THE PRACTICABILITY AND APPLICABILITY OF HIGH BURST
RANGING WITH THE 75-MM. GUN AND 155-MM. GUN
I. GENERAL REMARKS
HIGH bursts ranging is a comparatively recent development of a method
of adjusting fire upon a target which is hidden from the available
observation posts of the battery delivering the fire. As far as the writer has
been able to ascertain, from the rather limited amount of writings upon this
subject, nothing of this sort was used in the United States army before the
World War. The Germans seemed to have given this matter some attention
before the war or at least shortly after its beginning. This method of
adjusting fire was effectively used by them upon the allied forces. The
British and French, following the lead of the Germans, early in the war
developed effective methods of adjusting fire by high bursts, and in turn
these methods were adopted by the artillery of the American Expeditionary
Forces after the arrival of our army in France.
Several different methods have been developed and all give satisfactory
results within certain limitations of range and deflection. Some of the
earlier methods required rather elaborate organization and the use of
special instruments whose position were accurately located by surveying
methods. This in turn required personnel of special technical training. In
some cases the use of charts was required. These necessitated the services
of draftsmen and required considerable time and painstaking labor to
prepare.
Later development has had for its purpose the simplification of methods
of high burst ranging and these efforts have been in a large degree,
successful.
The greatest obstacle to simplicity has been the lack of a shell fitted
with a time fuse. The need for such a fuse is felt for other reasons than
that of high burst ranging alone. Most artillerymen agree without
question, that such a fuse would be of great value in many situations.
From the restricted point of view of high burst ranging alone, a time fuse
which when assembled with the shell gave it the same ballistic properties
as the percussion fuse, would be most desirable and give the highest
degree of simplicity in its employment. This thought leads naturally to
that of a combination time and percussion fuse. Whether or not such a
development takes place, is in the hands of the Ordnance Department. Further
474 PRACTICABILITY AND APPLICABILITY OF HIGH BURST RANGING
discussion of this particular point at this time and place is not desirable or
necessary.
As matters stand with regard to ammunition at present available, all
high burst adjustment must be made with shrapnel. If the nature of the
target requires the use of shell in the ensuing fire for effect, then it is
necessary to perform certain computations to determine the corrections
which must be applied to the firing data in order to secure the proper
trajectory for the shell.
II. METHODS OF ADJUSTMENT
It now appears best to describe and analyze the methods of high burst
ranging. No attempt will be made to discuss all known methods. The
methods developed at The Field Artillery School are in all respects the
simplest methods so far used and discussion will be limited to them. One of
the greatest advantages of these methods, three in number, is that all the
means for their accomplishment exist within a battalion of field artillery. At
times it might be advantageous to secure the aid of a flash ranging battery
but this is not at all necessary. This brings to mind the fact that any method
of fire adjustment which is independent of agencies outside of the battalion,
is of great value. At times it is necessary to enlist such aid, as that of the air
corps for instance. In theory coöperation between separate arms or even of
separate units of the same arm should offer no difficulty, but all will admit
that in practice such perfect coöperation is often difficult to realize.
Another advantage of high burst ranging is that it can be employed at night
with almost the same ease as in daytime. In hazy weather, excepting of
course when vision is completely impossible, except for very short
distances, it may be easier to adjust at night than in the daytime. This is
because a bright flash will show up well in the dark when white smoke may
be almost invisible in a light gray haze.
In order to keep clearly in mind just exactly what high burst ranging is,
it may not be amiss, here, to give a clear, concise definition of it. High
burst ranging is simply a special application of the principles governing the
use of an auxiliary target. The auxiliary target, in this case, is a point in the
air determined originally by the intersection of the lines of sight of two
observing instruments. This is the basis of all methods here discussed. The
various methods differ only in the manner of computing the corrections to
be applied after the adjusting point has been determined, in order to bring
the fire for effect upon the target.
The selection of the adjusting point is governed by certain
considerations. It must of course be visible to the observers and it
should be so selected as to be well above any obstacles that might be in
the vertical plane containing the line of sight. Its selection
475 THE FIELD ARTILLERY JOURNAL
is also governed by the limitations laid upon all auxiliary or witness targets;
i.e., it must be within 300 mils, in deflection, of the target and its range
must not exceed 4/3 of the range to the target nor be less than 3/4 of this
range.
The point selected may be directly above the target, but to insure
surprise effect it is better to have it at a different range or deflection or
both.
In all cases the position of the observers, that of the gun, the target and
the adjusting point must be accurately located on a firing chart or good map
of a scale of 1:20,000 or greater.
