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Discussion in 'Low Power Rocketry (LPR)' started by Hackazit, Aug 10, 2003.
I was emailed about this article once, it made I laugh!
Remember that the cause of most CATOs in black powder rocket motors is caused by "thermal cycling." That is, the motor has been exposed to extremes in temperature, and a debonding of case and propellant has occured. This opens up an additional, microscopic flame front, increases the chamber pressure, and something has to let go. That something is supposed to be the clay nozzle, but sometimes the propellant/delay/ejection charge go flying forward into the rocket, or if you're really unlucky, the casing splits.
Thermal cycling, if extreme, can affect all motors in a pack, but if mild, some motors can be affected, some not.
IIRC the range of temps that qualify for suspicion is 50 degrees fahrenheit. and the cato is most likely to occur at the cooler end of the range. For example, you store your motors in the basement at 60 degrees, but at the launch you leave them in the car all day. Temps can get higher than 110 in a closed car in the South. Outside temp is 80. so you pick a motor, install it in the rocket, and there's a small chance that motor could go boom. The higher the outside temp, the lower the chance.
Lets say you kept those motors till the winter, and you want to fly when the temp is 40 or below. Then you have a much better chance for the motor to blow.
But here comes another variable... if the motor has had enough time to stabilize -- say, a few months between extremes -- you may be ok.
Of course there are other causes for catos -- voids in the propellant caused by paper from the casing getting torn off during ramming by the machine, improper curing of the propellant (it is packed slightly wet and needs to dry a bit) which was the problem with the old Estes E15 motors, exposure to extreme humidity, and a few others.
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