Saturday, 25 February 2017

The Amazing Strv 103

The Strv 103, also called the S-Tank, is Sweden's trademark tank. A lack of turret, fixed gun with an autoloader, active suspension, three driver positions (one per crewman), these are only some of its unusual features. Swedish tank designers managed to surprise the world. However, the Strv 103 didn't come out of nothing, and a significant portion of the components that went into the tank came from experimental vehicles. How did Sweden assemble this industrial "Lego set"?


A Call for Centurions


Sweden suffered from "tank hunger" in the 1950s. Old designs were weak compared to tanks of potential enemies, but development of new designs wasn't working out. In an attempt to solve this problem or at least postpone it, the military was forced to consider buying foreign tanks. After several years of negotiations, they chose the Centurion Mk.3 British medium tank.

However, the Centurion was not a good match for the technical requirements formulated by the Royal Army Armament Department (KATF, Kungliga Arméförvaltningens Tygavdelning). The military wanted a 25-30 ton tank with a 75-105 mm gun and 20 hp/ton. The requirements also stated that the tank should be low.

Sven Berge, the creator of the Strv 103 and a key figure in Swedish tank design.

The 50 ton three meter tall Centurion with a top speed of 35 kph was well outside the required range of characteristics. However, it was already in production. This was the most important factor in the military's decision. The first Centurion Mk.3 arrived in Sweden in May of 1953.

Sans Turret

What about Swedish engineers? While this was happening, they were working on a tank called EMIL.

By this point, the project went through many transformations. The initial design called for a 28 ton vehicle with a 120 mm gun and an autoloader (two magazines with 8 shots each), 120 mm of front armour, and a top speed of 55 kph. As a result of discussions in 1952, the project was radically changed. It became noticeably larger and its mass grew to nearly 42 tons. Instead of a 120 mm gun, two new guns were designed: 105 mm and 150 mm.

This did not just delay the production of prototypes, but created a series of unsolvable problems. The tank itself, renamed Kranvagn (self propelled crane) in 1956, was not doing too badly. These changes shifted the prototype completion date to the fall of 1957, but overall, the tank was good. The problem was that it never received a turret or armament.

Overall diagram of the active suspension, sketch by Sven Berge. 

Sven Berge, who supervised the work on the KRV on behalf of KATF, knew the scale of the crisis that the prospective tank program was in. He started working on another combat vehicle concept in parallel with the Kranvagn. In many ways, it was similar to the initial tactical-technical requirements from the early 1950s. The tank was going to have a mass of 30 tons, a 105 mm gun with an autoloader, be highly protected and very maneuverable. The issue was that it was no longer possible to meet these requirements and stay under 30 tons in the mid-1950s.

Berge found an unusual way out: get rid of the tank's turret. The engineer began working on this in late August of 1956. He was planning on not only removing the turret, but fixing the gun in place. The entire tank was going to serve as a turret. This could be achieved with an active suspension that not only aimed the gun vertically, but could also "squat". Initially, Sven Berge proposed the Horstmann suspension, but modified so it could move up and down. A special rod would be inserted between the bogeys to serve as a foundation when lowered.

A demonstration of the advantages of a turretless tank.

Sweden Makes a Choice

Berge spent a weekend at work on his turretless tank. On Monday, he showed it to his colleagues in KATF. They shrugged, and only Erik Gillner, Berge's direct superior, showed any interest. Berge polished his idea from August to October, and it was shown to management on October 22nd, 1956. Despite skepticism from other engineers, Gillner decided to include Berge's vehicle in the overall plan.

It was not easy to choose between tank concepts.

Meanwhile, KATF began working on another concept for a potential tank. This happened right after the misfortune with the KRV's armament. Additionally, Sweden learned of a new tank developed by Germany. This tank also belonged to the 30 ton class. New work began in the spring of 1957, in tree different directions. All three tanks would be armed with a 105 mm gun.

The first direction, the Strv A (A: America), followed the British/American school of tank building. This would be a 42.5 ton tank with a decently large silhouette and 20 hp/ton. The second direction was the Strv T (Tyskland, Germany). The tank was also called French/German in correspondence, since the Europanzer program was launched around that time. This was a 32.5 ton tank with 30-35 hp/ton. The cost for low weight and high mobility was thin armour.

Finally, Sven Berge's concept was listed as Strv S (Sverige, Sweden). Thanks to highly sloped front armour, the protection of this tank was equivalent to the Strv A and mobility to the Strv T. Its mass was only 30 tons. Berge was in charge of all three projects, and their potential enemy was the Soviet medium T-54 tank, not heavy IS-3 or T-10 tanks. Sweden had not yet made a decision regarding which concept to follow.

