Saturday, 16 May 2020

T-26 as a Minesweeper

As armoured vehicles were built and improved, methods to combat them appeared as well. The race was only between shells and armour at first, but anti-tank mines appeared soon after. They could be used not only against tanks, but against other vehicles, and it was not necessary to completely destroy a vehicle, as a disabled tank was a perfect target for artillery. One of the countermeasures to this weapon was a mine detector installed on the tank. However, it was not that simple to implement this idea.

An American M3 halftrack disabled by a mine.

A new countermeasure against mines appeared soon after the mines themselves: mine trawlers. These devices were installed in front of a tank and triggered mines using their weight or some other method without causing harm to the crew. Later, devices that could detect mines from a distance, mine detectors, were invented. The hulls of anti-tank and anti-personnel mines were built from metal, and so metal detectors could be used. They discovered metal in the ground using electromagnetic induction and informed the engineer that was holding it. With time, anti-tank mines were built with wooden hulls, but these mines did not last long in the ground or in storage. Even now, in the age of plastics, many anti-tank mines still use metallic huls.

Be your own engineer

Most nations created mine detectors for infantry engineers before WWII. The USSR was no exception. Research took place since the late 1930s, and by 1941 there were several types of induction mine detectors in use: the VIM-210 and VIM-203. There was little information on mine detectors for tanks.

Engineers with VIM-type metal detectors.

The need for an effective anti-tank mine detector came up during the Winter War. Finnish minefields were often filled with homemade explosives, but still managed to throw off Soviet attacks and delay movement. The Finns managed to deploy mines in the Soviet rear, slipping through the forests.

Provisional tactical-technical requirements were composed on December 15th, 1939, by representatives of Voroshilov factory #174 by order of the Leningrad Military District, which led to the creation of the Tank Electromagnetic Mine Detector, the TM-1. The choice of factory was no accident. Factory #174 built most T-26 tanks, the dominant type of tank in the Red Army.

A T-26 tank, model 1937 production (Saint Petersburg Central State Archive).

According to the requirements, the frame of the metal detector was supposed to be located two meters in front of the hull and guarantee the detection of a metallic mine along the full width of the tank and within 30 cm of the tracks, which would allow the tank to maneuver. Mines could be detected at a depth of 1.3 meters. A dead zone up to 0.5 meters along the axis of the vehicle was allowed. The frame needed to be able to survive hits from a branch or shrub up to 3 cm thick and do not impede normal driving through obstacles like trenches and walls in second gear.

The mine detection equipment had to be located in a place impervious to bullets without stifling the crew's actions. The electric current came from the tank's on-board circuit. The mine detector had to be able to work for 8 hours without issues. In addition, it had to be simple and convenient to use. Installation could not result in removal of any accessories or parts. The mine detection lamp and any controls had to be located in front of the driver.

Representatives of factory #174 turned to the geophysics department of the Leningrad State University (LGU). The general principle of the metal detector was proposed by associate professor A.P. Krayev. Faculty members I.P. Ivanov and V.G. Zatsepin also worked on the TM-1. N.D. Zhuric from factory #174 also took part to consult when it came to installation in the tank.

The TM-1 metal detector consisted of a transmitting and a receiving element. The receiving element consisted of two frame antennas. The difference in electromotive force sensed by the two frames was sent through a transformer into a low-frequency amplifier that was connected to a neon lamp that indicated danger. The transmitter was powered by a 1000 Hz 100 W power source. Since the receive frames were different in size, a differential transformer was needed to compensate. Both the transmitter and receiver were mounted in front of the tank.

A diagram of the TM-1 circuit (Saint Petersburg Central State Archive).

The principle of the TM-1 was simple. The primary magnetic field caused by the transmitter created an inductive current in the hull of the mine, which would be accompanied by a secondary magnetic field. This field would cause an electromotive force inside the receiver, which would light up the neon lamp indicator.

Looking for a Finnish mine

The frames were built and went through trials, first in the electrometrics laboratory in the geophysics department, then at factory #174. The dimensions of the frame were 2 by 0.8 meters. This covered the required area without getting in the way when negotiating obstacles.

During trials the transmitter and receiver frames were not rigidly linked, but could be moved to allow for maximum sensitivity. As a result, they detected a mine 1.8 meters below them and 80 cm to the sides. Photographs show that the mine detector was tested on Panssarimiina m/36 mines (F-1 anti-tank mines in Soviet documents) captured in the winter of 1939-40.

A  Panssarimiina m/36 Finnish anti-tank mine.

The factory had to make the mounting rigid in order to install it on a tank. The transmitter was used as the foundation and the receiver frames were fitted inside and paraffin was poured in. This made it impossible to compensate for the EMF difference and mechanical compensation had to be replaced with electrical compensation. Since the frames were very different and there was no compensation equipment, the sensitivity of the detector fell significantly. It could now detect mines 0.8 meters underground and 0.2 meters to the side. The tracks of the tank were not fully covered. The size of the frames would have to be changed to improve the range, but this was not done.

