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BOK / Bureau of Special Constructions BOK-1 / TsKB-2
The project of the "stratospheric" aircraft (as they were commonly called at that time) SS or BOK-1 was conceived using as a base the long-range monoplane ANT-25 developed by the Sujoi brigade in the collective led by AN Tupolev. Basically the BOK-1 was an ANT-25 with reduced wingspan and fixed landing gear fairing and was the first work of the Bureau of Special Constructions (BOK) under the leadership of Chizhevski.
BOK-1 or SS (acronym for Stratospheric Plane from Russian Стратосферный Самолет) was a 1935 experimental aircraft designed to test flight capability at heights between 12000 and 13000 meters.
A single prototype was built which was successfully flown and the first pressurized cockpit manufactured in the USSR was testad.
Construction of the BOK-1 was started at the TsAGI's Experimental Construction Factory (ZOK) in late 1932. This factory had been created at the beginning of that same year, after the unification of the constructive capacities of the TsKB and the TsAGI.
In February 1933 the BOK was transferred to the TsKB of Factory No.39, under the direction of SV Ilyushin. The roughly 60 workers under Chizhevski's leadership became Brigade No.3 and BOK-1 received the TsKB-2 designation. A short time later, in the summer of 1934 the group was again transferred, this time to Smolensk, to the base of Factory No. 35 dedicated to automotive repair and maintenance.
Despite the low qualification of the personnel and the lack of management cadres, Chizhevski managed to concentrate the group towards the main task and in this way the production of the BOK-1, begun in Moscow in 1932, could be completed three years later.
The BOK-1 was conceived entirely in metal. The wing consisted of the centroplane and removable three-spar consoles. The removable consoles have 16 ribs, the upper edge of which is exposed on the wing surface. The first two stringers of the consoles up to the middle of the span were made of steel tubes with a chromium-molybdenum alloy and from there up to the wing end were made of duralumin. The third stringer, to support less load, was built entirely of duralumin. The wing skin was corrugated metal, similar to that used on the ANT-25. During the tests, the corrugated surface of the wing surface was covered with glued calico, which was then polished and painted.
The fuselage was made up of three sections: bow, center (built integrally with the center plane) and tail. The front of the aircraft could be easily removed to allow the installation of different power plants. The central section was built in such a way that the pressurized cabin could be installed from above.
The pressurized cabin was calculated to accommodate two crew members and conceived as an independent element, not forming part of the aircraft's structure. It was built in the shape of an oval cistern, made up of 11 metal frames and covered by a 2 mm sheet of duralumin. The pilots' access was through a hatch similar to those used in submarines, located in the upper part of the center of the aircraft that gave access to a cylindrical area. There it was necessary to leave the parachutes and then go to the cockpit, which was accessed through another hermetic hatch located in the rear wall. Space was tight and the cabin uncomfortable.
Structure of the pressurized cabin used in the BOK-1
Felt layers were placed between the top of the cabin and the fuselage skin. This coating functioned as a thermal insulator for the cabin, while providing acoustic insulation, so that the crew could converse without the need for headphones.
The cabin was located above the water cooling radiator of the engine. In flight, the temperature in the cabin as a result of the heating received from below reached 40ºC. To regulate the heating there was a small hatch governed from the cabin.
To guarantee a certain degree of visibility, seven windows similar to the skylights of the boats were fitted. The pilot had five windows and the observer had two. Each window was double glazed. The 15-16mm outer glass was tempered. The interior was 3-4 mm thick and was constantly blown with hot air. Mechanisms were created inside the cabin to absorb carbon dioxide produced by respiration and inject fresh oxygen at a constant rate of 120 liters per hour. The two crew members had oxygen masks for emergencies.
The landing gear of the BOK-1 was of the fixed and conventional type, with oleo-pneumatic dampers and drop-shaped aerodynamic fairings on the main landings. The wheels had dimensions 900 x 200 mm.
Structural drawing of the BOK-1 clearly showing the location of the crew in the cockpit.
The BOK-1 in its original version used an AM-34RN engine designed by Mikulin, capable of developing 800 hp at a height of 4000 meters. This engine was later replaced by turbocharged versions. The cooling radiator was located at the bottom of the fuselage and featured adjustable gates. A little forward was the oil cooler. During the first flights, a 4.0-meter diameter four-bladed wooden propeller was used, which was later replaced by a 4.35-meter double-bladed propeller and finally by a 4.1-meter diameter metal one.
The total fuel capacity reached 500 kg and was distributed in 4 cylindrical tanks located between the first and second spars of the centroplane.
The BOK-1 was painted in a combination of two colours: red and silver. In the empennage it wore the triangular outline, symbol of the BOK.
The 13 of December of 1935 the IF test pilot Petrov and chief engineer for testing (and one of the developers of the model) NN Kashtanov, made the first flight in the BOK-1.
