The maximum temp that the motor is allowed to reach is 392 F. I don't think there are many motors that get that hot. As for heat it will get warmest in the lower 1/3 of the motor mount - the nozzle heats up and radiates outward. As long as your have enough fin root and big enough (mass wise) fillets that aren't chock full of aluminum powder then they will not transfer heat fast enough to be of a worry when you are in the boost part of the flight. Most motors you can touch on the outside during firing. It is post firing heat soak that gets you, but you should be well out of mach and into recovery before that.

Edward

I'm considering swapping out the motor tube on my PR Comp3 for phenolic, justs to give another layer of insulation for such reasons.

Anyone's thoughts? I think we'll probably be fine. Maybe do the root epoxy with JB Weld + chopped FG?

-Ken

Paper would probably be best, less dense, more air pockets. Just depends on your preferences. Now, we are all worried about this weakening the fins - how many rockets have failed by this method? My guess would be zero. I've seen many MD rockets with G10 fins epoxied on. As for motor mounts in general, they are probably the strongest part of your rocket - it has an aluminum coupler inside 🙂 I generally use paper because it is light and loves epoxy.

Edward

I have often flown MD rockets with long burn motors, including the K250. Some thoughts:

- on a true moonburner or slot motor, I line the slot up between fins. The slot is the area that should get the hottest. My thought is that it is best to not bake a fin root 😯

- ProLine and AeroPoxy are far superior epoxies when heat is an issue

- Carbon is a pretty good insulator. While pricey, you might consider a carbon motor tube

If minimum diameter rockets hold up (and they do) then you will be just fine. I've flown perhaps two dozen L330s, K250s, K185s, J135s, etc. in minimum diameter and never had a problem. If you are looking for a bit more insurance, glass the fins tip to tip and you wil be very much protected.

From a thermodynamic standpoint putting the slot between two fins makes sense, but I figured that it actually would be best to line it up with a fin root to counteract the moment that all of that flowing propellant generates. Ie - you have it between two fins, it creates a moment and then the fin on the opposite side cannot help to aerodynamically correct for it. The fins on the side with the slot will, but they are so close that the lever arm is short.
If the slot is lined up with a fin root then you have the same moment generated by the propellant mass flowing down the core, but now you have two fins on the opposite side with a a larger lever arm - which should mean they have to move less to correct for the moment created by the propellant.

Edward

G10 is actually quite a good themal insulator. Good enough that real spacecraft use g10 to make standoffs to attach propulsion fuel lines to cold structures. In the fiber direction, carbon is more thermally conductive than aluminum. Spacecraft use carbon fiber plates as heat spreaders.

If you're concerned about the strength of hot epoxy, yoANCan make any epoxy more temperature resistant by giving it a post-cure cycle at elevated temperature. A car dashboard in the summer can get it up to 160-180F. Or you kitchen oven if your rocket is small enough. I have had good success with aeropoxy post-cured at 250 F.

Using Aeropoxy with both a Kevlar/CF hybrid, FG, and CF, I have never had an issue. Mix it up.