Antiaircraft Command - Directors and Height Finders

DIRECTORS

Mathematically, the antiaircraft problem was complicated. A gun could not beaimed simply at an enemy aircrafts present position. The plane would be longgone by the time the projectile arrived at that point. Antiaircraft artillerymenneeded a way to calculate where a hostile airplane most likely would be in the timeit would take their shell to reach that area. The projectile needed to be fired at aparticular point in space ahead of the airplane so, if all went well, the explosive andaircraft 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 throughthree dimensions of space at 300 miles per hour conjured the specter of algebra and trigonometry. The dilemmawas further complicated by the facts of real world artillery. Antiaircraft shells were subject to the effects ofatmospheric conditions, gravity and other basic laws of physics. These calculations were beyond the scope of mostservicemen. Besides, who had time to work math problems or consult firing tables when a hostile aircraft wasthreatening? Enemy airplanes needed to be engaged immediately and the window of opportunity was too short topermit time for the slide rule. Yes, good gunners could develop their abilities and learn to lead their prey, just like an adept duck or pheasanthunter. Indeed, many automatic weapons gunners (using 37mm, 40mm and .50 caliber guns) became quite skilledat 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 timedfuzes, direct fire sights would not work. Besides, antiaircraft doctrine demanded all possible accuracy from the bigguns on the first shot. By the time the detonation of that first round was observed, the engagement could well behalf over. There was little time to adjust fire visually. Fortunately, the heavy antiaircraft guns had available anaccurate technological solution - the director. A squad and their M7 director providing firingsolutions for a gun battery. Note the cabling running outthe right side of the pit.

On the left, the fuze dead time setter is adjusting forthe amount of time that elapses before a round is firedafter its fuze has been cut (1-4 seconds). In front of him,the elevation (far) and azimuth (near) trackers peerthrough elbow telescopes and follow a target by turningsmall handwheels at the base of the director. The rangeand altitude setters are standing at the front of the M7 tomake any needed corrections to ensure accuratetracking. The sergeant on the field phone is most likelythe chief of this batterys range detector section. Thedirector squad itself would commonly be under thecommand of a corporal.

It is interesting to see how this director crew kept theirmess kits and canteen cups handy, stored neatly andconveniently along the far wall of the pit.

Antiaircraft Command - Directors and Height Finders http://www.antiaircraft.org/directors.htm

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The director was essentially a mechanical computer designed to calculate firing solutions for and transmitpointing data to antiaircraft guns. Although this may seem quite advanced for the time period, the first antiaircraftdirector was adopted by the U.S. Army in 1917. Of course, during the interwar period and throughout the durationof the World War II, director technology continued to advance. By the later part of the war, electromechanicalcomputers 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 theenemy aircraft was determined or estimated, then entered into the director. Two observers would then track theaircraft through a pair of telescopes on opposite sides of the device. These trackers kept the airplane image on theirrespective crosshairs by turning handwheels, thereby providing the director with data on the aircrafts change inelevation and azimuth in relation to the director. As the mechanisms inside the director responded to the rotation ofthese handwheels, a firing solution was mechanically calculated and continuously updated for as long as the targetwas tracked. The director essentially predicted future position based on the aircrafts present location and how itwas moving. Directors soon incorporated correction factors that could compensate for ballistic conditions such asair density, wind velocity and wind direction. If the director was not located near the guns, a correction for parallaxerror could also be entered into the device to produce even more accurate calculations.

Directors transmitted three important solutions to the gun. The properfiring azimuth and quadrant elevation were calculated to determinewhere exactly to aim the gun. Also, since 3-inch and 90mm guns usedrounds with timed fuzes, the director supplied the flight time for theprojectile 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 oftentelephoned to the gun crew. Soon a system was developed to send thefiring parameters through cables to pointer displays on the gun by meansof selsyn motors. The gun crew would lay their piece simply by matchingtheir guns pointers to the data coming from the director. By 1942,directors were able to able to train the gun by a motorized remote controlsystem and transmit fuze times to an automatic fuze setter, thus speedingthe process and increasing firing accuracy.

