Adrian, I looked at it.. The new load is 91% of the impulse of the J570 and it flies in the same case. Basically more grunt, less NS, same casing as a J570.

The J570 is 1060NS and the J825 is 970NS. While there may be some nuances that would allow for a bit more altitude, my hunch is that the reduced NS and same amount of space needed probably point to the J570.

I'm 4 for 5 in J570 minimum diameter boosts. The one that failed got really wild at about 1500'. My hunch is that material fell into the nozzle under the zillion gees it was pulling, and thrust got redirected. I believe the J825 would be a greater challenge to fly, and the resultant altitude would be less... I may fly my J570 rocket with one sometime?

I agree that for an altitude shot, the J570 would be superior. But for a speed contest, or even just a personal speed record, the J825 would be pretty spectacular. My main question is, does it exist, and will it be available at some point?

8) Sorry, I missed this post... I have flown them for 6 months in my Talon 3. I have them in the box at all times 8)

Cool. I'd like to build a min-diameter carbon rocket around that sometime. 2000 mph would be a fun goal.

Cool. I'd like to build a min-diameter carbon rocket around that sometime. 2000 mph would be a fun goal.

I'd love to see that too. IF I could see it....... That might be too fast for me to keep track of.

2,000 miles per is the appx muzzle velocity of a 30.06 bullet. Pretty brisk, indeed...

Many years ago, a veteran flyer told me "the gremlins come out at mach 2". 2,000mph is closer to M3. That is movin'........

JW

Yeah, I would be concerned about the fins getting too hot and the epoxy softening too much. I heated up Barack-it to get the motor out (thanks for the tip, Warren) and was surprised how much more flexible the fins were. They were laid up with Bob's hobby fiberglassing resin, but still it got me thinking.

In a Mach 2.8 bird, the fins would be heated both by the motor and by the air. The motor could be insulated from the fin roots somewhat by using glass for the innermost layer of the tube layup. But I'm not sure how to protect the fins. Maybe some unidirectional carbon going back from the leading edge would spread the heat over the rest of the fin? High temperature epoxy? Aluminum core to absorb the heat?

My rocketry plate for the rest of the summer is going to be full repairing and flying Barack-it and building the next double-batch of Parrots, but a 38mm screamer will probably be my next project. (Or maybe a re-build of a 24mm G37?) Plenty of time to research these things.

I just did a sim. Yeesh. Over 100 Gs for a full second, and a top speed of about 2300 mph, assuming the fins stay on. There would be 80 lbs of drag, of which 20 lbs would be on the fins, or about 6.7 lbs per fin. That doesn't seem too bad, but that's assuming they're on perfectly straight and there's no wind shear to deal with. I don't think I have any idea what sort of forces would result from flying into a little crosswind at 2300 mph. 😯

Use Cotronics high temp epoxy on the leading edge of the fins and the tip of the nosecone. There is insufficient heat soak during a flight to cause significant softening of the epoxy although at the speed you're talking you'll probably get some ablative cooling as the paint and surface flakes away at the hot spots.

W

I think the gees would be a bigger problem than heat. You get all your speed all at once with this load. The heating of the fins due to airspeed would be very ephemeral... you'd be slowing down at t-plus 2? or so, and the air is much cooler as you ascend.

I think your challenges would be way tougher if you had a long burning motor... your time at velocity would be much longer, plus the motor would probably be a lot hotter. PLUS you would probably be flying a slotted motor, so one part of the motor would be *very* hot.

"Stratagem" has flown J570s several times just fine. Mayhaps I'll toss this motor in sometime?

Adrian, can you send me a sim file?

John,

When I get home I will, though you would have to upgrade to Rocksim 8 to read it.

Your responses on the heating are encouraging. I looked up cotronics, and they have an epoxy that looks good, with low viscosity, 10kpsi strength and 500F-600F working temperature. I'll have to contact them for prices.

Adrian, I have used Aeropoxy lately. I'm getting ready to see how good this stuff really is 😯 I'll know a lot more by early August.