I’ve long wanted to build a minimum-diameter 24mm rocket to use the amazing 6-grain 24mm motors CTI makes. I was originally going to make one of Wildman’s 24mm Blackhawks, but I waited too long (too many other projects) and it went oop. When it looked like SEMROC was going to close, the germ of an idea had formed in my head. I had built several of their Ready-To-Build rockets (to fly to over 1600 feet on Aerotech’s 18mm D motors) and realized that their #9 tube just fit inside their #10 tube. A #9 tube is a minimum-diameter tube, but it wouldn’t be strong enough by itself to survive a 6-grain G motor. However, a rocket made of concentric #9 and #10 tubes glued together could probably survive. I ordered four each of their #9 and #10 tubes, a number of #9 and #10 balsa tube connectors (BTC-9 and BTC-10), whatever #10 nose cones they had left, and a set of tabbed basswood fins (balsa fins would likely not survive a G-motor flight). If I recall they were SLS Javelin fins (FLV-16). I also bought an Aero Pack 24mm motor retainer. I know it adds drag, but the rocket was going to be near-minimum-diameter, not truly minimum-diameter, and I love the convenience. The Rocksim altitude was over 6000 ft, so dual deploy and GPS tracking made a lot of sense. The collection of parts has sat in my build pile for a long time. In the meantime, PerfectFlite released the StratoLoggerCF (SLCF), and Eggtimer Rocketry released the Eggfinder. They would both fit inside a 24mm tube. Yes, I know that Featherweight’s Raven will fit as well, at three times the cost of an SLCF. I prefer to destroy my Ravens (2 so far) in more expensive rockets. I have both an SLCF and a Raven for redundancy in my 54mm minimum-diameter Transonic II, which also has 4 custom-made (wire EDM) thermistor probes and processing electronics. It has measured the temperature profile at Lucerne Valley up to 12,500 ft AGL many times and the plots are available on-line.
Anyway, back to the 24mm rocket: The Eggfinder and the SLCF seemed the right choices. I finally got some time away from other projects to play with the parts and think through construction. Both the Eggfinder and the SLCF would fit in the tube, but how would I secure them and how would I power them? A standard 9V battery doesn’t come anywhere close to fitting. At Eggtimer Rocketry’s suggestion (in the manual) I looked up 300 mAh 2S 7.4V LiPo batteries. Venom makes one that fits very nicely in a 24mm tube, and I could pick up matching JST connectors on Amazon for a small sum. There would not be room for a mounting board or an allthread. What I decided on was a forward balsa bulkhead with a screw eye followed by the battery, followed by another balsa bulkhead for the screw switch, then the SLCF, and a rear bulkhead with a screw eye.
My final challenge: How to feed power for the ejection charges through the bulkheads. I considered lots of options including very small eurostyle connectors. I couldn’t figure out how to attach them to the balsa. While playing around with the #4 standoffs for my other e-bays, I wondered if there were ones long enough to go through the bulkheads. A quick search on McMaster led me to the needed parts, 91115A519, 3/4” long hex standoffs (round ones would be hard to keep from rotating). I cut one of the balsa bulkheads in half and trimmed each half to just over 3/4 inches, drilled slightly undersized pilot holes, and press fit the standoffs using my vise. I then sanded one end to just expose the standoffs. I then added the screw eye, being sure to avoid contact with the standoffs, and put a thick layer of glue on the outward facing end of the bulkhead. Balsa is porous and would actually let the pressure and gases from the ejection charge leak into the e-bay without the glue layer. The pictures below show the balsa tube connector with the cutting line, the standoffs and the sized bulkheads, and the finished bulkheads with feedthroughs. Believe me, the feedthroughs are not coming out.
Anyway, back to the 24mm rocket: The Eggfinder and the SLCF seemed the right choices. I finally got some time away from other projects to play with the parts and think through construction. Both the Eggfinder and the SLCF would fit in the tube, but how would I secure them and how would I power them? A standard 9V battery doesn’t come anywhere close to fitting. At Eggtimer Rocketry’s suggestion (in the manual) I looked up 300 mAh 2S 7.4V LiPo batteries. Venom makes one that fits very nicely in a 24mm tube, and I could pick up matching JST connectors on Amazon for a small sum. There would not be room for a mounting board or an allthread. What I decided on was a forward balsa bulkhead with a screw eye followed by the battery, followed by another balsa bulkhead for the screw switch, then the SLCF, and a rear bulkhead with a screw eye.
My final challenge: How to feed power for the ejection charges through the bulkheads. I considered lots of options including very small eurostyle connectors. I couldn’t figure out how to attach them to the balsa. While playing around with the #4 standoffs for my other e-bays, I wondered if there were ones long enough to go through the bulkheads. A quick search on McMaster led me to the needed parts, 91115A519, 3/4” long hex standoffs (round ones would be hard to keep from rotating). I cut one of the balsa bulkheads in half and trimmed each half to just over 3/4 inches, drilled slightly undersized pilot holes, and press fit the standoffs using my vise. I then sanded one end to just expose the standoffs. I then added the screw eye, being sure to avoid contact with the standoffs, and put a thick layer of glue on the outward facing end of the bulkhead. Balsa is porous and would actually let the pressure and gases from the ejection charge leak into the e-bay without the glue layer. The pictures below show the balsa tube connector with the cutting line, the standoffs and the sized bulkheads, and the finished bulkheads with feedthroughs. Believe me, the feedthroughs are not coming out.
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