Fire-Control Computer for Anti-Aircraft Artillery Using Simplified Geometry, 1938–1950: A So-Called “Angular-Velocity Device”

This document is an English translation of the original German-language article by André Masson.

Developed at the Swiss Federal Arms Factory W+F; Manufactured by HASLER AG, Bern

34 mm Flab Kan 38 W+F

amicale-dca.ch

Where Does the Cannon Aim? — The Basic Problem of Anti-Aircraft Artillery

During the flight time of the projectiles, the aircraft continues to move; one therefore does not aim at the aircraft itself, but at a point in the blue sky where the aircraft will also arrive by the time the projectiles get there: Where is that point?

In standard fire-control computers (approximately 1935–1960), the aircraft is surveyed, often by means of two telescopes (azimuth angle, elevation angle) and an optical range-finder to determine the distance. These data are fed into a mechanical computer, which continuously determines the elements of the flight path (position, altitude, course angle, speed, projectile flight time) and from these calculates the required firing angles of the guns and the fuze time.

There are also simplified versions, the so-called angular-velocity devices, which solve the same task with somewhat reduced precision but by measuring only the angular velocity and the distance. The position of the aircraft, its speed, and its flight path remain unknown. In Germany they were called “Kommandohilfsgeräte” (auxiliary fire-control devices). True fire-control computers were very expensive and always available in too few numbers. For these reasons (or in the event of failure of the main device), the simpler devices were also gladly used.

This article describes the fire-control computer of the 34 mm anti-aircraft cannon from the year 1938.

André Masson

Construction, Operation, and Computational Principles

Kleine Scheidegg, 1941 (Ref. 3, p. 162)

The operator at the telescope follows the aircraft and keeps it in the crosshairs. With both hands he rotates and swings the telescope mounting so that the rotation axis — driven by the fire-control computer — follows nicely along the total, spatially oblique angular velocity of the aircraft. The fire-control computer rotates the telescope autonomously, and at the handgrip the operator commands “faster” or “slower” until the automatically moving telescope follows the aircraft correctly in both direction and speed. The angular velocity is now correctly entered. The entire mechanics of the fire-control computer are contained in the base unit — but part of the computational task is also performed in the gun sight. The second operator (kneeling, the course setter) sets two arrow directions by eye: one arrow is to always point toward the aircraft’s position, the other is to lie continuously parallel to the aircraft’s course. These inputs are made by free-hand estimation with reduced accuracy. A third operator enters the range at a hand crank, which is continuously called out to him by the rangefinder crew (rear right). The photograph is from HASLER (Ref. 3, p. 162). It is not the only time HASLER attempts to make the required operating crew look optically small in photographs (likewise in the text, Ref. 3, p. 163: “…which could practically be operated by a single man”). In a contemporary photograph of the large fire-control computer by HASLER (for the 7.5 cm cannon) (Ref. 3, p. 166), the operator who enters the distances is not shown — but without range data one cannot fire.

The gunner and an assistant at the guns independently set in the gun sight — separately from the fire-control computer — the direction in which the aircraft appears to fly across the sky. A horizontal course of the aircraft, for example, appears in the elevation angle to be rising on approach but falling when departing. Parallel lines in the sight, which can be tilted in inclination, help to quickly set and continuously update the apparent direction of the aircraft.

The fire-control computer calculates, on the basis of the angular velocity and the distance to the impact point, the angular lead, which is transmitted to the gun sight. The sight is offset from the barrel axis by this angle so that the barrel fires ahead by the angular lead. Strictly speaking, to obtain the lead one would have to compute: angular velocity × projectile flight time. Since time never appears in any of the calculations, the device simplifies and computes the product: angular velocity × distance. This works well as long as the projectiles are not strongly decelerated, which is roughly the case at short ranges. The cam profile may also accommodate the deceleration of the projectiles.

The barrel elevation required to compensate for the curvature of the projectile trajectory (aiming higher than the impact point) is also calculated purely mechanically in the sight by means of a cam, based on the combination of the elevation angle and the distance to the impact point (see the illustration further below, page 7).

Finally, the fire-control officer can enter manual corrections at the fire-control computer and directly influence the placement of the shots. The projectiles have no time fuze and must score a direct hit to be effective.

