How does a rocket land itself?
No wings, no parachute, and an engine too powerful to hover with. How a booster gets from 8,300 km/h to standing still on a ship, in about six minutes.
5 min read
For the first sixty years of spaceflight, essentially every rocket booster ever built was used once and discarded. Watching one fly back and land upright still looks like a video played in reverse. Here is what is actually happening.
The starting position is terrible
When the two halves of a Falcon 9 separate, about two and a half minutes after liftoff, the booster is roughly 70 km up and travelling at about 8,300 km per hour — in the wrong direction for landing. It has no wings and no parachute.
First problem: heat
Coming back down, the booster runs into thickening air. Air cannot get out of the way fast enough, so it is compressed, and compressed air gets extremely hot. This is where returning spacecraft are usually destroyed.
The booster relights three engines and fires them into the oncoming air. This slows it down, but that is not really the point — the point is to push a cushion of its own exhaust ahead of itself, so the worst of the heating happens in the exhaust rather than on the rocket. It is called the entry burn.
Second problem: aiming
Four grid fins fold out near the top. They look like waffles and work like the flights on a dart, except they can move. They tip the booster left, right and around, steering it towards a target barely wider than the rocket is long — often a ship in the middle of the ocean.
They are machined from solid titanium and are left unpainted and uncooled, because they have to survive the heat of re-entry on every flight without being refurbished.
Third problem: the landing itself
This is the genuinely hard part, and it is a lovely piece of physics to share with a child.
The booster is nearly empty by now, so it is light. But even throttled all the way down, a single Merlin engine still pushes harder than the empty booster weighs. That means it cannot hover. Light the engine and it will not slow to a stop and hang there — it will slow, stop, and start going back up.
The legs swing out in the last few seconds, the engine cuts, and a 41-metre booster is standing on a ship about eight minutes after it left the ground.
Why bother?
Because the booster is the expensive part. A Block 5 booster is designed to fly ten times with only minor work between flights. Landing it turns the most costly component of a launch from a consumable into equipment.
If you want to feel how tight the timing is, the landing game in Coming Home gives you one engine and one chance, with the same problem: too early and you stop short and climb again, too late and you arrive far too fast.
Common questions
How does a Falcon 9 booster land?
It fires an entry burn to shield itself from re-entry heating, steers with four titanium grid fins, then lights a single engine for a precisely timed landing burn and deploys four legs seconds before touchdown.
Why can a rocket booster not hover before landing?
Even at its lowest throttle setting, one engine produces more thrust than the nearly empty booster weighs. It cannot balance, so the burn must be timed so that velocity and altitude reach zero simultaneously — a manoeuvre known as a hoverslam.
How long does the landing take?
On a typical flight the booster separates at about T+2:30 and touches down at about T+8:15 — roughly six minutes from separation to standing on the deck.