The dP/dt Failure in Propellants and Ignition Compositions
In 1962 I tested initiators for Thiokol at our lab in Sunnyvale, CA.
In firing these initiators at altitude in a vacuum chamber, the first unit
expelled the 2A pellets (B/KNO3/Polyester Resin) without igniting them.
This was a serious problem indicating that the second stage Pershing would
not ignite! The Thiokol engineer, Sam Zeman, happened to be on site. Confronted
with the problem he offered that the test conditions were unfair and that
the initiator should be fired into a closed volume, as it would be on the
missile. We added a plastic cap to the threaded initiator output and they
al fired successfully. Intrigued by the problem, I fired similar units
with black powder output charges. They scattered black powder into the
chamber without ignition.
The phenomenon is generally referred to as a ždP/dt FailureÓ. This refers
to the fact that a sharp pressure drop occurs, either at ignition or even
during burning. Why does that cause the material to quench?
One theory of solid propellant burning is the Summerfield Theory. In
that theory several layers are proposed which exist in burning propellant.
The first layer is the unburned propellant, next is the heated material,
then the products of vaporization or sublimation, then the combustion of
the vapors. The last layer is luminous (at least in the infrared) and is
pumping heat back into the other lower layers.
During an abrupt lowering of pressure, the vapor layer expands suddenly
lifting the luminous combusting layer to a greater standoff from the lower
layers. This stops heat transfer or minimizes it and the propellant ceases
to burn as vapor stops forming.
This phenomenon was useful in early rocket motor research as grains
could be ejected from the motor by failing the nozzle attachment. The ejected
grain would quench due to the dP/dt change and the partially burned grain
could be examined.
We built a detaching nozzle for UTC. On the first test I stood by at
one of the blockhouse windows. About 2 seconds into the burn the nozzle
blew, being fired by a timer, the grain ejected into a pile of wet hay.
It was still violently combusting. This is to say that not all propellants
will quench when undergoing a drop in pressure. This particular propellant
was highly aluminized. It is possible that the increased radiance from
the luminous zone was enough to keep up combustion.
What to do about the problem when using igntion material which are sensitive
to dP/dt? Confining the propellant until all burnt (in the case of an ignition
charge) would be one solution to the problem. Slowing the pressure drop
another. Using materials which are not sensitive to pressure drops would
be the best solution.
Unsubstantiated by any testing, I would recommend materials which have
high metals content. Mag/Teflon would be my first choice. T-1, Copper Oxide/Titanium
would be another. There is some evidence that indicates that ignitercord
or thermalite is not sensitive to pressure drops.
A good field for testing. Anyone out there willing to do the experiments?