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DIRECTORS
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Mathematically, the
antiaircraft problem was complicated. A gun could not be aimed
simply at an enemy aircraft's present position. The plane would be
long gone by the time the projectile arrived at that point.
Antiaircraft artillerymen needed a way to calculate where a hostile
airplane most likely would be in the time it would take their shell to
reach that area. The projectile needed to be fired at a particular
point in space ahead of the airplane so, if all went well, the explosive
and aircraft would arrive near the same position at the same time. |
This was not a simple
proposition. When pen was put to paper, the movement of an airplane
flying through three dimensions of space at 300 miles per hour conjured
the specter of algebra and trigonometry. The dilemma was further
complicated by the facts of real world artillery. Antiaircraft
shells were subject to the effects of atmospheric conditions, gravity and
other basic laws of physics. These calculations were beyond the
scope of most servicemen. Besides, who had time to work math
problems or consult firing tables when a hostile aircraft was threatening?
Enemy airplanes needed to be engaged immediately and the window of
opportunity was too short to permit time for the slide rule.
Yes, good gunners could develop
their abilities and learn to lead their prey, just like an adept duck or pheasant hunter. Indeed, many automatic weapons gunners
(using 37mm, 40mm and
.50 caliber guns) became quite
skilled at firing on aircraft at shorter ranges using gun sights and adjusting their
shooting by observing a stream of tracers. However, because of the much longer
reach of the bigger guns and their use of projectiles with timed fuzes,
direct fire sights would not work. Besides, antiaircraft doctrine
demanded all possible accuracy from the big guns on the first shot.
By the time the detonation of that first round was observed, the
engagement could well be half over. There was little time to adjust
fire visually. Fortunately, the heavy antiaircraft guns had
available an accurate technological solution - the director.
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A squad and their
M7 director providing firing solutions for a gun battery. Note the
cabling running out the right side of the pit.
On the left, the fuze dead time setter is
adjusting for the amount of time that elapses before a round is fired
after its fuze has been cut (1-4 seconds). In front of him, the
elevation (far) and azimuth (near) trackers peer through elbow telescopes
and follow a target by turning small handwheels at the base of the
director. The range and altitude setters are standing at the front
of the M7 to make any needed corrections to ensure accurate tracking. The sergeant on the field phone is most
likely the chief of this battery's range detector section. The
director squad itself would commonly be under the command of a corporal.
It is
interesting to see how this director crew kept their mess kits and canteen
cups handy, stored neatly and conveniently along the far wall of the pit.
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The director was
essentially a mechanical computer designed to calculate firing solutions
for and transmit pointing data to antiaircraft guns. Although this
may seem quite advanced for the time period, the first antiaircraft
director was adopted by the U.S. Army in 1917. Of course, during
the interwar period and throughout the duration of the World War II,
director technology continued to advance. By the later part of the
war, electromechanical computers were able to calculate highly accurate
firing solutions directly from data provided by
radar sets.
Directors, from the very
first models, were very similar in basic operation. The altitude or
slant range of the enemy aircraft was determined or estimated, then
entered into the director. Two observers would then track the
aircraft through a pair of telescopes on opposite sides of the device.
These trackers kept the airplane image on their respective crosshairs
by turning handwheels, thereby providing the director with data on the
aircraft's change in elevation and azimuth in relation to the director.
As the mechanisms inside the director responded to the rotation of these handwheels, a
firing
solution was mechanically calculated and continuously updated for as long
as the target was tracked. The director essentially predicted future
position based on the aircraft's present location and how it was moving. Directors soon incorporated correction
factors that could compensate for ballistic conditions such as air
density, wind velocity and wind direction. If the director was not
located near the guns, a correction for parallax error could also be
entered into the device to produce even more accurate calculations.
