The M-91 Vihor Main Battle Tank: The Tank with Russian and American DNA
One of the strongest industries of the Socialist Federal Republic of Yugoslavia (SFRY) was its defense industry. During the Cold War, the SFRY flirted with both the West and the East, procuring and developing indigenous weapons based on technology from both sides. Perhaps one of the more interesting success stories was the success of the SFRY’s armor industry and its ultimate product, the M-91 Vihor Main Battle Tank.
The story of the Yugoslavian tank industry begins in the late 1940s, following Tito’s spat with Stalin.
Due to the Soviet Union blocking exports of it’s then-current tanks to the SFRY, while also threatening to invade, Tito himself asked workers at the Petar Drapšin factory if they could make a tank. The workers agreed, and the factory was able to produce a few tanks (around half a dozen) based on a modified T-34/85 design, called Tip (Type) A.
However these were not enough, so the Yugoslavians struck a deal with the United States for military aid. They acquired a number of M4A3E4 Shermans, M47 Pattons, M18 Hellcats, and M36/M36B1 Jacksons. Tito then made amends with the USSR in the 1950s. This allowed the SFR to acquire T-55s, T-54As, SU-100s, and T-34 Obr. 1960s. These were replaced by the newer T-55As which were donated by the Soviet Union to replace the earlier models that the SFRY shipped to Egypt during the 1973 Yom Kippur War.
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This deal laid the foundations of good relations with the USSR for the purchase of the T-72, but this deal didn’t go smoothly. Czechoslovakia and Poland received licenses to produce the T-72 before Yugoslavia, and in the end, the license granted limited them to ten years of production or one thousand tanks, whichever came first and locked them into a purchase of a set number of T-72M and T-72MKs.
Factories from every Yugoslav republic contributed to the production of the Yugoslavian T-72, called the T-72MJ. The first T-72MJ rolled off the production line in 1983 and differed from other T-72Ms in that it featured a fully digital fire control system.
At this point, Yugoslavian engineers had devised many upgrades beyond the original T-72M. Accuracy was the primary area of improvement. A tall crosswind sensor was added immediately behind the main gun, the stabilization mechanism was improved with additional gyroscopes and a cant sensor and the all digital FCS was refined further. This resulted in the M-84, the Yugoslavian version of the T-72.
Then in 1987, the M-84A came out, and updated the armor layout to the T-72M1 standard while adding a one thousand brake horsepower V-46TK engine, dramatically increasing the speed and tactical mobility of the tank. However production of the M-84 and M-84A, despite the new names, was still governed by the one thousand tank cap set in the original T-72 license.
In order to avoid this cap, in 1987 the government began a new project called Novi Domaći Tenk (New Domestic Tank). It later received the designation M-91, becoming the M-91 Vihor (translated: Whirlwind). The Military Technical Institute at Belgrade spearheaded this effort. The timeline was to finish drivetrain testing in 1991, to produce a prototype series of ten tanks by 1993, and to enter serial production by 1994.
Due to the breakup of the SFRY in 1991, only three hulls and three turrets were completed (two cast, one welded). Croatia, being the destination at which final assembly was completed, mated one of these turrets to a hull to create a complete Vihor.
In many ways, the Vihor can be seen as a branching development of the M-84 series. While based on the same philosophy of a three-man, autoloaded, low profile tank, the Vihor’s fresh design diverged from the T-72 in many ways. The heart of the M-91 Vihor was the upgraded powerplant. While the one thousand V-46TK brake horsepower engine put in the M-84A was already an upgrade from the stock T-72’s V-46-6 780hp engine, the Vihor used a 1200 brake horsepower turbocharged multifuel diesel engine called the V-46TK1. This gave the Vihor superlative performance, with a max speed of around 75 kilometers/h and an acceleration of 0–32 kilometers/h in less than seven seconds, rivaling and even surpassing the performance of some gas turbine tanks of the same era.
