With its pusher engine, twin boom tail feathers and hugely glassed cockpit canopy, the S.21 looked right out of a pulp aviation comic of its era. This late 1930s/early 1940s Dutch single-seat fighter design was the brainchild of T. E. Slot, the former chief designer of Pander & Son. Of all-metal construction and powered by a German 1050hp DB 600Ga, as conceived, the S.21 was heavily armed with four fixed light machine guns and a curious 23mm Madsen cannon which could be directly handled by its pilot.
The construction of the prototype was initiated in the early 1939, and it was still uncompleted when the German invaded the Nederlands in May 1940. Seized by the conquerors, the prototype, still unfinished and unflown, was destroyed by them during some terminal structural tests.
Crew: 1 Length: 8,50 m Wingspan: 9 m Height: 3,50 m Empty weight: 2050 kg (4519 lbs) Max weight: 2750 kg (6062 lbs) Powerplant: Daimler-Benz type 601Aa with a starting power of 1260 HP, watercooled, 12 cilinder Maximum speed: 570 km/h Service ceiling: 9500
Max flight distance: 1400 km at 4000m at 300 km/h Machine guns: 4 forward facing 7.9mm FN machine guns / One fixed backward facing 7.9mm FN machine guns firing through the engine shaft Cannon: One 20mm Solothurn cannon, wich is able to angle down at 25 degrees
Bombs: Two 50 kg (110 lbs) bombs in the tail carrier between the two box-beams.tail carrier between the two box-beams.
Fly plane with left hand, aim 120lb cannon with left hand, watch were the plane is going with right eye, aim cannon with left eye, simple, what could go wrong?
Or did the pilot grip the stick with his knees and use both hands on the cannon?
By the cutaway drawings it looks like the cannon was below and in front of the pilot, and out of his reach.
Maybe some kind of servo was intended to control the depression of the cannon.
Anyway, it never flew, so how can we say how successful it was or wasn't, it's just another might have been.