CHAPTER 6
DESIGN EXAMPLES
Section I. GENERAL
the M4 initiator. The M3 catapult was one of the first
56. Purpose. This chapter provides examples of the
gas-actuated catapults to be standardized, and it is still
design of stroking devices and a gas-generating device
used in operational aircraft. The M3A1 thruster is a
which use the principles discussed in preceding
typical thruster with a bypass port at the end of its
chapters.
stroke. The M4 initiator is a delay initiator.
57. Scope. The devices chosen to illustrate these
principles are the M3 catapult, the M3A1 thruster, and
Section II. M3 CATAPULT
58.
General.
The M3 catapult is a three-tube
a. Stroke Length.
telescoping device, designed for upward ejection of a
(1) Using equation (1) or figure 20 and the
350-pound seat-man combination from an aircraft in the
values
specified
in
the
design
event of an emergency.
requirements, the approximate stroke
59. Design Requirements. The original requirements
length
necessary
to
meet
the
for the M3 catapult specified the following performance
requirements may be found.
The
and physical characteristics:
maximum stroke is often specified, since
a. Performance Characteristics.
the airframe does have a limit to the
maximum stroke which may be guided
Operating temperature range .. -65 to 160 †
F
and it is undesirable to attempt to eject an
Velocity ....................................... 80 fps min
aircraft seat without guiding its path out of
Acceleration...................................20 g max
the aircraft. If the maximum stroke is not
Acceleration change rate .............. 250 g/sec
specified, it may be arrived at in the
Weight propelled vertically .................350 lb
following manner: The overall length of
b. Physical Characteristics.
the catapult is 50 1/2 inches (specified),
Overall length ........................ 501/2 in. max
so the maximum stroke which may be
Structural loads:
obtained with a single stroking tube is less
Tension .....................................2,000 lb
than 50 inches. Referring to figure 20, it
Compression .............................6,000 lb
is obvious that a 50-inch stroke would
Firing mechanism .................. Gas actuated
acquire an acceleration greater than 20 g
Pressure to actuate........................ 1,200 psi
(the maximum acceleration tolerable to
Mounting:
the human body in upward ejection). The
Airframe .............. Trunnion at upper end
maximum stroke which may be fitted in
Seat ................ Male clevis at upper end
the overall catapult length specified using
two
stroking
tubes
would
be
60. First Order Approximations. Stroke length, stroke
approximately 90 inches, assuming that
time, peak pressure, charge weight, and grain geometry
each moving tube is 45 inches long (the
are calculated in the manner described in section II,
remainder of the
to the equations presented in chapter 4.
† At the time the M3 catapult was developed, the temperature range was -65 F. to 160 F. The upper limit of
later devices has been raised to 200F.
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