English translation
Early Aircraft Tracking by Mechanical Calculators in Anti-Aircraft Defense: Curved-Flight Calculators
This document translates the original German-language article “Frühe Flugzeugvermessung durch mechanische Rechner bei der Fliegerabwehr: Kurvenflug-Rechner” by André Masson (Langenthal, Switzerland), written October 2016 – January 2017.
Overview of This Work
Shortly before the founding of Contraves AG, Prof. Fischer wrote a “Memorandum on Active Air Defense” (31.12.1935) and submitted it to senior Army command posts. It argued that the targeting procedure for cannon anti-aircraft artillery had to be fundamentally re-examined, because the existing method of hit-point calculation (based on straight-line flight) could not solve the problem. This was before the establishment of Swiss anti-aircraft defense.
p. 2
A further document dated the same day is the “Memorial Luftschutz” (Air Defense Memorandum) of the General Staff section, which began the organization of the Swiss aviation and anti-aircraft forces. This work compares the perspective of senior military staff from the “Memorial” with the engineer’s ideas from the “Memorandum” — they differ considerably.
p. 7
Using two command instruments as examples, it is shown how the curved-flight extrapolation was later actually implemented in mechanical calculators. Described are the Swiss device Gamma-Hasler 43 with a somewhat simpler design, and the less well-understood, more complex Command Instrument 40 of the Wehrmacht, with the first known curved-flight calculation.
p. 15
Finally, a chronology of the first years of Swiss heavy anti-aircraft defense is assembled, primarily from the perspective of the technical instruments (up to 1945).
p. 23
A further chronology covers the early work of the private firm Contraves (early years).
p. 11
This work concerns cannon anti-aircraft defense, not aviation. An initial overview of the history of aircraft types used can be found at: http://www.glique.ch/Infrastruktur/Auswahl.pdf
Hit-Point Prognosis — The Fundamental Problem of Anti-Aircraft Defense
Where must one aim, so that after a long flight time (5 to 20 seconds) the shell is simultaneously at exactly the same location as the aircraft?
Initially, all command instruments (= fire-control computers) could only incorporate the future flight path by assuming a perfect straight-line flight: constant altitude and speed, fixed heading angle. Calculating a curved flight was initially impossible, and even when this became feasible from 1940 (Germany) and 1943 (Switzerland), it was still a regular curve, measured before the shot and then continued mathematically. The pilot could, of course, fly a different course — then the computer aims at a wrong point.
Tricking the anti-aircraft computers in order to survive — the pilots had probably figured this out themselves before the first instruction. A US training film advises pilots: at 12,000 feet altitude, never fly straight for more than 12 seconds, then make a marked course change (at 20,000 feet, every 20 seconds). The course change should be more than 20°. This most easily misleads the anti-aircraft computer’s prognosis. Pilots learn this quickly — in their own interest.
The command instruments were certainly high-tech computers, with many correction possibilities: wind could be input by strength and direction, also air temperature, even parallax (the deviation between the location of the computer and the cannons in distance, direction, and altitude) — but against unplanned movements of the aircraft they were helpless. This may have been a reason why so few aircraft were shot down by ground anti-aircraft (cf. p. 6).
The “Memorandum on Air Defense”
Author: ETH Prof. Dr. F. Fischer. Date: 31.12.1935
Focus is mainly on cannon anti-aircraft defense; no novel ideas are expressed regarding aviation (the aircraft cannot defend against attacks anyway).
Background: Several people knew each other from their studies and wanted to found a company. It must not compete with any existing firm in the difficult employment climate of the time; the subject area was to be military and ballistic (H. Bründli worked in the shooting-test section in Thun), and the topics of air defense were demanding, interesting, still completely unsolved, and of national importance by the assessment of the day.
The firm was soon founded and was called “Contraves,” meaning “against the birds.” Its ideas were new, demanding, at the limits of realizability. Collaboration with the military authorities during the entire Second World War did not go well; the tone was often almost hostile. The firm sailed financially on the edge of the abyss. The Army would not buy anything not yet developed. Only later in the 1950s and 1960s did the firm achieve a rapid rise with radar-controlled anti-aircraft devices that were in demand in the Swiss Army and also widely exported (or manufactured abroad).
Aviation
Own aircraft were not expected to provide much protection against air attacks — they could not guarantee the country’s defense. With the climb rates typical at the time and given the smallness of the country, there was too little time to reach high-flying bombers in time — half the country would already be overflown before the first possible contact could occur.
Quote: “The temporal conditions imposed by the country’s dimensions are so unfavorable for us that a solution to the task by our own aircraft cannot come into question. Doubling or tripling today’s number of aircraft would change nothing about this fact.”
Anti-Aircraft Defense with Cannon
It is shown clearly and emphatically in the “Memorandum” that cannon defense with the existing command instruments (about seven designs being commercially available) cannot fulfill its task once enemy aircraft no longer fly straight ahead. All existing devices at the time had only linear extrapolation; this may work for straight-line flights, but fails with curved flights.
Fischer does not merely show the fatal consequences of irregular curved flight. He demands an exact study of targeting possibilities, and honestly concludes that even a perfect curved-flight extrapolation would be useless if the pilot changes course just before or after the cannon fires. Fischer had the nerve to consider covering the entire theoretically possible space that the aircraft could still reach within the seconds before impact. He named the magnitude of the areas to be covered under certain assumptions of speed and turning radii — enormous areas! This would need to be studied carefully, with federal participation. Developing a viable method for engaging aerial targets was, in Fischer’s view, the most important task of national defense — this at a time when not a single anti-aircraft computer existed in the country, and probably nowhere in the world had even a normal curved-flight extrapolation been realized: the professor was already thinking further ahead.
