A follow-on project to Sunguidance in 1988 was Gimbaled engine guidance in 1989.
The gimbal mount was very simple, it pivoted at the top of the motor mount.
The heart of the pivot was a Dubro threaded Ball. As seen in the middle of the first photo (and link here).
JUST the ball, which was mounted onto a balsa post attached to the model. On top of the engine mount, I glued a 1/4" tall piece of 13mm body tube. I applied a little bit of vaseline to the Du-Bro ball, then mixed up a bit of 5-minute epoxy and poured the epoxy about 3/16" deep into the 1/4" tall piece of 13mm tube. Before the epoxy set, I placed the Du-Bro ball. When the epoxy cured, then the ball was embedded into it, but free to move at least 20 degrees.
Control horns were added in 2 places at 90 degrees. At the outer ends of the control horns, more Du-bro balls, and the pushrods used the corresponding sockets.
In two photos you see the mount by itself, and one shows the lower half of the entire gimbaled rocket with the servos, guidance, and ejection timer started at liftoff (the upper part contained a removable ejection charge and chute compartment). The fin unit slid on and off for easy access to the mount assembly. Unfortunately, the photos were not in very good focus.
In the last photo, the model as configured for flight #1. It was a very simple proof of concept test, powered by staged D12's. The rocket took off ballistic on the first stage, then the gimbaled mount of the second stage steered the rocket toward the sun when the upper D12-5 ignited. After that proved to be a success, then the remaining flights were single staged, once on an F15, the rest using an F10.
There is one drawback to having the mount gimbaled at the top. If the rocket made a severe side-maneuver, the mass of he motor would leverage itself on the top end of the mount to cause the control system to let the mount be a couple of degrees or so away from where it should be. But that was not necessarily a bad thing, since it would not feed upon itself. Also, only during an extremely bad flight should the model have any strong lateral (side) forces from its own maneuvering. But I did want to point that out. Indeed if someone did this for an HPR rocket, a more likely problem than a few degrees of wiggle would be over-stressing and therefore fracturing the gimbal assembly.
Here is a link with a bit more info on the Gimbaled engine R&D (I only recently found & scanned the attached photos and have not updated that page).
- George Gassaway