This question refers to the device which transfers the thrust force from the engine to the vehicle while allowing the engine to swivel - the "gimbal joint".

Both the F-1 and SSME have a bolt- or shaft-like device inserted (or able to be inserted) into the side of the gimbal joint. Obviously these engines share a common design heritage. (Curiously, the "male" and "female" sides of the joint are inverted between the two engines. In both cases the "shaft" goes into the "male" side.)

Is it used to lock the gimbal? To expand the mating parts so they fit properly? Neither of these "feels" right.

Answers with solid references only please!

F-1 (from "The Saturn V F-1 Engine" by Anthony Young):

enter image description here

enter image description here

SSME (from Rocketdyne SSME Pocket Data Book)

enter image description here

From Boeing SSME Orientation

enter image description here

  • 1
    $\begingroup$ I don't quite get the question. The shaft allows rotation around one axis, the block around the other, perpendicular. Just kinematically, that's how a gimbal works. $\endgroup$ Jun 26, 2018 at 4:23
  • $\begingroup$ It is certainly possible I do not understand how gimbals work. But in the 2nd picture if you drew a line from the word "seat" to the word "body", how could the engine pivot about that line with the shaft inserted? I don't think it could. Note in the 1st picture, the shaft is removed. $\endgroup$ Jun 26, 2018 at 4:32
  • $\begingroup$ As I wrote that I think I may be getting a glimmer of understanding. Thanks, I will think about that tonight. $\endgroup$ Jun 26, 2018 at 4:40
  • 1
    $\begingroup$ Note gaps around the shaft. They allow (limited) motion. Don’t have a reference for those engines, but in other applications the block is there to limit (“block”) upward motion, and the shaft is there to hold the block in place. $\endgroup$ Jun 26, 2018 at 5:01
  • 1
    $\begingroup$ I don't think the gimbal is removed in the first picture. I think what we see is a fastener holding it in place. The four holes would be for attaching a tool to apply torque to it. $\endgroup$ Jun 26, 2018 at 6:23

3 Answers 3


Let's take the second image for the F1 engine - the situation is similar for SSME, though everything is 'upside-down' in its pictures.

Gimbal schematics with elements, axis and slippage surfaces colored

The 'seat', 'body' and 'block' are three parts that turn against each other.

The block can turn around the shaft in X axis. The body can turn around the block in the Y axis.

The Z axis remains fixed, as the block can't turn 'across' the shaft, and the flat areas on its sides prevent body turning around the block.

That's all for rotation. Now there's translation, which needs to be constrained in all 3 axis, two directions each.

The highest load is compression load along the Z axis, as the engine fires. Thus the bottom surface of the block pressing against the seat, transferring the largest load. The opposite - tensile load, as the engine hangs before ignition on the bottom of the rocket, is transferred through the interface between the block bottom surface and body inner spherical surface, then from the block to the shaft.

X and Y translation is less of a concern, but still the seat-block interface prevents it - transforming it into a much weaker tensile load, that is either prevented by the shaft-block assembly, or completely negated by thrust of the engine exerting a compressive load.

  • 2
    $\begingroup$ Unfortunately, for my references I can only cite 2 semesters of mechanical engineering ;) $\endgroup$
    – SF.
    Jun 26, 2018 at 10:39
  • $\begingroup$ Thanks, great explanation. I thought it worked like a ball and socket joint. I didn't realize there was motion between the block and the body, I thought it was all between the body and the seat. Good thing I was AE and not ME :) $\endgroup$ Jun 26, 2018 at 11:09
  • $\begingroup$ Terrific explanation! $\endgroup$ Jun 27, 2018 at 15:15

The gimbal performs the following functions:

  1. Allows the engine to gimbal around a single point under thrust when driven by the thrust vector control actuators. It is desirable to minimize friction to minimize actuator force/size required.
  2. Prevents rotation of the engine around its thrust axis and prevents other components from taking that torsional load.
  3. Supports the engine weight during no-thrust conditions and prevents the engine from coming loose under the "rebound" condition experienced when the engine shuts off.
  4. Allows adjustment of the actual gimbal centerpoint relative to the thrust axis when the engine is installed in the vehicle.
  5. Has features to take out the torsion at the interfaces to the engine and vehicle.

You can see all of these features in the picture. The "seat" is the feature that takes the primary thrust load and around which the engine rotates per item 1. The "block and shaft" take the torsion and prevent rotation between the upper and lower halves of the gimbal per item 2. The lower dimple, as pointed out, combined with the block and shaft, provides the contact surface for reverse thrust per item 3. The adjusting screws on the sides allow the gimbal to be adjusted per item 4. The large tabs on the upper and lower faces transmit the torque into adjacent structure per item 5.

The "fabroid" is a low friction PTFE cloth application which reduces friction at the sliding contact points.


See exploded view here: ebay com/itm/Apollo-Saturn-V-F-1-Engine-Gimbal-Rocketdyne/

Feed lines of Kerosene and LOX are many feet off center, so rotation of the F-1 engine (roll) is restricted by the Shaft and Block to almost zero.
Note small loading pad on the 'bottom' of the Block, this supports hanging one of the four 18,000 pound gimbaled F-1 engines on the Saturn-V, the fifth center engine is rigid. The Block pad seats on a small concave pit inside the 10" hemisphere and is also at the same nodal-point as the thrust axis for yaw and pitch. The Block also has a Fiberglass-teflon sleeve pasted inside to control galing with the Shaft.

  • $\begingroup$ This should really be a comment. $\endgroup$ Sep 30, 2018 at 11:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.