PIBWL presents:

Polish 10TP cruiser tank

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	© Michał Derela, 2010

In the 1920s, a tank concept was still in its infancy, and many inventors were looking for their own ways to create an ideal fighting machine. A common fault of early tanks was their low speed, suitable for position warfare, rather than breaking raids. Several designers tried to overcome this problem by creating wheel-cum-track combat vehicles, combining best features of tanks, like off-road mobility, and armoured cars, like high speed on roads. The best known designs of a wheel-cum-track tank (and the only relatively successful ones), were the machines designed by J.W. Christie. The Polish designers followed his steps, creating the 10TP tank, which remained however a prototype.

Note: some photos may be enlarged.

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Background - the Christie tank

The Christie M1931 tank, bought by the US Army (note a small turret and MG armament). [1]

The American automotive inventor John Walter Christie started to develop his line of unorthodox tanks in late 1910s. Most of world's wheel-cum-track tank designs had separate retractable mechanisms of tracks and roadwheels. On contrary, Christie's designs had common set of wheels and could either ride on tracks, or without tracks, what simplified the concept a bit from mechanical point of view, although increased time needed to convert the vehicle between wheeled and tracked drive. He was convinced, that the future belongs to lightly armoured extremely fast tanks, difficult to hit, that could surprise the enemy like cavalry charges, and in 1928 he designed his novel tank, introducing so-called Christie-type suspension, on big, independent roadwheels, sprung by helical springs. The tank Christie M1928 (called in advance Model 1940 by the designer) developed almost unbelievable speed of 112 km/h (69 mph) on wheels, and a high speed of 68 km/h (42 mph) on tracks. Christie's firm - the US Wheel Track Layer Corporation in Rahway, New Jersey, started an advertising campaign, with a help from an enthusiastic press. Despite it, the tank M1928 was rejected by the US Army, and further Christie's tanks didn't make impressive sales as well. The Soviets bought only two tanks M1931 and a licence, which gave birth to long series of Soviet BT wheel-and-track tanks, and therefore, indirectly, to T-34 tank. The US Army bought only 7 M1931 tanks for testing, while Great Britain - one tank and a licence. However lightly armoured wheel-cum-track tanks appeared dead end, and riding without tracks was found highly impractical, but the Christie-type suspension remained an important input into the tank evolution, used in mass-produced Soviet and British tracked-only tanks of World War II era.

In late 1920s Poland was aiming at improvement of its armoured forces, in order to replace obsolete Renault FT tanks with a modern design, produced in the country. Christie first sent an offer of his early M1921 tank for a contest announced in Poland in 1926, but it was rejected, because it was late and too general. In 1929, Captain Marian Ruciński of the Military Institute of Engineering Research (WIBI) was sent to the USA to examine Christie's new M1928 tank. He was also shown blueprints of an improved model, later known as M1931. After his enthusiastic opinion, a special mission was sent to the USA on 26 February 1930, headed by the Chief of the Engineering Department, Colonel Tadeusz Kossakowski. In March, a contract was signed for a construction of one improved tank, to be delivered in June 1930. A price was $30,000 and $3500 for spare parts, and Christie received a pre-payment $10,000. There was also a further option to buy a licence for $90,000, after the tank's evaluation in Poland. However, when the tank was finally ready in October 1930, Christie refused to deliver it, demanding, that Poland buys a licence first, to prevent his tank from being illegally copied. When the Polish side refused to pay additional money, Christie broke the contract (it was contrary to version commonly quoted in Western publications, that the Polish side was responsible for breaking the contract). Just at the same time, in April, two tanks and a licence were sold to the Soviets (and delivered in secret as "agricultural tractors"). After negotiations, Christie later gave back the pre-paid money. Once again Christie tried to offer his new design to the Poles in 1936 (to be produced in the First Locomotive Factory in Chrzanów), but his offer was rejected by the Polish military then.

