Saturday, 4 March 2017

02SS Aerosan: A Tank in the Snow

The history of the aerosan in Russia and then the USSR is old and varied. The first mass produced aerosans were built in 1912, were actively used in WWI and then the Russian Civil War. Development of aerosans received great attention due to the geographical features of our country. It was not surprising that the Red Army had the largest aerosan fleet in the world during the Great Patriotic War. Meanwhile, the widely used NKL-26 reconnaissance aerosan was a pale shadow of what was initially planned for production. The wishes of the Soviet military materialized as the 02SS battle aerosan which had no equivalent in any country in the world.
Sled with Armour and Armament

Several groups worked on aerosans in the interbellum period. Since there was no urgent need for military aerosans, most designs were meant for civilian work. Of course there was some research into military applications, but none of it moved past experimental works.

Military aerosans were suddenly in demand in late 1939. The Winter War between Finland and the USSR became a proving grounds of sorts. Soviet heavy tanks were first used in battle here, as well as various novel military engineering solutions. It quickly became obvious that a vehicle that could quickly and surely move across snow was needed to perform a number of tasks. Tracked vehicles could not do this in that theater, so the aerosan found work.

Initially, there was no military aerosan in the Red Army, and civilian models were pressed into service. Typically, this was A.N. Tupolev's ANT-IV or N.M. Andreev's OSGA-6 (NKL-6) and NKL-16.

Aerosan units were given a wide range of objectives, including reconnaissance and evacuation of the wounded. NKL-16 aerosans were converted in the field; an aircraft turret with two ShKAS machineguns was mounted on the roof. However, the presence of armament alone could not make the NKL-16 into a patrol vehicle. The aerosan lacked armour, which made it vulnerable to even small arms.

TsKB-50 ended up building an aerosan that looked like a motorboat.

The experience of using aerosans in the Winter War was carefully studied by Red Army commanders. A decision was made to develop a special patrol aerosan. Aside from armament, this aerosan had to have reliable protection from small arms fire.

Several design bureaus began working on the patrol aerosan project in 1940. In the late 1930s, aerosans were within the jurisdiction of the People's Commissariat of Forestry, but some projects were curated by the People's Commissariat of Shipbuilding (NKSP). In addition, aircraft manufacturers (Tupolev's design bureau) and the Central Scientific Research Automobile and Automotor Institute (NAMI), as well as individual self-taught engineers like Grokhovskiy, were also working on aerosans.

The same aerosan from the front.

Even though work on armoured aerosans continued actively throughout 1940, the first steps to building one were made only towards the very end of the year. Decree #451ss was issued by the USSR SNK, which approved tactical-technical requirements for a 3.5 ton aerosled. According to the decree, a full size model would be built by December 25th, 1940, and a prototype a month later. TsKB-50 (Leningrad) was chosen as the designer. Its usual duties consisted of designing ships, including armorued motorboats.

A frame protected the propeller from branches.

N.A. Makarov was in charge of the TsKB-50's aerosan group. Nikita Afanasyevich dedicated his life to shipbuilding, and did not partake in building aerosans until 1940. However, he designed an amphibious motorboat in 1939, and it's possible that this was his starting point for aerosan work.

The choice of the producer of the experimental aerosan was no less surprising. The assignment was given to the NKVD shipbuilding factory #5 (modern day OAO "SF Almaz") in Leningrad. The People's Commissariat of Shipbuilding controlled the project. Given the unusual designer and producer, it's no surprise that the outcome was unexpected.

When Shipbuilders Build Aerosans

Correspondence in 1940 gave the index "02SS" to the aerosan. However, GABTU documents referred to it as "TsKB-50 armoured aerosan". The name of the project, "Armoured Aerosan (Tunguska type)" implied that the TsKB-50 design would be significantly different from any other design bureau's work.

The choice of the Tunguska sled (dog sled) as the layout was not the only distinguishing choice by Leningrad shipbuilders. Whenever a design bureau was tasked with producing a design outside of their previous experience, the result was rather original. It's not difficult to come up with an example. Tupolev himself was known as not only a designer of planes or aerosans, but also the G-5 torpedo boat, whose hull looks similar to a seaplane buoy.

Aerosan crossing a snow bank.

The tactical-technical requirements specified that the crew of an armoured aerosan consisted of three men, but it was reduced to two during the model building stage. The calculated mass grew from 3.5 to 4.7 tons by December 25th. The prototype was built on schedule, by January 25th, 1941. 400,000 rubles was spent on experimental work, and the cost of a production aerosan was estimated at 300,000. To compare, this was close to the cost of a T-50 tank, and its price was already excessive.

It would be more correct to compare the aerosan with the cheaper T-40 amphibious reconnaissance tank, which was closer in mass, armament, and purpose. Even the most expensive version of this tank cost less than half as much as the aerosan.

A special device for breaking the aerosan away from snow if it froze in.

The shipbuilding experience of TsKB-50 was plainly visible. The design looked closer to a motorboat on skis than an aerosan. The 7 meter long hull was completely armoured, aside from the plywood aerodynamic cap on the front which covered the batteries and toolbox. The crew entered the aerosan through doors on the sides. The hull was bolted to a frame which sat on wooden skis lined with stainless steel for better sliding.

Since the skis were rigidly attached to the frame, a special mechanism had to be used to steer, similar to a ship's rudder. Two rudders were installed behind each ski. The aerosan was steered with pedals and a steering wheel. The KRSTB radio was used to communicate, identical to the one used on tanks.

The turning mechanism was unmistakably naval in origin.

The liquid cooled M-103A aircraft engine installed in the rear behind an armoured bulkhead propelled the aerosan. The same 850 hp engine was used on SB bombers. The high cost of the aerosan is, in part, explained by this expensive engine. The cooling radiators were placed underneath, and large air intakes were installed in the side of the hull. The engine drove a three-bladed propeller, and a special frame was used to protect it from tree branches.

The rudders were lengthened during trials.

The armament of the TsKB-50 aerosan was no less original. It was common for armoured motorboats to receive tank turrets, since this was easier and cheaper than building new ones. The 02SS aerosan had a turret borrowed from the T-40 tank. Other elements of the T-40, such as observation devices and the driver's observation hatch, were also used in the design.

The armament of the aerosan was more impressive than that of the tank. Instead of a 12.7 mm DShK machinegun, the project used the 23 mm MP-6 (PT-23TB) autocannon designed by OKB-16 with a coaxial 7.62 mm DT machinegun. The crew also had personal weapons: 20 grenades and a PPD submachinegun. The submachinegun could be fired from five ports in the hull and turret.

It took multiple tries to traverse a country road, and with such a powerful engine!

Even though OKB-16 chief, Taubin, insisted that the MP-6 was in mass production, there was only one such gun. According to GABTU's plans, the MP-6 would be installed and tested in the T-40 tank in January of 1941, and the gun was already installed in the tank. Days before the trials were scheduled to start, the gun was removed and moved to the aerosan.

Several defects were discovered after the trials. The photograph shows damage to bolts and the weld seam between the cockpit and fuel tank armour plates.

The verdict of the acceptance commission seems interesting:
  1. The weight of the experimental prototype is 5360 kg, instead of the 4700 kg approved by the Model Commission.
  2. A used up MP-6 gun sent by OKB-16 is installed instead of a combat ready specimen.
  3. The TMFP optical sight is missing (the People's Commissariat of Armament never sent one).
  4. 36 magazines of ammunition are missing from the full ammunition loadout.
In other words, the gun was only good as a dummy weight, and gunnery trials could not be performed. This failure was directly connected with repressions of the OKB-16 management.

Too Heavy and Too Expensive

The first proving grounds trial of the TsKB-50 aerosan took place between February 10th and February 18th, 1941. The trials revealed that the cooling system design requires changes. A device made of two small skis that could lift up and tear the aerosan away from the snow was also requested. It took factory #5 five days to change the cooling system and add the starting device.

Damage to the wooden beam that connected the skis in the front of the sled.

