Why does the S-tank look like it does?
At the time when the first vague ideas about the S-tank started to take form (late 50’s), weapons technology was far more advanced than armor technology. It seemed prohibitively hard to design a tank that had sufficient protection against modern HEAT and APDS shells while also maintaining decent mobility and staying within reasonable weight limits. When the French and German governments cooperated on the Europa-Panzer project (what would become the AMX-30 and the Leopard 1), one of the stated design goals was to sacrifice armor strength in favor of mobility and weight savings. The British Chieftain was considerably better protected but especially in its early versions suffered from high weight and poor cross-country mobility.
In Sweden, however, the dream of creating a tank that had both good mobility and good protection lived on. British studies from WW2 (that the Swedes had gained access to when purchasing the Centurion) showed that the turret was twice as likely to take a hit as any other part of the tank and that hits below a meter from the ground were very unlikely. A taller tank also tended to take more hits. The S-tank lacked a turret and had a very low profile, which seemed very promising as far as not getting hit at all went.
Still, if you did get hit, you needed armor to survive. The S-tank seemed promising in this regard as well. By fixing the gun in the chassis, the entire front glacis of the tank could be sloped at an extremely shallow angle (12.5 degrees from the horizontal, or 77.5 degrees from the vertical). This extreme sloping enabled a comparatively very thin armor plate (40 mm, thinner than even then Leopard 1’s front armor) to effectively resist practically all known APDS projectiles of the time: they simply slid or bounced off the plate instead of penetrating. This didn’t work against HEAT, of course, but the fixed gun enabled a different solution for that. Since the gun couldn’t move, it was a simple matter to mount a lattice or “fence” in the front of the tank. This acted as spaced armor: HEAT projectiles would detonate against the fence, making the jet of molten metal lose a great deal of its penetrating power before hitting the glacis.
In this way, the designers accomplished a combination of low weight (the first production model S-tanks weighed 37 tons, less than the Leopard 1), excellent protection (from the front, the tank was practically immune to a lot of then modern weapons) and good mobility. The gun was also superior to most Western tanks of the time; it was a variant of the ubiquitous Royal Ordnance L7, but lengthened 10 calibers and fed by an autoloader that had a cycle time of 3-4 seconds. The lack of a turret was not seen as a problem, since the Swedish tank tactics of the time emphasized fighting from prepared locations, even when on the offensive.
Unfortunately, while the design was innovative, the S-tank turned out to be an evolutionary dead end. In the mid-70’s, new and longer APFSDS kinetic penetrators were entering service in many armies, and these no longer tended to bounce off of sloped armor. Anti-tank missiles with tandem HEAT charges mostly negated the “fence” and stabilization technology made firing on the move a much more attractive option than previously. Composite armor and improvements in engine technology made it possible to achieve excellent protection without sacrificing mobility. The S-tank, which had probably had had by far the best frontal armor protection of any Western tank in 1968, was mostly obsolete by 1980. While its contemporaries (Leopard 1, Chieftain, M60 Patton) received considerable upgrades over the years, budget constraints forced the S-tank to soldier on with only minor improvements until it was retired in the late 90’s.