The 10TP prototype in the Experimental Workshop. A background is retouched - click to enlarge or see the original scene. [2]

Since the Christie's design was considered as the most attractive of existing tanks, the Tank Design Bureau of the WIBI was ordered at the end of 1930 to begin preliminary works on own design of a similar wheel-cum-track tank, known under a working name: "a la Christie". It should be noted, that in 1929 the Poles also examined the Czechoslovak Kolohousenka KH-60 wheel-cum-track tank, but did not find it interesting. Also, in 1926, Poland bought two wheel-cum-track tankettes Saint-Chamond Chenillette M1921, but they were evaluated as useless. As the working name suggested, the Polish designers decided to model upon main features of the Christie's design, first of all, a suspension with big independent double-role roadwheels (for both riding on tracks and without tracks). The work was partly based on available advertising materials, as well as information and sketches obtained by Captain Ruciński, who was the main person in charge of the design. The design works were quite advanced by 1932, but soon it slowed down, because in a meantime, Poland decided to buy the British Vickers Mark E tanks, with a licence to manufacture. A priority was given to its development, leading to the 7TP tank design. When the WIBI was liquidated during a reorganisation at the end of 1934, most of "a la Christie" project documentation was destroyed as redundant - only to return to a wheel-cum-track tank idea a couple of months later.

10TP development

New works upon the wheel-cum-track tank started in the newly created Armoured Weapons' Technical Research Bureau (BBT Br.Panc.) from 10 March 1935. It was given a designation 10TP, and classified as a "pursuit tank" (czołg pościgowy, roughly equivalent of a cruiser tank). The designation followed a pattern set for the 7TP tank, and was supposed to mean 10-Ton Polish tank, despite its actual weight was about 12 tons. A design team was directed by Major Rudolf Gundlach, main designer was Jan Łapuszewski. It included, among others, engineers Stefan Ołdakowski, Mieczysław Staszewski, Kazimierz Hejnowicz, Jerzy Napiórkowski (Major Gundlach had already developed, among others, the armoured car Ursus wz. 29 and a widespread reversible tank periscope, later known as the Vickers Tank Periscope Mk IV).

The tank was considered to be an armament of four tank battalions in four planned so-called Motorised Units (Oddział Motorowy, OM) - they were motorised brigades of the kind of the later 10th Cavalry Brigade or the Warsaw Armoured and Motorised Brigade. Such battalion was to consist of a company of sixteen 10TP tanks, and a company of TK/TKS tankettes or new 4TP reconnaissance tanks. Therefore, it was scheduled to build at least 64 cruiser tanks. The 10TP was included in the programme of the Polish Army's upgrade for the years 1936-1942, which was approved by the Armament and Equipment Committee (KSUS) in January 1937.

Suspension of Christie M1931 ↑[1] and 10TP ↓[2]

The new tank shared general layout and an idea of suspension and drive mechanism with the Christie's tank, nonetheless it was an original design, not a mere copy (the Poles had no pattern to copy, anyway). It also incorporated many new technical solutions. First of all, it had a wider hull, making possible accomodation of a driver and a machine gunner side-by-side, and fitting a two-man turret. The turret was Bofors-designed, of a type used in the 7TP tank, armed with 37 mm Bofors anti-tank gun and a coaxial 7.92 mm MG (the original Christie's tanks were MG-armed). In fact, the prototype 10TP was fitted probably with the first experimetal turret, differing in details from series turrets, first of all: more slanted upper plate of a niche. The tank also had thicker armour, than Christie's. The Polish designers applied new, wider tracks, with smaller links, which improved off-road capabilities, were quieter, more durable and less prone to fall apart. The Poles developed also new drive sprockets and the way of link hooking, although it was complicated and not very reliable. When without tracks, front wheels were steered, like in the Christie's tank, but the second pair of wheels were raising with hydraulic servomechanisms, in order to make easier turning. A rear pair of roadwheels were driven by a gear, like in the Christie's tank (although the gear type was different). The tank steering system was quite advanced, using hydraulic servomechanisms (which, however, caused some teething problems during tests).

The 10TP fallen into a ditch due to a side clutch malfunction, near Waliły, April 1939. The tracks slipped from wheels. [2]

A construction of the first prototype started in 1937 in the Experimental Workshop (WD) of the BBT Br.Panc., located on the area of the State Engineering Factory (PZInż.) in Ursus near Warsaw. Works were delayed, among others, by of a lack of a proper engine, that was not produced in Poland. Finally, a 246 hp American La France engine was bought. Test in Poland showed, that it had a power output of only 210 hp, but it was decided to use it anyway. The prototype was completed in July 1938. The first longer ride took place on 16 August 1938. Captain Leon Czekalski of the BBT Br.Panc. reported:

