SB CANNON | MODEL CONSTRUCTION

Tupolev SB aircraft was conceived as a bomber, overtaking fighters. It was natural to create a heavy fighter, designed primarily to combat the bombers. Total produced three prototypes of the SB. According to the decision of the meeting of February 14, 1934, one of them got American motors “Cyclone”, another French Hispano-Suiza, and the third was going to keep in reserve in the form of a set of units. Three days later appeared a decree of the Council of labor and defense (STO), which stated that the third prototype of SB should be equipped with the type of engine, which is better to manifest themselves on tests.

November 22, 1934 the people’s Commissariat of defense, sent to the General Directorate of aviation industry (GUAP) a letter in which he expressed concern that bombers, ordered by the military in 1935, motors Hispano-Suiza is not enough. Offered to charge TSAGI to install the third copy of the French engines, the Gnome-rhône 14Krsd. It was a two-row radial air-cooled engines with a nominal power of 800 HP, which was developed for plant No. 29 in Zaporozhye under the designation M-85. If the alteration was successful, partial SB in the second half of 1934 it would be possible to produce with these engines.

But the production of a new bomber was delayed and the increased production of M-100 (Soviet copy of “Spanish-Suez”) is quite enough for the whole series. However, from November 1934 to AGOS TSAGI on the basis of the security Council began to design a heavy fighter ANT-46 (DI-8). On the corresponding appeal to the leadership of the Institute, chief of the air force of the red army commander Yakov I. Alksnis wrote a resolution: “Not to oppose the redesign of the aircraft SAT in ДИ8”.

January 10, 1935, the Commission has presented a full-scale model of the ANT-46. It was an almost full analogue pilot bomber SB-2ИС, but with engines of the company “Gnome-Rhone”. The crew consisted of three people-a Navigator, pilot and radio operator-gunner, placed as well as in the bomber. Bomb armament of the fighter was absent. Due to the bombs have greatly increased small arms. In the console housed an automatic denaturaciei (recoilless) guns APK-4 in caliber 76 mm. put Them so that the gas jet does not impinge on the tailplane. Each gun had 15 rounds, one in the trunk and the rest in stores. In the bow stood motionless heavy machine gun ShVAK (namely the 12.7 mm machine gun, 20-mm cannon changed it later) ammunition 150 rounds. Defensive weapons in the fuselage, DI-8 (ANT-46) correspond to the SB, that is, consisted of one ShKAS turret Tur-9 at the top and the second on the pin in the hatch. In addition to the fighter in the center section were two fixed ShKAS machine gun, firing back. Each of them had 500 rounds. For reconnaissance in the fuselage of the aircraft provided for the installation of the camera AFA-13.

The Commission approved the layout, making relatively few comments. Its members proposed to cut an additional window in the Navigator on the left side of the cab, mount the line pneumopathy between the Navigator and gunner, to place the bow machine gun ShVAK mobile unit to put in the rear fuselage of the launch tube for two parachute rockets PAR-13.

Denaturaciei guns like pre-test, setting them on SAT-2ИС, but the speedy finishing of the bomber had attached such importance, that this idea was abandoned.

Heavy fighter DI-8 (ANT-46) to factory tests

The prototype of the D-8 received movable machine gun in the bow cabin. August 1, 1935, M. Yu first raised it in the air. Having inherited the glider of the first embodiment of SB, the fighter suffered from the same drawbacks as experienced bomber – low track resistance, ineffectiveness of the ailerons, a tendency to spontaneous roll. And ways of dealing with it all was similar to that used on the experimental SB: installed a new console with u-turn and zoomed-in cross V, new tail.

But D-8 did not answer the main requirement of technical specification: he was Tehachapi bomber.

The maximum speed he was 388 km/h at an altitude of 4250 m. Worse was the rate of climb: altitude of 5000 m, the aircraft was collected at 11.4 min, yielding SB-2ИС about two minutes.

