Chamber of device: possible source of large energy losses and leakage. If a long ignition delay
is observed in initial testing, it may be minimized by using one or a combination of the
following remedies:
Use a "stronger" primer, i.e., one which will supply more energy in the form of hot gases or one
which will supply the hot gases at a faster rate.
Increase the energy released by the igniter by using a heavier charge or by changing to a
material with a higher impetus.
Use a coarser igniter granulation. (A coarse granulation provides more passageways for gas
flow and decreases the time required to ignite the ignition material.) Arrange the propellant
so as not to restrict the hot gas flow from the igniter.
Increase the strength of the cartridge case so that a higher pressure may be built up before the
case ruptures.
(c) The development of the correct combination of primer, igniter, and propellant charge to minimize
ignition delay while meeting the other ballistic characteristics requires careful consideration of the
factors which affect the delay and application of the proper remedies. The use of standard cartridges
and primers listed in chapter 4 will minimize ignition problems and these should be used whenever
possible.
(2) Rate of pressure rise.
(a) The rate of pressure rise is important in catapult applications because it determines the rate of
change of acceleration, which must be held within certain specified limits. The rate of pressure rise
will probably be highest at the high-temperature operating limit.
As the rate of change of acceleration has little effect on the terminal velocity, the objective is to stay
under the upper limit and not to necessarily match this limit.
(b) The following are factors which will affect the rate of pressure rise:
Ignition system: primer and igniter combination.
Chamber: initial free volume of device.
Mass to be accelerated.
(c) The following are techniques which may be used to lower an excessive rate of change of
acceleration:
Reduce the initial surface of the propellant by changing the grain geometry or by inhibiting some
of the surface of the grains.
Change to a plateau-type propellant or to a propellant composition which has a lower burning
rate exponent (n) over the pressure range obtained in the device.
Use a combination of different propellants or different webs.
Increase the initial free volume of the device.
Lower the peak pressure by increasing the piston area of the device.
For a given charge, a decrease in the mass to be accelerated will result in a lower rate of change
of acceleration.
(d) Usually as a last resort, some type of mechanical damper may be incorporated into the device to
control the rate of change of acceleration. The use of damped thrusters is quite common in some
applications requiring comparatively slow positioning of objects.
The mechanical design of these damped devices is discussed in paragraph 45.
(e) Operation at the low temperature extreme may also cause an excessive rate of pressure rise if the
grains are not restrained; they may be fractured upon striking the head cap of the devices, thereby
exposing excess surface area. A grain trap may be used to restrain the propellant, but care must be
taken in its design to prevent unwanted nozzling of the gasses. On the other hand, nozzling may be
used (a high-low device) to obtain the desired ballistic characteristics.
(3) Peak pressure. The peak pressure is important because it determines the maximum acceleration (which in
catapult applications is fixed by human tolerance limits) and the strength and, therefore, the weight of the
device. The object in this area is to match the
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