Matching pointers on a 3-inch gun in the1930s. Technology would only improve.

As radar technology developed at a seemingly exponential rate during World War II, its increasing capabilitieswere incorporated into director fire control technology, eventually rendering unnecessary visual observation of atarget. M4 AND M7 DIRECTORS The M4 and M7 fire control directors provided solutions for 3-inch and 90mm antiaircraft guns. The twodirectors were very similar in appearance and capability, with the M7 having a better ability to track diving orclimbing aircraft. The M7 also incorporated a correction for dead time, which is the interval between when aprojectiles fuze time is set and when the round is actually fired. In normal employment, a single directorsimultaneously provided firing data to all four guns of a battery. The M7 required a squad leader and fiveoperators, while the M4 managed with one less soldier. (The updated A1B2 versions of both directors required anadditional two men.) As with all directors, these boxes need to be oriented and calibrated properly. When handledcorrectly and provided with smooth tracking data by the operators, the M4 and M7 produced very satisfactoryresults.


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Crew from the 2d Coast Artillery demonstrating the M4 director to a crowd of engineers, industrialists and Army officers gathered atAberdeen Proving Ground in the fall of 1939. Note the substantial pedestal needed to support the directors 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 picturedon 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, productsof the Sperry Corporation, were capable of acceptingpresent angular height, altitude/slant range, and azimuthdata 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 M7was also modified with mechanisms designed to smoothinput data and enable calculation of more accurate futureposition information for gun laying. However, even thesedevastatingly effective new enhancements wereunsatisfactory by the end of the war as aircraft speed andcapabilities improved. The Army was soon requestingsystems capable of tracking aircraft flying at speeds up to1,000 miles per hour. (Right) M7 director squad at work in the New Guinea heat. Inside thebox was an array of gears, cams and differentials that mechanicallyreplicated gun firing tables and calculated a firing solution faster thanany human gunner could.

(Left) Detail of the parallax correction dialfrom the right side panel of an M7 director. The settings in this photo indicate that thedirector is about 79 yards north and 205 yardswest of the directing point of the gun battery. The up/down dial specifies the director is 30yards above the guns. Distances south, westand down were marked in red and the others inwhite. This offset adjustment allowed thedirector to calculate firing data as if it wereobserving aircraft from the geometric center ofthe four gun battery, even though the box waslocated 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.


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"Handle your director as gently as you do your watch. Care for it as thoroughly as you do your car. Rely on it asassuredly 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 werealso 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. TheM5 was much less complex that its heftier cousins for several reasons. Since automatic weapons used shells thatdetonated on contact, fuze time calculations were not required. The M5 was always set up within 15 feet of theweapon it controlled, so parallax correction was unnecessary. Furthermore, this model of director was not capableof receiving range data from external optical height finders or radar sets. Predictably, the M5 had fewer externalcable connections and was operated by a smaller crew of just three men. An M6 version of the director wasmanufactured 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 internalcams. Unlike the M4 and M7 directors that provided firing data for a four-gun battery, the M5 was designed totrain a single gun by remote control, necessitating one director for each gun.

M5 director (left) and M5A2 (right) for fire control of AAautomatic weapons. The large cylindrical device on the M5A2 is anintegrated range finder, that essentially performed the same functionas the height finder in a gun battery range section. This directormoved its gun by a remote control system, so there was no pointermatching required. The M5 weighed about 500 pounds and wasprized 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 inNorth Africa enthusiastically lauded the director and soldiers placed "great confidence" in the box. Otherautomatic weapons units were frustrated because they were barely able to emplace their director and gun beforereceiving orders to move. By the time First Army was ready to invade Europe on D-Day, that organizationsantiaircraft commanders decided to leave their M5 directors in England for the assault. First Army stressed that afield army required highly mobile antiaircraft units. They opined that the 500-pound M5 directors, along with theaccompanying generators and remote control equipment, had a great potential to slow the movement of 40mm fireunits, thus offsetting any gains in firing accuracy. Therefore, First Army opted for on-carriage control and favoredthe British Stiffkey Stick for gun aiming. Other commands, on the contrary, used the M5 director variants withvarying degrees of satisfaction for the duration of the war. As the war progressed, the M5 directors wereincreasingly set aside in favor of on-carriage fire control devices such as the M7 computing sight, also known asthe Weissight. However, it was common practice for an AA battery to have a portion of their 40mm guns utilizethe directors while the remaining pieces operated on sight control.