Ikv 103 SPG, modified to test an active suspension.

The Ikv 103 SPG was initially used to test the active suspension. Later, in the spring of 1959, a modified Medium Tank M4A4 was used. The idea of aiming the gun just by moving the hull was tested on both tanks. The use of such modifications reduced cost to a minimum: only 5000 kroner was spent on testing the Strv S concept.

The final discussion regarding the choice of concept took place in April-May of 1958. The discussion included new information about the Soviet T-10 tank and a thorough analysis of the worldwide state of tank building, as well as Sweden's long term need for tanks.

The new tank would be built as a part of a long-term rearmament program from 1965 to 1975 with a colossal budget for Sweden: half a billion kroner. Ultimately, the Strv S was selected, but with changes. For instance, the mass of the tank grew to 37 tons. The Strv A was also not yet outright rejected.

The idea that the Strv S was finally cemented as the main direction of Swedish tank building was confirmed by a patent submitted by Sven Berge for the active suspension. He received patent number US2966828 on January 3rd, 1961.

With Horns and Without

Bofors, not Landsverk, worked on development of the Strv S, although the latter also participated. KATF and Sven Berge personally oversaw the development. Bofors also worked on the Strv A in parallel for some time. The first conceptual designs were shown to the Armament Department in October of 1958. Bofors was mostly working on armament and, in case of the Strv A, the turret. Calculations showed that it should have 150 mm of armour sloped at 30 degrees and 60 mm thick sides. 30 rounds of ammunition would be held in the turret and 50 more in the hull. As for the Strv S, the total ammunition capacity was 50 rounds.

The initial look of the Strv S, August 1959.

The most interesting element of both designs was the gun. The customer told Bofors that the 105 mm gun designed for the KRV had insufficient penetration. The company took measures. The gun, called Kv 105 x 59 in the Strv S and KT 105 x 60 in the Strv A, was lengthened to 85 (initially 90) calibers. Thanks to this, the muzzle velocity of the armour piercing shell was increased to 1100 m/s, and the muzzle velocity of the subcaliber shell to 1455 m/s.

The Swedes still considered it necessary for the turreted tank to have an autoloader. The design was actively discussed after February of 1959. By initial calculations, the rate of fire of a gun equipped with this autoloader would be 20 RPM.

It was slowly becoming obvious that work on the Strv A was going into a dead end. The gun barrel was too long. It stuck far out of the turretless tank and even more so from the classical tank. Because of this, the Strv K was proposed as an alternative on March 23rd, 1959. The letter K meant that it was based on the KRV chassis. A British Centurion Mk.10 turret would be installed on it. The program was cancelled soon after when Sweden bought those same Centurions.

Layout of the tank.

Bofors presented the draft Strv S project in early August of 1959. It was already obvious that the tank would be extraordinary. The length of the hull was almost the same as the EMIL. There was little else in common aside from the number of wheels. The Strv S was 70 cm wider and the hull was 40 cm taller. Bofors estimated that its mass would be 31 tons.

The tank had a wedge shaped hull like on Berge's first drafts. The engine and transmission compartment was in the front. The Swedes decided right away that it should be combined, including a 300 hp diesel and a 200-300 hp gas turbine. Combined, they would offer 20 hp/ton and a top speed of about 60 kph.

Sven Berge's patent for a device to protect the gun from damage.

The armament of the Strv S was also unusual. The gun, fixed in the hull, was equipped with an autoloader that was fed from magazines in the rear of the hull. The requirement was for 50 rounds of ammunition. The biggest problem came from the gun. The 85 caliber gun was never installed, and a gun similar to the one developed for the KRV would take its place. However, it was still very long (67 calibers, about 7 meters). The gun barrel overhang was over 3 meters long. This became obvious in June.

As an alternative, KATF and Bofors decided to use a 20-pounder (84 mm) gun. Sven Berge also designed a special guard that would protect the gun when it collided with the ground and various obstacles. The guard was later also patented. The Strv S also received a 20 mm Automatkanon m/45B in a remote controlled turret. The turret and commander's cupola were borrowed from the PBV 301 APC which was also being designed at the time. In addition, a hull machinegun was installed in the left of the hull.

Reworked Strv S variant, August 1960.

After a discussion in August of 1959 it became clear that the excessively long gun has got to go. The simplest alternative was the British 105 mm L7 which was used on the Centurion Mk.10 (or Strv 101). However, the Swedes decided to not use it as is and design their own gun using the British one as a starting point. The length of the gun barrel of the Swedish version was 62 calibers.

