Tom Savoie and myself have both wondered a bit about whether heat from a rocket’s motor can weaken epoxy fillets. We exchanged e-mails and both called a friendly technician from West System epoxy named Brian, who was very helpful. The following is what we have learned, although the subject could be up for debate.
The situation we gave Brian was the following: a 54mm minimum diameter rocket using a K250 motor. The tube is phenolic, and probably wrapped in some sort of reinforcement, probably fiberglass. The fins are attached through the slotted airframe, but stop against the motor. They are filleted with West System epoxy, and then covered in a carbon/fiberglass combinataion. For those unfamiliar with the K250, it burns for 10 seconds, which should get the outside of the motor very hot, and should be as long a burn as almost all commercial motors. As per Tripoli certification standards, we told Brian the motor could get no hotter than 200°C (almost 400°F).
There has to be good heat transfer to affect the fillet. The phenolic tubing provides a small but important factor of insulation. One of the most important parts to the question is the length of the motor burn. Although 10 seconds is a long time for a rocket motor to burn, it is really not all that long. The fillet will probably barely heat up during the burn. This means the fins will be solidly attached while they are under the most stress. Once the rocket begins to coast, the motor is still hot, and will have time to transfer some heat to the epoxy. However, Brian does not feel that there is enough heat there to even weaken the epoxy momentarily.
West does hot oil bath tests of their epoxy, and they need to bring the oil near it’s flash point and leave the joint in the oil for awhile to see significant weakening. Even if the epoxy did reach a weak state, it would not get there until the rocket was well under chute. The fins would be experiencing little to no stress. However, it is very important to have some sort of cloth over the fins (fiberglass, carbon, etc.) Brian says that the fillet can become so hot that it is virtually useless, and the cloth will still have no trouble holding the fin in place. He also mentioned fillers: 404 and 406 are very heat resistant, while 407 and 410 can not be exposed to heat. The most reassuring news is:
West epoxy can not be permanently weakened by heat. It can become weak while it is hot, but once it cools down, it returns to 100% full strength. However, this is not true for hobby shop epoxy and wal mart (devcon) epoxy. This stuff can get very brittle and weak from being exposed to too much heat.
If you are building a rocket with a big motor and are concerned about heat weakening your epoxy, do the following: even in a through the wall design, glass any fin to motor tube joint. This will keep the joint right where it should be. Use West System epoxy. It is well worth it. No hobby shop stuff. Use 404 or 406 filler. 407 and 410 are definite bad choices for this application. Check the rocket when it gets back to make sure the fins have not moved. If not, even if the epoxy is goo, it will cool down and be just as strong as before.
Submitted by: Woody Hoburg