CHAPTER 3. BASIC DESIGN

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

c. Point A in figure 18 represents the instant the

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.

b. In the system shown in figure 17, a

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