Tupolev SB - Development

Development

In 1933 the Soviet Air Force ministry (UVVS) issued an outline requirement for a high-speed bomber. Work on this proposal at TsAGI began in January 1934. The SB was designed and developed in the Tupolev KB ("Design Bureau") by a team led by A. A. Arkhangelski. Two versions were planned – with Wright Cyclone radial engines (ANT-40 RTs), and with the Hispano-Suiza 12Y liquid-cooled V12 engines (ANT-40 IS). The skills gained in the design of the MI-3 and DI-8 aircraft were widely used. The first two prototypes were designed as ANT-40.1 and ANT-40.2. The Cyclone powered prototype flew first, on 7 October 1934, with the first Hispano-Suiza powered prototype (ANT-40₁), which featured a larger wing, flew on 30 December 1934, demonstrating superior performance.

The second Hispano-Suiza powered aircraft, the ANT-40₂ was considered a production prototype, and its performance was impressive. It was however plagued by teething problems, leading unhappy test personnel to cover the ANT-40₂ with placards listing the aircraft's defects prior to a visit by Sergo Ordzhonikidze, the Commissar for Heavy Industry. On seeing these placards, Ordzhonikidze summoned Tupolev to a meeting at the Kremlin to discuss these shortfalls. When Tupolev stated that most of the defects were trivial. On hearing this, Joseph Stalin said:

There are no trivialities in aviation; everything is serious and any uncorrected 'triviality' could lead to the loss of an aircraft and its crew.

The first production aircraft, designated SB, rolled off the production line before the end of 1935, and before ANT-40₂ had completed its flight test programme. The aircraft entered full production in 1936, and was produced in two plants, State Aircraft Factory No 22 at Moscow and No 125 at Irkutsk until 1941.

Despite the fact that the assembly lines were plagued with a constant string of modifications, some 400 SBs were delivered by the end of 1936 – a number of these being diverted to Spain – and 24 VVS squadrons were in the process of working up with the new bomber. Giving excellent performance in the Spanish Civil War, it acquired the popular name "Katyusha" (Catherine).

In 1937, negotiations were successfully concluded between the Soviet and Czechoslovak governments for the supply of SB bombers and a licence for local production in exchange for the right to produce the Skoda 75 mm Model 1936 mountain gun. The version of the SB to be supplied to, and subsequently license-built as the Avia B-71 was fundamentally the SB 2M-100A but fitted with the Avia-built Hispano-Suiza 12-Ydrs engine. A single 7.92 mm ZB-30 machine gun supplanted the twin ShKAS machine guns in the nose and similar weapons were provided for the dorsal and ventral stations.

Sixty aircraft were to be flown to Czechoslovakia by mid-1938. The planned licensed production program took a decidedly leisurely course, despite the increasingly dangerous political situation. By 15 March 1939, when the German Wehrmacht occupied Bohemia and Moravia, not one Czech-built aircraft had been delivered.

Development of the SB continued, meanwhile, with revisions being made to reflect the lessons of early operations in Spain. As problems were encountered in converting pilots to fly the SB, a trainer version, the USB was built in September 1937, with a modified nose with an open cockpit for an instructor and dual controls. Problems were also encountered with the aircraft's armament, with the nose guns having limited traverse and so being little use against head-on attacks. Later aircraft were modified with a better field of fire. From 1940 the dorsal gun position was replaced by an enclosed turret, while the ventral gun position, which was difficult to use effectively, was also modified.

The aircraft was also progressively fitted with improved engines. At first it was equipped with the Klimov M-100, a license-built version of the Hispano-Suiza 12Ybrs engine, but this was soon replaced by the more powerful M-100A, and from 1938 by the yet more powerful M-103. While the engine installation of the SB 2-M103 initially retained the drag inducing frontal radiators of the M-100 powered aircraft, an improved engine installation was developed with the radiators slung under the engines. On 2 September 1937 M.Yu. Alexeev set an official altitude record of 12,246 m (40,177 ft) with load of 1,000 kg (2,200 lb) in an M-103 powered SB. He had earlier set an unofficial record of 12,695 m (41,650 ft).

In an attempt to further improve the performance of the SB, which by 1939 was becoming obsolete, the development of two second-generation versions were authorised, a direct replacement for the SB and a specialised dive bomber. The level bomber, known as the SN-MN or MMN, had a new wing of reduced wing area and was powered by more powerful Klimov M-105 engines. Performance was little better than the standard aircraft, however, and it was abandoned. The dive-bomber, designated SB-RK (later redesignated Arkhangelsky Ar-2 after its designer, with Tupolev having been imprisoned and being in disgrace) was similar to the MMN but was fitted with dive brakes. Testing was successful, and it was ordered into production.

Even though the SB was no longer a state of the art aircraft, production continued to increase through 1939 and 1940, as the Soviet Union tried to build up the strength of their air forces to compete with the growing threat of Nazi Germany, with almost 4,000 being built in these two years. The SB was phased out of production in early 1941, being replaced by the Petlyakov Pe-2. A total of 5,695 were built at Factory No 22 at Moscow before it was evacuated to Kazan, while Factory No 125 built a further 1,136 at Irkutsk. Three prototypes were built at the Tupolev design bureau, while Aero Vodochody and Avia in Czechoslovakia built 45 and 66 respectively, giving a total of 6,945 built.