The illustrated fire-control computer is already significantly smaller in weight and volume than the one for the 7.5 cm cannon, which was likewise produced by HASLER. The simpler mechanics result in shorter production time and lower cost.

Operation of the Mechanical Gearing

In Ref. 1, the mechanical gear diagram is reproduced with only brief explanations of how it functions.

Normal text: How it must work — most of this is interpretation. Italic text: Still unknown and not understood. Underlined: These details are stated as such in Ref. 1.

The firm HASLER had more than ten years of experience in telephone technology. This is evident in the fire-control computer for the 34 mm cannon: many tasks are solved with an impulse technique that HASLER had long mastered — since the replacement of the “telephone ladies” by new automatic switching exchanges. Abbreviation used only in this text: TIT = Telephone Impulse Technique (Telefon-Impuls-Technik).

In the subsequently also HASLER-built large fire-control computer GAMMA-JUHASZ-HASLER for the 7.5 cm cannons there are indeed many electromagnetic switches, limiters, etc., but no TIT for data transmission.

Overview of the Mechanical Gear Diagram (from Ref. 1, p. 143)

Prominent in the center: the stepped gear for setting the angular velocity. The gear set on the left rotates at constant speed; the gear set on the right controls the telescope F via the worm at the top, which follows the aircraft at variable angular velocity. Upper right, lower left: two control groups that convert impulses received via TIT into gear movements. Strong magnets pull gear wheels away; spring force (not shown, presumed) returns them when de-energized. Lower right: the slender cam body determines, from the final firing range and the measured angular velocity, the angular lead, which is reported to the guns via TIT. Upper left: two cam bodies (FK) are used to compute a range correction from the elevation angle (aircraft) and the angular velocity (upper FK), and from the distance to the lead point and the angular velocity (lower FK), respectively: how much must be added to or subtracted from the rangefinder distance to obtain the distance to the impact point? For this, the aircraft’s motion must be known — i.e., at what angle it is approaching or receding from the gun position. This is provided by the two manually operated course arrows (see exterior view), whose engagement in the diagram is difficult to see, to the right of the upper cam body, in circles 14 and 15. The finished range correction is delivered to the gearing at the far right of the diagram via the long horizontal rod. The correction is added to or subtracted from the aircraft distance by a differential gear; the aircraft distance is continuously cranked in by a hand crank following the calls from the rangefinder crew, likewise at the far right. The finished distance to the impact point is then reported to the guns via TIT, probably also to the lower-left control group.

Details — Everything Examined More Closely

Control group 1 (upper right) contains an electric motor that rotates constantly at 50 Hz or 3,000 rpm. Its shaft passes below the control group and forms the drive for the second control group and for the left-hand gear set in the large stepped gear. When “faster” is requested manually at the telescope, one of the magnets at the lower contacts (left or right) of control group 1 is activated, causing a gear wheel to engage with the permanently rotating drive. This lowers the intermediate gear in the stepped gear by one stage, and the telescope follows at a higher angular velocity. Simultaneously, the probe at the lower-right cam body descends, driving the central gear roller. The TIT converter below it sends the new, now larger angular lead via TIT to the guns. Even with variable duration of commands from the telescope, it must be ensured (perhaps by electrical feedback) that only exactly whole steps are switched in the stepped gear. There must be a device to ensure that the angular velocity at the telescope can run “left-hand” or “right-hand” depending on the aircraft’s direction of passage. The upper two contacts of control group 1 appear to route the elevation angle of the aircraft (from the rangefinder) to the upper-left cam body — how this functions is not understood. There is no feedback from the fire-control computer to the rangefinder. The elevation angle rotates the upper cam body via a quarter-tooth circle shown in cross-section at the top left. The upper cam body corrects for angular velocity / elevation angle; the lower one for angular velocity / firing range. Together, the two probes with the complex linkage to their right determine the range correction, i.e., the conversion from aircraft distance to impact-point distance. Depending on the flight path and spatial position, this correction can be positive, zero, or negative. If the guns lie broadside (90°) to the horizontal projection of the flight path, the range correction is zero; if the aircraft flies directly toward the guns, the correction is maximum, with decreasing distance. If the aircraft is receding, the distance to the impact point increases.

