When to use T2T for MD rockets

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Well-Known Member
Oct 15, 2011
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Let me preface this by saying I don't have a whole lot of experience with minimum diameter HP rockets. I'm currenly L2 certified (have been for a few years) and am working on my L3, but all of those have been bigger birds. The closest I've come to MD before was perhaps 54mm birds with 38mm motors.

So last year I built a GoDevil38 to dip my toes into the realm of MD. Build was straight-forward, and the most challenging parts were fitting the darn electronics (single altimeter in the AV bay and Eggfinder into the nose). Did I mention I hate electronics integration in small rockets? Well, I do. Just ask Salvage-1. But that's beside the point of the thread, and I digress.

Fins on the GD38 are attached to the sustainer BT with Rocketpoxy, with significant, but not ridiculous fillets. Both surfaces were washed and roughened prior to assembly, of course (this isn't my first FG rocket). I did NOT do T2T FG on the fin can, as I didn't have any hardware or reloads that I'd think would stress the rocket significantly. Maybe a J825 (38/1080), if I was feeling really adventurous.

I was able to fly the rocket twice last season before I ran into logistical challenges (my fly-away rail guides broke): Once on a 2-grain 38mm H, that I can't quite remember at the moment), and once on an AT I-357. Nice altitude results, but these weren't really stressing the rocket. I had plans to move up to a J350 or J420, but didn't get around to those last year.

Now we get to the current situation. I've picked up some Loki hardware, including a 38/1200 case and some reloads: J1026 cocktail and K1127. These motors scare me. A quick physics calculation shows that either of these in the GD38 will accelerate it off the pad at somewhere between 80-100 G's. Mach flight isn't even really a question. "Teleportation" is a term sometimes used for itty-bitty rockets like Mosquito's and Quark's, but I think it could be accurately applied to the current flight profile for the GD38.

This is shred territory.

So, the question (sorry, long introduction)... At what threshold does T2T become advisable or even necessary? If my max motor size was a J420, or some other small J, I wouldn't be concerned. But with roughly triple the average thrust and double the total impulse...

Yeah, these motors scare me.

I'm not interested in making the rocket into a tank, but I want to take reasonable precautions to ensure it's safe.

Anyone else have experience (video's?) of their MD rockets on these motors they can share?

How thick are the fins?


just looked fin up ........ 1/16th. That will be your issue fin flutter. Had you used fins 3/32 or 1/8 & beveled, I would have no issue flying with just fillets.

BUT with 1/16th....T-T is highly advised. [unless they are carbon plate]

I built a 38 minimum with 1/16 carbon plate fins that survived a coupler folding at Max Q CTI J-530 I max.

I wouldn't think 2wice about sticking a K in it. Rocket survived just needed new coupler.
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... I built a 38 minimum with 1/16 carbon plate fins ...
And here I was... worried about 1/16th on a MD24 with G150 :facepalm:
Might go back to 1/32... :confused2:

I can't offer advice to the OP, but I will offer a hearty thanks for the timely thread and to Jim for his sage advice!