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Emplacing the M5 director for use was no easy task. Thecombined weight of the director and tripod exceeded 625 pounds.

Five men, often assisted by a sixth, were required to carry thedirector and set it on the tripod, as illustrated in this photo on theleft. Two porter bars or carrying poles were inserted intobrackets on opposite sides of the director to facilitate handling.

After the director was properly seated on the tripod, cableconnections were made to the fire units generator and to theguns remote control system. The soldier pictured on the rightside of this photo is busy unwinding the appropriate cables andpositioning their end receptacles at the proper places for quickand easy hookup.

When the director, generator and remote control wereoperational, the soldiers in this photograph assumed other dutiesserving 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 requiredoperators to estimate the slant range to an enemy aircraft, as the director was not designed to receive such datafrom an external source. The accuracy of the calculated firing solution depended on the skill of the humanestimator and his ability to adjust his approximation by observing a tracer stream. To offset this disadvantage, anM5A2 model was developed that mounted an integral 30-inch optical base range finder designed to feedcontinuous present slant range data to the director. This was essentially a miniature height finder attached to thedirector. Unfortunately, the improved device only made it to a handful of battalions before the war concluded. Unlike a gun battalions director, the M5 could not accept present position data from radar sets. This madeunseen 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 fieldtests 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 werestate-of-the-art four-component "computing systems" for use withheavy antiaircraft guns. The two directors were nearly identical,with the M9 being designed for 90mm guns and the M10 intendedfor the larger and more powerful 120mm piece. Developed by BellLabs, this next generation of director was electromechanical indesign and utilized a tracker that was similar in appearance to thetraditional director. However, instead of firing solutions beingcalculated by a mechanical model inside this box, present positiondata was sent electrically to another component - the computer(pictured at right). This computer received position information inthe form of DC voltages, performed the required calculationselectronically, then transmitted the solution data by selsyn motorsto the four guns of the battery. The other components of the M9 system were a power rectifierand an altitude converter. The rectifier converted AC power fromgenerators or commercial sources to the DC voltages required bythe computer. The altitude converter was necessary only when anoptical height finder or the earlier SCR-268 radar set was used toprovide altitude or slant range for the system, since both devices


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were incapable of transmitting the necessary information by DCvoltages to the M9s computer.

The M9 computer, power rectifier and altitude converterwere transported in the 2-ton canvas covered trailer M13 . The system components are shown here covered and securedfor travel. The computer is on the left. The altitude convertersits atop the power rectifier at the front of the trailer. Thecomponents did not have to be removed from the trailer foroperation. Another version of this trailer, the M14, wascompletely enclosed with steel sidewalls and a solid top. Sidewidows and a heater were other M14 upgrades.

M14 trailer from Battery A ofthe 125th AAA Gun Bn. The unitkept track of their antiaircraft"score" on the trailers walls.

(Edgar Lanham photo, used bypermission.)

Although this new system could be integrated with older ranging equipment, the true advantages becameevident when the M9 was tied directly to a radar set, especially the SCR-584 which was developed concurrentlywith 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 cloudceiling or at night without illumination from searchlights. It was the first light from the dawn of a new day in airdefense.


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Artists rendering of an M9 setup showing all of thesystems components. This equipment would be dug inand camouflaged in an actual combat position.

The optical tracker (seen in the left corner) resemblesa standard mechanical director. Firing computations,however, were calculated by the electrical computer inthe trailer. The tracker and optical height finder (in thecenter of the drawing) could be replaced by a radar setthat would send all position information directly to theM9s computer. In fact, this arrangement allowed ThirdArmy to never use its optical height finders in Europe andrecommend that the devices be eliminated from theequipment tables.