One observer should be posted so that his displacement from the line of
fire as viewed from the adjusting point is not in excess of 100 mils. This
gives him axial observation. The other observer should have a greater
displacement, the greater his displacement the better, up to 1600 mils. This
will reduce any errors made in plotting or in the setting off of the proper
azimuth on his instrument. The axial observer is best located when he is in
prolongation of the line of fire and close in rear of the gun.
Hereafter the axial observer will be referred to as O1 and the lateral
observer as O2.
The target, the base piece, O1, O2, the projection of the adjusting point
upon the horizontal and any reference points that it may be desired to use,
are accurately plotted upon the firing chart.
All the necessary altitudes and ranges can be taken from the map or
determined by topographical means. From these and the plotted reference
points, each observer can be given an angle of site and an azimuth
measured from some easily distinguishable reference point, which will
cause his line of sight to pass through the adjusting point selected. All
firing should be with the gunner's quadrant.
The next step is to obtain bursts within, and preferably near the center
of the field of view of the observers' instruments. When this is done, an
adjustment is obtained by trial fire, followed by improvement fire. O1
observes these bursts for height and direction and O2 for deviation in the
direction of range. Based upon their sensings a mean point of burst is
obtained. Any burst which will unduly influence the location of the mean
point of burst is disregarded and another round fired to replace it. At least
six, and preferably twelve, rounds should be used to determine the mean
point of burst. A more detailed discussion of the foregoing may be had
from Field Artillery School Notes, Book II, Chapter VII. (Revised 1927.) It
does not seem necessary or desirable to go into further detail here. Having
determined the mean point of burst, an adjusted elevation is obtained from it,
as well as an adjusted deflection. These of course apply to the shrapnel fired.
476 PRACTICABILITY AND APPLICABILITY OF HIGH BURST RANGING
The initial range for the shell and fuse it is desired to use, may be found by
one of the methods given below.
1. From the adjusted elevation, subtract the algebraic sum of the
mean point of burst, the false site,* and the actual site of the target. The
range corresponding to this elevation is the initial range to be used with
shell but must be corrected for the site of the target.
The above method as given applies only when the adjusting point
selected is vertically above the target. If it were at a different range and
deflection, the following additional corrections would have to be applied to
the adjusted elevation for shrapnel:
(a) The adjusted elevation must be decreased by the number of mils
necessary to bring the mean point of burst to the same altitude as the guns.
Make a ratio of the range corresponding to this elevation over the map
range to the adjusting point. Apply this ratio to the map range of the target
and then correct it for the site of the target. This will give the elevation to
be used in firing for effect on the target.
(b) In changing the deflection to fire for effect, the shift must be made
from the mean point of burst and must be equal in amount but opposite in
direction to the shift made from the direction of the target to the adjusting
point. This automatically takes care of any deflection corrections
necessitated by the atmospheric conditions existing at the time. If the
adjusting point is at a different range than the target, the difference in drift
for the two ranges must be taken into consideration.
It then becomes apparent that it is simplest to use a point directly over
the target. However, it is often undesirable to do this. The next simplest
method is to select the adjusting point at the same range as the target but
having a different deflection.
The above method is often called the "False Site Method." Its application
is based upon the principle of the rigidity of the trajectory. As is well known,
this principle is not absolutely correct, but holds within very close limits for a
small displacement of the trajectory, especially for flat trajectories.
This method is the simplest and easiest to apply and has given very
satisfactory results with the 75-mm. and 3-inch guns at ranges up to 6000
yards.
2. The second method might well be called the "Corrector Method."
The mean point of burst is determined as in the preceding case. Now
determine the height, in mils, of the mean point of burst above
* False site as used in this article is defined as the vertical angle between the plane
of site of the target and the plane of site of the adjusting point.
477 THE FIELD ARTILLERY JOURNAL
the guns. This is done as follows: First, take the algebraic sum of the false
site and of the site of the target. Second, increase or decrease this sum by
the number of mils the mean point of burst is above or below the adjusting
point. The result will be the number of mils the mean point of burst is
above the guns.
Now from the firing tables take the range corresponding to the quadrant
elevation of the piece. Determine the range midway between this range and
the map range of the adjusting point. With this as an argument, take from
the tables the displacement of the height of burst in mils, for a change of 5
points on the corrector scale. Divide this into the number of mils the mean
point of burst is above the guns. Multiply this result by the number of yards
the burst is displaced in range by a change of 5 points on the corrector
scale, using the same range as argument as was used in taking from the
tables the displacement in height of burst. The result is the distance in yards
beyond the projection of the mean point of burst upon the horizontal plane
through the guns at which the trajectory pierces this plane.
