A link to the document explaining CATOs and temperature cycling for black powder
I've stored it many places, including the YahooGroup for our local club. I can
cut & paste
the text if that helps....
Here is why temperature cycling can cause Catastrophic failures (CATOs). The
propellant and the casing all expand and contract at different rates. Since the
so small, this is only a problem if the temperature that the motor "sees" swings
wide extremes. When this happens, we see several effects:
1) The propellant and the clay nozzle develop a crack at their interface. This
results in *Lower*
peak pressure and peak thrust because the motor can begin the
end-burning earlier than it should (never forming the "big dome" of burning
that we should get at normal peak thrust).
2) The casing and the propellant can de-bond. They aren't really bonded in a
but the mechanical bond is weakened from the stretching and contraction. (For
motors, there may be a tiny glue-like "bonding", but the cycling will break that
The flame can propagate along the entire inside of the casing and propellant
result in a huge overpressure. This leads to a casing split (if the delay is
"grabbing" the casing tightly) or a "blow through" which is like a Roman Candle.
The two of these can combine to form different CATO scenarios:
a) Blow through at ignition or just after ignition (on the pad/rod). Clearly a
sign of a
nozzle/propellant interface crack allowing the flame front to reach the debonded
propellant interface at or just after ignition.
b) CATO above the pad (like 50 feet up). Clearly there was no crack along the
propellant/nozzle interface and the flame front had to wait until it naturally
casing wall and then propagate up the de-bonded propellant/casing interface.
A final scenario is the cracked propellant grain. These can go BLAM (or KA-PLOW)
spectacularly since they really overpressurize the casing big-time and can
happen with a
perfect casing to propellant bond. A defective tool used to form the centerbore
propellant can cause these. The C5-3 had such a problem when a tool was
I believe the root cause was a lack of radius on the tip, which formed a sharp
led to cracking. Motors also could be cracked if any contaminant got on the tool
or in the
propellant during ramming, but dropping or rattling will not cause a crack!
As for the temperature cycling - avoid firing a motor at a temperature 75
degrees F lower
than the highest temperature it has ever seen. If fired while too cold, the
will be contracted away from the casing and it will probably fail. Folks
launching in cold
weather can do so if they store their motors in their warm car or in their
inside pockets. (Is that an F100 in your pocket or are you just happy to see me?)
Why would a normally stable rocket fly unstable when using a motor that it flew
Did you look at the nozzle? We have had several VERY scary "flights" where the
little thrust and/or veered into cruise missile mode. After crashing and putting
brush fire, we examine the nozzle and find that it is either too wide (wider
at the throat) or it is eroded asymmetrically. The asymmetric erosion is bad and
clearly see the exhaust residue all over the missing area of the nozzle
indicating that it
disappeared at ignition or shortly thereafter.
All unstable flights with Estes motors from years "A" and "B" and maybe "C" need
inspected and if the motor/nozzle is the cause, a M.E.S.S. form filled out and
manufacturer notified. The least that will happen is a package of replacement
motors and a
kit. The most that will happen is an improvement in materials used in
manufacturing and a
product that performs like we remember for decades and decades.
I hope this info helps folks.
-Fred Shecter NAR 20117
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