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Directors transmitted three important solutions to
the gun. The proper firing azimuth and quadrant
elevation were calculated to determine where exactly to aim
the gun. Also, since 3-inch and
90mm guns
used rounds with timed fuzes, the director supplied the flight time for the
projectile so the fuze could be set to detonate at the proper point in
space. With the very first generation of directors, the firing solution
was often telephoned to the gun crew. Soon a system was developed to
send the firing parameters through cables to pointer displays on the gun
by means of selsyn motors. The gun crew would lay their piece simply
by matching their gun's pointers to the data coming from the director.
By 1942, directors were able to able to train the gun by a motorized remote
control system and
transmit fuze times to an automatic fuze setter, thus speeding the process
and increasing firing accuracy.
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Matching pointers on a 3-inch gun in
the 1930s. Technology would only improve. |
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As radar technology
developed at a seemingly exponential rate during World War II, its
increasing capabilities were incorporated into director fire control
technology, eventually rendering unnecessary visual observation of a
target.
M4 AND M7 DIRECTORS
The M4 and M7 fire control
directors provided solutions for 3-inch and 90mm antiaircraft guns.
The two directors were very similar in appearance and capability, with the
M7 having a better ability to track diving or climbing aircraft. The
M7 also incorporated a correction for dead time, which is the interval
between when a projectile's fuze time is set and when the round is actually fired.
In normal employment, a single director simultaneously provided firing data to all four guns of a
battery. The M7 required a squad leader and five operators, while
the M4 managed with one less soldier. (The updated A1B2 versions of both
directors required an additional two men.) As with all directors, these
boxes need to be oriented and calibrated properly. When handled
correctly and provided with smooth tracking data by the operators, the M4 and M7 produced
very satisfactory results.
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Crew from the 2d Coast Artillery
demonstrating the M4 director to a crowd of engineers, industrialists and
Army officers gathered at Aberdeen Proving Ground in the fall of 1939.
Note the substantial pedestal needed to support the director's weight of
over 700 pounds.
Many M4 directors were eventually upgraded to include
refinements found on the later M7. Compare these photos to the M7
units
pictured on this page. You should
immediately notice the similarities between the two models. The M7
was adopted in November 1941.
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As the war unfolded, the M4
and M7 directors, products of the Sperry Corporation, were capable of
accepting present angular height, altitude/slant range, and azimuth data
from radar units SCR-268,
SCR-545 and SCR-584. This advancement made firing on
unseen targets possible, much to the detriment of many enemy aircrews.
The M7 was also modified with mechanisms designed to smooth input data and
enable calculation of more accurate future position information for gun
laying. However, even these devastatingly effective new enhancements
were unsatisfactory by the end of the war as aircraft speed and
capabilities improved. The Army was soon requesting systems capable
of tracking aircraft flying at speeds up to 1,000 miles per hour.
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(Right) M7
director squad at work in the New Guinea heat. Inside the box was an
array of gears, cams and differentials that mechanically replicated gun
firing tables and calculated a firing solution faster than any human
gunner could. |
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(Left) Detail of the parallax correction
dial from the right side panel of an M7 director. The settings in
this photo indicate that the director is about 79 yards north and 205
yards west of the directing point of the gun battery. The
up/down dial specifies the director is 30 yards above the guns.
Distances south, west and down were marked in red and the others in white.
This offset adjustment allowed the director to calculate firing data as if
it were observing aircraft from the geometric center of the four gun
battery, even though the box was located a few hundred yards away.
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Other corrections that could be set on the
M7 directors included adjustments for non-standard ballistic conditions,
such as air density. Wind was an important consideration. Wind
direction and speed were resolved into north-south and east-west
components and entered into the director through the dial shown in the
photo on the right. |
"Handle your director as gently as you do your
watch. Care for it as thoroughly as you do your car. Rely on
it as assuredly as you do your rifle."
Antiaircraft Artillery Field
Manual FM 4-136
M5 DIRECTORS
Automatic weapons units had the
option of several types of on-carriage sights for their weapons.
