Several years ago Slovenia started a programme to upgrade its old 100 mm armed T-55 MBTs to a new standard locally known as the M-55 S1.

This upgrade work is being carried out in Slovenia and the first upgraded M-55 S1 MBTs were returned to their units late in 1997/early 1998. '

While some of the sub-systems have been designed and manufactured in Slovenia, especially in the area of optics, other parts have been supplied by foreign countries, including Israel.

The exact number of T-55 MBTs to be upgraded to the new M-55 S1 configuration is not known although it is estimated that Slovenia has about 55 of these vehicles in service.

The overall layout of the upgraded T-55 S1 MBT is virtually identical to the Russian T-54/T-55 MBT covered in detail in an earlier entry.

The upgrade carried out in Slovenia covers the three main areas of the vehicle, armour, mobility and firepower.

The standard 100 mm D-10T rifled gun has been replaced by a 105 mm L7 type rifled tank gun which can fire a wide range of ammunition types including High Explosive Anti-Tank - Tracer (HEAT-T), Armour Piercing Fin Stabilised Discarding Sabot - Tracer (APFSDS-T) and High Explosive Squash Head - Tracer (HESH-T).

The 105 mm L7 gun is fired electrically as is the 7.62 mm machine gun mounted coaxially with the main armament. The roof mounted 12.7 mm machine gun is retained.

The 105 mm barrel has been fitted with a three part thermal sleeve which is said to reduce barrel bending by over 70 per cent so helping to increase accuracy of the weapon.

A new computerised fire-control system has been installed in the M-55 S1 MBT which is claimed to allow stationary and moving targets to be engaged with a high first round hit probability while the tank itself is stationary or moving.

The fire-control system has three modes of operation, automatic, semi-automatic and manual. In the first mode all the crew have to do is manually insert the type of ammunition to be used.

The gunner is provided with the Slovenian Fontana SGS-55 two axis stabilised day/image intensification sight with a laser range-finder which feeds information to the fire-control computer. As an option, a thermal version of this system is also available.

The vehicle is fitted with a Slovenian COMTOS-55 commander's take over set which enables him to override the gunner and carry out target tracking, ranging, target selection and to fire the main gun/ coaxial machine gun. This is mounted under the existing TPN-10 sight which is retained with the picture of the target coming from the gunner's sight by a charge coupled device.

The driver is provided with a Fontana CODRIS combined day/night driver's periscope which allows day (80° field of view) or night (78° field of view) simply by flicking a switch.

The standard T-55 has a turret and hull of welded and cast steel armour but for added battlefield survivability the upgraded M-55 S1 has been fitted with explosive reactive armour (ERA) to the frontal arc of the turret, glacis plate and upper hull sides.

This armour is believed to be the latest Israeli Super Blazer system which gives a high level of protection against High Explosive Anti-Tank (HEAT) attack but cannot be activated by small arms fire.

According to Slovenia this ERA provides the following protection levels:

(1)Over an arc of 50° left and right from RPG-7 HEAT warheads

(2)Over an arc of 40° against HOT HEAT missile warheads

(3)Over an arc of 30° against TOW HEAT missile warheads

The upper part of the suspension is protected by a five part rubber skirt which provides two functions. The first is to help reduce dust and the second to activate explosive warheads before they hit the main armour of the vehicle.

For improved cross-country mobility the suspension of the vehicle has been replaced and now each side consists of five rubber-tyred road wheels, drive sprocket at the rear, idler at the front and four return rollers. The new 580 mm wide track has been fitted with replaceable rubber pads and to reduce driver fatigue a hydraulic steering system has been installed.

The standard T-55 MBT is fitted with a V-12 water cooled diesel developing 580 hp but this has been replaced by a 600 hp diesel engine. As an option a new power pack that includes a MAN 850 hp diesel can also be installed. With a combat weight of 40 tonnes, this power pack would give a good power-to-weight ratio of just over 21 hp/tonne.

The upgraded M-55 S1 does not only rely on its improved armour for battlefield survivability and is also fitted with Fontana LIRD (Laser IRradiation Detector and warner) system. This consists of a roof mounted detector head unit, indicator unit and harness.

The system has a full 360° coverage in azimuth and from -20 to +60 ° in the vertical plane. It can alert the crew with a pulsed laser from a laser range-finder or a laser illuminator/designator. This allows the crew to take action such as moving to a new position or activating onboard devices such as the grenade launchers.

Mounted either side of the turret is a bank of six Israeli Military Industries IS-6 CL-3030 instantaneous self screening system for combat vehicles - installed on Israeli armoured vehicles for many years. This launches grenades to a range of 40 m and a 60 m wide smoke screen is laid which lasts for about two minutes, depending on wind conditions. The tank can also lay a smoke screen by injecting diesel fuel into the exhaust outlet on the left hand side of the hull.

The engine and crew compartments are each fitted with a fire-detection and suppression system. Each of these systems comprises optical detectors, cylinders filled with Halon 1301 gas and a control device which can be operated automatically or manually.

A brand new communication system is installed in the M55 S1 which includes two new antennas, new radios, VIC intercom system and an externally mounted telephone. The telephone is fitted at the rear of the vehicle on the right side and allows the accompanying infantry to talk to the tank crew.

The older Russian-type lighting system has been replaced by a Western system including new headlamps, side lights, turning indicators and a roof-mounted flashing warning lamp for peacetime use.

Stowage is a problem on many MBTs, especially those manufactured in Eastern Europe as these have always been more cramped than their larger western counterparts.

Mounted on the turret rear is a large stowage box where the items that have been displaced by the installation of vertical hull side armour have been relocated. Equipment such as kit, rations and a camouflage net can be stowed in this basket.

Upgrades underway for Slovenian Army.