are direct functions of the rate of change of pressure and maximum pressure within the system. The
problem then is to obtain the desired pressure-time curve in the devices by proper selection of propellant
and propeller grain design.
(2) Ballistically there are several limiting factors in the selection of an "optimum" propellant charge design.
The mechanical design and ignition characteristics of catapults and removers limit the degree-to which
grain geometry and propellant formulations can be varied to obtain the desired pressure-time curves.
When the igniter burns, the propellant cartridge ruptures and grains travel down the inner tube and strike
the head cap where some are randomly fractured. Also, ignition of the grains is not instantaneous nor as
uniform as desired. In addition, the selection is limited by the charge geometries that are available in the
desired size. Because of the small required webs and the very low charge weight, and therefore small
overall grain dimensions, only certain configurations are commonly extruded and available for
development and production purposes.
(3) Two basic grain designs have been used for the most part in propellant actuated devices.
These are uninhibited, single-perforated grains and uninhibited, seven-perforated grains.† Propellants with
high burning rate exponents can be supplied as single perforated grains. As the surface-time history of an
uninhibited, single-perforated grain is approximately constant, the increasing rate of gas production is
caused by an increase in the burning rate of the propellant as the pressure in the propellant actuated
device builds up. Plateau propellants with low burning rate exponents can be supplied as uninhibited,
seven-perforated grains. The pressure rise in the propellant actuated device does not cause as large an
increase in the burning rate of seven-perforated propellant as does the increasing surface area of the grain
(made available as the web decreases). This increasing surface area provides the increase in the rate of
gas evolution.
(4) Other grain designs may be usable in propellant actuated devices, but may not be feasible from production
and cost standpoints. These designs could include externally inhibited single or multiperforated grains or a
grain in the shape of a right triangular prism externally inhibited except for a small surface along the three
edges of the sides. It is also possible to combine several propellants and grain designs within one charge.
(5) The development of the desired surface-time history for a given application is discussed later in this
chapter.
b. Use of Experimental Pressure vs Time Curve to Refine Propellant Charge Design. Four aspects of the pressure-
time curve are of prime importance in any experimental pressure-time curves obtained from any propellant actuated
device. They are ignition delay, rate of pressure rise, peak pressure, and the integral of the pressure-time (P-t) curve.
These four aspects are not independent of each other, and any modification of one to improve its ballistic characteristics
may result in modification of one or all of the remaining aspects.
(1) Ignition delay.
(a) The aim in design of most devices is to make the ignition delay as short as possible. (This may not
be true in some devices when a delay of a few seconds is necessary for proper sequencing of
operations, but in this case, a delay train is incorporated into the device.) The ignition delay time is a
function of the conditioning temperature, i.e., as the conditioning temperature is lowered, the ignition
delay increases. For this reason, any modifications to the ignition system to minimize ignition delay
usually are tested fit the low temperature operating conditions.
(b) The following are factors which may affect ignition delay time:
Primer: total energy or rate of energy release.
propellant in the case; size of the orifice between the primer and igniter.
† A single-perforated grain is a cylindrical grain with a single perforation along its axis. A seven-perforated grain has one
perforation on the axis and six additional perforations equally spaced on the common radius around the center hole.
58
