ignite before the bullet begins to move. Refer to my earlier articles, The 6mm Shortly &Bringing the Short Fat Case to 1000-Yard Competition.
Correspondences
and Distinctions between Cartridges and Solid-Fuel Rocket Engines
Now,
some facts, and a bit of conjecture about comparisons between solid-fuel
rocket engines and cartridge cases. First, propellant granules differ
dramatically. Typical rocket engines use no more than a few granules
(called grains), which are typically designed to burn one at a time,
with constant energy production, and thereby to maintain constant
chamber pressure as produced gases jet through the exhaust orifice,
with the only significant impediment being the nozzle throat; typical
cartridges use many hundreds to many thousands of granules which,
as demonstrated above, produce best performance when all have ignited
before the bullet begins to move and which are designed to push against
a relatively heavy exhaust (bore) impediment (the bullet) and where
practical barrel length is limited.
These factors lead to dramatic differences in burn times and pressures rocket engine burns typically last many seconds and generate pressures of several hundred to several thousand psi small arms cartridge burns typically last less than 0.002 seconds and generate breech face pressures that can exceed 75,000 psi. However, I see a more important distinction.
In a rocket engine, trapped air pockets can be devastating. There, such pockets often result in secondary ignitions that multiply total combustion area and thereby catastrophically skyrocket chamber pressure remember all those spectacularly explosive failures in the early US space program?
In the cartridge, trapped-air pockets are unavoidable but are also relatively tiny, except, perhaps, in a partially filled case. At issue is a factor related to latent heat of compression, which manifests as a compression-related temperature increase within interstitial voids as volume shrinks, gas molecules move faster (temperature increases). Compression of pockets, large or small, to any given pressure generates the same pocket temperature. In rocket engines and cartridges alike, pocket temperature often exceeds kindling point. However, total heat depends upon pocket volume.
Critically, it takes both heat and temperature to achieve ignition in adjacent granule surfaces. As heat is transferred from the hot gas to the cool granule, the gas cools. If the pocket is too small, it simply will not contain enough energy to heat adjacent surfaces sufficiently to cause ignition, despite the extremely high temperatures that can occur within the compressed gas voids. By and I contend that such heating occurs so rapidly that although an exothermic reaction could be initiated in the surface layer, the underlying cool layers will quench the reaction before sufficient additional heating can occur to result in a sustained reaction as was discussed above. (Precision Shooting's resident "Rocket Scientist", Randolph Constantine, also points out that much or all of the heat in such voids could be dissipated in the endothermic heat absorbing decomposition of the adsorbed anti-oxidants in the surface layers.)
Thus, in a normal cartridge load, the myriad, relatively tiny, gas pockets seem unlikely to cause secondary ignitions. However, it is quite certain that this effect will expedite subsequent granule ignition. When those pre-heated surfaces are finally exposed to the combusting propellant cloud, hotter areas will ignite faster, which is a critical factor. My point here: Owing to issues of scale, not all rocket-to-cartridge comparisons are equally applicable but the basic concepts do cross over.