There were also many complaints regarding the tank itself. It was clear that it was too small to fit both tank components and a crew. The design was radically altered and a reworked variant presented in August of 1960. Its combat mass grew by 4 tons. The hull grew by almost a meter in length and 15 cm in height. The overhang of the gun was reduced to 2 meters. The danger of hitting something and damaging the barrel was still high enough to require a special guard. The amount of hull machineguns was increased to four, all in the upper front plate.

Modified KRV testing the new suspension.

Modifications of the Strv S did not end here. In parallel, engineers began working on the active suspension concept. Its initial variant took up too much space. Instead of vertically moving bogeys like on the Centurion, the Swedes began working on a hydropneumatic suspension. The new system was much more compact and simple: only the first and last road wheel were used instead of all four. The suspension elements were hidden inside the hull. The KRV prototype was modified to test the system in the second half of 1960. The tank lost its first and last road wheel, the suspension was modified , and a gun was added in the front. Trials confirmed that the idea was correct.

A full sized model of the Strv S, 1960. The tank would be built in metal looking like this.

By late 1960, the Strv S took its final form. Trials swept away all suspicions that the tank would hit obstacles with its gun barrel. The full sized model no longer had a guard around the gun. The tank was approved for mass production. The vehicle inherited some elements from the KRV, like track links.

On the Way to Production

According to the contract signed between KATF and Bofors in November of 1959, the development of the Strv S included building two experimental prototypes. These prototypes were scheduled for 1961. The budget for the turretless tank program, adjusted in 1960, was 15 million Swedish kroner. This is a sizeable sum, but it does not compare to other prices of the era. This is about how much it would cost to buy less than two dozen Centurions. Speaking of the Centurion, the Strv S1 and S2 borrowed their road wheels from the Centurion Mk.3.

Trials of the Strv S1, summer of 1961. The hull is covered in sheet metal to preserve secrecy.

Sven Berge approved a plan to slowly evolve the design of both of his prototypes on January 2nd, 1961. The Strv S1, the first prototype, was a demonstration of the suspension. It received an electric motor instead of the stock engine and the L7 gun as its armament. This is how it was equipped for its first trials. The S1 was the only tank from the Strv S family to receive a turret with a 20 mm automatkanon m/45B gun. To preserve secrecy, the hull was covered in sheet metal.

Changes made to the Strv S after the first trials. The 20 mm AA gun turret is already missing.

The Swedes kept working on the armament and engine in parallel with the tank itself. Experimental tanks were supposed to receive the 6.5 L Rolls-Royce B81 230 hp engine. However, a decision was made in mid-January of 1961 that tanks from the "zeroth" batch (starting with the third vehicle) would receive other engines. The same was true for experimental tanks. The S1 would keep the B81 engine until the middle of 1962. The S2 received a new engine even earlier, in mid-1961. In both cases, the engine would be used alongside a Boeing 502-10 gas turbine, used on American helicopters. As for the armament, the trials went poorly for the 20 mm gun. The tank only carried 40 rounds for it and insufficient aiming angles meant that it soon disappeared from the Strv S.

Strv S2 on trials, winter of 1961. The tank already has a 62 caliber long gun, but no fume extractor.

The second prototype, the Strv S2, differed from its brother. It received the full power plant and a commander's cupola with no gun turret. This tank was built for trials of both the suspension and the armament. The 105 mm L7 gun was quickly replaced with a 62 caliber gun. The tank went through trials in fall-winter of 1961 with this gun. The first prototype also joined mobility trials in later 1961. Both tanks were constantly modified. For example, the Kulspruta 58 (Swedish version of the FN MAG) light machinegun was added to the second tank's commander's cupola instead of the AA gun. Various changes continued until spring of 1963. The Strv S1 was now called Strv S1C and Strv S2 was Strv S2B.

The first tank from the zeroth batch was supposed to come off the assembly line on October 1st, 1963. This time, even though changes were continuously made, the deadline was met.

Evolution of the zeroth batch of the Strv S, early 1962.

Bofors presented a draft project for the initial batch on February 9th, 1962. The tank once again received a gun guard and a muzzle brake, but these were not the most important changes. For several reasons, the hull of the tank had to be redesigned. Its height grew to 1900 mm and the shape changed. One of the causes was that a decision was made to install amphibious equipment on the tank.

The underwater driving equipment tested on the Strv m/40 and Strv m/42 was rejected. Instead, the Swedes designed a system similar to the one created by Miklós Straussler during WWII. The tank was equipped with a collapsible box structure that gave it buoyancy. The tank could be controlled by any of the three crewmen, and both the commander and driver could aim and fire. Another version of the tank was proposed, without a muzzle brake or a protective guard. This version went into production.

