One of the ways that the Falcon 9's attitude is controlled is through thrust vectoring of the main Merlin engine(s?). In images of older thrusters i've seen hydraulic or electric linear actuators to adjust the angle of the nozzle such as in this vernier motor however I can't find any concrete info on how this is achieved in the Merlin class of engines. The closest I could find was an entry on wikipedia which states that:

Propellants are fed via a single shaft, dual impeller turbopump. The turbo-pump also provides high-pressure fluid for the hydraulic actuators, which then recycles into the low-pressure inlet. This eliminates the need for a separate hydraulic drive system and means that thrust vectoring control failure by running out of hydraulic fluid is not possible.


The Merlin engine for Falcon 1 had a movable turbopump exhaust assembly which was used to provide roll control by vectoring the exhaust

I'm not quite sure what to make of these entries. Does this mean that the thrust vectoring is achieved through 3 hydraulic pistons similar to the 3 electric 'pistons' in the Vernier Motor? Also can all Merlin engines thrust vector and what's the thrust vectoring capability of the capable Merlin engines (in deg)?


1 Answer 1


The Merlins are gimbaled hydraulically, with two pistons per engine. The actuators are the white (diagram) or light blue (photo) cylinders at the top of the long gray linkage rods, labeled "TVC actuator". This is Merlin 1C rather than the current Merlin 1D, but I believe the actuators are similar.

schematic and photo of Merlin 1C engine

I'm not sure of the gimbal range, but it's probably close to 5-10º in any direction. It may be a little larger on the first-stage center engine (which is offset longitudinally from the others), as it's the most critical to landing maneuvers.

The Merlin engine for Falcon 1 had a movable turbopump exhaust assembly which was used to provide roll control by vectoring the exhaust

For a single-engine stage, it's not possible to control roll by gimbaling the engine mounted on the centerline -- you need a source of torque offset from the center. For multi-engine stages, you pivot engines on opposite sides in opposite directions. Falcon 1 got around this problem by pivoting the turbopump exhaust, which is off-center from the main engine. The single-engine second stage of the Falcon 9 has a reaction control system using compressed nitrogen to control the attitude and roll of the stage, so doesn't need this scheme, so the turbopump exhaust on current Merlin engines is fixed, saving weight and complexity.

  • $\begingroup$ I'm not sure, and I don't know much at all about hydraulics. If they aren't equally powerful in extension and contraction, you'd definitely want opposed pairs, though. $\endgroup$ Commented Nov 10, 2017 at 0:41
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    $\begingroup$ Oh, wait, you're right - the actuators are the short light gray cylinders, not the long gray linkage rods. Editing. $\endgroup$ Commented Nov 10, 2017 at 0:53

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