CHAPTER 3
BASIC DESIGN
initiator generates gas which flows through the hose to
11. General. a. Propellant actuated devices are
the thruster. When sufficient gas pressure is exerted on
basically simple devices containing a minimum number
the thruster firing mechanism, the thruster cartridge is
of parts. They are light in weight, yet strong enough to
fired. As the propellant burns in the thruster, the
withstand the maximum pressure created by burning the
pressure in the thruster chamber increases and causes
propellant they contain. The materials selected for use
the thruster piston to extend, moving a body (load). A
in these parts are compatible with the propellant, igniter,
curve showing the relationship of pressure to time within
and primer at various temperatures and in the functional
the thruster is presented in figure 18.
and storage conditions to which the parts are exposed.
b. Constant awareness of basic concepts must be
maintained when designing propellant actuated devices.
initiator functions and point B represents the moment
Certainly the most important concept is that of reliability.
the thruster firing mechanism is operated. Point C is the
Determination of how these devices will operate in
time internal gas pressure is first noted in the thruster,
conjunction with other components in a system must be
and point D is the time peak pressure occurs. Point E
established along with a reliable method of initiation and
represents the instant the thruster piston is fully
a simple but sure method of installation.
extended.
c. Standard parts are used wherever possible, and
d. If it is assumed that piston motion starts at point
when special parts are necessary, they are designed so
C (when pressure is first developed in the thruster), the
that they are manufactured easily.
All parts of
actual work cycle of the device extends from point C to
propellant actuated devices are interchangeable
point E. However, thrusters normally have initial locking
between similar units, and under no conditions may the
arrangements (to prevent the piston from extending
functional reliability of a device be dependent upon the
prior to the time the thruster cartridge fires), and the
selective fit of any or all parts. Propellant actuated
initial lock is released when the pressure reaches some
devices are designed for ease of proper assembly and
intermediate point between C and D.
wherever possible, parts are made nonreversible so that
it is impossible to assemble a component backwards.
12. Motion or Time Functions . a. The function time
for propellant actuated stroking devices is the time
interval from initiation of operation to the completion of
a stroke. This interval may vary from as little as a few
milliseconds (some thrusters) to a quarter of a second
(some catapults).
Thrusters containing damping
devices may take more than one-half a second to
complete a stroke.
mechanically operated initiator is connected to a thruster
by a length of hose. When the lanyard is pulled, the
Figure 17. A simple PAD system.
initiator cartridge is fired. The burning propellant in the
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