In TSAGI began to think how to correct the situation. In particular, it was proposed the installation of the motors M-34Ф (not yet available). The rejection of the use of the Dynamo-reactive cannon with their low rate, low projectile speed and poor accuracy of the battle led to the development of alternative options for weapons D-8. It was proposed to install in the wing four machine guns ShVAK or place under the fuselage removable container with five such guns.

But it was not embodied in metal. Factory testing of the ANT-46 was until June of 1936, on state tests the car did not pass. Early in 1937, all work on DI-8 ceased.

But in the spring of 1936 in two different places in parallel led to the development of two other modifications of the SB cannon, closer to a serial bomber.

In April 1936 the Artillery scientific research Institute (AANII) the red army began to explore the possibility of weapons of SB rapid-fire automatic guns. Chose AZP cannon caliber 25 mm (sometimes called AZP-25), created in this very RAD – powerful but quite heavy and with the length of the barrel. Doing estimations and 20-mm cannons ShVAK, which had smaller dimensions and weight. Three options have been considered their placement in the consoles of the wing outside the disk metania screw in the root of the wing, under the fuselage and in the fuselage with the passage of the guns under the pilot’s seat.

The first option was quickly discarded. For gun and ammunition feed mechanism there is clearly not enough space. In addition, the small internal volume of the wing is not allowed to place large ammunition. The strength of the console is considered insufficient, the gun in its recoil could break the spars. A relatively small rigidity has led to a large scatter of shells when firing. But the console in the evolution of the aircraft also “breathes”, bobbing up and down. A considerable distance from the axis of the fuselage to the place of installation of the gun when fired, created a turning point. Uneven firing could lead to a yaw of the aircraft, a further dispersion of the shells, making it difficult for the pilot. Removing guns from the cockpit complicates the fire control system and recharge. Access to guns in the consoles provided only on earth to fix something in flight was impossible.

From the ventral installer refused, arguing that it would increase aerodynamic drag. In addition, it was feared that this would become more likely freezing automatics guns.

ANT-46 to fixed gear ski gear, winter 1935/36 G. Guns APK-4 shrouded

Professionals AANII did not like, and the option of placing the breech of the cannons in front of the bomb Bay. Trunks when it was under the floor of the cockpit, passing on the sides of the space Navigator. This arrangement provides maximum stiffness of the mast, but the ends of very long guns went through the front Windows. When shooting celluloid, most likely, would be dissipated. Glazing can be replaced with a metal sheet, but then the Navigator would lose forward visibility.

Stopped on a variant with installation of guns AJP in the root of the wing in its median plane at the sides of the fuselage. To push them away was impossible, as the clearance from the ends of the blades before plating was equal to only 300 mm. Long barrels to increase the rigidity in the two areas held through couplings, are attached the brackets to the Board. Aerodynamic drag has decided to reduce the closure of shafts and their mounting fairings from dural sheet. Little speakers from the fairings trunks ended at the nose of the plane. The Board was going to protect the linings of stainless steel with a thickness of 3.5 -5 mm. Ammunition intended to be placed in front of the bomb Bay. These approaches have had limited access to guns and feeder of bullets from the cab radio operator-gunner.

From October 1936 began a detailed study of the draft version of SB with two guns AJP. This work led by military engineer 2nd grade Ryazanov.

For this aircraft engineer Alexandrov developed the original power design of guns ammunition. They are not fed from the tape and from the store, and large boxes of cassettes, where were placed rows. The spring pushes the cartridges one by one on a horizontal feed conveyor, ending reel with the notches. They picked up the cartridges and laid in the breech of the gun. It reminded kartechnitsy or rotative modern Gatling gun. The drive of the feed mechanism provided with an electric motor. Cassette stood vertically above and below the conveyor. Their height was different, in accordance with the cross section of the fuselage. Boxes and feeder were taken approximately 400 mm of the length of the bomb Bay. The spent cartridge case was assigned to trays in the back of the bomb Bay.

I must say that the decision of the placement of weapons demanded significant changes in the design of the aircraft. Was planning to reinforce the front spar by replacing the set braces by a continuous wall with a hole for the installation of the gun. The impact could be reduced by mounting the muzzle brake but the barrel was too close to the hull of the bomber; a jet of hot gases could damage it. The root ribs needed to be redone, providing a window for ammunition handling and removal of cartridges. Also going to do the rib hatch for access to the breech of the gun from the bomb Bay.