Tests of strips welded onto the upper front plate. These strips turned out to be an effective means of "braking" shells.

Meanwhile, the Swedes were also working on improving the tank's armour. The armour thickness of the experimental tanks and initial batch were the same. The upper front plate was only 40 mm thick, but sloped at 78 degrees. The lower front plate had the same thickness and was sloped at 72 degrees. Trials of special welded on strips were performed in July-August of 1963. It turned out that these strips improve armour protection, but they were not implemented right away.

The first vehicle from the zeroth batch.

The 10 tanks from the zeroth batch, built between fall of 1963 and early 1964, were constantly modified. Initially, they had no fording equipment, commander's cupolas identical to the prototypes, and only two hull machineguns. A high caliber machinegun was planned for installation in the right side of the hull, but this idea did not move past models. The tanks also received an additional gun lock, which was modified at least once.

The tanks had road wheels from the Centurion Mk.10 and new track links with rubber pads. Meanwhile, KATF and Bofors continued discussions regarding further improvements to the tank. Instead of the British engine, the military wanted the Volvo TD96 diesel from the Volvo Titan truck. The idea was rejected since the engine was larger, yet weaker.

This tank already has the rear toolbox and a model of a high caliber machinegun (or autocannon) on the right.

Further investigations and trials meant that the look of the Strv S changed constantly. Of course, there was nothing as major as a new hull, but minor changes were made. For example, trials showed that the commander's cupola was not ideal. The new cupola also received a mount for a light machinegun. The idea of two hull machineguns was rejected, and they were replaced with toolboxes. Extra toolboxes were also added in the rear.

Other countries began showing an interest in the tank. For example, Great Britain wanted Strv S tanks for trials. Norway went even further, and it wished to buy these tanks in 1966. However, trials of the Leopard 1 were happening around this time, and Norway eventually chose them. The choice might have had something to do with the price: the cost of one Strv S was enough to buy two Leopards.

Swedish Know-How

Work on improving the Strv S finally ended in March of 1966. A year later, in September of 1967, the tank was accepted into service. It received the index Strv 103, although Bofors documents kept calling it Strv S for some time. Ten years of work were not wasted: the army received an unusual, but technically excellent vehicle, ideally suitable for fighting in Scandinavia.

Strv 103A. Tanks from the first production batch had no fording equipment.

The order for the first batch, indexed Strv 103A, included 70 tanks. These tanks were sent to the P2 and P5 tank regiments. After all the modifications, the mass of the tank grew to 37 tons. The engines remained the same as those on the zeroth batch. The increase in mass had its effect on mobility, and the top speed of the tank dropped to 50 kph. This state of affairs did not go unnoticed by KATF. Even before the Strv 103 entered production, the search for a new engine began.

Of course, the production tank had many changes compared to the pre-production ones. The boxes containing machineguns and tools were changed. The lights and their guards were also changed. The biggest change was the addition of a "rake" above the engine compartment. The increase in mass was no accident: it was the cost for improving armour. The tanks from the first production batch never received fording equipment.

Trials of anti-HEAT screens.

The Swedes began testing another improvement with these tanks. Work on anti-HEAT shields began in 1966 during the search for additional protection. Initially, the idea was to install them on the sides, but began changing later. The Strv 103 received screens on its sides, as well as a two part screen in front. Later, it evolved into an unusual system of pins inserted into the front of the hull. This system could effectively defeat HEAT shells. The design was accepted into service, and only declassified in 1992. For this reason, the screens do not appear in official photographs taken before the 1990s.

Strv 103B, the main version of the tank.

A list of requested changes was composed as a result of use of the Strv 103A. These changes were implemented in the Strv 103B. This was the main tank in the series; 220 were produced. Shipments began in 1970 and Strv 103A tanks made earlier were modernized to the same standard. The Swedish army was completely equipped with Strv 103 tanks by the end of 1971.

Deployed fording system.

The Strv 103B was much more mobile. The 300 hp Boeing 502-10, the 490 hp Caterpillar 553 gas turbine was used. The mass of the tank grew to 39.7 tons, but its effective power grew to 18.4 hp/ton. The tank also received a full set of fording equipment. The design of the track links changed: they returned to the KRV design, but had removable rubber pads. Additional armour of the rear plate was added later. 

Overall layout of the Strv 103B.

The lifespan of the Strv 103 indicates how good of a tank it was. The tanks were phased out of service in 1997, 30 years after their adoption. The tank was modernized many times, but a bad tank would still not last that long in any army. One must take their hat off for Sven Berge who proved that a tank with no turret can still be an excellent fighting vehicle.

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