They were also allocated a smaller, simpler, and somewhat lighter director for
off-carriage fire control of these guns. Standardized in January
1941, the M5 director was an Americanized version of the British Kerrison
Predictor. The M5 was much
less complex that its heftier cousins for several reasons. Since
automatic weapons used shells that detonated on contact, fuze time calculations
were not required. The M5 was always set up within 15 feet of the weapon it
controlled, so parallax correction was unnecessary. Furthermore, this model of
director was not capable of receiving range data from external
optical height
finders or radar sets. Predictably, the M5 had fewer external cable connections and was
operated by a smaller crew of just three men. An M6 version of the
director was manufactured for use with British guns and equipment.
The M5 director could be
set up for use with either the 37mm or
40mm antiaircraft guns by changing internal cams.
Unlike the M4 and M7 directors that provided firing data for a four-gun
battery, the M5 was designed to train a single gun by remote control,
necessitating one director for each gun.
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M5 director (left) and M5A2 (right) for
fire control of AA automatic weapons. The large cylindrical
device on the M5A2 is an integrated range finder, that essentially
performed the same function as the height finder in a gun battery range
section. This director moved its gun by a remote control system, so
there was no pointer matching required. The M5 weighed about 500
pounds and was prized by some units, but eventually set aside by others.
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During stateside training, it was stressed that
the M5 director improved firing accuracy by a 5:1 ratio. Naturally,
its use was emphatically encouraged. In the field, however, feelings
were mixed. Early reports from AAA units in North Africa
enthusiastically lauded the director and soldiers placed "great
confidence" in the box. Other automatic weapons units were
frustrated because they were barely able to emplace their director and gun
before receiving orders to move. By the time First Army was ready to
invade Europe on D-Day, that organization's antiaircraft commanders
decided to leave their M5 directors in England for the assault. First Army stressed
that a field army required highly mobile antiaircraft units. They
opined that the 500-pound M5 directors, along with the accompanying
generators and remote control equipment, had a great potential to slow the
movement of 40mm fire units, thus offsetting any gains in firing accuracy.
Therefore, First Army opted for on-carriage control and favored the
British Stiffkey Stick for gun aiming. Other commands, on the contrary, used the M5
director variants with varying degrees of satisfaction for the duration of the war. As the war
progressed, the M5 directors were increasingly set aside in favor of on-carriage fire control devices such as the M7 computing sight, also known as the Weissight.
However, it was common practice for an AA battery to have a portion of
their 40mm guns utilize the directors while the remaining pieces operated
on sight control.
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Emplacing the M5 director for
use was no easy task. The combined weight of the director and tripod exceeded
625 pounds.
Five men,
often assisted by a sixth, were required to carry the director and set it
on the tripod, as illustrated in this photo on the left. Two
porter bars or carrying poles were inserted into brackets on opposite sides of
the director to facilitate handling.
After the director
was properly seated on the tripod, cable connections were made to the
fire unit's generator and to the gun's remote control system. The
soldier pictured on the right side of this photo is busy unwinding the
appropriate cables and positioning their end receptacles at the proper
places for quick and easy hookup.
When the director,
generator and remote control were operational, the soldiers in this
photograph assumed other duties serving the 40mm cannon or their
accompanying machine
guns.
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One of the chief criticisms
of the M5 and the self-synchronous M5A1 design was that the director required operators to
estimate the slant range to an enemy aircraft, as the director was not
designed to receive such data from an external source. The accuracy of the
calculated firing solution depended on the skill of the human estimator
and his ability to adjust his approximation by observing a tracer stream. To
offset this disadvantage, an M5A2 model was developed that mounted an
integral 30-inch optical base range finder designed to feed continuous present
slant range data to the director. This was essentially a miniature
height finder attached to the director. Unfortunately, the improved device
only made it to a handful of battalions before the war concluded.
Unlike a gun battalion's
director, the M5 could not accept present position data from radar
sets. This made unseen firing and night firing impractical for
automatic weapons units, except under very unusual circumstances. A
new T25 experimental radar director for automatic weapons was in
development, but was just undergoing field tests by a single AAA group
during the waning stages of the war.
