July-August - Air Defense Artillery

The propellant temperature is measured
by use of a special powder temperature
thermometer at each battery p0sition.
Corrections for variations are computed
and applied to the present director
as an effect on muzzle velocity.
Of all of the principal factors affecting
the internal ballistics of a projectile,
only the chamber capacity is unknown,
or is not measured. Assuming uniform
projectile loading, which we should
achieve consistently by use of an automatic
rammer, the chief cause of variation
in chamber capacity is due to wear
and erosion; and wear and erosion of a
gun bore are inherent in any type of
artillery fire. The effect of an increase
in chamber capacity, that is, wear of the
chamber and bore, is to decrease muzzle
velocity.
Since information concerning expected
muzzle velocity is so vital to successful
gunnery, let's see how this information
is obtained now. It could be obtained
from a field chronograph. However,
in nine years of service in antiaircraft
artillery, I have never encountered
the special teams to perform this operation;
so the value of this method in a
practical sense must be questioned. Also,
since wear is constantly changing the
chamber capacity, the problem of chronographing
is a recurring one. Another
method of determining battery developed
muzzle velocity is by analysis of
fire. Application of this technique is far
from universal, however.
Unfortunately, muzzle velocity data
is all too often determined by a process
known as the "educated guess" method.
The difference between this guessed
muzzle velocity value and the true battery
developed muzzle velocity was one
of the greatest of the unknown errors
eliminated by trial fire corrections. If
trial fire is prohibited, then we need another
sound method of determining the
muzzle velocity.
A SoLUTION
IHAVE attempted to develop the idea
that, in reality, trial fire correctigns are
not to eliminate "unknown errors," but
are corrections which, in the main, eliminate
the effects of variations from standard
of conditions affecting the internal
ballistics of a projectile. I have also attempted
to show that of these conditions
which might affect the internal ballistics
of a projectile, all are kept standard, or
variations from standard are known or
can be measured, with the single exception
of that of variation in chamber capacity
due to wear and erosion. It follows
then that trial fire may be substituted
for where necessary by the following
general procedure:
1. Perform all steps of preparation for
fire with the greatest accuracy in
order to eliminate personnel errors.
2. Make available to each battery a
simple "pull-over" type bore gauge
to measure the chamber and bore
diameters.
3. Use tables prepared by the Ordnance
Department to relate a given
chamber measurement to an expected
muzzle velocity. If data for
these tables is not already available,
it could easily be obtained
from experimental firing.
4. From the muzzle velocity values
then available for each gun, select
an average muzzle velocity value to
be applied to the director. It is apparent
that the problem of selecting
the average muzzle velocity would
be greatly simplified by arranging
to have guns of approximately equal
wear in the same battery, and by
keeping wear approximately equal
within the battery by ensuring that
the number of rounds fired from
each gun remains roughly the
same.
FORT BANKS REACTIVATED BY ARMY
5. Correct the average muzzle velocity
value for variations from standard
in propellant temperature and projectile
weight.
SYSTEM AS USED BY ROYAL ARTILLERY
..
THE gunnery system outlined above
has been in use with the British Antiaircraft
Artillery for several years. Corrections
for external ballistics are applied
directly to the predictor as in our Service.
The muzzle velocity setting is dependent
upon the wear of the tube and
the propellant temperature, both of
which are measured. Variations in
weight of projectile are ignored. The
wear of the tube is measured by a pullover
gauge issued to each AAA troop.
The bore measurement is converted to a
muzzle velocity value through reference
to a table in the firing table. If the muzzle
velocities of all four guns do not
cover more than a 25 ftjsec band, the
mean muzzle velocity is taken as the
battery developed muzzle velOCity.If a
gun exceeds this 25 ftjsec limit on muzzle
velocity difference, it is regarded as
an "odd" gun; its muzzle velocity value
is not used in determining a predictor
setting; compensation for this greater
variation is made by special corrections
in elevation and fuze at the gun in a
manner comparable to our calibration
corrections. Once determined, the mean
muzzle velocity is corrected for propellant
temperature, and is then applied to
the predictor.
CONCLUSION
,
Trial
possible.
fire should be used whenever
Since trial fire will frequently be impracticable
in war, the gauge method of
determining battery developed muzzle
velocity should be incorporated into
US antiaircraft artillery teaching as an
alternate means to trial fire in the preparation
fur fire.
Immediate reactivation of Fort Banks at Winthrop, Massachusetts,
was announced by the Department of the Army.
FortBanks, which will be used by antiaircraft artillery units, was declared
excess to Army needs on January 31, 1950.
JULY-AUGUST, 1951 37