Engines · Rocket · Demo
Pick a real engine, load a stage with propellant, and burn it — then watch one logarithm decide whether you reach orbit.
Saturn V (5 engines) · RP-1 / LOX, gas generator. Five F-1s lifted the 2 900 t Saturn V with a liftoff TWR of about 1.2 — the biggest single-chamber engine ever flown, burning 2.6 t of kerosene and oxygen per second each.
The on-screen burn is compressed; the physics clock next to the rocket runs at the stage's real burn time. Scrub the slider to inspect any instant.
That last row is the tyranny in one number: doubling the propellant at the same dry mass never doubles Δv, because the logarithm pays out less for every tonne you add — and the new tonnes must themselves be accelerated. The escape from the log is not more fuel; it is throwing empty tanks away.
In plain terms
A rocket is the only machine that carries everything it pushes against. It throws its own propellant backwards at enormous speed, and momentum conservation shoves the vehicle forwards. That simple trade has a cruel bookkeeping rule — the rocket equation — which says speed is bought with exponentially more propellant. This demo runs the honest numbers for four real engines.
No air, no road, no water — thrust is pure momentum exchange: F = ṁ·ve, mass flow times exhaust velocity. That is why rockets work in vacuum, and why every gram thrown backwards matters twice: once as push, once as weight you no longer carry.
Tsiolkovsky’s equation, Δv = ve·ln(m0/m1), says the speed a stage can gain depends only on its exhaust velocity and its full-to-empty mass ratio. Not on thrust, not on burn time, not on size. Everything else in rocketry is a fight to improve one of those two numbers.
Because the mass ratio sits inside a logarithm, doubling the propellant never doubles Δv — the extra tanks full of fuel must themselves be accelerated. Reaching orbital speed on one kerosene stage demands a vehicle that is ~95% propellant, which is why rockets stage.
Δv does not care about thrust — but gravity does. A rocket must out-lift its own weight (T/W > 1) or it simply sits there burning. Hydrogen engines have superb Isp but modest thrust; that is why hydrolox stacks so often leave the pad strapped to solid boosters.