M9 AND M10 DIRECTORS
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Appearing in the spring of 1943,
the M9 and M10 units were state-of-the-art four-component "computing systems" for use with heavy
antiaircraft guns. The two directors were nearly identical, with the
M9 being designed for 90mm guns and the M10 intended for the larger and
more powerful 120mm piece.
Developed by Bell
Labs, this next
generation of director was electromechanical in design and utilized a
tracker that was similar in appearance to the traditional director.
However, instead of firing solutions being calculated by a mechanical
model inside
this box, present position data was sent electrically to another component
- the computer (pictured at right). This computer received position information in the
form of DC voltages, performed the required calculations electronically, then transmitted
the solution data by selsyn motors to the four guns of the battery.
The other components of the
M9 system were a power rectifier and an altitude converter.
The rectifier converted AC power from generators or commercial sources to
the DC voltages required by the computer. The altitude converter was
necessary only when an optical height finder or the earlier
SCR-268 radar set was used to
provide altitude or slant range for the system, since both devices were
incapable of transmitting the necessary information by DC voltages to the
M9's computer.
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The M9 computer, power rectifier and
altitude converter were transported in the 2-ton canvas covered trailer
M13 . The system components are shown here covered and secured for
travel. The computer is on the left. The altitude converter
sits atop the power rectifier at the front of the trailer. The
components did not have to be removed from the trailer for operation.
Another version of this trailer, the M14, was completely enclosed with
steel sidewalls and a solid top. Side widows and a heater were other
M14 upgrades. |
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M14 trailer from
Battery A of the 125th AAA Gun Bn. The unit kept track of their
antiaircraft "score" on the trailer's walls.
(Edgar Lanham
photo, used by permission.) |
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Although this new system
could be integrated with older ranging equipment, the true advantages
became evident when the M9 was tied directly to a radar set, especially
the SCR-584 which was developed concurrently with the M9. In this
scenario, the tracker and altitude converter were completely bypassed, the
cabling simplified, and the system provided with a continuous stream of
easily digested, accurate position information from the radar.
Visual spotting of an aircraft was no longer necessary. Targets
could now be engaged from below a low cloud ceiling or at night without
illumination from searchlights. It was the first light from the dawn
of a new day in air defense.
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Artist's
rendering of an M9 setup showing all of the system's components.
This equipment would be dug in and
camouflaged in an actual combat position.
The
optical tracker (seen in the left corner) resembles a standard mechanical
director. Firing computations, however, were calculated by
the electrical computer in the trailer. The tracker and
optical height finder (in the center of the drawing) could be
replaced by a radar set that would send all position information
directly to the M9's computer. In fact, this arrangement
allowed Third Army to never use its optical height finders in Europe
and recommend that the devices be eliminated from the equipment
tables. |
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HEIGHT FINDERS
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One
vital piece of information required by all fire control directors was
the altitude or slant range of an enemy aircraft, a variable necessary
for its antiaircraft firing calculations. The director itself
was unable to determine altitude, so an external ranging device was
essential. Developed in the 1920s, the telescope-like optical
height finder was the solution. As the war progressed, altitude
information was increasingly provided by radar. However,
even though the new radio technology was more accurate and less
limited by visibility than the optical method, height finders could
still be useful. Once the radar had assisted the director crew in getting "on target" visually,
the optical height finder could be switched in to provide altitude
data while the radar was then freed to search for other targets.
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The Army's war-era M1 and
M2 height finders were stereoscopic range finders, working on the same
principle that enables a pair of human eyes to perceive depth.
