In the beginning, there were technical requirements
The pre-history of Swedish heavy self propelled artillery began in 1949 when Bofors was ordered by the Army to create a working prototype of a 155 mm gun for an SPG. This SPG had several names in documents: 15 cm kv fm/49, Akv 1949, and VK 152 S 49 (the last index was used internally within the company).
It was planned that the new SPG would weigh up to 30 tons and production of two units per month would begin in the spring of 1956. In the end, everything went differently: the production SPG entered service in 1967 and its mass grew to 52 tons. The increase in mass can be explained by the increasing need for more armour and protection of the fighting compartment and ammunition from weapons of mass destruction.
As for the significant delay, the problem here was the choice of an appropriate chassis. Additionally, as stated in a memo on April 1st, 1960, delays were caused by constant changes of requirements, a lack of qualified engineers, and doubt regarding the viability of the project as a whole.
Initially, three variants were considered from the tracked chassis: the existing Strv m/42 with a Volvo A8B engine supercharged to 450 hp and a VL 420 gearbox, and two more options, neither of which existed at the time. The first was an experimental project based on the suspension of the American M4 Sherman tank. The second was an unnamed future heavy tank. Perhaps this was a project by the Royal Administration of Military Factories (Kungliga Arméförvaltningens Tygavdelning, KAFT), later named EMIL, but it was a paper project at the time; its development began only in 1949.
A drawing of the Krv EMIL tank.
Among various plans for guns of many calibers in minutes of a meeting held at Bofors on May 30th, 1951, there are plans for a 15 cm SPG. According to them, design work would finish in 1952, a model will be build in 1954, and trials would begin in 1956. These minutes are interesting, as Bofors was working on many medium and high caliber artillery systems. The company was flooded with major new projects to the point that the Swedish armed forces had to purchase 96 French M50 155 mm howitzers to cover their needs for heavy artillery. In the Swedish army, this foreign guest earned the name Fransyskan: "French woman".
There is another interesting fragment of the minutes, evidence of an "IS-phobia" that gripped Europe at the time. Technical specifications for a future 105 mm AT gun (one of the options for EMIL's armament) included not only hypothetical millimeters and degrees, but "Joseph Stalin-vagn" in person.
Fragment of meeting minutes at Bofors, May 30th, 1951.
In early 1952, Bofors chose their chassis: the Strv m/42. The project schedule written on November 24th, 1952, contains the requirements for the SPG in detail. In general, they were the same as the ones given by the military in 1949. The main armament of the vehicle would be a semi-automatic 152 mm gun with hydraulic recoil brakes and a rate of fire of 15 RPM. It also had to retain the ability to load the gun by hand. The length of the gun would be 50 calibers, or 7.62 meters. The gun would be equipped with a muzzle brake that absorbed 25% of the recoil. The maximum range with a 51 kg shell was 25 kilometers, with the elevation angles ranging from 0 to 45 degrees.
The tracked chassis needed to reach a speed of 30 kph off-road, and a maximum speed of 40 kph. The same Volvo A8V was used as the engine, but the ordinary 420 hp version. The overall mass of the vehicle would be under 33 tons, which gave it an effective horsepower of 12.7 hp/ton. The 55 cm wide tracks provided a ground pressure of no more than 0.65 kg/cm^2.
As you could have guessed, attempts to mount a heavy and rapid firing gun in an armoured turret on a medium tank chassis remained on paper. However, one other variant described in 1949 had more luck. Work on the aforementioned EMIL went on as planned. The development of this tank was high priority. The project was classified as "secret, vital importance to the safety of the state", while the 15 cm SPG documents were just "secret". Due to the secrecy, the heavy tank wasn't indexed Strv (Stridsvagn, tank), but Krv (Kranvagn, self propelled crane).
A model of one of the EMIL variants.
The tangled history of the EMIL deserves its own article. In short, the Swedes initially did not wish to build their own tank, opting instead to purchase a batch of Centurions. A request for a purchase was sent to Great Britain. The British replied that they could provide the tanks, but with a caveat: they would do it only after covering their own army's needs. This could take anywhere from five to fifteen years.
