I have hinted at this project in a couple of other threads. As they say, “Show us the photos or it didn’t happen.” So here we go!
I have been thinking about a VTS for quite a while now. After thinking about what I want to achieve with this project I have defined the goal. It is to make an effective Vertical Trajectory System using off-the-shelf purchased electronics. I don’t particularly want to go down the track to produce a fancy, all-singing, all-dancing, high-performance unit with custom firmware. I really want to make it happen with electronics destined normally for the RC aircraft world.
Along the way I found out Cryoscum and Strud were also working on similar systems (with fancy gain scheduling and all sorts of wonderful features), so I had a bit of a chat with them as well. Studying the aerodynamics is non-trivial and took up a lot of time also. It also convinced me to go for something that was relatively tame to improve my chance of success. My control system theory also told me the same thing. I will also endeavor to keep it simple to hopefully add to the success of the project.
Lots to think about: Control moments, flight envelope, servo torques, bending moment diagrams, material properties, drag, lift, aerodynamic center movement… The list goes on.
I was originally going to go with levers to multiply the torque from the servos, but due to complexity (and backlash which is a killer in this application) I went for direct connect to the servo splines. The excursion of the fins will be mechanically limited to keep the torque required within what the servos are capable of providing.
ON WITH THE PROJECT!
I have an autopilot that I used in my fixed-wing training model. Lots of different stabilisation modes, so lots of possibilities. After thinking though how all the modes work I figured that there was a mode that was workable in a rocket. So I whipped up a quick proof-of-concept with some cheap servos and an RC receiver for a pulse generator.
- VTSProofOfConcept.jpg (224.39 KiB) Viewed 4871 times
Once connected up it showed that the operation mode was as I expected and all actuators behaved as I expected. I should be able to extract roll, yaw and pitch control with the autopilot and three servo channels.
So onto SolidWorks and produced a mount for four servos, and a fin design.
The fin design (double wedge) is suited for supersonic flight and has quite a predictable aerodynamic center in the supersonic part of the envelope where servo torques become more important.
- Fin1.png (101.72 KiB) Viewed 4871 times
Mechanical stops are built into the fin hubs to prevent any sudden control system excursion ripping a fin or three off.
- VTSServoMount.JPG (216.66 KiB) Viewed 4871 times
A mount for the servos was printed (ABS) then tapped for the servo mount screws and airframe screws. Plugs were printed to go into the cavities in the mount. These provide pilot holes to guide a holesaw at a later stage.
The fins were printed horizontally to give better strength along the plane of the fin. These fins get screwed to hubs that attach to the servo splines.
I had considered using a long (hand-made) bolt through each fin to hold the fin to the servo. You can see the hole in the top of the fin for that provision. I have gone away from this idea as the thread (for going into the servo) would be cut with a die, rather than rolled. This results in a stress raiser at the root of the thread, and with a long lever arm would be a likely failure mode. It would have been nice to use these bolts as the fin would be more in compression radially, adding to the strength of the unit.
The mount was fitted to the airframe and illuminated from within to locate the mounting holes and approximate the locations of the fins. 10mm holes were drilled at the approximate location of the fin hub centers.
The assembly was fitted (along with the temporary guiding plugs) into the airframe and the hub holes drilled to 35mm using a Sutton holesaw with ¼” centering drill guided by the pilot hole in the temporary plugs. Care had to be taken to ensure the filament-wound fiberglass did not splinter badly when the saw broke through inside.
- VTSAirframeReadyForHole.JPG (110.69 KiB) Viewed 4871 times
- VTSHoleSaw.JPG (122.2 KiB) Viewed 4871 times
Once the holes were cut the edges were given a quick coating of Loctite 401 CA glue to keep the edges from fraying. It was then sanded and painted (signal red over white undercoat).
Hubs were fitted in preparation for the fins to go on.
Each fin was attached using M2.5 capscrews (qty4). My printed NC (off my Nike Apache) was then put on just to get an idea what the finished product will look like.