" ...The tank departed from the WD area at 9:25 and arrived to Pomiechówek at 1 p.m. so it travelled 57.3 km in 2 hours and 20 minutes... The following average speeds were achieved: hardened road - 34.5 km/h, dirt road - 20.6 km/h... On the way back a right lower fan bracket was broken... The tank is excellent to drive on dirt roads, negotiating any bend with easiness... it features a well-sprung ride on potholes and I believe it would offer good characteristics for firing. The front fenders are too short and they haven't any sandguard from inside that results in dust making the tank driving very difficult for the driver... It consumes too much oil in the gearbox... The tank negotiated a steep hill (approx. 35°- 40°) quite easily without any hitting against the ground, forded several times the Wkra river getting immersed by several centimeter above the floor level... On the way back it got into a deep roadside ditch in Łomianki (some 2 m deep), cutting a tree near the road... The tank rode back from the ditch on the reverse gear, without any difficulty... A reason was a temporary brake failure... General observation: result of the first roadtest along a distance of 111 km was satisfactory in terms of speed and negotiating difficult off-road distances".

10TP hauled by C7P tractor, Radzymińska Street in Warsaw, 25 April 1939. An accident was caused by a motorcycle rider, who cut in the column, and the tank driver turned into a ditch, losing tracks. After hauling the tank down to the ditch bottom, and putting the tracks on, the tank drove out by itself. ↑[1] [2]→

Until the end of September 1938 the tank underwent successive trials in different conditions, interrupted by minor faults. Then the tank was sent back to the WD for modifications. On 16 January 1939 the tank was tested on a way to Łowicz (154 km). Then, in spring, between 22-25 April 1939, it undertook a longer 610-km raid on a way Warsaw - Białystok - Grodno - Vyalikaya Byerastavitsa - Białystok - Warsaw, with a good average speed 36 km/h.

After this ride, the tank was stripped down and its drive mechanism and transmission components were examined. It rode some 2000 km by then. After an overhaul, the tank was demonstrated in May to representatives of the highest military authorities (along with the 4TP and PZInż.130 prototype tanks). Unfortunately, a further fate of the 10TP prototype is not known (probably it wasn't fit to fight - there is no information in publication on its armour, but presumably it was made of ordinary steel plates, and it was lacking some equipment). It also could have been stripped down by September 1939, to use some components in the 14TP tank construction.

In a final evaluation, there were revealed faults, like insufficient engine cooling system, too quick wear of a gearbox and clutches, too quick wear and damages to roadwheels and tracks, too small fuel tank due to a higher fuel consumption, inconvenient access to some transmission or engine parts. A hydraulic steering system was found interesting and recommended for further designs. Riding without tracks was found impractical - its only use was on hardened roads, and the tank on wheels could not go off-road without geting stuck. Putting the tracks on was taking the crew 30-45 minutes. Therefore, a development of the 10TP as a wheel-cum-track tank was closed. The 10TP was, however, the most advanced of tanks constructed in Poland, and it gave the designers a valuable experience. It could also be a base for next purely tracked tanks, with improved armour and armament. The first of such designs was to be the 14TP tank.

14TP tank design

The 10TP prototype. [2]

As soon, as around 1936-1937, there started works upon a purely tracked cruiser tank, known later as the 14TP. Thanks to getting rid of a complicated gear driving rear wheels, and systems steering the front wheels and raising the second pair, some weight and space could be saved. It made in turn possible increasing armour thickness and fuel tanks capacity (a similar conclusion was taken by the Soviets, who developed the BT wheel-cum-track family into A-20, and then famous T-34 tanks). The 14TP design was ready by the beginning of 1938. The Experimental Workshop started works upon a prototype, and by the end of 1938 some 60% of parts were ready. Completing of the prototype was scheduled on March 1939, but unfortunately, further work was halted by a lack of a proper engine. The American La France engine used in the 10TP was considered too weak, and the Polish 300-400 hp engine was only in development. The Poles found an appropriate engine 300 hp Maybach HL108 and tried to buy 1 or 2 examples. An idea to buy engine in Nazi Germany just before the war was surely too optimistic, and, despite attempts at buying it via Sweden, most probably the deal hasn't been finalized by the war outbreak (earlier, in 1937, Maybach agreed to sell a party of 8-10 engines only, but the Poles could not afford - and didn't need all at that time).

A fate of an unfinnished prototype is not known, probably it was captured by the Germans. Unfortunately, there are no photos nor documentation of this vehicle and its final look and specifications are not known for sure. Early reports, from January 1939, showed a machine similar to the 10TP, what would be most logical. On the other hand, the German intelligence produced a sketch of the Polish tank "T39", showing a low profile machine with 5 pairs of Chritie-type roadwheels and a different turret. The sketch probably has nothing in common with the 14TP.

