Here is my standard BP motor failure explanation document cut & pasted
Here is why temperature cycling can cause Catastrophic failures
(CATOs). The nozzle, propellant and the casing all expand and contract
at different rates. Since the motors are so small, this is only a
problem if the temperature that the motor "sees" swings between wide
extremes. When this happens, we see several effects:
1) The propellant and the clay nozzle develop a crack at their
interface. This actually 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 surface area that we
should get at normal peak thrust).
2) The casing and the propellant can de-bond. They aren't really bonded
in a "glue" sense, but the mechanical bond is weakened from the
stretching and contraction. (For wet rammed motors, there may be a tiny
glue-like "bonding", but the cycling will break that bond). The flame
can propagate along the entire inside of the casing and propellant
interface and result in a huge overpressure. This leads to a casing
split (if the delay is still "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 casing to 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 reached the 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) quite 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 of the propellant
can cause these. The C5-3 had such a problem when a tool was
mis-manufactured. I believe the root cause was a lack of radius on the
tip, which formed a sharp edge, which 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
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 propellant 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 toasty
parka 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 stable with before?
Did you look at the nozzle? We have had several VERY scary "flights"
where the rocket had little thrust and/or veered into cruise missile
mode. After crashing and putting out the brush fire, we examine the
nozzle and find that it is either too wide (wider than normal at the
throat) or it is eroded asymmetrically. The asymmetric erosion is bad
and you can clearly see the exhaust residue all over the missing area
of the nozzle indicating that it disappeared at ignition or shortly
All unstable flights with Estes motors from years "A" and "B" and maybe
"C" need to be inspected and if the motor/nozzle is the cause, a
M.E.S.S. form filled out and the 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
17 years ago