Section III. DESIGN STRENGTH CALCULATIONS

Therefore:

W= 1.4 X 2 X 10-4 X 1,800 X 0.210

W=0.11 inch

The actual web is 0.110 inch.

This equation may be applied to catapults, removers,

and thrusters, remembering that it is only a first order

approximation.

25. Cartridge Case Volume. a. The cartridge case

volume is usually estimated in one of two ways,

depending upon the size of the individual propellant

grains. If the grains are "small" and will be oriented

randomly when loaded into the cartridge case, the

loading density is taken to be about 30 in.3/lb of

propellant. For an example, suppose it is estimated that

8 grams of propellant in the configuration of a cylinder

0.25 inch long by 0.10 inch outside diameter are

required for a thruster application. The case volume

required would then be estimated as:

30 in.3/lb X 8 gm x lb/454 gm = 0.53 in.3

Additional volume also must be provided for the case

cap and igniter charge retainer.

This must be

Figure 22. Curve for approximating propellant

determined after estimation of the igniter charge volume

charge for thrusters.

by a preliminary design of the head cap.

b. If "large"' grains are to be used, they may be

(2) To determine the charge weight for a

loaded iii some definite geometrical armament. The

thruster, calculate the energy expended in

grains are then stacked in the cartridge with their

doing the required work (in inch pounds).

centerlines parallel to the centerline of the case. The

Then find the intersection of that

case volume is then estimated by the size and number

particular abscissa with the curve

of the grains and their geometrical arrangement. This is

representing the propellant to be used,

discussed in section II, chapter 6, in the design example

and the ordinate of that intersection

on the M3 Catapult.

represents the approximate charge (in

26. Igniter Charge. The igniter used in most propellant

grams).

actuated devices developed up to the present time has

24. Propellant Web. The web is the minimum

been black powder. A rule of thumb that has evolved to

thickness of the grain between any two adjacent

estimate igniter charge weight is the use of about 40

surfaces of the propellant and the initial propellant web

grams of black powder per pound of propellant. This

in inches, w, may be estimated from equation (6):

estimated igniter charge may have to be increased or

w=1.4 C'Ptm

(6)

decreased depending upon results of firings between -

where C'=pressure coefficient of linear burning rate of

65°F. and 200°F.

the propellant (of the order of 2 X 10-4 in./sec/psi). As

before, P is peak pressure (1,800 psi) and tm is stroke

time (0.210 sec), for the M5 catapult.

Section III. DESIGN STRENGTH CALCULATIONS

commonly are used. Critically stressed port ions of

27. General. a. In most propellant actuated device

components of propellant actuated devices are designed

applications, minimizing the weight of assemblies is a

so that material is used efficiently.

primary consideration. For this reason, materials which

possess a high strength-to-weight ratio, such as heat-

b. Resulphurized steels are never used, since

treated alloy steels and high-strength aluminum,