O.Thetford, P.Gray German Aircraft of the First World War (Putnam)
Ago S I
Two of these single-seat aircraft were built, but were not completed before the war ended. They were designed for low-level attack, and probably had some degree of protective armour. Armament was given as two Spandau machine-guns and one 2 cm. cannon, although how the cannon was installed is not known. The photograph is the only one known to exist and, although poor, it does show well the split undercarriage chassis, which was considerably ahead of its time. Engine: 260 h.p. Basse und Selve BuS III.
J.Herris Development of German Warplanes in WWI (A Centennial Perspective on Great War Airplanes 1)
The increasing important of anti-tank weapons lead to Idflieg creating the S-type. The S-type was basically an evolved J-type whose primary purpose was ground attack, especially destroying tanks. The requirement was for a two-seat armored biplane with 20mm Becker cannon fitted for anti-tank duties. The gunner also had a flexible machine gun to defend the aircraft against enemy fighters. The Ago S.I, the only S-type apparently completed before the armistice, was too late to go into production or operation.
J.Herris Otto, AGO and BFW Aircraft of WWI (A Centennial Perspective on Great War Airplanes 37)
The growing number of Allied tanks was an ever-increasing problem for the German Army as the war continued. Germany was unable to implement the obvious solution, building a large number of their own tanks, due to lack of industrial capacity. So it addition to new infantry weapons and tactics to counter tanks, Idflieg created a new category of aircraft, the S-type, for anti-tank warfare.
The S-type was an armored tank destroyer armed with a 2 cm Becker cannon. The Allied tanks were armored against rifle-caliber fire from machine guns and rifles and could be easily penetrated by the heavier shells from the 2 cm Becker, especially as the fire would be against the thin armor on the top of the tanks.
The final Ago design was the Ago S.I. It was extrapolated from the armored J-type in that the engine and cockpit were similarly armored against enemy rifle and machine gun fire. However, it had an innovative design to enable the gunner to attack tanks with a downward-firing 2 cm Becker cannon. To allow this the undercarriage was designed so there was no axle between the wheels.
The configuration of the armor and cannon are shown in the adjacent sketch. Two machine guns were intended to be fitted,- one was for the gunner to use to protect the S.I from fighter attack. The other machine gun was likely intended as a fixed gun for the pilot.
References differ on the engine used. One mentions a 300 hp Basse St Selve BuS.IVa engine and the other mentions a 500 hp Benz, - this would need to be the Bz.VI or Bz.VIv, both of which were 500 hp V-12 engines with a 60° angle between cylinder banks. In any case two prototypes were built in late 1918.
The Ago S.I was the only S-type completed by war's end and development was too late for production. However, with its armored airframe and downward-firing anti-tank cannon it could be considered the great-grandfather of the contemporary anti-tank helicopter.