Ironically, I had that happen with the old 18 mm Quest black powder motors. I think the casings were thinner than the Estes motors, and I remember the motor casings “felt” hotter. The white paint on the body tube over the CASING both bubbled and browned.
I’ve seen this video before of rocket motors filmed on a test stand
there is a flame that extends out the forward end of the motor after the charge goes off. Or at least in a static motor test, WHEN THERE IS AIR AROUND THE MOTOR, that may not be there after the charge goes off.
But it also hit me, the motor CASING for black powder motors is made of paper. And the inside of the motor has to be the hottest part of the rocket for the whole flight. So a tightly wrapped paper cylinder can clearly take a lot of heat before it catches fire or even fails. Plastic and nylon chutes seem much less tolerant of even short exposures to heat. Kinda wonder if they made a really thin lightweight Nomex chute if that would work, wouldn't even need wadding.
Tried to look at stuff on flashpoints for paper and black powder (good call from
@Daddyisabar on Fahrenheit 451, I'd forgotten the rationale for the number in the title---although I didn't think it was THAT depressing a book. I liked Ray Bradbury, but he was a bit dark at times.) In any case, the flashpoints for paper and black powder aren't that different, so that's not it.
I think the points raised above are all factors, so thanks guys for the ideas.
A big one is TIME. Assuming the nose cone, chute, cord, and wadding all come out promptly, the duration of exposure to the heat is extremely short. I am not sure why black powder lights so fast (maybe because it has it's own oxidizer?), but I've seen igniters butted up against cardboard and at most it makes a char mark, but doesn't physically light the cardboard on fire, while certainly the same thing DOES ignite the motor. So it takes more time to catch paper on fire then it does for black powder.
Concentration also may be a factor. I've done some pop pods where I used an 18 mm motor inside a BT-20, put a couple BT-20 to BT-5 centering rings inside as a combined motor block and adapter, and run the BT-5 the length of the rocket. (Aside,
@lakeroadster , I really had a hard time packing recovery gear between a -5 and a -20, and even struggled a bit with -20 to -50, maybe I was trying to use to large a chute). Anyhow, while a BT-5 doesn't seem to have a problem with charring with 13mm motors, it DOES have a problem when you use it to vent an 18 mm. After 3 flights (sometimes after 1!) I could see something wasn't right, and when I cut them open the inner tube had been burned through. Same thing when I tried this for a 24mm motor venting through a BT-20. My solution has been to put a rolled up aluminum can section inside, which always annoyed the heck out of der Micromeister (sorely missed still!), since it was metal and, in his opinion, violated the safety code.
Also interesting is the comment about the smoothness of the inside of the tube. In my experience, and I think in some of the pics
@Ronz Rocketz has shown, when there is chute charring often it is very localized, like a chunk of hot clay/propellent from a non-zero delay motor hit it and stuck to it for a moment. Some of my early "no-motor-mount" Balsa helicopters would get localized burn marks on the inside, and two of them actually DID catch fire just forward of the motor position, so now I put Mylar tape or aluminum foil over the exposed internal balsa for these models=== AND I can see where the hot spots are over the tape and foil.) So the burning particles of a non-zero delay motor don't "catch" on the tube.
Other comment was on oxygen, likely the hot gases and propellant particles attached to the clay use up most or all of the oxygen in the tube on their way out, and there isn't much force to suck fresh air back INTO the tube after deployment, so the internal tube environment after deployment is NOT conducive to "catching fire." This is in contrast to my helicopter balsa blades, which upon deployment are actually IN the wind.
If there is something helpful to these thoughts, it might be the Kevlar shock cord failures. I've never liked the TriFold mounts, mainly because they put a "bump" on the internal tube surface that may "catch" the chute or wadding on their way out the front. So I LIKE motor mount attachments, but it DOES place the shock cord nearly directly in the path of the ejection charge (as opposed to the tube walls which are NEVER DIRECTLY in the path of the ejection charge.) I have yet to emulate the smart people who put heat shrink tubing around the tail end of the cord, but seems like a good idea.
