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Is having an engine mounted on a gimbal or having some ailerons better for turning a rocket that is still in the atmosphere.

What I mean is why does SpaceX use some grid fins, which are a type of ailerons, and not simply a engine on a gimbal for their vertical landing??

enter image description here

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  • $\begingroup$ You do realise that they are only deployed on descent right ? $\endgroup$ – Antzi Feb 15 '17 at 1:11
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    $\begingroup$ @Antzi : Please re-read the 4 last words of my question. $\endgroup$ – Dat Ha Feb 15 '17 at 1:13
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They need to aim the vehicle the whole descent

The engines only burn three times during landing: once to boost back, once to slow down and once to land at the end. In between these burns they still need to aim the vehicle, that's where the grid fins are used.

enter image description here

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Grid fins are useful over a limited duration of the return trajectory, but are efficient (require no consumables), add to the vehicle's stability, and can be used to make small changes to the trajectory (no "suicide burn").

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SpaceX uses both. In the landing videos, you can see the center main engine gimbaling to rather large angles during the final approach. In the initial landing attempts, they found that the combination of gimbaled main engine plus nitrogen thrusters at the top of the stage was not sufficient for the last phase of the landing. So they had a choice: add larger thrusters (in groups or gimbaled) to the top of the stage, or add aerodynamic control surfaces.

As Eric said, the grid fins offer some advantages over thrusters.

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  • $\begingroup$ Iirc there was a problem with roll control which single engine cannot counter at all and the thrusters don't have enough authority to control in atmosphere. The fins extend farther from the axis of rotation to make it easier. $\endgroup$ – jkavalik Feb 15 '17 at 17:53

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