DIRECTORS

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.        

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.

     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.

     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. 

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.

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.

(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.

	(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.
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. 

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.

     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.

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.

     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

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.

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.)

     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.  

Artist's rendering of an M9 setup showing all of the system's components.  In an actual combat position, the equipment would be dug in and camouflaged.      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.

HEIGHT FINDERS

     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 various models of radar.  In fact, Third Army AAA organizations never used their optical height finders in Europe and recommended their elimination.  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 while the radar was then freed to search for other targets.

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.         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.

     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.  

RADAR-OPTICAL HEIGHT FINDER

     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 round trip time of the pulse.  Altitude data was calculated and sent to the director by a repeater.  The unit could measure a maximum altitude near 30,000 feet, but this was dependent on the aircraft being visible.  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. 

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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