Meanwhile, across the Baltic sea, phantom hordes of IS tanks still shimmered in the distance. After the British refusal, KAFT began developing a Swedish heavy tank to repel them. The EMIL had three possible guns: a 105 mm L/67 rifled gun and 120 or 140 mm L/40 smoothbore guns. Regardless of the gun, ammunition would be loaded by a loading mechanism from two drums: one with armour piercing and the other with high explosive rounds.
Development of a heavy tank is not a fast process, especially when you have no prior experience and the end result has to be good. In parallel, negotiations were going on with France about purchasing the light but well armed AMX 13 tanks. Sensing the loss of Swedish money to the French, the British tank builders changed their priorities, and Great Britain agreed to sell the Centurions in December of 1952. Many excuses had to be made to the French, who were the first to answer Sweden's requests without any gotchas, and a difficult time began for the EMIL project. The first Centurion MkIII tanks arrived in 1953, and by 1954 work on the Swedish heavy tank ceased.
The consortium made of Bofors, Landswerk, and Volvo, who were developing the EMIL together, proposed it for the 1958 purchase, but the tank lost again to the cheaper S-tank.
Front part of an EMIL hull with an IS-3 style pike.
Nevertheless, the especially important project as not fruitless. Until the blueprints were shipped to the archive, two EMIL hulls were built. One chassis (with four road wheels per side) was reminiscent of the IS-3 and IS-7 due to the characteristic pike nose.
The second chassis had six road wheels per side and had a more traditional shape of its front armour. Both chassis had a rear transmission. Trials of the turretless EMILs showed decent performance, and the second, larger, chassis was chosen for the 15 cm SPG base. It was changed radically: the front and rear switched places, putting the drive sprocket in the front and the fighting compartment in the back. The engine and transmission were placed in the middle of the hull, and the front housed the driver and his assistant. The idlers were equipped with a mechanism to lower them during firing so they could absorb a little bit of the recoil.
SPG like a Rocket Launcher
The features of the suspension weren't what made this SPG exceptional, the armament was.
Since the building of the prototype Artillerikanonvagn 151 (this happened around 1960), the gun caliber changed. Now the requirements were for the 155 mm NATO caliber. The main features of the 155 mm gun was the automatic loading mechanism. The designers placed it in the oscillating part of the turret, hanging between two armoured compartments for crewmen on the turntable.
Rear of the Artillerikanonvagn 151 during magazine installation. Note the lowered idler.
Here, in the rear of the turret, the 14 round replaceable magazine was housed. It took several minutes to load a new magazine, but the Artillerikanonvagn 151 could empty it in 45 seconds. As mentioned above, the ROF requirement was 15 RPM. If one shell was loaded into the barrel, the full ammunition load was indeed 15 rounds, and the Akv-151 was capable of firing them all in one minute.
The mass of the prototype was 51 tons. The 700 hp engine allowed it to accelerate to a speed of 55 kph. The crew consisted of 7 men. The only assembled prototype of the Artillerikanonvagn 151 can be seen at the Arsenalen military museum near the small city of Strängnäs.
Artillerikanonvagn 151 SPG. The gun and idlers are in travel position.
The Artillerikanonvagn 151 was not destined to make it to mass production. By the time the prototype was built, work was already underway on the famous turretless S-tank, whose unique chassis was suitable for the Bofors artillery system. The idea of unifying parts of the tank and the SPG was tempting, and the Bandkanon 1 SPG on the Strv 103 chassis went into production.
The resulting vehicle surpassed its prototype in mass: the Bandkanon 1A weighs 52 tons and the Bandkanon 1C weighs a tone more. The engine was the same as the 39 ton tank, which was not enough. The effective power of 10-11 hp/ton allowed it to accelerate to 28 kph. Nevertheless, the exceptionally fast large caliber gun capable of firing 15 47 kg shells per minute at a range of 25 kilometers remained in service from 1967 to 2003. 70 vehicles of this type were planned, but the budget was only enough for 26.
Swedish heavy SPG Bandkanon 1A.
When talking about Swedish SPGs, rumours that surround them must be addressed. There are opinions that they were created, first and foremost, as WMD delivery platforms. On one hand, the Swedes certainly had a serious domestic nuclear armament program. In addition, both the US and the USSR had nuclear shells of similar calibers (152 and 155 mm). However, this hypothesis is not supported in documentation, unlike Swedish rocket or aircraft based delivery systems.