Saturday, March 13, 2010

The Thunderbolt 38mm build continues!

Work on the 38mmThunderbolt kit has continued now that a new supply of gloves and epoxy has arrived. This rocket uses JB weld and 30 minute Devcon epoxy, the latter never being very popular among the "pro amateurs" but has always been my choice, and will continue to be for ease of use and reliable performance.

The hardpoint anchor was assembled, with a bead of CA applied to the threads to lock them. The Kevlar recovery line was tied in place, wrapped in thread, and finally "potted" (soaked) with a thin CA. The knot will take the stress of recovery, with the CA potting simply used to keep the knot secure over the years. The hardpoint anchor is like a small aluminum coupler, and it formed the base of a 38mm stack which was secured with a zip tie to ensure a tight fit. Ideally the hardpoint will be strong enough alone, but in this system the coupler also takes on recovery forces transferring them to the upper and lower airframe sections. All surfaces were sanded with a very coarse, 60 grit sandpaper which 30 minute epoxy seems to love.

During some downtime, I selected some extra brass motherboard standoffs (fairly heavy) to use as noseweight. There is no known stability issue with this rocket, even with J motors, because it is so long (see below). However, this is a particularly light nosecone. My concern was that without some extra inertia, the nosecone would eject but not help pull out the recovery system. 38mm rockets are always hard to pack, particularly with so much thick kevlar. There is a very real risk that the parachute will get stuck in the rocket, or as seems to often be the case, halfway out of the airframe. By adding an extra oz or so, I hope to help prevent this. Do you think this matters? Please comment below if you have some ideas about this issue.

Finally, the two airframe sections have been glued together. Even with some sanding and rotating, there was a substantial gap and what looks like a slight bend. These are probably both small enough to ignore at this point. My acme fin-can rockets always seem to have a nice spin to them anyway, which is fun. The joint will be filled and possibly slightly reinforced with JB weld next.

After adding the rail guides, and sanding down the nosecone to fit, I only need to drill a small vent hole near the top, and the rocket is ready to fly! No need to paint it just yet, fiberglass has a sexy look to it anyway (until it gets black inside from ejection residue that is!)

This isn't exactly as crazy as my 72 inch x 38mm model from back in the day, but boy it is still pretty long. This thing will be great on little H motors.

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