Little Dog Dual-Deploy

To get some experience flying with electronic deployment, I'm planning a new dual-deploy rocket based on the Performance Rocketry Little Dog Dual Deploy model. This "kit" consists of only the fiberglass portions, I get to design the rest of the rocket. I think it makes a good transition from a full kit build to a full custom design. It's also made entirely of G10 fiberglass, which I'd find hard to machine myself.

Here's the pieces I'm planning on using for this model:

  • 1 Performance Rocketry 54mm Little Dog (Dual Deploy)
  • 1 Perfect Flight MAWD flight computer
  • 1 Perfect Flight DT2U USB data transfer cable
  • 1 Beeline TX 70cm beacon
  • 20 feet Kevlar cord
  • 1 Slimline 54-38mm Ogive tailcone motor retainer
  • 1 Slimline Motor Adapter 38-29mm
  • 2 ACME Conformal Rail guides
  • 1 48" parachute
  • 1 18" parachute
  • 2 Top Flight TF-FCP-06 6" Nomex Chute Protector
  • 2 Swivels
  • 4 1/8" Quick links
  • 4 U-bolts

I'm planning on making the initial flights using the MAWD flight computer. When the Telemetrum is ready, I'll fly that as well to test it, with the goal of using just the Telemetrum eventually.

The kit came with a short motor tube, measuring only 8.125" long. Aside from failing to provide much support for a longer motor case, when I slide it back to fit into the tailcone, the motor tube doesn't reach the forward edges of the fins. To resolve this, I bought 3' of 38mm tubing from Performance Hobbies. I'll cut off 16" and use that for the motor tube.

Build Plan

I'll cover the proposed build plan in separate stages:

  • Fin can
  • Payload bay
  • Electronics sled
  • Recovery systems

Fin can

The pieces involved here are:

  • Aft body tube with pre-cut fin slots
  • Three G10 centering rings
  • 16" of G10 38mm motor tube
  • Slimline Ogive tailcone motor retainer
  • 10' 1/8" Kevlar cord

Here's a proposed sequence of build steps:

  1. Dry fit the centering rings around the motor tube and inside the body tube. Two of the rings are a tight fit around the motor and one is loose. All three fit nicely inside the body tube.

  2. Cut the motor tube to length using a hand miter saw.

  3. Locate the forward and aft edges of the fins on the motor tube. These need to account for the length of the tailcone, which is 3/4", and must be quite accurate.

  4. Measure back 1/2" from the forward end of the motor tube.

  5. Rough up the motor tube at these locations using 60 grit paper.

  6. Draw a line around the body at both locations.

  7. Glue the middle centering ring to the motor tube. Use one of the tight fitting rings to avoid mis-alignment. Make fillets on both faces of the ring where it meets the tube.

  8. Glue the forward centering ring to the motor tube. Make a fillet on the forward face of the ring where it meets the tube.

  9. Using a dremel routing bit, cut a slot in the outside edge of the forward centering ring, large enough for the kevlar cord.

  10. Tie the kevlar cord around the motor tube, aft of the forward centering ring. Run the cord through the slot and tape it to the motor tube ahead of the centering ring.

  11. Tighten the cord around the motor tube and push it up to the rear face of the forward ring. Apply plenty of epoxy around the tube and over the cord. Glue the cord into the slot in the ring, making sure that it does not protrude beyond the outer edge.

  12. Ease the forward inner corners of each fin to pass over the epoxy fillet on the aft side of the middle centering ring. Make sure they fit all the way around as it will be hard to know which part of the fillet will end up under each fin slot.

  13. Rough up the inside of the body ahead of the fin slots, behind the fin slots and where the forward centering ring will meet the body tube.

  14. Smear epoxy just ahead of the fin slots. Drive the motor tube down the body until the middle centering ring lies just ahead of the fin slots. Clean the fin slots of epoxy. Set the body tube on its forward edge and let the epoxy cure.

  15. Apply epoxy to the forward centering ring around the outer perimeter. Make sure no epoxy gets inside the motor tube. That shouldn't be too difficult as there will be a well between the motor tube and body tube ready to accept epoxy.

  16. Prepare a fin alignment jig using a piece of cardboard.

  17. Adjust each fin slot so the the fins can be inserted easily.

  18. Apply a thin layer of epoxy to the root of each fin and stick the fins to the motor tube. Use the fin alignment jig to hold all of them in position.

  19. Apply fillets to both sides of each fin on the motor tube, both sides of each fin on the inner face of the body tube.

  20. Mix up some epoxy and talc (50/50 epoxy and talc). Apply that as fillets on both sides of each fin where it meets the outer face of the body tube

  21. Glue on the aft centering ring using JB weld.

  22. Check the tailcone fit. Sand down the motor tube until the tail cone fits tight against the body tube.

  23. Rough up the end of the motor tube and inside of the body tube using 60 grit paper.

  24. Glue on the tail cone using JB weld