_initial / m_final), where v_ex is the exhaust speed and the m's are the masses before and after the burn. The message is stark: to go much faster you need a huge mass of propellant, because the speed gain grows only with the logarithm of the mass ratio.

This is why real rockets are almost entirely fuel and why they shed empty stages on the way up — dropping dead weight makes the remaining mass ratio kinder. Everyday jet engines share the same principle, though they also gulp in outside air, which a true rocket does not.