The reason for the failure was the quality of the product. The frame was made from any available materials. The mount for the frame was made from wood, which did not make it rigid enough, and a copper ribbon instead of wire (like in the calculations) was wrapped around it. The receiver frames were made with different gauge wire. The differential transformer prevented the use of resonance in the receiver coils and the capacitors used were leaky amateur grade ones.

The theoretical and laboratory work, production, and testing were finished by early February of 1940. Despite all of its drawbacks, the TM-1 was accepted for installation on a T-26 tank and mobility testing. An old T-26 tank with a cylindrical turret and rail antenna with no armament installed was used for testing, likely having just arrived from repairs.

A T-26 tank with a TM-1 mine detector in the factory #174 courtyard (Saint Petersburg Central State Archive).

The installation contained some improvisations. The wiring was laid as it fell and the equipment was not fixed in place. Due to an absence of a 12 V transformer, a battery was installed behind the driver with a 24 V transformer and a converter on top. Resistances for an electric EMF compensator were not picked, instead an amplified and a variable resistor were used. All of this was attached to a board and placed on top of a DT magazine rack. The frames were installed in the front on a special mount made from metallic girders. It was not rigidly fixed and the height could be adjusted with cables.

Limited engagement

The first 500 meter test run was done with the presence of the factory representatives, the group of developers, and two military representatives from the Red Army ABTU on February 3rd, 1940. Tests showed that mines could be detected 1 meter down and 0.3-0.4 meters to the side. After 15 minutes of work, a voltage spike caused by the stock generator caused one of the low frequency amplifier vacuum tubes to burn out and one of the regular capacitors that was installed instead of a heavy duty capacitor to burst. The trials ended.

A second set of trials was held on February 7th with the same commission present. It also ended in failure. The EMF difference could not be compensated and the TM-1 didn't work.

TM-1 mine detector from the side (Saint Petersburg Central State Archive).

Third trials were held on February 12th. The receiver frames still did not have identical parameters. The mine detector worked, but not as expected. One frame's indicator light lit up when the other's went off. The different parameters could still not be compensated for or evened out. The devices and the frames themselves were improvised and just too different. The mobility trials ended with this.

In their conclusions, the collective that worked on the device considered that the TM-1 could give a 3x2x1.3 meter detection zone given the following conditions:
  • The frames must be produced in factory conditions at one of Leningrad's electrotechnical factories in full accordance with calculations and technical requirements developed by LGU specialists.
  • The frame mount must be rigid and made from duralumin and not wood.
  • EMF compensation and calibration must be performed by the LGU electrometrics laboratory, after which it would be checked and accepted by a commission with representatives of the LGU, ABTU, and factory #174.
Factory #174 would also have to develop a mount for the frames that would allow them to be lifted up when not in use. A proposal was made to give the geophysics department electrometrics laboratory some experimental models and TM-1 equipment to conduct experimental research.

The ABTU military representatives had their own opinions. Military technician 2nd grade G.V. Gomolitskiy and military technician 3rd grade S.N. Odintsov concluded on February 23rd, 1940, that the device has a number of significant drawbacks. Their report stated that it was too early to decide the degree of effectiveness and the presented model is only useful to determine the value of the concept of detecting metal in front of the tank, regardless of if it was a mine or some other metallic object. They also indicated that the device and its installation were improvised. The coils were also not protected from small arms fire and made it impossible to drive the tank faster than in second gear. The ABTU representatives regretted not being able to test the range at which the TM-1 can detect other mines.


TM-1 metal detector frame, seen from above. (Saint Petersburg Central State Arhive).

The final decision was as follows: one prototype must be built where all the drawbacks are resolved and all the issues are fixed.

On March 5th, 1940, the documents with the conclusions of the military representatives and the signatures of chief designer of factory #174 S.A. Ginzburg and head of the KB-2 design bureau K.P. Gavrut were sent to the senior ABTU military representative at factory #174, Binkov. He concluded on March 6th that the prototype is very unfinished and does not satisfy tactical-technical requirements "...as a result of which, it is impossible to determine the value of this device. I consider it necessary to propose any factory to complete the work..."

On March 8th the letter with signatures of the director, chief engineer, and chief designer of factory #174 was sent to the secretary of the Leningrad municipal committee A.A. Kuznetsov. The document stated that work should continue and the factory asks for help from Leningrad's electrotechnical factories to produce three prototypes that could be tuned an sent to the armed forces.

The response didn't end up with the documents studied by the author, but presumably the request was granted. On March 19th the ABTU 8th department asked factory #174 to send a representative to Moscow to sign an agreement for production and testing of mine detector prototypes. A response followed only on April 28th, 1940, stating that the factory is overloaded with special work and can't agree to this request. The factory proposed to make this agreement with the LGU, only accepting the work to install and test the device on a T-26 tank once the design, production, and tuning were complete.

The work didn't resume until the start of the Great Patriotic War. The USSR returned to induction mine detectors installed on vehicles much later. The LGU geophysics department's work was not in vain. Their invention turned out to be in demand by the Navy, where it was used to find sunken ships and submarines.

Based on personal experience with metal detector induction coils, the author can state that the rigidity of the mount, consistency of the coil, and prevention of the loops from shifting all have a significant effect on the performance of the device.

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