Last week @SteelyEyed updated his old Nuclear Crowbar thread, which was the first time I saw it. Reading through it, I immediately knew I wanted one. Even better, they shared their Rocksim file. The original design is obviously HPR, with a 54mm mount. I don't have any spare tubes big enough, so I looked at what a downscale would look like. I started with assuming I would downscale the ringtail to a BT80, which is the biggest tube I have available right now. Scaling everything down in Rocksim worked out pretty much perfectly to a main bodytube of BT55, with a transition to BT50 for the rear. I was torn between having a 24mm mount and having a motor retainer. I cleaned up the sim, removing the things I wouldn't need for a LPR build, swapping materials, etc. The quick sims showed it would get to ~400ft on a C, ~600ft on a composite D. Good enough for me.
The next step was to rebuild (most of) the rocket in Fusion 360 to get the CAD files I wanted for my laser cutter and 3D printer.
Once I had the files sorted, I started by cutting slots in the tubes. My laser cutter already had the rotary setup from a previous project, so it was easiest to start there. I hate cutting tubes on the rotary, but it usually gives good results and saves a lot of time aligning fins. This time it went great for the BT55, but the BT50 had some issues I didn't find until after I had taken down the rotary, and I was too lazy to set it back up.
Next I cut out the fins, centering rings, and engine block (plus some spare engine blocks because I had room left). I also created a marking jig for the MMT, with little grooves for a pencil, that precisely locates the centering rings. As you can see, one of the centering rings is slotted, which mates to the TTW fin tab.
If you look closely at the top of the root edge of the fin, there is a little TTW fin tab that's used just for alignment. I remember reading a thread about rockets like the Arapahoe that have long strakes, and how difficult they are to position, so these alignment tabs are my solution, and it's worked very well on the 2 rockets I've built using them.
Motor mount cut to length, and marked using the guide. The aft end of the guide has a little angle that makes a corner. The tube is just pressed into the corner and rotated while you hold a pencil in the notch. Overkill for most things, but it worked well for this.
The 2 forward centering rings are then glued in place, and I attached about 6 feet of kevlar, which goes through a notch in the top centering ring. What I should have done was built a baffle, and attached the shock cord to that, but I keep forgetting to add them to my builds. Some day...
I printed the transition and nosecone out of PLA. It's not ideal, but I figure the transition won't see much stress, and a nosecone is easily replaceable. I slid the transition into the BT55 and snapped in a fin, but there was a gap between them. When I lined the laser up to the rear of the tube, it was a little long.
A little sanding closed the gap nicely.
Once I had that all set, I removed the transition, and glued the MMT in the BT55. The fins were inserted into the the slotted centering ring to make sure everything stayed properly aligned, and a Qualman fin guide was used to keep the fins in place.
After that dried, the transition and rear centering rings were glued into place.
The BT50 on the aft end of the rocket was a nightmare from start to finish. My cut conditions on the laser cut through the tube in a couple places, I had to patch it up with CWF and superglue. Once I was able to put it together on the rest of the rocket, I realized that I had cut the tube too short, and there would be a 3/16" gap between the tube and transition on the transition coupler. I considered just filling it with CWF, but eventually decided to just cut a filler ring slightly larger than the gap and slide it on first. Then I snuck up on the fit by sanding down the laser cut tube.
After that I discovered that one of the slots was slightly off. I enlarged that slot until I could get the Qualman fin guides on and it looked straight (spoiler: it wasn't). Then I positioned everything one final time and used thin superglue to lock the band and BT50 tube in place. I burnished the edge and ring while the superglue set to try and reduce the edge. Then I applied some CWF to the tubes, but the BMS tubes aren't that bad.
Up to this point I wasn't sure I was going to have enough clearance to use micro rail buttons, or if I would have to use a launch lug. I inserted the fins and the ring tail, then balanced a short section of micro rail. There's definitely enough clearance for the rail, although I may have to modify the buttons a little if it's too tight.
I found the center between 2 fins, and drew a straight line. I drilled 2 starter holes with the tip of my knife and hardened them with some super glue. After everything is painted I'll clear them up and attach the rail buttons.
And with that done, it was finally time to attach the fins. I glued them in one at time, using the Qualman fin guides and the ring fin to keep everything in alignment.
After the glue dried I checked the fin alignment. The deflection in the one fin is very evident when put next to a straight edge. Oh well, looks like it's gonna be a spinny boi.
When I designed the nosecone, instead of printing the shoulder to engage directly with the body tube, I reduced it's diameter to the ID of a BT55 coupler. I cut a short piece of coupler, and laser cut a disk to fit inside + plus a small hole for a hook. This will create a little cup that I'll eventually glue to the nosecone after everything is painted.
