Need some help

[Barracuda]

I built a 49"BT-80 that flew pretty well but I made an ejection baffle(I thought)with the 9" motor tube and some "choreboy"steel wool in it and glued on a cap then cut holes in the cap.well the chute and all was fine but it burnt the end of the cap off I still have about a 7" motor tube but could someone tell me what I did wrong because I know you will.Maybe the mt too thin or too long (I've only flown it on D12-5,but built it for E28 reloads)

[burkefj]

Did it really burn the end of the cap, or was there just too much resistance(not enough holes) and it blew the end cap off?

Frank

[MarkII]

I'll take a stab. First, understand that an ejection baffle has to do two things: trap burning particles, and cool down the gases that are used to pressurize the recovery section. A simple mesh-style baffle traps the burning particles that get blown forward when the motor's ejection charge goes off, but unless care is taken to design and construct it right it does nothing to cool down the gasses. ("Cool down" in a relative sense, of course.) No doubt the chore boy did the first task; it contained and held onto the ferociously hot fire. The mesh didn't burn, but the fiber board or lite ply end cap did. No surprise there, since I'm not sure that your design was able to fulfill the second task. More thought has to be put into designing scratch baffles to insure that they accomplish both tasks. You can't just slap one together on the fly and expect it to hold up to repeated use. I'm not a fan of mesh baffles to begin with, though. At the very least, they have to be designed so that the mesh is accessible after assembly, because you will need to pull it out and shake out the debris or else totally replace it periodically. Since they trap nearly all of the particles on the surface of the near side, mesh baffles can clog and become totally blocked. There is a risk of this happening suddenly, without warning, during any deployment event.

Where you install the baffle is important, too. Putting it too close to the top of the motor can compromise it's effectiveness. A baffle should be installed as far forward as practical while still leaving enough room for the recovery system. Even a well-designed baffle will have a short life if it is installed too close to the motor.

I never use mesh baffles myself; I only use cold-air baffles in my rockets. Labyrinthine baffles (the "half moon" style is a classic example) also work well. So do "sifter" style baffles; these have a bulkhead on the motor side that is perforated with holes in the center, an empty holding/cool down chamber above it, and then another bulkhead on the parachute side that is perforated with holes around the outer edge. To extend the life of a baffle, coat all surfaces with a durable substance. I use JB Weld epoxy, mixed up and then thinned with alcohol to the point where I can "paint" it on with a tin brush, but I have heard that even a good coating of all surfaces with wood glue will help a great deal.

[Barracuda]

To answer the first q..yes it burnt the end off..answer to the 2nd I understand and will redesign or use blown in insulation(is that what ppl call dog barf).Plus would the fact that the date on my D12-5s' were 6-95,they worked ok just a little swelled on the end about an 1/8" from humidity.Thanks again I've just never seen a sim or equiv. that shows how much pressure or how hot the eject.chrg gets..........now I know