Decrease the range corresponding to the adjusted elevation by the
amount just determined. It will be the initial range for shell. The elevation
for shell corresponding to this range must be corrected for the site of the
target.
This method assumes that the trajectory is a straight line from the mean
point of burst to the point in which it pierces the horizontal plane of the
guns. This of course is not true. This method will always give a correction
which is slightly too great. Some idea of the degree of accuracy of this
method may be gained from the following:
Assume that we have determined that the mean point of burst is 19
mils above the guns and that the map range to the mean point of burst is
4000 yards. The actual elevation of the piece is the elevation for 4000
yards plus 19 mils, disregarding corrections for the moment. This, as will
be seen by consulting the firing tables, is the elevation for 4400 yards.
Hence the distance to the point where the trajectory pierces the horizontal
plane through the gun is 400 yards beyond the projection of the mean
point of burst on this plane. Using a range of 4200 as an argument, we
find that the displacement in height of burst for a change of 5 points on
the corrector is 4.22 mils and that displacement in range for the same
change is 89.8 yards. Applying the rule given above we have (19 ÷ 4.22)
× 89.8 = 404.3 yards approximately. This is very close to 400 yards. If
the interpolation were carried to the nearest tenth of a mil, the result
would have been 407 instead of 404.3 yards. As the probable error at
4000 yards is 17 yards, it is evident that this method gives quite accurate
results.
478 PRACTICABILITY AND APPLICABILITY OF HIGH BURST RANGING
3. The third method of obtaining the initial elevation for the fire for effect
could be called the "Position Correction Method." The procedure is as follows:
Determine the difference in altitude in feet between the adjusting point and
the gun. From the firing tables, take out the position correction that must be
applied in order to cause the trajectory to pass through the point immediately
below it and in the horizontal plane of the gun. Determine the adjusted
shrapnel range and subtract from it this amount. Correct this range by the
position correction for the target and the result will be the initial shell range.
This method is perhaps more laborious than the others but it is apparent
that it is the most accurate. It is based upon position correction tables which
take into consideration all changes in elevation due to position. It does not
introduce the inaccuracies involved by the application of the principle of
the rigidity of the trajectory or those mentioned under the discussion of the
"Corrector Method."
For longer ranges this method would be the most appropriate when the
greatest possible accuracy is required.
It will have been noticed that heretofore no mention of corrections for
meteorological conditions has been made. Such corrections would be
unnecessary if the projectile used in adjustment and fire for effect were the
same. However, when they are different all of the above methods may be
refined by applying meteorological corrections to the adjusted shrapnel
range, using this to find the initial range for the shell and then applying
meteorological correction to this.
When adjusting by high burst, as in all cases when the fire for effect
cannot be observed, a range one fork or one-half fork over and short of the
initial range should be included in the fire for effect. Just what limits
should be used depends upon the accuracy of all the data involved.
Likewise a deflection of 5 or 10 mils on each side of the adjusted deflection
should be included.
The difference in drift between shrapnel and shell should always be
included in the final deflection even when meteorological corrections are
not made.
High bursts may also be used to establish a point which may be used as
a witness target. This method may be of value when the terrain is such that
no good witness target exists upon it, or when it is desirable to have a
witness target that can be used at night. The procedure of establishing such
a point follows:
Assume that an adjustment has been secured upon a target invisible
from terrestrial observation posts and that the adjustment was made
with shell. All arrangement as to observers and all data except that
pertaining to the adjusting point must be known and
479 THE FIELD ARTILLERY JOURNAL
plotted as described before. Increase the elevation of guns and fire a round
of shrapnel with a fuse range corresponding to the adjusted shell range and
with a corrector setting near the middle of the scale. Thereafter change only
the site in order to obtain bursts sufficiently high to be seen by both
observers. The observers should have their instruments referred to some
plotted reference point and when they see the first burst they clamp their
instruments so that the crosshairs intersect upon it. Following this a series
of six or twelve rounds is fired without change of data and the deviations of
each burst recorded. O1 observes for height and direction and O2 for
deviations in the direction of range. The azimuths of both instruments must
be recorded.
When it is again desired to fire upon the same objective the instruments
are set up in all respects exactly as when the referring adjustment was
made, with the exception that corrections are made to place the
intersections of the cross-hairs upon the mean point of burst as determined
in the referring adjustment instead of upon the point of burst of the first
round of that series. From this point the procedure is exactly the same as
that of an original adjustment.
