Gimbal mount

[dcshrum]

I'm working through a rocket that will guide itself straight up using thermopile sensors and ardupilot.

I'd like to find some sort of off the shelf gimbal to mount the motor to.

Suggestions?

 

[bobkrech]

I'm working through a rocket that will guide itself straight up using thermopile sensors and ardupilot.

I'd like to find some sort of off the shelf gimbal to mount the motor to.

Suggestions?

Their aren't any of the shelf gimbels.

This has been done before using the Futaba Horizon sensor that you can purchase for $50. You need to mount two micro servos above the engine orthogonally to push the top end of the motor tube around, and build a simple gimbel mount from centering rings or motor mounts at the aft end of the motor tube.

Bob

 

[rocketsmith]

Look into heim joint type spherical bearings. I know I have seen some large ones but I don't remember where. It would need to have some mechanism for returning to center. Remember, though, it will be heavy and cheap or light and very expensive. Look at industrial and aerospace suppliers websites. Good luck.

 

[bobkrech]

The picture on the left is a gimbel mount. It consists of 3 rings with the outer ring (half ring in this example) with two opposing axles (in the horizontal plane here) that connect to the middle ring. The middle ring has two axles 90 degrees off from the other axles that connent to the inner ring.

The pictured vertical orientation of the gimbel can be compared to a horizontal rocket airframe so you can see exactly how it would work in a rocket. The outer ring is the airframe, the middle ring is a short length of smaller airframe or a centering ring, and the inner ring is the motor tube.

When attached to the rear of the rocket, the forward end of the motor tube moves like the top of a joystick. You simply use two microservos mounted 90 degrees apart ans in line with the axles to shove the top of the motor tube around. If you used the Futuba you can use it to drive the servos.

This is lightweight and cheap to make.

Bob

 

[georgegassaway]

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).
https://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXEMG0

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).
https://homepage.mac.com/georgegassaway/GRP/RandD/Sunguidance.htm#gimbal

- George Gassaway

 

[luke strawwalker]

I'm working through a rocket that will guide itself straight up using thermopile sensors and ardupilot.

I'd like to find some sort of off the shelf gimbal to mount the motor to.

Suggestions?

Use a metallic U-joint for RC monster trucks-- should be available at the LHS or online... check out Traxxas stuff...

I saw John Pursley's gimbal stabilized rocket using the horizon sensor system-- it looked really good. Used regular RC servos to control the gimbal, 90 degrees apart to control pitch and yaw. Roll control isn't a big deal with no fins, at least not for the length of time our rockets typically fly...

You can build your own gimbal if you want, from plywood and pins-- just make a U-joint like a car driveshaft so it can pivot in pitch and yaw...

They also make plastic/phenolic U-joints for RC monster trucks-- one of those would probably work as well and be a little lighter, depending on the size motor you're planning to use...

later and good luck and post pics! OL JR