Ye Olde Rocket Forum - View Single Post - 1972 D12-3. Should I fly it?

Quote:

Originally Posted by rocketguy101

I thought the issue with thermal cycling was the differential diametrical change between the case and the propellant slug (due to different coefficient of thermal expansion of the materials) which allowed the propellant to de-bond from the paper case. During the burn, when it reached the OD of the slug there is a sudden large increase in surface area, causing a pressure spike which leads to CATO.

I don't see how this could self heal...but I've wondered if you could put an epoxy fillet at the back of the motor to keep the slug in, like the old EnerJet motors???

No. Understand that the bonding that occurs between the casing and the propellant (and previously pressed nozzle) occurs within seconds under great pressure.
The de-bonding occurs, as you say, from the change in diameters. If fired within a few months after this occurs, you'll get flame propagation through that microscopic separation. My theory is that if the motor now is kept in normal storage conditions, the bonding reoccurs as a function of pressure over time. After all, there is still pressure being applied, as the diameter the casing was made at, is smaller than the chunk of propellant. This might explain why a lot of the early 90's Estes E15's, which were taken from the market due to a high probability of catos, (anecdotally) work perfectly now. The cause of their problems was slightly different. They worked fine when fired within a few months after manufacture, but 'dried out' too much, and started catoing later. But it seems the longer they're just left alone, they "settle in" to their casing.

By the way, I think your idea about the epoxy fillet would just lead to either a nozzle blow out or a split casing.

But it's still just a theory