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MOTION · B1 · 003 · Newton's second law

A rocket gets faster the lighter it gets.

Newton's second law: acceleration = force ÷ mass. The engines keep the same push while the rocket burns off its own weight, so every second the same push moves less rocket.

Try itSame push, less rocket

This is a Saturn V first stage with its real numbers. Burn the fuel and the push stays the same, but the mass drops, so the crew gets pushed harder and harder. Turn the engines down to keep them under 3 g.

Fuel burned 0%

Engine push 100%

Rocket mass–

Engine push–

Crew feels–

Crew limitFREE

NumbersHow big is it?

Saturn V numbers from its published stage data. The sim leaves out air drag and the small rise in push as the air thins.

≈ 90%

Share of a Saturn V that was fuel at liftoff: about 2,600 t of propellant in a rocket of about 2,900 t.

33,000 kN

First stage push at liftoff. On 2,950 t that is only about 1.1 g for the crew. Same push near the end of the burn: almost 4 g.

3 g

Space Shuttle crews were held at 3 g for the final minute of the climb to orbit, with the engines turned down as the tank emptied.

HowWhat happens, step by step

The engines push with about the same force the whole time.
The push depends on how much fuel goes through the engines each second, not on how full the tank is.
Every second, tonnes of fuel leave as exhaust.
The rocket the engines are pushing gets lighter all the way up.
Same force, less mass, more acceleration.
Acceleration = force ÷ mass. Near the end the push would crush the crew, so engines get turned down.

Same in your car: five people and luggage, and it pulls slower than when you're alone. Same engine, more mass.

Check3 quick questions

Bonus

That's why rockets drop their empty stages. Less dead weight, more speed. Empty the tank in the sim: the 873 t left is stage structure and everything above it, all still being pushed.