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Zveno project
Zveno (Russian: Звено, lit. 'Flight') was a composite aircraft developed in the Soviet Union during the 1930s. It was the brainchild of the aviation engineer Vladimir Vakhmistrov. It consisted of a Tupolev TB-1 or a Tupolev TB-3 heavy bomber mothership and two to five parasite fighters. Depending on the variant, the fighters either launched with the mothership or docked in flight, and they could refuel from the bomber. The definitive Zveno-SPB using a TB-3 and two Polikarpov I-16s, each armed with two 250 kg (550 lb) bombs, was used operationally as a strategic weapon system with good results against targets in the Kingdom of Romania during the opening stages of the Eastern Front campaign in World War II. The same squadron later carried out an attack against a bridge on the Dnieper River that had been captured by German forces.
In June 1931, Vladimir Vakhmistrov from the Scientific Test Institute of the Air Force (Nauchno-Issledovatel'skiy Institut Voyenno-Vozdushnykh Sil, abbr. NII VVS) started work on combinations of fighters rigidly attached to heavy bomber aircraft. The system was envisioned to serve several purposes:[page needed]
In all Zveno configurations, all aircraft were piloted and all had their engines running—it was expected that the combined thrust would more than offset the weight and actually improve the performance of the mothership compared to conventional bombers. The fighters were rigidly attached via pyramid-shaped metal frames, with both latches controlled by the fighter pilots. On Zveno-1, the front latches were controlled by the bomber crew, while the back latch was controlled by the fighter pilot; this was changed to all-fighter-pilot control in the next version. The original design included umbilical fuel lines which allowed the fighters to use fuel from the bomber while attached, although this was not fully implemented in practice.
The first successful flight of Zveno-1, using a Tupolev TB-1 mothership and two Tupolev I-4 fighters mounted on top of the wings, took place on 3 December 1931. An error in the sequence of opening the latches by the bomber crew resulted in one of the fighters prematurely separating, but the TB-1 – with an I-4 still attached to one of the wings – remained in controlled flight and the second fighter was soon uneventfully deployed. After latch control was fully moved onto the fighters, the normal launch procedure consisted of fighter pilots opening the tail lock and then pulling on the control stick to open the front locks and separate from the bomber. As predicted, the presence of docked fighters had a minimal impact on performance of the mothership, and Zveno-2 Tupolev TB-3 carrying three Polikarpov I-5 fighters handled the same as an ordinary bomber. To mount the aircraft on the wings, they were pushed up special ramps but the centerline aircraft had to be lifted on top of the fuselage by hand. This was so cumbersome that the centerline I-5 became a permanent fixture on top of the Zveno-2 TB-3 and never started in the air. At one point, the centerline I-5, still with a pilot at the controls to operate the engine, had the wings and the tail surfaces removed and was used purely as a fifth powerplant for the bomber mothership.
The Zveno-3 in which a TB-3 carried two Grigorovich I-Z fighters under the wings presented a different challenge—the I-Z was a monoplane with fixed landing gear which touched the ground while it was suspended under the mothership. To accommodate uneven ground during takeoffs, the fighters were attached via a floating frame that permitted vertical movement relative to the TB-3. However, immediately after takeoff the I-Z pilots had to push forward on the control sticks to lock the frame in the fixed bottom position—if the fighter-bomber was not held rigid in flight, the bomber became extremely difficult to control. During one of the test flights, one I-Z pilot, Korotkov, incorrectly timed the locking maneuver and the lift generated by his fighter broke the docking frame and he crashed him into the underside of the mothership's wing. The bomber then attempted an emergency landing with both fighters still attached. At the relatively slow landing speed of the TB-3 there was a loss of lift for the I-Z which fell away, killing Korotkov. This was the most serious accident of the entire Zveno program despite the inherent complexity of carrying as many as five aircraft, performing mid-air launches and dockings, and damage to the motherships by propellers and landing gear of the fighters. Tests with various combinations of aircraft determined that top-mounted configurations presented the most difficulty in docking due to turbulent airflow coming off the mothership's wings. The problem of safe underwing attachment was solved with the introduction of the Polikarpov I-16 fighter with retractable landing gear, permitting the use of the same rigid mounting frames as for the top-mounted aircraft.
The mothership and its fighters were unofficially nicknamed the "Vakhmistrov's Circus" (Russian: Цирк Вахмистрова).
The military ordered 40 carriers to be split between the navy and army, but the Soviets were worried of an invasion by the Germans, and production was shifted toward proven weapons. Only five carriers were ever delivered.
In 1938, Vakhmistrov devised the Zveno-SPB (Sostavnoi Pikiruyuschiy Bombardirovschik, lit. 'Combined Dive Bomber') which consisted of a Tupolev TB-3-4AM-34FRN mothership and two Polikarpov I-16 Type 5 fighters. Each of the fighters was armed with a pair of 250 kg (550 lb) FAB-250 high-explosive bombs. Although an I-16 Type 5 could get airborne on its own with no more than 100 kg (220 lb) of bombs, once hoisted in the air by the TB-3 it could reach 410 km/h (220 kn; 250 mph) at 2,500 m (8,200 ft), had a service ceiling of 6,800 m (22,300 ft), and could dive at up to 650 km/h (350 kn; 400 mph).[page needed] Once the bombs were dropped, the SPB-launched I-16s performed like conventional Type 5s. The three-aircraft Zveno-SPB had a total takeoff weight of 22,000 kg (49,000 lb), a top speed of 268 km/h (145 kn; 167 mph), and a range of 2,500 km (1,300 nmi; 1,600 mi).[page needed] The use of a mothership increased the range of the I-16s by 80%.[page needed]
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Zveno project
Zveno (Russian: Звено, lit. 'Flight') was a composite aircraft developed in the Soviet Union during the 1930s. It was the brainchild of the aviation engineer Vladimir Vakhmistrov. It consisted of a Tupolev TB-1 or a Tupolev TB-3 heavy bomber mothership and two to five parasite fighters. Depending on the variant, the fighters either launched with the mothership or docked in flight, and they could refuel from the bomber. The definitive Zveno-SPB using a TB-3 and two Polikarpov I-16s, each armed with two 250 kg (550 lb) bombs, was used operationally as a strategic weapon system with good results against targets in the Kingdom of Romania during the opening stages of the Eastern Front campaign in World War II. The same squadron later carried out an attack against a bridge on the Dnieper River that had been captured by German forces.