To free up space under the instrument, it was necessary to reduce the width centralnogo gas tank. The volume was decreased by almost half. The cannon from the tank must be separated by fire partition. The top and bottom of the wing skin was going to cut hatches for access to the gun.

Mount cannon in the wing was designed by an engineer Grigoriev. The first version, made in late 1936, recognized a failure. Rigidity, according to the calculations, was lower than required. In early 1937 began a new design. It turned out to be much tougher, but has become much harder: 55 kg against 24 kg. This weighting was considered unacceptable. Used in the design of the gun carriage forgings replaced riveted nodes. It is possible to reduce weight up to 31.5 kg Around the same time came to the conclusion that the trunks necessary to push over the nose of the aircraft, abandoning the protective steel plating.

Since the gun was mounted motionlessly, the targeting was provided by the pilot, turning the aircraft. When planning to use semi-automatic optical sight PPN-3 and PPN-4. The unit is semiautomatic, which was corrected, was supposed to be the Navigator, and sighting pipe would mount two – pilot and Navigator. Why the second is not clear, because the Navigator could not change the position of the machine to restore it on the target.

But the modification is SB with guns AJP never came to physical implementation. Work on it ceased in late 1937 or early 1938 More successful was a competing project with cannons ShVAK, which was created in the 7th division of TSAGI under the leadership of engineer Schmidt. The first version called 5-Tokarev (aka 5-SWISS), prepared in 1937, the Number “5” indicated the number of guns. All five guns were in the bomb Bay, their trunks slightly beyond the contour of the sheathing and closed a total fairing. To protect the plating on the trunks davincibles steel pipe – covers.

Ammunition in detachable metal tape housed in boxes and filing sleeves. Each gun had one box with 100 rounds. In the sleeves because they had a different length was from 20 to 50 rounds per gun.

To protect the lining of the forward fuselage in front of the trunks, made of anodized aluminum was replaced by stainless steel thickness 0,8 – 1 mm. Reloading of guns was carried out with compressed air, fire control – electric.

The model ANT-46 denaturaciei without guns, instead of with an extra machine guns

The installation weighed 500 kg. According to the original plan 5-ShVAK was mounted when necessary on any SB, turning it into a multi-seat fighter or heavy bomber.

System 5-ShVAK was installed in the fuselage section SAT and shot back in the dash since April 11, 1937 To April 28, fired only one gun at a time, and 7 September for the first time fired a volley with the whole battery of five guns.

Development of 5-ShVAK was a 4-ShVAK. Guns it was one less – removed the one that was in the center. 4-ShVAK was also distinguished by the improved performance of several nodes. Weight turned lower and are quite reasonable – about 470 kg with ammunition. Ammo slightly decreased to 100 – 130 rounds per gun. 4-ShVAK actually mounted on the aircraft, which was taken to the landfill NIPAW in Noginsk. The guns were adjusted so that the converging gave the fan runs. In Noginsk fired on the ground from January 13, 1938

System 5-ShVAK in the dash, 1937

The total rate reached 3000 RDS./min, second salvo ensured the defeat of any aircraft of the time.

Factory testing installation on NIPAW ended successfully. Representatives of the air force required a significant increase in ammunition – up to 250 – 300 rounds per gun.

But further work on 4-ShVAK stalled. Perhaps this was due to the rapid progress in aviation. High-speed bomber SB, for which the system was created, by 1938, had already begun to lag in its data from the new fighters and some bombers of the West. The rate began to do the new project heavy fighters with higher performance data.

The last case of equipment of the bomber SB cannon armament refers to the November 1940 IN NIPAW the aircraft used as a flying test bench for 23-mm gun MP-6 (Taubina-Baburin). The gun was located in a fairing under the fuselage. Thus equipped the two cars and the end of 1941 were tested on the ground and in the air. Their further fate is unknown.

V. KOTELNIKOV

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