How fast is the world's biggest rocket?

A rocket's size doesn't determine its final speed, but rather the mass it can carry to that speed. Starship, at 123 m tall with about 7,500 t of thrust at liftoff, is the largest and most powerful vehicle ever to fly, but its goal is the same as any orbital launcher's: accelerate its payload to about 7.8 km/s, around 28,000 km/h. A light rocket like the 18 m Electron reaches exactly the same speed with a 300 kg payload; Starship aims to do it with more than 100 t. The difference lies in the mass and in reusing both stages, not in the speedometer.

In the test flights, the Super Heavy booster's 33 Raptor engines push for about two and a half minutes until the stack reaches about 5,500 km/h at 65 km altitude. After hot-staging, with the ship's engines igniting before it separates from the booster, the Starship vehicle keeps accelerating to about 26,000-27,000 km/h. SpaceX has so far kept it slightly below orbital velocity so the trajectory ends over the Indian Ocean without needing a braking burn. Meanwhile, the booster slows down and returns to the tower.

Until 2023, the size record belonged to the Apollo program's Saturn V, at 111 m tall with 3,400 t of thrust. Its profile was similar: the first stage shut down at about 9,900 km/h at 67 km altitude, and the third stage left the spacecraft in a parking orbit at 28,000 km/h. It then reignited for the trans-lunar injection, which took the astronauts to about 39,000 km/h, the highest speed humans have ever traveled.