In June 1931, Vladimir Vakhmistrov from the Scientific Test Institute of the Air Force (Nauchno-Issledovatel'skiy Institut Voyenno-Vozdushnykh Sil, abbr. NII VVS) started work on combinations of fighters rigidly attached to heavy bomber aircraft. The system was envisioned to serve several purposes:[page needed]
In all Zveno configurations, all aircraft were piloted and all had their engines running—it was expected that the combined thrust would more than offset the weight and actually improve the performance of the mothership compared to conventional bombers. The fighters were rigidly attached via pyramid-shaped metal frames, with both latches controlled by the fighter pilots. On Zveno-1, the front latches were controlled by the bomber crew, while the back latch was controlled by the fighter pilot; this was changed to all-fighter-pilot control in the next version. The original design included umbilical fuel lines which allowed the fighters to use fuel from the bomber while attached, although this was not fully implemented in practice.
The first successful flight of Zveno-1, using a Tupolev TB-1 mothership and two Tupolev I-4 fighters mounted on top of the wings, took place on 3 December 1931. An error in the sequence of opening the latches by the bomber crew resulted in one of the fighters prematurely separating, but the TB-1 – with an I-4 still attached to one of the wings – remained in controlled flight and the second fighter was soon uneventfully deployed. After latch control was fully moved onto the fighters, the normal launch procedure consisted of fighter pilots opening the tail lock and then pulling on the control stick to open the front locks and separate from the bomber. As predicted, the presence of docked fighters had a minimal impact on performance of the mothership, and Zveno-2 Tupolev TB-3 carrying three Polikarpov I-5 fighters handled the same as an ordinary bomber. To mount the aircraft on the wings, they were pushed up special ramps but the centerline aircraft had to be lifted on top of the fuselage by hand. This was so cumbersome that the centerline I-5 became a permanent fixture on top of the Zveno-2 TB-3 and never started in the air. At one point, the centerline I-5, still with a pilot at the controls to operate the engine, had the wings and the tail surfaces removed and was used purely as a fifth powerplant for the bomber mothership.
The Zveno-3 in which a TB-3 carried two Grigorovich I-Z fighters under the wings presented a different challenge—the I-Z was a monoplane with fixed landing gear which touched the ground while it was suspended under the mothership. To accommodate uneven ground during takeoffs, the fighters were attached via a floating frame that permitted vertical movement relative to the TB-3. However, immediately after takeoff the I-Z pilots had to push forward on the control sticks to lock the frame in the fixed bottom position—if the fighter-bomber was not held rigid in flight, the bomber became extremely difficult to control. During one of the test flights, one I-Z pilot, Korotkov, incorrectly timed the locking maneuver and the lift generated by his fighter broke the docking frame and he crashed him into the underside of the mothership's wing. The bomber then attempted an emergency landing with both fighters still attached. At the relatively slow landing speed of the TB-3 there was a loss of lift for the I-Z which fell away, killing Korotkov. This was the most serious accident of the entire Zveno program despite the inherent complexity of carrying as many as five aircraft, performing mid-air launches and dockings, and damage to the motherships by propellers and landing gear of the fighters. Tests with various combinations of aircraft determined that top-mounted configurations presented the most difficulty in docking due to turbulent airflow coming off the mothership's wings. The problem of safe underwing attachment was solved with the introduction of the Polikarpov I-16 fighter with retractable landing gear, permitting the use of the same rigid mounting frames as for the top-mounted aircraft.
The mothership and its fighters were unofficially nicknamed the "Vakhmistrov's Circus" (Russian: Цирк Вахмистрова).
The military ordered 40 carriers to be split between the navy and army, but the Soviets were worried of an invasion by the Germans, and production was shifted toward proven weapons. Only five carriers were ever delivered.
In 1938, Vakhmistrov devised the Zveno-SPB (Sostavnoi Pikiruyuschiy Bombardirovschik, lit. 'Combined Dive Bomber') which consisted of a Tupolev TB-3-4AM-34FRN mothership and two Polikarpov I-16 Type 5 fighters. Each of the fighters was armed with a pair of 250 kg (550 lb) FAB-250 high-explosive bombs. Although an I-16 Type 5 could get airborne on its own with no more than 100 kg (220 lb) of bombs, once hoisted in the air by the TB-3 it could reach 410 km/h (220 kn; 250 mph) at 2,500 m (8,200 ft), had a service ceiling of 6,800 m (22,300 ft), and could dive at up to 650 km/h (350 kn; 400 mph).[page needed] Once the bombs were dropped, the SPB-launched I-16s performed like conventional Type 5s. The three-aircraft Zveno-SPB had a total takeoff weight of 22,000 kg (49,000 lb), a top speed of 268 km/h (145 kn; 167 mph), and a range of 2,500 km (1,300 nmi; 1,600 mi).[page needed] The use of a mothership increased the range of the I-16s by 80%.[page needed]