design for any particular set of components and cartridge performance level. Critical variables include: powder column length-to-diameter ratio, case shoulder angle, bullet mass compared to cross sectional area, bullet base configuration (boattail versus flat base, etc.), effective friction load (neck & barrel), seating depth, case filling ratio and ratio of case volume to bore cross-sectional area. Generally, such a design will represent a unique accommodation to three factors: first, minimization of interior surface area; second, minimization of distance from initiating primer blast to furthest powder granules; third, a suite of shoulder design considerations, which have several critical influences.
Military small arms and artillery designers have some incentive to produce the most efficient case shape. However, that research information is not readily available and, after all (excepting a few weight-critical applications, such as helicopter armament), adding a bit of steel to a gun or material to a cartridge case or powder to a propellant charge is of little relative importance. Furthermore, for most applications, cartridge design must also accommodate handling and chambering considerations.
On the other hand, in rocketry, weight considerations dominate – with current technology, it can cost $20,000 to place one pound of anything into orbit and, critically, every pound added to the engine results in a significant payload loss. In other words, in rocketry, designers have the ultimate incentive to design the most efficient (lightest) engine possible. Those pushing the envelope of cartridge design have precisely the opposite motivation; for them, cost (weight) is not an issue – only results matter.
While considerably more energetic, solid rocket fuel is, for practical purposes, significantly similar to conventional smokeless powder. Furthermore, criteria for designing the most efficient solid rocket engine turn out to be germane to the issue of designing the most efficient and ballistically consistent cartridge case. Before making any general conclusions, and without entering into boring detail, I would like to review this subject and visit a few facts from various discussions between By and myself.
Background, Clarification and a Bit of Review
The
primer blast does several things to the powder charge. First, it drives
the base of the charge forward and it can also create an axial hole
to some depth. Second, it directly ignites some granules, either by
condensation heating (as nascent combustion gases condense onto surfaces)
or through contact heating (as incandescent particulates penetrate
surfaces). Third, through compressive shock, it can significantly
heat granules that do not ignite. Fourth, it can partially fluidize
unignited granules, causing some degree of granule-to-granule fusing
– this significantly changes the nature of individual granules and
the propellant mass.
Adiabatic Heating
While compressive (shock) heating
probably does not lead to direct granule ignition, in some cartridges,
it is of significant importance because, by raising granule temperature,
it can dramatically reduce "ignition delay" – when those granules
are subsequently further heated by the producing energetic propellant
cloud.
Also, during the combustion phase, adiabatic heating of air within the charge mass can significantly heat adjacent surfaces. This heating occurs as initially unignited granules deform plastically. First the smaller openings are sealed, which eliminates porosity. Then gas in the remaining voids is further compressed. As confining pressure increases, void volume shrinks and temperature increases.