40. Firing Mechanism. a. General. The firing
by the piston, discontinuity at the trunnion,
mechanism initiates the primer which ignites the
and bending effects. Provision must be
propellant in a device. Firing mechanisms are classified
made for stopping the piston at the end of
into three general types: (1) gas operated, in which the
its stroke. A common means of stopping
driving force for the firing pill is derived from gas
the piston is to provide an interference fit
pressure from an initiator or by-pass port, (2)
on the last portion of travel of the piston.
mechanically operated in which the firing pin is driven
(3) In a catapult or remover, the body or
by a compressed spring, and (3) electrically operated, in
housing is referred to as the outside tube.
which electric current fires it special primer directly.
This member is provided with a complex
(1) Firing pins.
closure at the one end, which includes
trunnions, firing and release mechanisms,
and a cartridge. A simple cap closes the
both gas and mechanically operated
other end. The design principles involved
firing mechanisms, and their design
are similar to those described for a
is critical. Binding of the firing pin in
thruster, except that bending forces
its guide must be avoided, and one
developed during the stroking may be
method of achieving this is by
significant and tubing sizes may be
maintaining a length-to-diameter
dictated by standard commercial sizes.
ratio of 2 to 1 or more, although
ratios as low as 0.9 to 1 leave been
(4) The body designs of special purpose
used successfully. The surfaces of
devices are not considered since they
the firing pin and guide must be
usually are similar to that of a thruster
finished for protection against
(closed system) or remover (open system)
corrosion and to minimize friction.
already described.
In addition, the tolerances for the
39. Piston. a. The function of a piston in a propellant
clearance between firing pin and
actuated device is to transmit the gas pressure
guide must be as small is possible.
developed in the chamber to the load to be moved. In
some devices the piston is simply a rod (most thrusters),
diameter ratios and the clearances
while in others, one or more tubes may form the stroking
used in some existing devices.
member (catapults and removers).
b. Stresses developed in pistons or moving tubes
Table X. Firing-Pin Ratios and Clearances
are caused by gas pressure and reactive forces resulting
from moving the load. If the load is guided along a track
Length/
Firing pin and
or runway, the stresses in the piston are pure tension or
guide
compression, depending on whether the piston pushes
Device
diameter
clearances
or pulls the load. More involved stresses in pistons or
ratio
tubes result when the load is guided only partially, which
Catapults M3, M4, M5 ................
2.5
0.003 to 0.007.
is the case for most removers and catapults. With
Removers M1, M3......................
2.5
0.003 to 0.007.
partially guided loads, any eccentricity of the load
Initiators M3, M4 ........................
0.9
0.003 to 0.007.
produces bending stresses in the stroking member.
Initiators M5, M6, M10 ...............
1.5
0.002 to 0.006.
Whenever possible, the slenderness ratio (length-to-
Thrusters M1, M2, M5 ................
1.0
0.001 to 0.005.
diameter ratio) of compression loaded designs should
Thruster M3 ...............................
1.5
0.002 to 0.006.
not exceed 20 to minimize bending effects.
c. For pistons loaded in compression, Eulers'
column formulas may be used. The formulas are not
presented in this text since they are dependent on end
conditions which must be established for each
application. For example, a thruster's piston may be
pinned to a load or connected by a trunnion, in which
Figure 32. Firing pins.
cases the column (piston) is considered to have a
pinned connection. Pistons occasionally are threaded
directly into the load; the column here would be
considered to have a "built-in" or "fixed" connection.
38