The position of the two round guide slides determines how strongly the range correction comes into play. There is a position of these guides at which the inner probe produces no effect at the output despite displacement. The two fork pieces 14 and 15 (standing toward the viewer, poorly visible) engage with the two course arrows, which are manually tracked above the fire-control computer housing (see photograph, page 11). Since the two forks in the diagram and at position 14 according to historical photographs (Ref. 4) can only be shifted linearly back and forth, but the universe of possible aircraft courses is rotationally symmetric around the fire-control computer, the two course arrows must already be combined with each other in the upper part of the housing (i.e., before engaging the mechanics of the fire-control computer). How this works exactly is not understood — one would have to open a device. Fork 15 according to the diagram effects a correction of the elevation angle at the upper cam body when a climbing or diving flight is set with the course arrow (see photograph, page 11).

Output of Corrected Distance and Angular Lead

The information from the two cam bodies, modulated by the course-direction setting, reaches a bell-crank lever that carries the middle contact of an electrical “micrometer.” Here begins an “electrical follow-up,” i.e., a copy of the information onto a new shaft that can bear mechanical loads. The two outer contacts are fixed on the movable toothed quarter-circle. When the inner contact moves in one direction, a coil is magnetized, which shifts gear wheels and sets them into continuous rotation. This moves the quarter-circle, which is “followed up” to the inner contact — the range-correction information is thereby continuously copied onto a new shaft that carries it all the way to the right, where it is added to or subtracted from the aircraft distance via a differential gear. Subsequently, a pulse generator X2 converts the distance to the impact point via TIT into a pulse train sent to the guns in increments of 100 meters. There, from the corrected firing range and the elevation angle, the barrel elevation is calculated with its own cam body — i.e., the angle by which the cannon must aim higher than the impact point (curvature of the projectile trajectory due to gravity). On a second TIT channel, the information for the angular lead is fed to the sight in steps of 8′; this mechanism is not clearly documented at the cannon side. It appears that on the one hand the entire telescope is deflected, but within the field of view there is still a lead to be observed — this is not understood, unless in Ref. 1 a mixing of different aiming procedures has occurred. A separate drift correction (probably for windage) is also installed (rotation of the projectiles).

Manual Corrections

The fire-control officer can manually correct three quantities directly. To do so, he presses the “up” or “down” keys at the correction device. These impulses do not go into the fire-control computer but bypass it directly to the equipment vehicle to which the three guns are connected.

The possible corrections are:

Azimuth angle: The telescope base plate shifts laterally by ±1′ per pulse.
Higher/lower: The firing range changes by ±100 m (approx. 1′).
Lead: The lead angle changes by ±8′ (amounts to approximately 24 m at 3 km range).

Assessing how to correct the tracer trajectories will not be entirely simple, since an incorrect lead or an incorrect azimuth angle during a lateral pass look similar or the same; the same applies to altitude and lead during a direct overflight.

Reset

Mentioned for the manual corrections in Ref. 1, and presumed for the fire-control computer settings: at some point the changes accumulated so far must be reset so the system is ready for the next aircraft. No information on this has been found to date.

Construction and Deflection of the Gun Sight Telescope (from Ref. 1, p. 144)

Lower half, Fig. 277: This original sight is no longer present in the Flieger-Flab Museum in Dübendorf. The second observer at the gun uses an auxiliary telescope (not shown) to set the apparent direction of the aircraft by means of his own reticle plate, which he must rotate. This determines the direction in which the sighting telescope is deflected from the barrel axis. The TIT pulses from the fire-control computer determine the angular lead (deflection per dense hatching). In addition to the telescope deflection, the gunner must still observe a lead within the telescope field of view — this is not understood. Perhaps two aiming methods were mixed… In addition, a lateral deflection by the fire-control officer’s manual correction acts directly on the telescope base plate, plus a drift correction according to firing range. The telescope base plate is adjusted vertically diagonally to maintain the firing angle (aiming higher than the impact point due to trajectory curvature), as well as by direct manual correction.

Upper half, Fig. 276: Below the telescope base plate, the firing-angle setting is adjusted purely mechanically by means of a cam body, from the elevation angle of the cannon (set by hand with force via a massive handwheel, displacing the long horizontal rack) and from the firing range (transmitted by the fire-control computer, rotating the cam body).