An innovative idea from Fischer’s memorandum: a “ground pilot” would operate a secondary cockpit connected to the aiming computer — with rudder and throttle — simulating the future flight path of the target aircraft, allowing the system to cease firing when a hit could no longer be expected. This concept was original but rather unclearly formulated.
The “Air Defense Memorial”
Author: Hans Bandi, General Staff. Date: 31.12.1935
Signed: Heinrich Roost, Chief of General Staff. Aviation receives stronger emphasis here than ground air defense.
Background: In the First World War, individual aircraft were used mainly for auxiliary and observation tasks, assisting ground troop operations. This was now changing, as air forces assumed independent tasks. Switzerland was neither prepared nor equipped — one rubbed one’s eyes and looked at what was happening and what it meant for the country.
Aviation (14 pages)
Great respect is expressed for surprising air attacks. Fortifications are no longer adequate; attacks behind front lines are possible; the element of surprise. Development of aircraft and their weapons: single-seat aircraft with fixed guns, two-seaters with flexible rear and side machine guns, large air cruisers. Faster fighters protect bombers. Acquisition and maintenance costs are very high.
Switzerland’s own fighter aircraft were outdated and unsuitable. Means would never suffice to build air fleets equivalent to those of neighboring countries. The same conclusion as Fischer’s memorandum: own aircraft have no real chance of preventing aerial attacks. But this insight fades away in the “Memorial” without great consequences.
Active Ground Defense with Cannon
There were major differences of opinion:
“One group wishes to assign the entire active civil air defense to ground defense, while the other prefers to forego defensive air defense (fighters) and ground defense means in favor of a strong offensive air fleet.”
Cannons had not advanced strongly in recent years, unlike command instruments, searchlights, and sound locators. If bombers flew at 6,000 m, the effectiveness of ground defense fell to zero. In favor of ground defense: high readiness day and night, but requiring a large number of trained personnel.
Central Air Defense Command (10 pages)
Bandi clearly advocates unified central command of all air defense. The observation and alarm system cannot be separated into civilian and military domains. All air defense forces should be placed under unified leadership.
Comparison: “Memorandum on Air Defense” and “Air Defense Memorial”
Two leading minds thought through the future of air defense in 1935 — an interesting starting point!
Both were clearly aware of the enormous threat posed by surprising air strikes deep in the interior of the country. Six years before Pearl Harbor, both saw that terrible catastrophes had to be expected. Air defense was not a secondary matter.
The ETH professor F. Fischer showed, based on the missing minutes, that the air force had no chance of preventing such strikes. Therefore the only chance lay in far-reaching cannon anti-aircraft — and here all fire-control computers failed with curved flight. A new technique was urgently needed for extrapolating the flight path better into the future.
General Staff officer H. Bandi spoke of the organizational aspects of future air forces and anti-aircraft defense. Occasionally one senses that Bandi was carefully formulating due to the sensitivity and need to balance many interests — walking on eggshells (Fischer showed no such care, formulating uncompromisingly and bluntly). Technical questions about hit probabilities were not addressed.
If the aircraft had no time to climb, and if the heavy cannon anti-aircraft was barely viable because of “complicated instruments,” then a great open gap remained that was never explicitly addressed. The anti-aircraft defense, at least against high-flying bombers, simply did not function. The recognized danger remained without a technical means of combating it — and Fischer’s proposal remained without effect.
Chronology of Early Swiss Heavy Anti-Aircraft Defense (excerpt)
31.12.1935: Memorandum on Air Defense (Fischer) and Air Defense Memorial (Bandi), both dated same day.
March 1936: Founding of Contraves AG; operations begin May 1, 1936.
August 1936: First patent application by Contraves for data delay on steel tape (used in Oionoscope).
September 1937: Switzerland orders 15 command instruments from Gamma-Juhasz.
February 1938: Letter from Lt. Col. Kraut from Budapest reports on the “Gamma course” and Stefan Juhasz’s preliminary studies on curved-flight extrapolation (intention, not yet realized).
1943: Swiss curved-flight calculation realized in Gamma-Hasler 43.
End of 1944: 43 heavy anti-aircraft batteries, 71 ELASCOP devices, 120 searchlights.
1945 (total war, Switzerland): 218 total enemy aircraft crashed or made emergency landings; 10 shot down by anti-aircraft cannon of all calibers; 12,094 rounds of 7.5 cm fired, 6,824 rounds of 34 mm, 5,791 rounds of 20 mm.
Appendix 1: Brief Description of Early Contraves Anti-Aircraft Computers
Oionoscope: Continuously monitors targeting error without real firing. The angles computed by the command instrument are recorded, stored in a central facility delayed by the flight time of the shells, and the resulting shell detonation point is recomputed and compared with the new, current position of the aircraft. A large, complex installation requiring three theodolites in full operation.
Stereomat: During actual firing, two theodolites photographically record the aircraft and the detonation cloud simultaneously. The angular data on the film are used to determine in a separate resistor-network computer the three-dimensional deviation between aircraft and shell burst. Only one unit was ever built.
Verograph: Determines distance to aircraft precisely via two theodolites 1–2 km apart, compared with continuous readings from four rangefinders (training situation). Distance errors are continuously recorded on paper tape.
All three devices have nothing to do with combat operations — they serve only for quality assurance on the training ground. Single examples of the Stereomat and Verograph survive (Museum Full, Museum Meisterschwanden).
Author: André Masson, Langenthal, Switzerland
Written: October 2016 – January 2017
Note: This is the eighth work in a series on mechanical anti-aircraft fire-control computers of the Second World War era.
[Translation covers the first 28 pages (the complete document); this translation covers the full text.]