How does a rocket steer with no wings?
There is nothing to push against and no rudder that would work. Rockets turn by pointing their engines — and on the way home, by using four titanium waffles.
5 min read
An aeroplane turns by moving flaps that push against the air. A rocket spends most of its flight where there is almost no air to push against, and the parts of the flight where there is are the parts where it is going too fast for flaps to survive. So how does it point where it wants to go?
It points the engines
The main answer is gimballing. Each engine is mounted on a joint that lets the whole engine swivel a few degrees, so the thrust no longer points straight through the rocket’s centre of mass. That off-centre push rotates the vehicle.
It is the same idea as pushing a shopping trolley from one corner rather than the middle. A small angle is enough: Falcon 9’s engines move only a few degrees, and the rocket is long, which gives the leverage.
Why not fins?
Fins work only in air, and only up to a point. Above about 60 km there is nothing for them to bite. They also add weight and drag for the whole flight in exchange for helping during a couple of minutes of it.
Some rockets do carry small fixed fins low down for stability at launch — Saturn V had four — but the steering was always the engines.
Coming back down is a different problem
A returning booster is nearly empty, its engines are off for most of the descent, and it is falling through thickening air. Gimballing is no use when nothing is burning.
This is where the grid fins come in. Four flat lattice panels fold out near the top of the booster and bite into the airflow, tipping it left, right and around. They look like waffles because the lattice gives a lot of surface in a package that folds flat and keeps working at supersonic speed.
Falcon 9’s are machined from solid titanium and left unpainted, so they survive re-entry heating on every flight without needing to be replaced.
And in space?
Once the air is gone and the engines are off, the only option is to throw something. Small thrusters — Dragon has sixteen, producing 400 N each — puff gas in one direction to rotate the spacecraft in the other.
It is Newton’s third law again, exactly as at liftoff, just very much smaller.
Common questions
How do rockets steer?
Mainly by gimballing: the engines swivel a few degrees so the thrust no longer points through the centre of mass, which rotates the vehicle. In space, small thrusters puff gas to turn the spacecraft.
Why do rockets not have wings?
Wings only work in air, and a rocket leaves the atmosphere within minutes. They would add weight and drag for the entire flight in exchange for helping during a small part of it.
What are the grid fins on a Falcon 9 for?
They steer the booster on the way back down, when the engines are off and gimballing is unavailable. Four titanium lattice panels fold out and bite into the airflow to aim the rocket at its landing spot.