maximum pressure is recalculated. Sotc that the
PIMP is the effective pressure from the poiiit cf
maximum pressure back to the breech. Artillery
tube designers currently compute the values for
PIMP curve directly without first resorting to the
computed pressure-travel curve.
i)3. Hasically the design approach of recoillcss
weapons is similar to that for artillery tubes, hut
refinements based on statistical treatment of results
of actual firing data are then introduced. These re-
finements arc necessary due to the demand for
minimum weight while still retaining safety in firing.
Therefore, following the preliminary weapon design,
a test giiii is produced, having the desired interior
contour, but with wall thicknesses substantially in-
creased over those calculated as being necessary.
With this gun a propelling charge is established to
give the desired muzzle velocity with gun aiid charge
at 70 °F. A sufficient number of rounds arc fired
to give adequate statistical data for establishing the
Normal Operating Pressure (NOP) which is the
average of pressures measured in a test barrel firing
an established round to produce the rated muzzle
velocity with the temperature at 70°F. The NOP
then replaces the CMP for final prototype weapon
design and the giui tube pressure design curve is
revised accordingly.
Permissible Individual Masimum Pressure
(PIMP) is determined more exactly than for artil-
lery tubes. R'ith the firing data from the test weapon,
and with the NOP established, a new design pres-
sure curve is plotted. The methods for computing
NOP and PIMP appear later with the detailed
discussion on design procedures for the respective
tubes.
ill. for small arms tube design, pressures for com-
puting wall thickness are obtained from pressure-
travel curves already established by firing tests
or computed by interior ballisticians. As with re-
coillcss and artillery tubes, the maximum design
pressure extends back to the breech. Chamber pres-
sures corresponding to the position of the projectile
are used rather than the actual pressure at the base
of the projedile. The1 pressure at any point in the
bore by being somewhat lower than in the chamber
introduces a small factor of safety for stress com-
putation.
2. Strength Requirements
95. Having determined the pressures which will
act along the various scctioiis of the gun tube the
required wall thicknesses can now be determined
for entire tube length. For this purpose, an allow-
able stress is assigned, above which no part shall be
stressed. This figure must not be greater than the
elastic limit of the material at the temperature
existing in the material. An exception involving
small arms is discussed later (Section 10a). The wall
thickness so determined will be minimum arid may
be increased for handling or machining reasons. The
allowable stress assigned to a particular tube de-
pends on the nature of its service. For relatively slow
fire artillery weapons it is the clastic limit at 70°F,
which is defined as being 10,000 psi less than the
yield poiiit. For recoillcss tubes, it is necessary to
consider the temperature attained in the tube
material aiid desigii the tube on the basis of the
yield point at that temperature. Figures öG and 58
give properties of steels used in gun tubes at all
temperatures of interest. As discussed later in
Section 10a, small arms tubes ha^e been assigned
an allowable stress of 75,000 psi for approved steels.
90. As a convenience in design, ihe term Elastic
Strength Pressure (ESP) has been established. It
is defined as the pressure which produces an equiva-
lent stress in the section wall equal to the allowable
stress of the giui tube material at 70°F. In a gun
tube of changing cross section then- is an ESP
corresponding to each type of section. For recoillcss
gun tube design an additioiial term is introduced,
called ESP,,,,,, the clastic strength pressure of a
hot tube. Formerly its value of 0.80 ESP was used
to compensate1
for the degradation' of material
strength at elevated temperatures. Now it is the
gas pressure at a point in the tube which produces
an equivalent stress at that point equal to the yield
point of the material at the permissible maximum
tube temperature.
Kith more experience and additioiial design data,
a more exact approach is available for the design
of prototype recoillcss gun tubes. Two conditions
make this possible. The first is an ESP,,ol computed
from the PIMP that is based on the anticipated
chamber pressure for firing at I2.")°F, the upper
limit of the ambient temperature range for recoillcss
gun opera! ioii. The second is the known degraded
yield strength of the tube material at elevated tem-
perature, Yh. The allowable maximum tube tempera-
ture is usually made to correspond to a given rate
of fire or the allowahlc maximum temperature is
specified and the rate of fire adjusted to it. The tube
is designed in accordance with the reduced strength
of the material at these elevated temperatures.
39