An image of the enemy aircraft was produced by two lenses 13.5
feet apart. This image was matched to a reticle by the stereoscopic
observer (at his center observation position on the telescope) until both
airplane and the reticle appeared to be at the same distance in his
viewfinder. The
readings at this point
accurately calculated the range to the target. Range was converted
to altitude by simple trigonometry. The observed range (slant range)
was the length of the hypotenuse of a right triangle. The vertical leg of this
triangle corresponded to the aircraft's altitude. The elevation of
the height finder (angular height) provided the angle needed to solve for
altitude. The height finder continuously resolved the calculation
and automatically sent the data to the director by means of a cable
connection.
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A squad trains on
an M1 height finder at Ft. Sheridan. The proper operation and
maintenance of the telescope was the responsibility of the height finder
sergeant, who directed a squad of five men. As a much needed part of
the range section, the height finder squad traveled with the director
crew.
Standing on the
far side of the telescope (from left to right) are the azimuth tracker,
the stereoscopic observer, and the elevation tracker. On the near
side of the finder is the altitude setter. The sergeant standing
next to him is providing instruction on the operation of the device.
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The height finder
was filled with dry helium gas to combat moisture and prevent
excessive stratification of the gases inside the telescope, which
could induce an error due to light refraction. In hot and sunny
climates, a canvas sunshade was placed over the finder to shelter the
gas inside from overheating and possibly introducing errors from
thermal-induced stratification. In extremely cold weather, a
thermostat-controlled electric cover could be fitted around the
instrument to keep the precision mechanisms from freezing up.
Neither cover interfered with the operation of the height finder.
Here, the height finder squad of
Battery B, 745th AAA Gun Battalion has the end pieces of their canvas
sunshade in place as they wait for action in the Pacific Theater.
The height finder sergeant is standing at his proper position of
stereoscopic observer in this photo. |
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Altitude or Slant Range?
Altitude, of course, is
simply how high the target airplane is flying above the ground. Slant range
is the straight line distance from the gun position to the aircraft.
Altitude was preferred in calculating firing solutions because it
was the variable least likely to change when an attacking aircraft
was making an approach to its target. However, height
finders had difficultly determining altitude for low-flying aircraft
(below 1,650 feet) or distant aircraft low on the horizon. In
those instances, slant range was substituted. It was the
responsibility of the sergeant in charge of the height finder to
inform the director crew whether he was transmitting altitude or
slant range data to the director.
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A detailed view of
an M1 height finder from the altitude setter's position. The
elbow telescope to the left is for azimuth tracking. The
eyepiece mounted on top of the finder to the right is the main
viewing assembly used by the stereoscopic observer. The
rectangular window on the front of the telescope displays the
measuring drum which indicates the altitude or slant range of the
target as calculated by the height finder. The altitude setter
sends this reading to the director crew by turning the small wheel
on the transmitter, which is the box-like device below and to
the the right of the drum window. The handwheel is turned
until the reading on the transmitter dial matches the indication on
the measuring drum. |
RADAR-OPTICAL HEIGHT FINDER
SCR-547
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The SCR-547 was a hybrid
radar-optical unit designed to provide directors with altitude or
slant range. The target aircraft first needed to be spotted in
the optical scopes of the SCR-547, where it was visually tracked in
azimuth and angular height. Once the enemy aircraft was properly
sighted, the unit transmitted a narrow radio pulse on a 10 centimeter
wavelength. This pulse reflected off the target aircraft back to
the SCR-547. Slant range was determined by the round trip time
of the pulse. Altitude data was calculated and sent to the
director by a repeater. The SCR-547 could measure a
maximum altitude near 30,000 feet, but this was dependent on the
aircraft being visible to the trackers through the unit's telescopes
at that range. |
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SCR-547
in operation on Espiritu Santos, New Hebrides |
Since the
SCR-545 and
SCR-584 radars were able to send all present
position information to directors for both seen and unseen aircraft, the
advantages of the SCR-547 height finder were short lived.
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Front and rear views of the SCR-547 height
finder.
One circular reflector antenna was for
transmitting and the other for receiving.
The SCR-547 was often called "Mickey
Mouse", for obvious reasons.
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