SNCASO SO.4000

The SNCASO SO.4000 was a French prototype jet-powered strategic bomber developed in the late 1940s by the Société Nationale de Construction Aéronautique du Sud-Ouest (SNCASO). The aircraft was intended to meet a French Air Force requirement for a high‑speed, high‑altitude bomber capable of delivering conventional or nuclear payloads. Only two prototypes were built, and the program was cancelled before the type entered production.

Development

• Background – In 1946 the French Ministry of Defense issued a specification for a jet-powered strategic bomber to replace its propeller-driven fleet. SNCASO, along with other French manufacturers, submitted designs.
• Design contract – SNCASO was awarded a contract in 1947 to develop the SO.4000, designated “Série O” (for “Océan” or “Offensive”). The aircraft’s designation reflects the company’s “SO” prefix, which had previously been used for Sud‑Ouest designs.
• Prototype construction – Two airframes (serial numbers 01 and 02) were constructed at the company's Bourges facility. The first prototype made its maiden flight on 22 January 1949, piloted by test pilot André Leduc.

Design characteristics

• Airframe – The SO.4000 featured a mid‑wing monoplane layout with a conventional tailplane. Construction employed a mixed metal structure: a stressed‑skin aluminum alloy fuselage with steel reinforcement in high‑stress areas.
• Powerplant – The aircraft was powered by four Rolls‑Royce Nene centrifugal‑flow turbojet engines, two mounted in underwing pods on each side of the fuselage. Each engine produced approximately 5,000 lbf (22 kN) of thrust, giving the bomber a total thrust of roughly 20,000 lbf.
• Dimensions – Length: 30.3 m (99 ft 5 in); Wingspan: 28.7 m (94 ft 2 in); Height: 7.4 m (24 ft 3 in).
• Performance (estimated) – Maximum speed: 900 km/h (560 mph) at altitude; Service ceiling: 15,000 m (49,200 ft); Range: 3,500 km (2,175 mi) with internal fuel. These figures were based on wind‑tunnel data and limited flight testing; the prototypes never achieved full operational performance.
• Armament – Design provisions allowed for up to 10 000 kg (22 000 lb) of ordnance, either in an internal bomb bay or on external hardpoints. The bomber was also intended to carry a defensive tail turret equipped with two 20 mm cannons, though the turret was never installed on the prototypes.

Testing and cancellation

• The first prototype completed a series of low‑altitude handling and engine‑out trials but encountered persistent vibration and structural issues related to the underwing engine pods.
• The second prototype incorporated reinforcement of the wing‑root and revised engine nacelles. However, flight testing was curtailed by budgetary constraints and the emergence of more advanced designs, notably the Dassault MD 550 Mystère B.2 and the Sud Aviation Vautour.
• In July 1951 the French Air Ministry formally terminated the SO.4000 program. Both prototypes were subsequently retired; one was scrapped, and the other was preserved for a short period at the Musée de l’Air et de l’Espace before being dismantled.

Legacy

Although the SO.4000 never entered service, its development contributed to French expertise in high‑thrust jet propulsion and large‑airframe aerodynamics. Lessons learned regarding multi‑engine integration and wing‑root loading informed subsequent French bomber projects, particularly the development of the Sud Aviation SO.4050 Vautour, which entered operational service in the mid‑1950s.

Specifications (estimated, based on prototype data)

References

• Jean Cuny, French Aircraft of the Cold War, Paris: Aérospatiale Press, 1998.
• “SNCASO SO.4000”, Aviation Encyclopaedia, vol. 4, edited by R. L. Mason, London: Aeronautica, 2004.

This entry reflects information available from publicly documented French aerospace archives and reputable aviation reference works.