Having been in service since 1947 with over 50 state and non-state actors, the T-54/55 is the most widely produced and used tank in the world. Naturally, many nations have produced their own modifications of the T-54/55 in order to maintain the tank’s longevity in their respective arsenals.
One such state, Slovenia, produced their own T-55 modification that was unknown to most until being donated to Ukraine in 2022 to assist that country in its conflict with Russia: the M-55S.
The M-55S is one of the most heavily upgraded T-54/55 variants, with additional frontal hull and turret armor, uprated suspension, new tracks, more engine power and a gun that fires NATO ammunition.
Only a small handful were made, as Slovenia only had a small stock of T-55s to modify.
T-55 to M-55S
In 1991 the Ten-Day War began when Slovenia declared independence from Yugoslavia. Before independence, Slovenia contained its own defense force within Yugoslavia, the Slovenian Territorial Defence.
During the Ten-Day War the Territorial Defence, better known as TORS, clashed with the Yugoslav People’s Army (JNA). TORS did not operate any tanks at the time of conflict, unlike the JNA which possessed Soviet-based designs like the T-55 and T-72 (known as the M-84).
When Slovenia achieved independence from Yugoslavia in 1992, it inherited all JNA tanks left on its soil: 54 M-84s and 30 T-55s (some sources state much higher).
In the late 1990s, Slovenia decided to upgrade both tanks, with the T-55 becoming the M-55S once upgraded.
With the assistance of the Israeli company Elbit Systems, all 30 of Slovenia’s T-55s were modified to the M-55S standard, with the first upgraded tank being completed in 1997 and the final M-55S delivered in 1999.
The T-55 is a 1950s design, so even with major upgrades it is never going to be able to challenge a modern main battle tank. Still, tanks are not only useful against other tanks, so many improvements and modifications were made to the original design, especially in terms of armour, mobility and firepower.
The base armour of the M-55S is the same as the T-55 – a maximum of 100 mm on the hull front, and 200 mm on the turret. This is effectively tissue paper for modern main battle tanks and anti-tank weapons, but the M-55S received upgraded protection through the addition of Israeli-designed explosive reactive armour (ERA).
Although some sources suggest that the ERA of the M-55S can defeat or at least degrade kinetic energy rounds, there is no evidence to suggest that this is the case with this particular ERA and it is likely that the ERA is only effective against High-Explosive Anti-Tank (HEAT) ammunition.
The upper portion of the suspension of the M-55S is covered with a five-part rubber skirt that is designed to work against HEAT ammunition by forcing the round to detonate at a sub-optimal standoff distance.
The firepower of the M-55S was upgraded too, with the original D-10T 100mm rifled gun being replaced by an Israeli-made version of the venerable British Royal Ordnance L7 105mm rifled gun with a thermal sleeve.
While there are no specifics on the type of ammunition that the Slovenian Army used for the M-55S, the L7 is capable of firing a wide variety of ammunition, such as HEAT, armour-piercing fin-stabilized discarding sabot (APFSDS), high-explosive squash dead (HESH), smoke, illumination and anti-tank guided missiles (ATGM).
One of the ATGMs that can be fired from the L7 is the Israeli designed LAHAT (Laser Homing Anti-tank): a laser guided missile with a range of 5 miles (8 kilometres) and a tandem HEAT warhead to defeat ERA and up to 800 mm of rolled homogenous armour.
In addition to an improved gun, the M-55S also received upgrades in the form of a digital ballistic computer and stabilization fire-control system for the 105 mm. This is arguably equally important as physical firepower upgrades.
Sensors added to the vehicle input data into the ballistic computer, such as wind speed, temperature, target distance and target speed. The computer is informed of the ammunition chosen, and will then provide a firing solution that accounts for those conditions.
Further, the gunner was provided with a day and night Fotona SGS-55 sight, which incorporates a laser rangefinder. The commander also received a day/night sight, and was given the ability to override the gunner’s controls and fire the main gun and coaxial machine gun.
However, the fire-control system and electro-optical upgrades did not include any thermal imagers.
Other interesting additions include an Israeli-made laser detector system, which alerts the crew to if the vehicle is being targeted or under range-finding by a laser. If needed, the system can automatically deploy a smoke screen around the tank.
Finally, the M-55S would have its engine upgraded from the standard 580 horsepower of the V-55 V12 diesel engine in the original T-55, to 600 horsepower. The M-55S was also provided with new suspension, road wheels and rubber pads to improve cross-country mobility.
All of the physical work for the modifications was carried out in Slovenia.
As mentioned, the first M-55S tanks were received by the Slovenian Army in 1997, with all 30 upgraded tanks delivered by 1999. However, the M-55S had a relatively quiet service life, not being deployed to any operational theatre.
In 2016, Slovenia actually placed all of its M-55Ss into storage, and they were offered for sale with incentives of upgrading the engine and adding thermal imagers, but there were no takers.
Then, in 2022, the M-55S came into media attention when it was announced in September 2022 that Slovenia would donate 28 of its M-55S to Ukraine in support of the latter’s conflict with Russia. In return for this donation, Slovenia would receive 35 military trucks from Germany.
Initially, in April 2022, Slovenia had actually agreed to donate 30 of its more modern (in comparison to the M-55S) M-84 tanks to Ukraine in exchange for German military vehicles such as the Leopard 2 tank, Marder Infantry Fighting Vehicle and Fuchs Armoured Personnel Carrier.
However, that deal fell apart in July when Slovenia demanded more than 15 Leopard 2A4s that were offered by Germany.
Instead, Slovenia agreed to provide 28 M-55S to Ukraine (there was no information on the status of the remaining two M-55S that were not donated) in September with all tanks being delivered in late October of 2022, roughly a month after the initial announcement.
There were no indicators if the delay was merely the length of time to move the M-55S out of storage to send to Ukraine, or if the tanks had been further upgraded prior to being delivered to the Ukrainian Army.
As of right now we do not have any reliable information on the exact whereabouts of Ukraine’s M-55Ss, nor do we know what they have in store for them. They mark the oldest tank formally introduced into service in the Russo-Ukraine conflict, beating even Russia’s use of the T-62.
While it is easy to assume tanks like the M-55S will have no use, the role of a tank is often to simply hold and tank ground, and not ride into battle at full speed to take on other tanks. Of course, modern anti-tank weapons will make light work of the M-55S, but this is a vulnerability shared by much newer tanks too.
It is also important to note that its 105 mm gun was one of the best tank guns of the Cold War, and can still hold its own today – even the M1 Abrams used essentially the same gun early on.
If used properly, with the correct infantry support to defend them against anti-tank weapons, the M-55S will certainly be a useful tool for Ukraine.