On contrary to contemporary British and German designs, the 14TP probably had a more advantageous sloped front hull plate - a further development of the 10TP profile. During further development, the tank was meant to be fitted with newly-developed Polish 47 mm anti-tank gun and Model C air-cooled machine-guns. In that case, a big hull machine gun's housing, being a weak element in the 10TP front, would probably be smaller. The driver would probably receive reversible periscope to improve his vision.

A projected characteristics of the 14 TP were:

• Weight: 14 t
• Crew: 4
• Armament: 37 mm wz.37 Bofors gun and two 7.92 mm wz.30 machine guns (to be replaced with 47 mm AT-gun and air-cooled Model C MGs).
• Armour (of welded rolled plates): 35-50 mm.
• Speed: 50 km/h
• Power to weight: 21.4 hp

Technical description of 10TP tank:

Construction:

The tank had a classic layout, with a rear drive. In front there was a driver's compartment, not separated from a combat compartment, which had a turret upon it. An engine compartment was at the rear, with a longitudinal engine. The crew was four. In front there was a driver's seat on the left, and a machine gunner's post on the right. A commander and gunner had their stations in the turret, on seats turning with the turret. A forward hull plate was sloped, but its profile was spoilt by a machine gun housing and two side cases of unclear purpose. The water-cooled hull machine gun was mounted in a big housing, and it had a heavy-looking armoured mantlet and armoured water radiator (in next tank designs it was expected to fit new air-cooled machine guns). A two-part driver's hatch was in a front slanted plate. Hull sides were vertical. Rear plate above the engine was quite slanted. The turret had a one-part hatch in the roof, and a rear niche for a radio or ammunition. In turrets' sides there were vision slots and pistol loopholes. The commander had a reversible observationperiscope wz.34 in the turret's roof.

The armament was one 37 mm anti-tank gun wz. 37 Bofors and 7.92 mm wz. 30 coaxial water-cooled machine gun in a common armoured cover in the turret, in addition to a hull-mounted 7.92 mm wz.30 MG. A turret traverse was 360°, manual, mechanical. A gun elevation was from -10° to + 20°. Ammunition: 80 gun rounds (APHE and HE) and 4500 MG rounds. The turret weapons had a telescopic sight and periscopic sight.

The armour was made of riveted and welded rolled plates. Thickness: hull front, sides, back: 20 mm, bottom and top: 8 mm, turret front and sides: 16 mm.

Engine - petrol 4-stroke V12 American la France, displacement: 12,358 cm³, catalogue output: 240 HP, actual output at 2800 rpm: 210 HP, water-cooled. There were probably two radiators on both sides of the engine, like in the 7TP tank, with inlet holes in an upper plate and outlet holes in an upper rear slanted plate. Capacity of fuel tanks was 130 l.

Transmission and steering - dry multi-disc main clutch, mechanical gearbox, 4 forward gears, 1 reverse gear. Hydraulically-controlled side clutches with drum brakes, mechanical final drives transmitting power to drive sprockets and the last pair of the roadwheels. The gear powering rear wheels was inside its rocking arm.

Chassis - 4 pairs of big double roadwheels with rubber rims, sprung independently on rocking arms and vertical helical springs with tension adjustment (the springs of the front pair of wheels were horizontal). Drive sprocket was in the rear, idler in the front. The front pair of roadwheels was steerable, on swivel axles. The second pairs of wheels was hydraulically lifting up for driving without tracks. Single-pin, single-spur metal tracks, each made of 65-67 links, link width approx. 350 mm, pitch approx. 170 mm, length of track on the ground approx. 334 cm, track approx. 224 cm. When dismounted, the tracks were put onto fenders and fixed with stripes.

Electric installation - 12 V single-wire, generator 300W, battery 120 Ah. A projected communication means were N2C radio in a turret niche and an intercom.

The 10TP prototype without tracks, showing raised second pair of wheels. [2]

Camouflage

The prototype was painted in a standard Polish camouflage scheme of three colours: greyish sand and dark brown over brown-green (base color). The patches were airbrushed, with soft transitions, the shapes were horizontal mainly. The interior was painted in sand.

Drawing of the 10TP - Copyright © Thierry Vallet - Kameleon Profils - courtesy of Thierry Vallet.

Specifications: