Zveno project Zveno ( Russian : Звено , Chain link or a military unit " Flight " ) was a parasite aircraft concept developed in the Soviet Union during the 1930s . It consisted of a Tupolev TB - 1 or a Tupolev TB - 3 heavy bomber acting as a mothership for between two and five fighters . Depending on the Zveno 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 - 16 s , each armed with two 250 kg ( 550 lb ) bombs , was used operationally with good results against strategic targets in Romania during the opening stages of the German - Soviet War . The same squadron afterwards also carried out a tactical attack against a bridge over the River Dneiper that had been captured by advancing German forces . Development . In June 1931 , Vladimir Sergeyevich Vakhmistrov of the NII VVS ( " Nauchno - Issledovatel ' skiy Institut Voyenno - Vozdushnykh Sil " – scientific test institute of the air force ) started work on combinations of fighters rigidly attached to heavy bomber aircraft . The system was envisioned to serve several purposes : 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 bomb 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 pilot , this was changed to all - pilot control in the next version ) . The original design included umbilical fuel lines which permitted 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 attached to one of the wings remained in controlled flight and the second fighter was soon uneventfully deployed . After latch control was fully moved into the fighters , the normal launch procedure consisted of 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 fifth powerplant for the bomber mothership . The Zveno - 3 in which 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 the 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 rigid in flight , the bomber became extremely difficult to control . During one of the test flights , I - Z pilot Korotkov incorrectly timed the locking maneuver and the lift generated by his fighter broke the docking frame and crashed him into the bottom of the mothership ' s wing . As the bomber came in for an emergency landing with both fighters still attached , the slow landing speed of the TB - 3 resulted in 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 appearance of the Polikarpov I - 16 fighter with retractable landing gear . This permitted 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 " ( Цирк Вахмистрова ) . Operational history . In 1938 , Vakhmistrov devised Zveno - SPB ( SPB : Sostavnoi Pikiruyuschiy Bombardirovschik , 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 knots , 255 mph ) at 2500 m ( 8 , 200 ft ) , had a service ceiling of 6800 m ( 22 , 310 ft ) , and could dive at up to 650 km / h ( 350 knots , 405 mph ) . 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 22000 kg ( 48 , 500 lb ) , a top speed of 268 km / h ( 145 knots , 165 mph ) , and a range of 2500 km ( 1 , 350 NM , 1 , 550 mi ) . The use of a mothership increased the range of the I - 16s by 80 % . The SPB first flew in July 1937 , with TB - 3 piloted by Stefanovskiy , and I - 16s piloted by Nikolayev and Taborovskiy . Following the successful test program in 1938 , Zveno - SPB was accepted into service . By 1 February 1940 , Soviet Air Force was supposed to receive 20 TB - 3s and 40 I - 16s , with the same number going to the Soviet Navy . Vakhmistrov was also asked to investigate the possibility of using Tupolev TB - 7 , Tupolev MTB - 2 , and GST ( PBY Catalina ) as the motherships , as well as arming I - 16s with 500 kg ( 1 , 100 lb ) bombs . By 1939 , the government support for the project had waned , the Navy canceled all of its orders , and the Air Force reduced the number of fighters from 40 to 12 . However , Soviet military observers noted the success of the Luftwaffe Junkers Ju 87 dive bomber s in the opening stages of World War II . As the Soviet Union had no dive bombers , it was decided to resume low - scale work on the Zveno - SPB . Testing of the first production Zveno began in June 1940 . It differed from the prototype in using the much more powerful I - 16 Type 24 fighters . A total of six mothership - fighter combinations ( six TB - 3s and twelve modified I - 16 Type 24s ) were completed . All were attached to the 2nd Special Squadron of the 32nd IAP ( Fighter Regiment ) of the 62nd Aviation Brigade of the Black Sea Fleet Air Force stationed in Eupatoria . Mirroring the nickname of the Zveno experiments , the squadron was dubbed " Shubikov ' s Circus " ( Цирк Шубикова ) after its commander Arseniy Shubikov . Zveno - SPB saw limited but successful combat use during the German - Soviet War . In the opening stages , the Black Sea Fleet Air Force was tasked with destroying industrial targets in Nazi Germany - allied Romania . The most important of these was the King Carol I Bridge over Danube which carried the Ploieşti - Constanţa oil pipeline . After several failed attempts to destroy the heavily protected bridge with conventional bombers , the task was given to the Zveno squadron . As a combat test , it was decided to first attack the Constanţa oil depot . On 26 July 1941 , two Zveno - SPB aircraft performed a successful attack on the depot in broad daylight with no losses . The fighters disconnected 40 km ( 22 NM , 25 mi ) from the target and returned to the home airfield under their own power . The first of the two bridge raids took place on 10 August 1941 . For this mission , the I - 16s were fitted with additional 95 - liter ( 25 US gal ) underwing fuel tanks for an additional 35 minutes of flight time . Of the three Zveno - SPBs , one had to turn back due to mechanical problems . The other two launched their fighters 15 km ( 8 NM , 9 mi ) from the Romanian coastline . The fighters successfully dive - bombed from the altitude of 1800 m ( 5 , 900 ft ) and returned home with no losses despite heavy anti - aircraft fire . The second raid took place on 13 August 1941 . This time , all three Zveno - SPBs reached the target . The six fighters scored five direct hits on the bridge and completely destroyed one of the spans . On the way back , the fighters strafed Romanian infantry near Sulina and returned to Eupatoria with no losses . Following the successful sorties , two additional Zveno - SPB were brought to operational status , bringing the total to five . The main limiting factor was the lack of high - output Mikulin AM - 34 FRN engines , as the other versions were not powerful enough to get the aircraft airborne . On 16 August 1941 , Admiral Kuznetsov asked Joseph Stalin for additional AM - 34FRN - engined TB - 3s from the Air Force so they could be converted to Zveno - SPB carriers , but the request was denied as the Air Force had suffered heavy losses in the opening days of the war . In the meantime , the five aircraft continued flying operational sorties , destroying a dry dock in Constanţa on 17 August and a bridge across the Dnieper River on 28 August , losing one I - 16 in the process . During the repeat attack the next day , four Zveno - launched I - 16s engaged several Messerschmitt Bf 109 s , shooting down two . Despite the high success rate , Zveno missions ended by 1942 due to high vulnerability of the obsolete TB - 3s and I - 16s in the face of enemy air superiority . It is estimated that Zveno - SPB flew at least 30 combat missions .