Not Included in This Azimuth-Angle Geometry

(In the KdoGt. GAMMA-JUHASZ-HASLER for the 7.5 cm cannon, all of these corrections are taken into account)

Parallax lateral correction for gun position
Elevation correction for gun position
Wind correction (longitudinal and transverse)
Air temperature, air density
Propellant temperature, muzzle velocity

The device therefore computes the lead entirely without information about the true aircraft speed, altitude, or course angle, without projectile flight times, and without trajectory extrapolation. A constant vertical velocity of the aircraft will be contained in the total angular velocity as a first approximation.

Chronology

1937: From the phrasing in Ref. 3, p. 163, it is evident that the Swiss Federal Arms Factory W+F had the lead role in developing the fire-control computer. HASLER manufactured it. In February 1938, two 34 mm guns and one fire-control computer are first used in a recruit school (RS). A third gun was initially still needed by the KTA for shooting trials (ammunition testing). The shooting results in Zuoz were “excellent.” Ref. 2, pp. 116–118, 152. Already during these first shooting trials with recruits, visitors came from abroad: the “Chief of Staff of the Inspection of the German Anti-Aircraft Artillery in the Reich Aviation Ministry” observed the anti-aircraft shooting in Zuoz for one day. The 34 mm cannons shot the target bag down already during the first pass. Ref. 2, p. 152. — In Ref. 1, p. 146, from Fig. 280 it is evident that the German guests were even allowed to study the device with it open. Otherwise, fire-control computers were a subject of the highest secrecy — was this shown to friend or foe? On amicale-dca.ch, the visit is recorded only in the year 1939, in contrast to Ref. 1 and Ref. 2.

In December 1938, the chief of arms of the air and anti-aircraft forces wished to block the planned order of 60 guns. He considered the effect of the projectiles with direct-impact fuze to be hopeless. The mobility of the gun was also considered insufficient. The guns were ordered anyway (Ref. 2, pp. 119–121, 157). By the end of the war in 1945, 397 34 mm guns were deployed in Switzerland, mainly in site defense (Ortsflab), partly at airfields (information from the Dübendorf Museum).

1942: Conversion to the Contraves-Kern sight; the fire-control computer and the optics of the aiming apparatus at the guns were no longer able to cope with the increased speeds of aircraft. The fire-control computers were phased out (according to the Flieger-Flab Museum; according to Ref. 1: end of the 1940s).

From 1950: The 34 mm guns again received a computing sight, analogous to the 20 mm. The rangefinder-reported distance and the freely estimated speed had to be set. Ref. 1, pp. 33, 106, 152.

1968: End of firing with the 34 mm after an accident involving a burst cannon barrel (from memory); according to amicale-dca.ch, after “several shooting accidents.”

In Germany, at a secret meeting in Berlin on 6 November 1942, “the necessity of abandoning the angular-velocity devices” was established. Ref. 5, p. 68. In Ref. 5, pp. 65–66, photographs are reproduced of a Zeiss high-tech device of angular-velocity type.

Sources

Herrmann Schild: Fliegerabwehr. Bewaffnung und Ausrüstung der Schweizer Armee seit 1817. Verein der Freunde der Schweizerischen Luftwaffe (VFL), Dübendorf.
Oberstbrigadier H. Born: Die geschichtliche Entwicklung der Flab. Avia-Flab / Huber & Co., Frauenfeld. Second updated edition 1969.
Hundert Jahre Hasler, 1852–1952. Jubilee volume. Guggenbuhl und Huber, Zurich, 1952.
Historical photographs from 1940 on the website of the Museum für Kommunikation (which has taken over parts of the HASLER collection): www.mfk.ch — under Sammlung, Datenbank und Recherche, search “Kommandogerät Hasler” to find 89 historical images (May 2016).
Jenaer Handbuch zur Technik- und Industriegeschichte, Vol. 11, 2008. Verein Technikgeschichte in Jena.

André Masson, Langenthal. April/May 2016.

This is the fifth article on the mechanical computers of the anti-aircraft artillery of the Second World War era.

Article	Topic
First	Fire-control computer SPERRY
Second	Fire-control computer GAMMA-JUHASZ-HASLER
Third	Precise distance determination, check and training devices, Oionoscope, CONTRAVES
Fourth	Computing with cam bodies