Some parts of engines might have flexible areas like tubing that need to deform with the rotation of the engine gimbal. Setting this up can be somewhat tedious, but it’s nothing new: just make sure your coordinate system is lined up properly.
The gist is that you’ll need to make an armature for deforming, consisting of a “fixed” bone, and a gimbal bone in lieu of a gimbal empty. Rather than parenting your engine nozzle and thrustTransform to a gimbal empty, you’ll parent them to the gimbal bone instead.
Create a new bone. In edit mode, name the initial bone fixed.
Create a new bone underneath by duplicating that first bone, and name it gimbal. This will be the gimbal object referenced in the config’s gimbal module. This should be oriented in the same way a gimbal empty would be, with Y+ facing downwards.
To help orientation of bones, you can turn on axes display in the Object tab of the armature:
The name of the armature object doesn’t matter, but make sure it is parented to the overall engine part empty.
For the mesh you wish to deform, create two vertex groups, named fixed and gimbal; or otherwise matching to the names of the two bones of the armature. Parent this mesh to the armature, using Armature Deform.
Once that’s done, it’s time to weight paint. This is mostly down to user preference, but manually assigning the weights across five mesh loops (100%, 75%, 50%, 25%, 0%) tends to work well enough. For the gimbal deform vertex group, 100% should be the part in contact to or otherwise the closest to the gimballing component, tapering away.
The fixed vertex group should be the inverse. One way to do so quickly is to duplicate gimbal (and renaming it appropriately to fixed), and then inverting the weights.
From edit mode, go to Mesh ➡️ Weights ➡️ Invert to invert weights.
You can switch to weight paint mode to check your work: below, the two vertex groups are perfect mirrors of each other.
As a final check, enter pose mode and move the gimbal bone around. Your mesh should deform as expected.
With the mesh deforming properly, we need to make sure the rest of our gimballing engine (like our engine nozzle/bell) still does rotate properly.
Luckily, the process is quite similar: just parent anything that needs to rotate to the gimbal. Be sure, however, that it is parented to the bone, and not the armature object! You’ll need to do this in pose mode, and probably by selecting your relevant mesh(es) in the outliner.