Background edit
Red Air Forces considered the concept of a dedicated dive bomber since 1934, but only as a distant second alternative to tactical level bombers.[1] In 1936 Nikolay Polikarpov initiated preliminary design for a three-seat SVB dive bomber powered with two M-100 engines.[1] Prototype SVB drawings completed in February 1937 did not materialize, even in a prototype airframe; instead, Polikarpov reworked them into Polikarpov VIT-1 anti-tank ground-attack aircraft.[1] Bombers were Tupolev's exclusive territory, and Polikarpov made a backup proposal to convert the VIT-1 airframe into a heavy fighter, code-named MPI (Russian: многопушечный истребитель, multi-cannon fighter).[1] Air Forces rejected the VIT concept and advised Polikarpov to concentrate on the MPI heavy fighter.[1] They made it clear that MPI was considered a technology demonstrator rather than a candidate for mass production.[1]
VIT-1 to VIT-2 edit
In July 1937 Polikarpov received a go-ahead to build two twin-engined fighters around M-100 engines.[2] His factory in Khimki[1] rolled out the first prototype of the original Polikarpov VIT-1 in heavy fighter configuration in October;[2] flight test continued until February 1938 when VIT-1 was at last rejected, citing "failure to meet design targets".[2]
On October 13, 1937 Polikarpov submitted a different proposal for a multi-role aircraft built around M-103 engines, Polikarpov VIT-2, that made possible three combat configurations: short-range, high-speed level bomber (BSB), anti-tank ground attack (VIT) and heavy fighter (MPI).[2] The prototype, completed out on May 1938, was tested by Valery Chkalov and Boris Kudrin with M-103 engines, reaching 498 km/h level speed with 6166 kg takeoff mass (508 km/h with 5310 kg).[2]
Polikarpov made a decision to replace M-103 with the new, more powerful M-105 that had not yet reached mass production - a decision that ultimately killed his twin-engined designs, including the SPB.[2] Polikarpov's factory became a test and rework shop for the defective M-105s, but nevertheless VIT-2 took off in August 1938, showing only a marginal increase in maximum speed (to 513 km/h).[3] Independent tests in Zhukovsky revealed that even this number was inflated by 15 km/h.[3] A redesigned VIT-2, delivered in February 1939, barely made 500 km/h - the minimum design target set in 1937 yet far ahead of any competing designs and mass-production models.[3]
Naming ambiguity edit
The Commissariat for Aircraft Industry suppressed Polikarpov's project, to the point where it banned presentation of VIT-2 to Air Forces generals.[3] M. Nersisyan, engineer in charge of VIT-2, managed to get through to Kliment Voroshilov to get a permission for a show flight.[3] Air Forces chief Yakov Smushkevich, fascinated by the show, took over the sole prototype for a thorough examination by Air Forces test pilots.[3] Two runs of test flights, in September-October 1938 and in February 1939, persuaded Smushkevich in the feasibility of a fast dive bomber, thus VIT-2 program was renamed SPB (Russian: skorostnoy pikiruyushchy bombardirovshchik, high-speed dive bomber).[3] Polikarpov disagreed and proposed a different acronym, SBP (Russian: skorostnoy bombardirovshchik, pikiruyushchy, high-speed bomber with dive capability).[3] His design was optimized for speed; conversion to a true dive bomber required structural reinforcement that, Polikarpov feared, will lead to unacceptable loss of speed.[3] For a brief period in the first half of 1939 the three names - SBP, SPB and VIT-2, were used concurrently; by the end of 1939 SPB prevailed.[3]
Producing the prototype edit
March 29, 1939 Vyacheslav Molotov authorized small-scale production run of SBP (sic) at the Plant No. 124.[3] Polikarpov objected: his prototype needed substantial changes, and the chosen plant was not capable of building advanced aircraft.[3] Production was moved to Plant No. 22, then building Tupolev SB; Polikarpov was ordered to deliver SPB (sic) based on revised VIT-2 on Janyary 1, 1940 and a dedicated PB-1 dive-bomber, with different engines, on July 1, 1940.[4] What looked like a successful contract became an end of Polikarpov's firm.[4] In November 1939, while Polikaprov toured German aircraft facilities, Commissar for Aircraft Industry Mikhail Kaganovich created a new design firm, headed by Artyom Mikoyan (later Mikoyan-Gurevich OKB, today's Mikyan) and transferred 80 of Polikarpov's staff to Mikoyan.[4] Polikarpov continued working without a proper production workshop and without his key personnel, managing to roll out the first SPB prototype (numbered SPB 1/0, zero standing for "prototype") on the last day of 1939.[4] On December 14, 1939 he submitted another proposal based on the same airframe - Polikarpov TIS, a heavy escort fighter (on the same day Mikoyan proposed a competing Mikoyan-Gurevich DIS).[5]
