The French Air Force (Armée de l'Air) judged the design a failure despite some records being set and cancelled plans to put it into service in 1951.

[1] As built, the aircraft was a metal-skinned mid-wing monoplane powered by a 22.2-kilonewton (5,000 lbf) Rolls-Royce Nene turbojet engine licence-built by Hispano-Suiza.

The canopy of the cockpit was unusually tall because the Air Force decided to provide the pilot with a parachute despite the Heinkel ejection seat.

[3] The second prototype was scheduled to make its first flight on 15 August 1948, but this was delayed until 16 September 1949, possibly due to the need to revise the air intake to improve the flow to the engine.

Flight testing showed that there was little improvement in the supply of air to the engine and that it was not very maneuverable as a lot of force was required to move the rudder.

[6][7] The proposed production variant, the SO.6021, was lightened in the hopes of improving its performance by reducing the amount of armor plate carried and shrinking the canopy.

By this time, the Air Force had already decided to use the aircraft to support the SO.9000 program by testing various small turbojet engines on wingtip mounts.

[10][5] Data from The Complete Book of Fighters;[15] X-Planes of Europe II: Military Prototype Aircraft from the Golden Age 1946–1974[16]General characteristics Performance Armament