Small fillets added to the fins. The nose cone has been sanded, and a thin coat of CWF applied. The ring tail isn't glued on at this point since painting the inside is a pain. I'll tape off where the tail touches the legs and glue them together once everything has been painted.
And that's where it stands for now. It's going to rain all week here, but Friday it may get up to 50F, so I'm hoping to find a minute to spray some primer down, but I may not make any more real progress until it warms up here.
The next step was to rebuild (most of) the rocket in Fusion 360 to get the CAD files I wanted for my laser cutter and 3D printer.
Once I had the files sorted, I started by cutting slots in the tubes. My laser cutter already had the rotary setup from a previous project, so it was easiest to start there. I hate cutting tubes on the rotary, but it usually gives good results and saves a lot of time aligning fins. This time it went great for the BT55, but the BT50 had some issues I didn't find until after I had taken down the rotary, and I was too lazy to set it back up.
Next I cut out the fins, centering rings, and engine block (plus some spare engine blocks because I had room left). I also created a marking jig for the MMT, with little grooves for a pencil, that precisely locates the centering rings. As you can see, one of the centering rings is slotted, which mates to the TTW fin tab.
If you look closely at the top of the root edge of the fin, there is a little TTW fin tab that's used just for alignment. I remember reading a thread about rockets like the Arapahoe that have long strakes, and how difficult they are to position, so these alignment tabs are my solution, and it's worked very well on the 2 rockets I've built using them.
Motor mount cut to length, and marked using the guide. The aft end of the guide has a little angle that makes a corner. The tube is just pressed into the corner and rotated while you hold a pencil in the notch. Overkill for most things, but it worked well for this.
The 2 forward centering rings are then glued in place, and I attached about 6 feet of kevlar, which goes through a notch in the top centering ring. What I should have done was built a baffle, and attached the shock cord to that, but I keep forgetting to add them to my builds. Some day...
I printed the transition and nosecone out of PLA. It's not ideal, but I figure the transition won't see much stress, and a nosecone is easily replaceable. I slid the transition into the BT55 and snapped in a fin, but there was a gap between them. When I lined the laser up to the rear of the tube, it was a little long.
A little sanding closed the gap nicely.
Once I had that all set, I removed the transition, and glued the MMT in the BT55. The fins were inserted into the the slotted centering ring to make sure everything stayed properly aligned, and a Qualman fin guide was used to keep the fins in place.
After that dried, the transition and rear centering rings were glued into place.
The BT50 on the aft end of the rocket was a nightmare from start to finish. My cut conditions on the laser cut through the tube in a couple places, I had to patch it up with CWF and superglue. Once I was able to put it together on the rest of the rocket, I realized that I had cut the tube too short, and there would be a 3/16" gap between the tube and transition on the transition coupler. I considered just filling it with CWF, but eventually decided to just cut a filler ring slightly larger than the gap and slide it on first. Then I snuck up on the fit by sanding down the laser cut tube.
After that I discovered that one of the slots was slightly off. I enlarged that slot until I could get the Qualman fin guides on and it looked straight (spoiler: it wasn't). Then I positioned everything one final time and used thin superglue to lock the band and BT50 tube in place. I burnished the edge and ring while the superglue set to try and reduce the edge. Then I applied some CWF to the tubes, but the BMS tubes aren't that bad.
Up to this point I wasn't sure I was going to have enough clearance to use micro rail buttons, or if I would have to use a launch lug. I inserted the fins and the ring tail, then balanced a short section of micro rail. There's definitely enough clearance for the rail, although I may have to modify the buttons a little if it's too tight.
I found the center between 2 fins, and drew a straight line. I drilled 2 starter holes with the tip of my knife and hardened them with some super glue. After everything is painted I'll clear them up and attach the rail buttons.
And with that done, it was finally time to attach the fins. I glued them in one at time, using the Qualman fin guides and the ring fin to keep everything in alignment.
After the glue dried I checked the fin alignment. The deflection in the one fin is very evident when put next to a straight edge. Oh well, looks like it's gonna be a spinny boi.
When I designed the nosecone, instead of printing the shoulder to engage directly with the body tube, I reduced it's diameter to the ID of a BT55 coupler. I cut a short piece of coupler, and laser cut a disk to fit inside + plus a small hole for a hook. This will create a little cup that I'll eventually glue to the nosecone after everything is painted.
Small fillets added to the fins. The nose cone has been sanded, and a thin coat of CWF applied. The ring tail isn't glued on at this point since painting the inside is a pain. I'll tape off where the tail touches the legs and glue them together once everything has been painted.
And that's where it stands for now. It's going to rain all week here, but Friday it may get up to 50F, so I'm hoping to find a minute to spray some primer down, but I may not make any more real progress until it warms up here.
