Chapter 4. DESIGN TECHNIQUES

CHAPTER 4

DESIGN TECHNIQUES

Section I. INTRODUCTION

17. General. This chapter provides a basic knowledge

c. Method of initiation.

of the preliminary design of propellant actuated devices.

d. Minimum mounting strength (structural loads).

Methods of approximating parameters not generally

e. Open or closed type system.

given in design requirements are presented. Materials,

f. Initial and/or final lock requirements.

safety factors, and methods of calculating wall strengths

g. Propelled mass.

and selecting tube sizes to be used in propellant

h. Velocity.

actuated devices are discussed.

The design of

individual components of the devices is described, and

i. Maximum acceleration.

the use of protective finishes and dissimilar metals is

j. Maximum rate of change of acceleration.

outlined.

k. Ignition delay.

18. Design Requirements. The customary starting

l. Resisting or assisting forces.

point in the design of propellant actuated devices is the

m. Strokes (must be compatible with h, i, and j

requirements which list in detail the size, weight,

above).

strength, and performance of the device. A typical list

of design requirements includes the following.

n. Special requirements such as bypass and buffer

or damper requirements.

a. Maximum envelope dimensions.

b. Maximum weight.

Section II. METHOD OF FIRST ORDER APPROXIMATIONS

20. Stroke to Separation. a. The requirements for

19. General. Not all significant parameters are defined

catapults, removers, and occasionally thrusters specify

in design requirements. The design requirements may

terminal velocity, v, maximum acceleration, am, and

specify acceleration, rate of change of acceleration, and

velocity, but, not the stroke necessary to satisfy these

maximum rate of change of acceleration, a° The

requirements. Stroke and velocity but not acceleration

stroke, S, of such devices can be estimated by using the

may be specified for thrusters.

The envelope

following equation, the derivation of which is contained

specifications may give exterior dimensions but not the

in chapter 5.

internal volume and expansion ratio nor the propellant

charge and cartridge size. The unspecified parameters

must be determined by the designer in conjunction with

b. The M5 catapult, for example, has the following

the ballistician. Methods of approximating stroke, stroke

performance characteristics when fired at 70° .:

time, working pressures, and propellant charges are

presented here. The last parameter is treated in greater

detail in chapter 5.

† The quantities used here are the actual quantities measured during the development of the M5 Catapult. They are used in

these equations to illustrate the usability of the equations. The acceleration and rate of change of acceleration are usually given as

specified maxima. The values of these two quantities to be used in these equations for preliminary design are chosen by the designer

from experience. Chapter 6 gives step-by-step use of the equations for some examples.