Friday, 12 February 2016

Turretless Swede: Strv S

In the mid-1950s, Swedish tank building was at a crossroads. The Swedish military needed a tank that was most appropriate for the local theater of war. This tank had to be small and light, have a powerful gun, good armour, and the ability to quickly move around the battlefield.

Swedish tank designers spent a lot of time working on a heavy tank known as the KRV. The result was an unusual vehicle that would have had a 155 mm gun with an automatic loading mechanism in the turret, similar to the French AMX 50. The design was too heavy and too expensive, and only experimental chassis with no armament were built. It turned out to be cheaper to buy Centurion Mk.3 tanks from the British, adopted as Strv 81. However, the Centurions did not entirely satisfy the military. These tanks were about as large and heavy as the KRV, and even later Strv 101 (Centurion Mk.10) tanks with an improved 105 mm L7 gun did not entirely satisfy firepower requirements.

Sven Berge headed a project for a new tank. The first blueprints are dated 1956, and by 1957, three new potential designs were ready:
  • Strv A: 42.5 tons, well armoured, with a loading mechanism, comparable in size with the American M60, and rather slow.
  • Strv T: 32.5 tons, similar to the Leopard 1 in design. Effective power was about 25-30 hp/ton, armour was no greater than 75 mm.
  • Strv S: a completely new concept of a turretless tank. Its combat mass was estimated at 30 tons and it had extremely good protection due to its low silhouette.
The S design was picked as most suitable. Work started in that same year on elements of this new tank that were tested on various vehicles. In 1959, a KRV prototype with one road wheel per side removed and a 105 mm gun in the front was tested. Experience gained while working on the KRV was applied as much as possible on this project.

By 1961, the outline of the new tank formed. A fully sized wooden model was built, and the first prototype was finished a while later, the Strv S1. It was mostly a test bed for the tank's chief feature: an active suspension, which allowed the tank designers to leave out a vertical aiming mechanism. In addition, the tank could "crouch down", which is very important in Swedish terrain. The Strv S could hide behind a small stone wall, which are still a very common sight in Sweden.

The Strv S2 was ready by the end of 1962. The second prototype was no longer a test test bed, but a fully fledged fighting vehicle that implemented all new ideas from the turretless tank. This prototype was armed with a 105 mm L7 gun, later replaced with a 105 mm L/62 Bofors gun. The gun was equipped with an autoloader. Initially, a remote controlled machinegun turret was planned for the Strv S, but the Strv S2 did not have it. On later vehicles, it was replaced with two machineguns in the front of the hull.

The tank was equipped with a Rolls-Royce K60 engine and a Boeing GT502 gas turbine. The front armour was only 40 mm thick, which seems quite little, but it's enough to see what angle it's positioned at, and all complaints disappear. The mass production tank, indexed Strv 103, was also amphibious. Buoyancy was achieved using a system similar to the Duplex Drive, designed by Nicholas Straussler. Overall, the result was an interesting and original fighting machine, adapted to the region in which it was going to fight.

Two experimental Strv S tanks survive to this day. The tank that this article is dedicated to sits at an entrance to the base of an army unit south of Karlsborg. Since the base is active, access to the tank is restricted to weekends only. Another prototype spent a lot of time at a scrapyard for Swedish military vehicles. In late September of 2015, the tank took up an honourable spot neat the Beredskapsmuseet museum near Helsingborg. The two prototypes differ slightly, but not even Swedish historians know which one is an S1 and which one is an S2. During installation, some removable parts of both vehicles were lost, and air intakes welded shut.

The Strv S differs not only from the mass production Strv 103, but even from the pre-production batch of vehicles. Even the shape of the hull is somewhat different. There is a characteristic "step" on the right side, behind which the muffler is placed. A large headlight sits in the center of the tank, with smaller indicators on the sides (the indicators only remain in the rear part of the tank). The machineguns were supposed to be on the left side of the upper front plate. The box containing them was lost, but you can see where it used to be attached to the tank.

On the left side, the Strv S is similar to the production vehicle, but there are still many differences. The rear is the most distinct. In addition, there is a cutout near the drive wheel which is absent on production tanks.

Initially, the Strv S had a 105 mm L7 gun, later replaced with a longer 105 mm Bofors gun. This gun is currently installed on both prototypes.

The rear plate migrated to production tanks without changes. Wide hatches are present to load ammunition. A small hatch is present at the top to eject spent casings.

A characteristic part of the Strv S is the lower front plate with wide mud scrapers in front of the tracks. The final drive covers also bulge noticeably. In the pre-production batch, the mud scrapers were removed, and the bulges were reduced in size. The tow loops also migrated from the upper front plate to the lower, as they were interfering with fording equipment.

According to the official version of events, the Strv S suspension largely reuses components from the KRV, but this is not so. Only tracks were reused. The drive wheels and idlers were made from scratch, and road wheels were taken from the Centurion tank. Aside from simplicity of production, this decision was inspired by a drive for parts unification.

The track links migrated without change from the KRV tank. Pre-production tanks had a new track link with a rubber pad, more suitable for use on paved roads.

The production Strv 103 upper front plate has little in common with the Strv S. The prototypes shared its gun placement design and the general idea of engine access hatches, as well as the machinegun placement. Large hatches to the left and right of the gun provided access to the engines, in the production version, the hatches are much smaller. Another sign of it being a prototype is the vision devices placed in the connection between the upper front plate and the roof.

The roof also has several major differences. The crew placement, and therefore the position of the hatches, is the same, but otherwise there are many differences, especially in the rear.

The commander's cupola was borrowed from the Pbv.301 APC. Initially, it was to be used without changes, including the 20 mm M/45B autocannon. The autocannon was present on the model, but the Strv S1 did not have it. The commander's cupola migrated to the pre-production batch of Strv 103s, but was later replaced with an improved version.

Another tell-tale sign of the Strv S is the presence of separate hatches for driver and his assistant. In the first Strv 103s, these hatches were replaced with one big one with a clever system for closing and opening it.

Detailed photos of the Strv S exterior can be seen here. (click the mauve arrow on the right side of the screen)

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