Firing Mechanism

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,

(a) Firing pins (fig. 32) are contained oil

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

Table X shows the length-to-

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.