The Tupolev SB (development TsAGI-40), was a Soviet high-speed bomber, twin-engined, three-seat monoplane bomber which flew for the first time in 1934.
Its inception can be traced to the Tupolev ANT-40, studies of which began in early 1930s with speed given very high priority, as it was considered the best defence against fighters.
In 1933 UVVS (Soviet Air Ministry) issued a requirement. In January 1934, TsAGI (Central Aerodynamic Institute) had gathered enough information to make that project real.
The SB was developed at Tupolev KB (design bureau) by a team led by Aleksandr Arkhangelski.
There were two versions planned, one powered by the Wright-Cyclone radial engine (called ANT-40RT) and another powered by the Hispano-Suiza 12Y liquid-cooled V-12 engine (called ANT-40IS). The experience gained with the design of the ANT-21 heavy fighter was put into practice.
The first prototype, ANT-40.1RT (Wright powered) made its first flight on 7th October 1934, while the second one ANT-40.2IS (Hispano-Suiza powered) flew on 30th December 1934 and showed superior performance. The Hispano-Suiza variant was actually a Klimov M-100, which was the Soviet copy of the Hispano-Suiza 12Y engine.
The ANT-40.2 was considered for production thanks to its good performance numbers for the time. However, it had consistent teething problems which lead to unhappy personnel to cover it with placards, listing its defects before the visit of Sergo Ordzhonikidze (Soviet commissar for heavy industry). When seeing those placards, Ordzhonikidze summoned Tupolev to a meeting at the Kremlin to discuss those shortfalls. In that meeting, Tupolev stated that those defects were trivial, to which Stalin answered:
"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 was completed before the end of 1935 (when the ANT-40.2 was still under testing) and it entered full production in 1936. It was manufactured in two plants. Plant No 22 at Moscow and plant No 125 at Irkutsk (Siberia) from 1937 until 1941.
As stated, there was a second prototype of the ANT-40IS, called ANT-40IS 2. It featured various improvements such as enlarged tail unit, engine nacelles moved slightly forward and a trim tab added to the right aileron. This prototype was tested with a fixed ski landing gear and defensive armament installed (a 7.62 mm ShKAS machine gun in the rear cockpit). After testing of this second prototype was completed, it was sent to plant No 22 to incorporate such improvements into production aircraft.
The ANT-46 was a heavy-fighter variant, directly derived from the ANT-40.1RT but powered by two French Gnome-Rhône 14K radial engines rated at 800 hp each. It was armed with two 76 mm APK-4 recoilless guns placed in the outer wings. It was also equipped with two fixed rearward-firing 7.62 mm ShKAS machine guns installed in the inner part of each wing. They were controlled from the gunner cockpit, who was also provided with the usual ShKAS in his cockpit that every SB had.
There was also provision (although it was never installed) for a 12,7 ShVAK machine gun placed in the nose.
In November 1934 the project received the ANT-46 denomination from Tupolev and the official DI-8 denomination from Soviet authorities.
The prototype of this variant was completed in the summer of 1935 and flew for the first time in August 1935. During winter 1935-1936 it was tested with a ski landing gear and the engine cowling was changed.
The prototype of this variant was completed in the summer of 1935 and flew for the first time in August 1935. During winter 1935-1936 it was tested with a ski landing gear and the engine cowling was changed.
It was tested by Tupolev KB until June 1936. Those tests revealed stability problems, for which it was considered to replace the tail unit with the one of the ANT-40IS 2, however, it was never completed. Another considered change that was never made was to replace the radial engines with Mikulin AR-34RN ones.
Anyway, the project for the APK-4 recoilless guns was cancelled due to the arrest of its creator, Leonid Kurchevsky, so the ANT-46 project was left on hold. It was considered to replace them with two 20 mm ShVAK cannons in each wing or with a gun pack containing five 12,7 mm ShVAK machine guns placed in the bomb bay, like the British Bristol Blenheim Mk.IF. However, interest was lost, the ANT-46 was never presented to state trials and in 1937 Tupolev was also arrested, so his initials were banned from aircraft denominations and the planes were officially called TsAGI-40 and TsAGI-46.
Once in production, the SB 2M-100 (the denomination for the first production variants, powered by two Klimov M-100 radial engines) saw combat in the Spanish Civil War, from where some feedback was collected to improve the type's defensive armament. These experiments included the installation of an MV-3 turret or a new enlarged glazed cockpit in order to improve lateral visibility.
Those experimental variants were completed during the course of 1936 and 1937 and, while most of them were unsuccessful, some defensive arrangements were carried over for later variants.
Sources:
1st https://en.wikipedia.org/wiki/Tupolev_SB
2nd https://massimotessitori.altervista.org/sovietwarplanes/pages/sb/tapani/prototypes/ant-40-46.htm
3rd https://massimotessitori.altervista.org/sovietwarplanes/pages/sb/tapani/prototypes/defensive.htm
4th Signal Squadron - Aircraft In action 194 - Tupolev SB in Action
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