The BOK-1 “stratospheric” aircraft, equipped with an AM-43FRN engine with a three-bladed propeller, was the first Soviet aircraft to have a pressurized cabin. During the first flight it was found that at 9000 meters the pressure inside the cabin corresponded to a height of 2 - 3 km above the ground and the temperature oscillated around 25ºC.
The flight program was continued in 1936. IF Petrov was forced to constantly move from Moscow (where he worked at the NII VVS) to Smolensk for the development of these tests, so the decision was made to continue them with PM Stefanovski, who in the summer of 1936 moved to Smolensk on secondment.
During the tests the 25 of June of 1936 Stefanovski and Kashtanov reached a height of 10,875 meters flying. At this point, two problems were manifested: the exterior glass of the windows was completely fogged, impeding the vision and the wing ailerons were frozen, losing all effectiveness.
The BOK-1 was transferred to Moscow, to the NII VVS test airfield . The fight for the dominion of the heights entered a new stage. It was a question of the tests to determine the maximum ceiling. To this end, the plane was especially lightened. On Stefanovski's first flight on the NII VVS a ceiling of 14,100 meters was reached.
In the fall of 1936 the BOK-1 flights on the NII VVS were canceled. The atmosphere was getting too cold. By that time, 42 flights had been made reaching speeds of 230 km / h at sea level and 242 km / h at 400 meters with a normal takeoff weight of 4162 kg.
The results obtained so far were recorded in the report presented by the head of the NII VVS Korobov on December 10, 1936.
These results are summarized in the following:
- The feasibility of using the pressurized cabin for high-altitude aircraft was verified.
- The design of the selected cabin was shown to be functionally correct, so that it was possible to go from experimental construction to the development of specific objectives.
- The BOK-1 experimental aircraft proved to be an excellent research device that could be used as a flying bed for future experimental developments of pressurized cabins, equipment and weapons tests in the conditions of the stratosphere.
- The tasks to be solved in the short term were:
BOK-1 flight ceiling increase
Fixing the problem with glass hatches
Improved visibility
Weapons installation and testing
Creation of defensive firing points operable from the watertight cockpit
Armor of the cabin to prevent its de-sealing due to the impact of projectiles.
As conclusions, the Narkomat of the Defense Industry was asked to create military aircraft with a hermetically sealed cabin, improve the working conditions of the BOK, develop the BOK-7 version in a military version as a high-altitude bomber and force preparation in the TsIAM of the AM-34TN turbocharged engine to locate in the BOK-1.
On December 22, the report was signed by the head of the VVS Ya. I. Alksnis, who requested to raise the ceiling to 20 thousand meters.
The new AM-34RNB-TK engine was installed in the BOK-1 in the spring of 1937. After the ground set-up tests, the test flights were continued. In the period between June 6 and October 1, the new engine worked for 5 hours 40 minutes on the ground and 24 hours 1 minute in the air.
The 27 of August of 1937 Stefanovski, with Reno as an observer, reached a ceiling of 14100 m before the turbocharger is damaged. Repairs continued until February 1939. In April 1939, a new AM-34FRNV engine with TK-1 turbocharger was installed in the BOK-1, with which several flights were made without exceeding 14,000 meters.
In the second half of the 1930s, the BOK-7, BOK-8 and BOK-11 models were created on the basis of the BOK-1, in which the concept of a pressurized cabin and the use of turbochargers were developed. Some of these models were designed with remotely controlled weaponry, which was a major technological achievement.
The BOK-1 development versions include:
BOK-7 - Originally conceived as a stratospheric reconnaissance aircraft, it ended up being developed as a record-setting aircraft. It began to be projected in 1935 as a development of the OKO-1. It was used on the ground for crew training for the BOK-15.
BOK-8 - Conceived as an armed development of the BOK-7 (the second copy). It was not built, but it became the BOK-11.
BOK-10. Exploration version project on the BOK-7. High altitude bomber developed on the BOK-7. It was used on the ground for crew training for the BOK-15.
BOK-12. Project for a twin - engine version of the BOK-7 conceived as an experimental flying laboratory. A large aperture camera for exploration was planned to be installed in the BOK-12 fuselage. It was never realized.
BOK-13. High altitude passenger plane project with capacity for 6 people developed on the basis of the BOK-7.
BOK-15. A powerful stratospheric plane conceived in 1937 to set a new world record for flight distance. Two copies were built in 1939. The start of the Great Patriotic War brought about the cancellation of flights.
Retired in 1939, the BOK-1 served as the basis for a series of experimental developments.
Engine: 1 x Mikulin AМ-34RN 830 hp
Wingspan: 30.00 m
Wing area: 78.80 m²
Length: 12.86 m
Height: 5.50 m
Wheelbase: 5.19 m
Empty weight: 3481 kg
Normal takeoff weight: 4600 kg
Maximum takeoff weight: 4880 kg
Maximum speed SL: 240 km / h
Maximum speed at 9000 m: 315 km / h
ROC: 500 m / min
Landing speed: 85 km / h
Practical ceiling: 12000 m
Maximum ceiling (lightened): 14,100 m
Seats: 2

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