The performance was also achieved within a small space, only 3.4 cubic meters (to compare, the 1500 brake horsepower engine of a Leopard 2 takes up seven cubic meters). Overall, this gave the Vihor a very good hp/ton ratio of 30. The Vihor’s suspension was also improved to handle this performance. The road wheels were given a new design, and the vertical travel of the suspension was increased to 350 millimeters from 280mm on the T-72, allowing for improved offroad performance (up to fifty kilometers/h offroad). This entire drivetrain was extensively tested, the prototype hull (mated with an M-84 turret) was driven over fifteen thousand kilometers during testing.
Armament wise, the Vihor’s armament was an evolution of the 2A46M gun that armed the T-72. A new thermal sleeve was developed, and a muzzle reference sensor to measure muzzle droop after thermal changes (as found on western tanks). A quick change mechanism was also implemented. The autoloader was also improved from the T-72’s AZ-72 autoloader, allowing bidirectional movement similar to the T-64 autoloader to improve loading efficiency and times (up to ten rounds per minute). Accuracy was also improved by a weather station and a suite of sensors that measured properties of the ammunition such as temperature and exact weight of propellant charge. These additional parameters can be seen on the control boxes for the Vihor’s FCS. The turret drive was upgraded from the hydraulic horizontal turret drive to all-electric, with a traverse speed of 50 degrees per second, more than twice faster than a basic T-72. The ammunition was no slouch either, with 554 millimeters of RHAe penetration at 2500 meters, enough to deal with most of its contemporary threats.
An innovation here was the use of a bakelite obturator (a ring of material that seals a round against the barrel) on the round’s sabot instead of a copper obturator to reduce fouling. The Soviets transitioned to an aluminum obturator later on, instead of bakelite.
The sighting systems were on par with Western sighting systems, featuring digital zoom, last rangefinder, third-generation night vision, and thermal sights. However, the commander only had a day/night sight without thermals, although he had a screen that allowed him to see the gunner’s thermal view and the ability to rotate the turret to engage a target he was tracking through his sight. A laser-warning receiver (LIRD) was also installed on the turret. The LIRD system possessed a degree of soft APS functionality, it could automatically rotate the turret towards a threat and deploy smoke. The LIRD system was also exported successfully to other countries. Rounding out the commander’s systems, the Vihor was equipped with an encrypted, frequency hopping radio, similar to the American SINCGARS system.
The armor of the Vihor follows the same design philosophy of its Soviet contemporaries. It utilized composite armor arrays on the cast turret and hull, comprised of a mix of silicium carbide, aluminum, rubber, and an alloy honeycomb filled with quartz sand mixed with epoxy. This new armor was 3.5 tons heavier than the armor on the M-84A, which was analogous to that on the T-72M1. KAO M-99 ERA was also planned to be attached to the Vihor, however this program was only completed in 1998 so most photos of the Vihor are missing the ERA. This ERA was estimated to be between the Soviet Kontakt-1 and Kontakt-5 in effectiveness, with limited protection against kinetic energy rounds.
Due to the breakup of the SFRY, the Vihor never entered mass production. Due to the disparate nature of its production across all of the republics of Yugoslavia, many countries used components from the Vihor program to upgrade their fleets of M-84s during the Yugoslavian Civil War. Croatia even assembled one welded turret and a hull into a complete Vihor, finishing it up with some M-84 parts.
It’s rumored that this tank took part in an operation to capture a JNA outpost in Degman. There lies the Vihor’s final legacy. Following the end of the civil war, Croatia developed the Vihor into the M-95 “Degman” tank. Croatia added additional composite armor and new electronics to modernize the Vihor for the twenty-first century. Two M-95 Degmans were built and currently reside somewhere in Croatia.
The primary source for Vihor and M-84 development was Modernization and Intervention: Yugoslav armored units 1945–2006, by Bojan B. Dimitrijević”
The author would like to thank Luka Puzigaća, MD for assistance with research.
Charlie Gao studied political and computer science at Grinnell College and is a frequent commentator on defense and national-security issues.
Image: Wikimedia Commons