HEIGHT FINDERS

One vital piece of information required by allfire control directors was the altitude or slantrange of an enemy aircraft, a variable necessaryfor its antiaircraft firing calculations. The directoritself was unable to determine altitude, so anexternal ranging device was essential. Developedin the 1920s, the telescope-like optical heightfinder was the solution. As the war progressed,altitude information was increasingly provided byradar. However, even though the new radiotechnology was more accurate and less limited byvisibility than the optical method, height finderscould still be useful. Once the radar had assistedthe director crew in getting "on target" visually,the optical height finder could be switched in toprovide altitude data while the radar was thenfreed to search for other targets.

The Armys war-era M1 and M2 height finders were stereoscopic range finders, working on the same principlethat enables a pair of human eyes to perceive depth. An image of the enemy aircraft was produced by two lenses13.5 feet apart. This image was matched to a reticle by the stereoscopic observer (at his center observation positionon the telescope) until both airplane and the reticle appeared to be at the same distance in his viewfinder. Thereadings at this point accurately calculated the range to the target. Range was converted to altitude by simpletrigonometry. The observed range (slant range) was the length of the hypotenuse of a right triangle. The verticalleg of this triangle corresponded to the aircrafts altitude. The elevation of the height finder (angular height)provided the angle needed to solve for altitude. The height finder continuously resolved the calculation andautomatically 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 ofthe telescope was the responsibility of the heightfinder sergeant, who directed a squad of five men. As a much needed part of the range section, theheight finder squad traveled with the director crew.

Standing on the far side of the telescope (fromleft to right) are the azimuth tracker, thestereoscopic observer, and the elevation tracker. Onthe near side of the finder is the altitude setter. Thesergeant standing next to him is providinginstruction on the operation of the device.

The height finder was filled with dry helium gas tocombat moisture and prevent excessive stratification of thegases inside the telescope, which could induce an error dueto light refraction. In hot and sunny climates, a canvassunshade was placed over the finder to shelter the gas insidefrom overheating and possibly introducing errors fromthermal-induced stratification. In extremely cold weather, athermostat-controlled electric cover could be fitted aroundthe instrument to keep the precision mechanisms fromfreezing up. Neither cover interfered with the operation ofthe height finder. Here, the height finder squad of Battery B, 745th AAAGun Battalion has the end pieces of their canvas sunshade inplace as they wait for action in the Pacific Theater. Theheight finder sergeant is standing at his proper position ofstereoscopic observer in this photo.

Altitude or Slant Range? Altitude, of course, is simply how high the target airplane is flyingabove the ground. Slant range is the straight line distance from the gunposition to the aircraft. Altitude was preferred in calculating firingsolutions because it was the variable least likely to change when anattacking aircraft was making an approach to its target. However, heightfinders had difficultly determining altitude for low-flying aircraft (below1,650 feet) or distant aircraft low on the horizon. In those instances, slantrange was substituted. It was the responsibility of the sergeant in charge ofthe height finder to inform the director crew whether he was transmittingaltitude or slant range data to the director.


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A detailed view of an M1 height finder from the altitude setters position. The elbow telescope to the left is forazimuth tracking. The eyepiece mounted on top of the finder to the right is the main viewing assembly used bythe stereoscopic observer. The rectangular window on the front of the telescope displays the measuring drumwhich 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 devicebelow 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

The SCR-547 was a hybrid radar-optical unitdesigned to provide directors with altitude orslant range. The target aircraft first needed to bespotted in the optical scopes of the SCR-547,where it was visually tracked in azimuth andangular height. Once the enemy aircraft wasproperly sighted, the unit transmitted a narrowradio pulse on a 10 centimeter wavelength. Thispulse reflected off the target aircraft back to theSCR-547. Slant range was determined by theround trip time of the pulse. Altitude data wascalculated and sent to the director by a repeater. The SCR-547 could measure a maximumaltitude near 30,000 feet, but this was dependenton the aircraft being visible to the trackersthrough the units telescopes at that range. 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 bothseen 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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Antiaircraft Command - Directors and Height Finders

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