velocity and acceleration can be controlled most
effectively. Without acceleration control, the maximum
velocity normally occurs at time E, the end of the stroke.
i. Acceleration and rate of change of acceleration
of a device are controlled by the selection of internal
ballistic parameters. Occasionally, further control is
effected by the addition of a buffer or damper. External
dampers were used in earlier propellant actuated
devices, and internal dampers have been used
successfully in several recently designed thrusters.
thruster. The spring acting against the floating piston is
compressed or extended as the buffer fluid reacts to
temperature changes. When the thruster is fired, the
expanding gas drives the floating piston against the
fluid, exerting pressure on the main piston. The main
Figure 18. Pressure-time curve for system of
piston begins its stroke when the pressure buildup is
sufficient to shear the locking pin (fig. 5). The fluid
surrounding the main piston is then forced through the
e. Every effort is made to minimize the time from
orifice into the volume between the floating piston and
point A to point C. The exception to this is in the design
the main piston. The velocity of the main piston is a
of the delay initiator. The time from point A to point B is
function of the viscosity of the buffer fluid, the orifice
increased intentionally to establish a specific sequence
area, and the difference in force due to the same
of operations. The delay function of the initiator is a
pressure acting against a large area on one side of the
major consideration in designing elaborate systems.
main piston and on a considerably smaller area on the
The span of time from B to C is referred to as ignition
opposite side.
delay (the interval between actuating the firing
j. Motion is not only controlled by grain design and
mechanism and the beginning of sustained rise of
by the addition of dampers but may also be controlled
pressure in the propellant chamber).
ballistically by metering the flow of propellant gas
f. The C to D span may be the most important for
through an orifice. A high-low system is an example of
it is during this pressure rise time that such important
ballistic control. In a high-low system, the propellant is
performance characteristics as rate of change of
burned in a chamber and the gas is bled from this
acceleration are determined. Furthermore, this C to D
chamber into a much larger chamber through an orifice.
span affects the selection of propellant, propellant
Thus, the propellant is burned at high pressures (which
geometry (perforations and web, etc.), internal volume,
are conducive to good burning) while the stroke is
and expansion ratio (the ratio of final internal volume to
controlled by the low pressure gas in the large chamber.
initial internal volume). The time from C to D varies
k. A pressure relief valve can also be used to
from a few milliseconds for releases and initiators to 100
control the motion of a propellant actuated device
or more milliseconds for some catapults. The ballistic
ballistically by dumping the gas that would cause
design chapter 5 of this manual discusses this important
excessive acceleration. Throughout the stroke, the
interval in detail.
valve opens and closes to maintain a nearly constant
g. Peak pressure (point D) also is important since
pressure within the device.
it determines maximum acceleration, charge weight,
13. Load. The load experienced by the piston of a
and the working pressure the unit must withstand. This
thruster is the total of all forces acting on the thruster.
working pressure affects the piston size, the wall
Propellant actuated devices may be subject to loads
thickness, material selection, and overall weight.
which assist as well as resist motion. These loads
h. Finally, the interval from point D to point E
include the inertia forces of the mass of the body being
represents the remaining time required to complete the
propelled and the moving parts of the propellant
piston stroke. Most of the piston movement occurs
actuated device, initial and final locks (if used), friction
during this interval, and so it is during this time that
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