Why does a rocket fly?

Inside the combustion chamber, fuel and oxidizer react and generate gas at high pressure and temperature. That gas escapes through a bell-shaped nozzle, which accelerates it to speeds of 2,500 to 4,500 m/s depending on the propellant. Expelling mass downward produces an equal force upward on the vehicle: this is thrust. The greater the gas flow rate and its exit velocity, the greater the thrust, with no involvement at all from the surrounding air.

To lift off, thrust must exceed the rocket's total weight, loaded mostly with propellant: in a typical launch vehicle, more than 85% of the mass on the pad is fuel and oxidizer. At first the acceleration is modest, barely above gravity, which is why rockets rise slowly in the first few seconds. But every second the vehicle burns tons of propellant and gets lighter, so the same thrust produces ever greater acceleration. By the end of a stage the load can reach four or five times gravity.

Reaching orbit requires about 7.8 km/s, and no single-stage rocket achieves that with a useful margin, because empty tanks and engines are dead weight. The classic solution is to divide the vehicle into stages: when the first exhausts its propellant, it separates and the next stage ignites its engines already starting from a high speed and a much smaller mass. The Tsiolkovsky equation, formulated in 1903, sums up this logic: final velocity depends on the exhaust velocity of the gases and the ratio between initial and final mass. For a rocket, flying means managing that ratio with precision.