Four and a half ideas for guidance actuators

Adrian A

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  1. Two or more canards ahead of the CG: This is the most well-known/well-proven approach. I used to do a lot of RC slope soaring with flying wings, so I'm familiar with the potential for 2 control surfaces to control pointing in all 3 directions. Control roll to align the canards with the desired pitch axis and then pitch up. The canards can be directly connected to the servos, which is simple and avoids backlash, but is not compatible with narrower airframes unless the canards are offset along the length of the airframe (which might be o.k.). Linkages to allow the servos to be stacked vertically are possible, but add complexity and maybe some backlash. If the canards are offset from the fins to prevent control reversals during the flight, they are in the way of tower rails, and so rail guides (possibly detachable) would be required. Canards add drag throughout the flight, and aren't effective during the earliest part of the boost while the rocket is moving slowly.
  2. Full-airframe thrust vector control (FATVeC): This is what I have in mind for steering in a 38mm airframe. Connect the upper and lower halves of an airframe with a central pivot that takes all the axial loads from the boost, and then use 2 servos to drive pitch/yaw lead screws to tweak the alignment between top and bottom parts of the airframe. This is for pitch/yaw only; something else is needed for roll control. The advantages here are that there is almost no additional drag, it produces control torques at any velocity as long as there is an acceleration, and also aerodynamic steering when the rocket is moving. No interference with the tower. The downsides are a more complicated and heavier mechanism, and it's unproven whether the servos could be fast enough and strong enough. Here's a photo of some hardware I bought to play with. A 6mm lead screw, and two each of servo clamping couplers and lead screw nuts from Servo City, and two of McMaster's smallest ball rod ends. Pen for scale.
    IMG_0199.jpg
    The lead screw and nuts are very smooth, low-friction, and free of backlash. Impressive value since all 5 parts combined for about $26 plus shipping.
    Some mini servos that have a chance of fitting into a 38mm airframe are on their way. This idea also requires low roll rates or the servos will have no hope of keeping up. Which brings me to the half system:
  3. Roll control with one servo for 2 canards: @Finicky inspired this idea with his retractable canards. Similar to Finicky's system, the angle of attack of the control surface is fixed but the amount of the control surface that is exposed is variable. The idea here is to put one flat, angled control surface that is directly connected to the servo output and entirely inside the airframe when the servo is centered. When the servo is active it sticks the surface out a slot on one side or the other to provide roll torque (and some pitch as a side effect). To minimize undesired pitch moments, this would work best near the CG of the rocket, which is convenient anyway because that's where the FATVec system would be. If the servo is small enough, then the control surface can be centered in the tube and overlapping the servo. Otherwise, it would need to be ahead of or behind the servo. Sorry I don't have a sketch for this yet.
  4. Active 2-stage coupler: I'm in the early stages of planning for a 54mm/38mm 2-stage rocket for BALLS next year that would be designed to go over 100k before I get my L3 cert. The 38mm/54mm coupler presents an opportunity for a different variation on the FATVeC idea. The 38mm rocket's motor is the inner part of the coupling. It's surrounded by 38mm airframe tube connected to the end of the 54mm airframe for the stage coupling. If that 38mm airframe can pivot at the front end, then there would be some nice leverage at the back for 2 servos to provide pitch and yaw actuation. I think I could get two servos side-by-side in the airframe and move the end of the tube with simple ball end links. This would only add about 2" to the length of the airframe, not counting the roll control that you'd also need. I'm not sure about the pivot at the front end of the coupler yet, but I'm thinking about something like a 2" derlin ball bored out for the 1.5" airframe and riding inside a 3d printed cup.
  5. If the FATVeC system turns out to be too fragile or weak, a more robust approach that would work while the rocket is accelerating and decelerating is to move around some ballast weight inside the airframe rather than the whole airframe. I suspect the mass to get decent control authority would be substatial, and you wouldn't have the aerodynamic control that you would with the FATVeC approach, so I have put this one on the back burner for now.
 
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Finicky

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Adrian,

Thanks for putting this list together. It's nice to see a kindred spirit trying to cram as much mechanism as possible into a very small space :)

I'm trying to improve upon my system to make it more amenable to Mach+ flights. My challenge is that everything still needs to fit into the upper part of the nose (in addition to the camera), so I've got a volume that is quickly tapering to nothing. The current system uses two servos to deploy the canards, but really only needs one since I don't have any interest (yet) in countering roll of the main body of the rocket by differential opening of the canards.

