DRP/APK recoilless cannon

Designed by L. V. Kurchevskij. The designations are standing for Dinamo-Reaktivnaya Pushka (dynamic-reactive, i.e. recoilless, cannon) and Avtomaticheskaya Pushka Kurchevskogo (automatic cannon by Kurchevskij) respectively. Some sources read the "A" as "Aviatsionnaya" ("Aircraft") instead of "Avtomaticheskaya".


18k b/w photo of 76.2mm APK-4 cannon under wings of a Grigorovich IP-1 from "The complete Book of Fighters" by W. Green and G. Swanborough.

In the late 1920s it became more and more obvious, that the era of rifle-caliber machine guns as primary aircraft weapons was coming to an end. This led to several approaches for larger-caliber weapons. One of the solutions tried in the USSR were recoilless cannon for aircraft use, one prominent supporter being Marshal M. N. Tukhachevskij. With the exception of some experiments in the USA during the First World War (Davis gun) this was something absolutely new in those days. The Soviet pioneer in this field was L.V.Kurchevskij. In his recoilless guns a quite simple principle was used: They fired two projectiles simultaneously, which left the gun in opposite directions. The live bullet left the muzzle for the target and the compensating bullet exited through the rear end of the barrel. So there were no recoil forces and no high stresses for the airframe. The only disadvantage lay in the free space needed to the rear for the exiting compensating bullet and flames. And the early DRP designs were single-shot devices which could not be reloaded in flight.

The experiments with DRPs for aircraft can be traced back until 1923. In that year were conducted tests of different DRPs (not necessarily designed by Kurchevskij) with a caliber of up to 6 inches in a Junkers F 13. In the same year Kurchevskij also introduced his DRP at the OVI-VSNKh and tested them on the proving ground at Rzhev. But in 1924 Kurchevskij was arrested and sent to the Solovki prison camp (an island in the White Sea). There he thoroughly reworked the construction of his DRP. In 1926 those guns were built and demonstrated. In the same year the first DRPs were also introduced to the inventory of the Soviet infantry.

In 1928 Marshal Tukhachevskij ordered L. V. Kurchevskij to install the DRP in aircraft. In 1929 Kurchevskij introduced such weapons with a caliber of 3 and 4 inches respectively for "the use from aircraft against slowly maneuvering targets like dense bomber formations". This idea was favored by the NTK AP member Nadashkevich and the guns were built two months later under the designation APK-1.

In December 1931 tests were conducted with an I-4 fighter equipped with two 76.2mm DRP-76 guns under the upper wing, outside of the strut anchorage points. Firing against ground targets was successful, but during those tests one of the cannon exploded. Nevertheless the experiments continued until the retirement of this machine in 1935/ 1936.


47k b/w photo of the Sukhoj I-4 with two 76.2mm DRP-76 under the upper wing from "OKB Sukhoj - A History of the Design Bureau and Its Aircraft" by V. Antonov, Ye. Gordon et al.

While the first DRPs could fire only one single shot, they were soon developed into weapons which were reloadable in flight with a capacity of six to ten shells. This obviously led to the designation APK instead of DRP (the DRP-76 thus becoming the APK-4 with the same 76.2mm caliber).

In the following years tests were continued with a number of different APK models, ranging in caliber from 37mm to 102mm (see table below). They were installed in a number of aircraft designs, most of them specially designed for their use. But the experiments were only partially successful, on some occasions the guns exploded and their accuracy against aerial targets left much to be desired. Nevertheless in 1934 series production of the APK-4 is told to have been initiated at Factory No. 7 at Leningrad. A second production batch is told to have been produced by Factory No. 8 at Podlipki and "Bolshevik" at Leningrad.

It seems that about in 1936 the further development and testing of the DRP/APK guns as an aircraft armament was finally abandoned. It is known that in January of that year Molotov received a report from the High Command of the Navy stating that "... the events during the trials conducted by the People's Commissariat for Defence in the last few years to find a way of using the APK-4 have led to the conclusion, that it is totally unsuitable as an aircraft armament. Due to the low muzzle velocity of its projectiles the hitting accuracy is rather low. Calculations of the Command of VVS-RKKA, based on firing trials at Yevpatoria, showed that those guns have a hitting rate of only 0.3% against heavy bombers in a distance of 1km." Additionally, the introduction of the RS-82 rockets, which could be carried by any fighter without major modification of the aircraft, made the DRPs the more unnecessary. Its designer Kurchevskij was arrested again in 1937 and remained in prison until January 1939.


42k b/w photo of Tupolev ANT-29 with 102mm APK from "The complete Book of Fighters" by W. Green and G. Swanborough. Note small insert photo of the aircraft's tail with the exhaust tube of the gun.

Detailed technical data about those weapons have yet to be found. The following table lists the DRPs found in the available documents in chronological order.


Gun Caliber Year Installed in
? up to 152mm 1923 Junkers F 13
DRP-76 76.2mm 1926? I-4?, I-Z?
APK-1 76.2mm, 102mm 1929 ?
APK-4 76.2mm 1930? I-4, I-12, I-Z, IP-1
APK-6bis 65mm 1931 I-4
APK-11 (or APK-37) 37mm 1932 I-4, I-14bis, I-Z
APK-36bis ? 1933 I-4
APK-4bis 76.2mm 1933 I-Z
APK-11 45mm 1934 DI-8, IP-4
APK-4M 76.2mm 1935 I-Z
APK-5 75mm 1935 ANT-17, ANT-31
APK-8 102mm 1935 ANT-29
APK-100 100mm 1935 DI-8?
? 80mm 1936 BICh-17

Some more pictures:
I-14bis with wing-mounted 37mm APK-11.
Rear end of 37mm APK-11 under wing of I-Z.
DI-8 with wing-mounted 45mm APK-11 (or 100mm APK-100?).
I-12 with boom-mounted 76.2mm APK-4.
BICh-17 with wing-mounted 80mm APK.

References:
"History of aircraft construction in the USSR", Vol.1 and 2;
"History of aircraft industry in the USSR" by Ivan I. Rodionov, found at A. Grechikhin's Site;
"Sowjetische Jagdflugzeuge" ("Soviet Fighters") by W. Kopenhagen (in German);
"The complete Book of Fighters" by W. Green and G. Swanborough;
"OKB Sukhoj - A History of the Design Bureau and Its Aircraft" by V. Antonov, Ye. Gordon, N. Gordyukov, V. Yakovlev, V. Zenkin, L. Carruth and J. Miller;
"Aero" magazine, different volumes (in German);
All-new design September 25, 2000
by Thomas Heinz;
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