For simplicity's sake the method just described is that for a point
immediately above the objective. It is not, however, necessary that the
referring point be so limited. Any point within the limits of range and
deflection prescribed for a witness target in general may be used.
The necessary changes are quite obvious and nothing further need be
said about them except in one particular. If the range of the referring point
is different from that of the target, it is necessary to obtain the ratio of the
adjusted range to the map range in each instance that the witness point is
used. Under varying weather conditions this ratio will not always be
constant and the ratio used upon one occasion will not of necessity be the
correct one to apply upon a subsequent occasion.
III. APPLICABILITY OF HIGH BURST RANGING METHODS
All of the foregoing description and discussion has been confined to an
exposition and analysis of certain methods of high burst ranging. It is now
necessary to consider when, where and to what type and caliber of gun they
are applicable. The discussion would not be complete without
consideration also, of when the methods could be applied, but also when it
would be preferable to apply them in preference to other methods of
adjustment.
It is an invariable rule that when observed fire can be placed upon a
target, that this kind of fire and no other should be used. By observed fire is
meant fire that can be observed both during adjustment and fire for effect.
480 PRACTICABILITY AND APPLICABILITY OF HIGH BURST RANGING
Unfortunately the conditions which permit of observation of fire are
very often lacking. The best conditions obtain when the individual actually
conducting the fire and immediately commanding the fire unit also does the
observing. Lacking this, the next best condition is that in which the battery
commander receives reports as to location of bursts from some other
individual, such for instance as an auxiliary observer on the ground or an
aerial observer.
There are, however, many inherent difficulties in observation from
airplanes and balloons, particularly in the former case, and at best neither is
entirely dependable over any great period of time. For these reasons then it
is necessary to have some scheme upon which to fall back when all means
of direct observation fail. Such a scheme presents itself in the methods of
high burst ranging.
Again it may be desirable to use high burst ranging methods even when
fire for effect can be observed. This would be particularly so when the
element of surprise is paramount. By adjusting upon a point not directly
over the objective, a rapid burst of fire at an effective range might be
placed upon a target with a highly devastating effect. This would be
especially so in case the fire for effect was conducted with gas shell. The
moral effect in any case would be very great.
The value of the methods of high burst ranging for use at night have
already been mentioned and need no further discussion.
Like all means of conducting fire other than by direct observation by
the battery commander, there are certain essential conditions which must
be fulfilled in order to permit adjustment by high bursts. In the first place,
an accurate topographical map must be available and preferably of a scale
of 1:20,000 or greater. In the absence of this, the means must be at hand for
obtaining the topographical data which ordinarily would be taken from the
map. One essential which would be most often lacking is the necessary
time involved in such procedure. Granted sufficiently accurate
topographical data, all the other essentials would ordinarily be at hand.
These consist of two properly adjusted battery commander telescopes, a
firing chart and communication between observation posts and the battery.
These always exist within a battalion of field artillery.
The desirability of using the same projectile for adjustment and effect
has already been discussed sufficiently. It has been shown also that the lack
of such a projectile is by no means an unsurmountable obstacle. It therefore
follows that any gun which fires both shrapnel and shell can be used for
high burst ranging provided that firing tables containing the necessary data
are at hand. There is also another provision that must be included here
from a practical standpoint. The variation in the height of burst of the
shrapnel must not be so great as to require an excessively large number of
481 THE FIELD ARTILLERY JOURNAL
rounds to be fired in order to determine a mean point of burst with the
desired degree of accuracy and the zone of dispersion must be sufficiently
small to be included within the field of view of the observing instruments.
All these conditions are satisfied by both the 75-mm. gun and the 155-
mm. (G.P.F.) gun. A number of problems have been fired with the former
at The Field Artillery School and the above theoretical conclusions have
been well substantiated in its case.
Only one problem, in which the position correction method was used,
has been fired there with the 155-mm. gun. This was at a range of over
10,000 yards. The center of impact of the fire for effect was 12 yards over
and 2 yards left. This seems to indicate that from a practical standpoint this
weapon is well adapted to the employment of high burst ranging methods.
This confirmatory evidence is scant of course and should be substantiated
if possible by additional problems.
IV. CONCLUSION
Heretofore the subject of high burst ranging has been regarded by many
as a highly involved and complicated process. This is no doubt due to some
of the earlier methods employed and the method used to present them. It
has been shown that satisfactory methods are in existence which are in no
sense complicated. All artillerymen should be conversant with these
methods and understand the relative merits of each so that in any situation
he can determine whether or not to employ high burst adjustment and
which of the methods to use.
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