THE COAST ARTILLERY JOURNAL - Air Defense Artillery

190 THE COAST ARTILLERY JOURNAL
7S-mm. gun, with a muzzle velocity of 550 meters per second, firing H. E. shell
armed with the 24/31 fuze, with the 3-inch, 2600 foot-second, M-1 gun, firing
H. E. shell armed with the Mk. III fuze. Assume a vertical circle struck with
a slant range of 6000 yards from the gun. Then the following tabulation wiII
show the time-of-flight differences of the two guns at that slant range and at
the various altitudes listed:
Altitude
yards
5500
4000
2000
o
Time of flight
seconds
75-mm. gun M-l gun
27.0
23.8
21.2
19.7
14.0
13.8
13.5
13.2
Decrease in
time of flight
seconds per cent
13.0
10.0
7.7
.6.5
If, to make the story complete, the two dead times be added so that predicting
interval rather than time of flight is shown, the results would be:
Altitude
yards
5500
4000
2000
o
Predicting interval
seconds
75-mm. gun M-l gun
35.0 14.5
31.8 14.3
29.2 14.0
27.7 13.7
48
42
36
33
Decrease in
predicting interval
seconds per cent
20.5 59
17.5 55
15.2 52
14.0 51
.If the improvement in fire, as represented by these percentages, seems large,
as indeed it is, consider further that our own IOS-mm. gun, with a velocity of
2800 foot-seconds and a heavier projectile than the 3-inch gun, has a predicting
interval that varies from about 10 per cent less at low altitudes to over 40 per
cent less at high altitudes, as compared with the M-l gun. In time of war the
IOS-mm. gun would be fired at 3000 foot-seconds which would result in still
more of a decrease.
These enormous time improvements do not affect the probabilities of gunfire-they
do affect vitally the probability of prediction. In time of war an
aviator is not constrained to adhere to the predicted course and there is no
power available that enables us to predict on the aviator's mind. However, the
shorter the predicting interval, the less opportunity the aviator has to change
his mind and his course before the prediction culminates in a group of bursts.
For this reason the decreases in time mentioned above wiII be probably the
greatest factor, in the event of war, in producing an increased number of hits
over past records.
A second decided materiel progress is in the perfection of Case III methods
of fire by means of follow-the-pointer dials for laying the gun in azimuth and
elevation. Captain K. M. Loch, R. A., in his 1927-28 "Duncan" Gold Medal
Essay (Journal of the Royal Artillery, July 1, 1928) states that "experts abroad
regard it as a method at least 100% advance on any other." Captain Loch, an
inveterate student of antiaircraft matters and a chronic winner of the "Duncan"
essay, is cautious about going that far himself, but he strikes the nail squarely