Hover-Slam

This short video is a rocket launch and landing from the point of view of the rocket booster. It’s pretty cool.

The rocket in question is a SpaceX Falcon 9 which is a partially reusable launch vehicle that specializes in low-cost transport of people and payloads to Earth orbit.

The rocket has two stages, a large booster (mostly a fuel tank) with 9 sea level optimized engines and a small second stage with one vacuum optimized engine.

The booster, with its 9 engines doesn’t make it all the way to orbit. It separates at about 50 miles altitude and then returns to Earth to be re-used.

The second stage with its single engine pushes on to reach orbit, and after deploying its payload it typically de-orbits, mostly burning up with any remains falling into the sea.

In this video, you can see the Falcon 9 booster’s “grid fins” deploy, and along with the cold gas thrusters, they maintain attitude control. You’ll also see two re-ignitions of the engines. The first one uses 3 engines to slow the booster before it hits the denser parts of the atmosphere preventing burn-up, and the second one uses a single engine to slow the booster for landing.

The landing is really even more exciting than it looks. The rocket engines on the Falcon 9 are so powerful that with even just one of them burning and throttled all the way down, they can’t hover. If Earth wasn’t in the way, the booster with one engine lit would slow its descent down, briefly pause, and then begin to ascend again. That means they have to time the burn such that the rocket reaches the surface at precisely the moment of that brief pause. If they light the engine too soon, the rocket would take off again before reaching the surface and if they light the engine too late, it’ll be destroyed as it slams into the ground (or the barge — they land at sea most of the time.) This is called a “suicide burn” or in SpaceX language, a “hover-slam”.