Polikarpov's other competitors produced their own prototypes for fast bombers: Kocheryegin OPB, Petlyakov 100, Tupolev 103, Yakovlev BB-22, Arkhangelsky B-2.[4] A state commission held on November 18, 1939 reviewed these prototypes and recommended Petlyakov (first choice, although expensive)[5] and Yakovlev[5] for mass production, yet somehow failed to notice already existing SPB.[4]
Crashes and inquiries edit
The second prototype SPB (SPB 2/0) was completed on January 10, 1940.[5] SPB 1/0, piloted by Boris Kudrin, flew on January 18; SPB 2/0 on March 26, the first serial production SPB 1/1 on April 17 and SPB 2/1 on April 26.[5]
On April 27, 1941 serial SPB 2/1, piloted by Pavel Golovin, crashed in an unrecoverable spin after a thirty-minute level flight around the Khodynka Field.[5] All SPBs flown to this date suffered from troubles with M-105 engines, usually ending in bursts of motor oil.[6] On the previous day Golovin, who flew the same aircraft from the factory field to Khodynka, reported increased (but not critical) temperature of the right engine.[5] Investigators did not even consider engine failure; their inconclusive ruling blamed the spin either on insufficient longitudinal stability of the aircraft on crossing the cloud front, or the pilot error.[5] A contemporary review of the crash by Perov and Vladimirov named engine failure the most likely cause of the crash.[5] The commission allowed further tests of SPB 2/0, 1/1 and 3/1 and grounded SPB 1/0l[5] Polikarpov was ordered to run a full set of wind tunnel tests at TsAGI and obtain a TsAGI review of the aircraft stability.[6]
Soon after the crash Smushkevich reported to Stalin that the choice of production aircraft, either SPB or 100, will be made "in the nearest days" of May 1940.[6] The opinions of Air Forces generals were changing rapidly from full support to SPB on May 15 to rejecting it on May 25.[6] By the end of May the decision was made against Polikarpov, although the tests continued.[6] On June 2, 1940 test pilot Mikhail Lipkin experienced another in-flight engine failure; landing with his right engine out, he clipped a parked Tupolev SB and crashed but survived.[7] Test continued; in the same month Polikarpov obtained an order for fifteen more serial aircraft although his OKB kept on losing engineers and workers to other firms.[7]
Another crash occured on June 30, 1940, killing Lipkin and his flight engineer. Lipkin was instructed to test the airframe for wing flutter at 600 km/h.[7] Withesses on the ground saw the aircraft disintegrating in midair, in level flight at an apparently high speed; the investigators blamed the crash on wing flutter[7] and personally on Polikarpov's deputy Zhemchuzhin who did not adjust, as instructed, trim tabs on the ailerons. TsAGI commissioners believed that SPB was prone to flutter upon reaching 500 km/h.[7] Yet twenty days later TsAGI issued a report stating that SPB could not succumb to flutter until reaching 800 km/h.[7] Engine and airscrew failures, again, were not considered at all.[7]
On July 29, 1940 the new chief of aircraft industry Alexey Shakhurin grounded all test flights of SPBs.[7] Polikarpov was cut off the information loop, denied access to investigation reports.[7] By the end of August Shakhurin has written off the project completely; the sole surviving SPB was donated to Moscow Aviation Institute, the factory tooling reused for Pe-2 production.[7]
Reference edit
Sources edit
- Vladimir Perov, Nikolay Vasilyev (in Russian). Zagadka SPB ili pochemu pikiruyushchi bombardirovshchik ne poshol v seriyu («Загадка СПБ, или почему пикирующий бомбардировщик не пошел в серию»). Krylya Rodiny magazine, June 1997.