For much higher velocities I want to get away from 3D printed shells and get back to standard FW noses. I also want to make the deployable canards truly aerodynamically "invisible" when they are retracted. The current system needs to have oversized slots since the canards rotate out of the nose. This means some part of the slot is always open to airflow regardless of canard position. That can't be good for minimizing drag.

I've been toying with various prototype mechanisms. I've also looked at small lead screws just like you are. One thing I don't like about hobby servos is that since there is no access to a rotary encoder signal it's not possible to know if the system stalled during operation by looking at the post-flight data. I suppose I could put a current sensor on the servo power line to infer stalling ..............

Anyway, here's a video of a canard deployment mechanism I put together. You can tell by the shape that it's meant to fit into a nose on an angled plane. This concept could be driven by either a hobby servo or a lead screw.

From the 10+ test flights I've made with my system, one thing that has surprised me is just how little canard surface area is necessary at high-ish velocities (>150m/s) in order to make course corrections.

Cheers !

Steve G.
 

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dhbarr

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Battery and motors in the nose, pulling against anchors in the neck. Keeps the mass at the proper end, shouldn't make it any thicker, can be easily transferred to another airframe. Samantha of course, b/c nose wiggling. Be careful not to completely overshadow the fins.
 

waltr

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Great ideas The videos of the finicky system flights are something elese.

Am wondering how any of these methods will work at Mach+.
 

Adrian A

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Adrian,

Thanks for putting this list together. It's nice to see a kindred spirit trying to cram as much mechanism as possible into a very small space :)

I'm trying to improve upon my system to make it more amenable to Mach+ flights. My challenge is that everything still needs to fit into the upper part of the nose (in addition to the camera), so I've got a volume that is quickly tapering to nothing. The current system uses two servos to deploy the canards, but really only needs one since I don't have any interest (yet) in countering roll of the main body of the rocket by differential opening of the canards.

For much higher velocities I want to get away from 3D printed shells and get back to standard FW noses. I also want to make the deployable canards truly aerodynamically "invisible" when they are retracted. The current system needs to have oversized slots since the canards rotate out of the nose. This means some part of the slot is always open to airflow regardless of canard position. That can't be good for minimizing drag.

I've been toying with various prototype mechanisms. I've also looked at small lead screws just like you are. One thing I don't like about hobby servos is that since there is no access to a rotary encoder signal it's not possible to know if the system stalled during operation by looking at the post-flight data. I suppose I could put a current sensor on the servo power line to infer stalling ..............

Anyway, here's a video of a canard deployment mechanism I put together. You can tell by the shape that it's meant to fit into a nose on an angled plane. This concept could be driven by either a hobby servo or a lead screw.

From the 10+ test flights I've made with my system, one thing that has surprised me is just how little canard surface area is necessary at high-ish velocities (>150m/s) in order to make course corrections.

Cheers !

Steve G.
Kindred spirit indeed, though you and @JimJarvis50 are way ahead of me with working systems. Nice canard mechanism. Doing my own 3D printing is definitely on my to-do list.
 

David Schwantz

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Imbed the female spline in the canard, servos are screwed right to airframe. Narrowest possible setup. No linkages and direct drive to canards.
 

Adrian A

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Imbed the female spline in the canard, servos are screwed right to airframe. Narrowest possible setup. No linkages and direct drive to canards.
Yes, I agree this is the way to go if your airframe diameter is more than twice the height of the servos you want to use. For a 38mm airframe, the only servos I can find that would fit that way are the tiny sub-micro 2.5 gram variety
 

Finicky

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Kindred spirit indeed, though you and @JimJarvis50 are way ahead of me with working systems. Nice canard mechanism. Doing my own 3D printing is definitely on my to-do list.
Just FYI - For getting unusual mechanism stuff for prototyping I find places line AliExpress to be useful. Here's a few links to stuff I've bought in the past. I'm not sure any of it will ever make it into a rocket, but I find it helpful when designing.




 

Adrian A

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Just FYI - For getting unusual mechanism stuff for prototyping I find places line AliExpress to be useful. Here's a few links to stuff I've bought in the past. I'm not sure any of it will ever make it into a rocket, but I find it helpful when designing.

Thanks for the links.
For my full-airframe TVC, I only need a few mm of travel, so I got this:
to attach the lead screw directly to the servo. The lead screw has 8mm per rotation, so with a 180 degree servo motion I'll get 4mm of travel. +/- 2 mm with about 15 mm distance from the lead screw axis to the centerline pivot would make about +/-7.5 degrees of rocket bend, which I think would be a lot more than I would need. I'm planning to attach the running nut with some Bellville spring washers clamping on both sides of the bulkhead in an attempt to accommodate the misalignment between the lead screw and the bulkhead it's pushing/pulling.
 
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