A second stage of factory trials was conducted between February 24th and March 3rd. The second stage of proving grounds trials began on March 4th and continued until March 21st. Based on their results, the chief of GABTU, Lieutenant-General Fedorenko made some unfortunate conclusions on April 24th, 1941:

"The trials revealed a number of serious design defects caused by the project authors (TsKB-50) not accounting for experience in designing, producing, and using civilian aerosans.
The experimental prototype of the armoured aerosan has the following drawbacks:
  1. Heavy weight: 5360 kg instead of 3500 kg in the tactical-technical characteristics. The 5-7 mm of armour protects from a normal bullet only at a range of over 70 meters and from an armour piercing 7.62 mm bullet from a range of over 700 meters.
  2. High ground pressure. If the pressure is evenly distributed, it reaches 0.13-0.15 kg/cm². whereas the ground pressure of normal aerosans does not exceed 0.04-0.06 kg/cm². The center of mass of the prototype is too far back, causing the rear part of the skis to sink into the snow more than permissible, which causes excessive friction during motion.
  3. Poor maneuverability. When moving across snow with an ice crust, the turning radius is 150-200 meters. On roads, the aerosan is almost completely uncontrollable.
  4. The 860 hp M-130A aircraft engine is irrationally used to achieve a speed of 50-60 kph, which provides a power to weight ratio of 150 hp/ton, 8-10 more times than that of any tank. At the same time, the aerosan can only drive across a level field with a grade of no more than 10-15 degrees. The aerosan cannot cross a snow bank over one meter tall.
  5. In addition, the prototype does not have:
    1. A backup engine starter.
    2. Brakes.
    3. Reverse gear.
    4. Starting from standstill.
    5. Fixation of components, etc.
Based on the above, I consider that the authors of the project (TsKB-50) did not solve this problem in a satisfactory manner. It is not reasonable to improve and produce a design with such a large number of major defects. It is difficult to build such an aerosan, therefore I consider it more correct to assign the design to two design bureaus and factories, namely one prototype from TsKB-50 and factory #5, and the other from the Izhor factory. The latter is currently working on an aerosan on its own initiative.
The due date for the design should be established as November 1st, 1941, with the goal of completing factory and proving grounds trials by January 1st, 1942."

A number of bolts that attached the hull to the frame broke during the trials. There was also damage to the aerodynamic cap, starter mechanism, and bolts connecting the front part to the crew compartment. TsKB-50 built a heavy and expensive vehicle that simply didn't take off. It was not surprising that the aerosan received a poor evaluation during a meeting on May 24th, 1941, between specialists from the People's Commissariat of Aircraft Building, GABTU, NKSP, and TsKB-50. Management of TsKB-50 did not argue with this evaluation.

Attaching the hull to the frame with bolts proved unreliable.

The "Armoured Aerosan (Tunguska type)" was not the largest and most ambitious project. TsKB-1, headed by B.I. Levkov, proposed an armoured aerosan hovercraft in the spring of 1941. Its advantage was that it could travel across swamps, rivers, ice, and snow, not only in the winter, but also in the summer.

Sadly, no sketches of the design were preserved, but the overall specifications made it clear that this was a continuation of Levkov's hovercraft design. The three-man 8-9 ton vehicle would be equipped with two M-62 aircraft engines and have the same armament as the 02SS aerosan.

Damage was also noticed in the rear of the aerosan, around the rudders.

TsKB-50 representatives supported Levkov's proposal, but B.N. Yuryev and A.A. Arkhangelskiy, engineers representing the NKAP, criticized the proposal. According to calculations, the design was expensive (development and construction would cost 1.5 million rubles), the engines were poorly protected from Molotov cocktails, and were also rare and expensive. Simple logic dictated that a reconnaissance vehicle that cost as much as three KV-1 heavy tanks was excessive. Nevertheless, a draft decree for the development and construction of the aerosan was prepared. Two prototypes with different armament would be built at factory #445 in Moscow.

Requirements for a more humble vehicle were composed in parallel. Its mass was 3.5-4 tons, the crew, armour, and armament were the same as that of the TsKB-50 design. However, work did not proceed past the design stage. The start of the Great Patriotic War required new simpler and cheaper ways of equipping the Red Army with patrol aerosans.

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