Design Overview of the
Cruiser Tank Mk.I (ISTM-1)

by: G. Wolfe

Introduction

Cruiser tank Mk.I was a Nordenfeld medium tank developed in the 718 after the Medium tank Mk.II proved unsatisfactory during the Shevali civil war. The tank was jointly developed by the Royal Shevali Army and the Imperial Land Army, initially disguised as the Indigenous Shevali Tank Model 1 or ISTM-1. The Cruiser tank Mk.I would distinguish itself as the first among the long line of the Nordenfeld Cruiser tank family.

Background

Although primitive armoured fighting vehicles had been used as early as the Partition war in the form of improvised armoured cars and tractor-mounted artillery pieces, armoured warfare was still in its infancy until the turn of the 7th century. Lack of historical account regarding warfare in the 20th century AD left military planners of the era to wonder how armoured vehicles should fight, and be fought, in the increasingly lethal modern warfare.

The predominant school of thought at the time believed that armoured vehicles should be utilized as infantry support, providing accurate, direct fire support that cannot be provided by the field artillery. Therefore, the maximum speed of the armoured vehicles was constrained by the pace at which the infantry unit could advance.

The infantry support nature of armoured vehicles also necessitated a relatively low-velocity gun to lob High Explosive Fragmentation shells against entrenched enemy positions, as well as multiple machine gun positions to defend the vehicle against assault by enemy infantry. Any threat of enemy armoured vehicles should be dealt with by towed high-velocity gun firing Armour Piercing rounds, that followed the armoured spearhead as organic anti-tank platoons within each armoured regiment.

These ideas formed the backbone of Imperial Land Army armoured warfare doctrine that was prevalent throughout the late 6th century and was evident in its contemporary tank design. Slow, heavily armoured behemoth armed with a large-calibre main gun and numerous ball-mounted machine guns sticking out from the hull were common features seen in tanks of the era, such as ILA's first modern tank, the Heavy Tank Mk.A, or its multi-turreted successor the Mk.B, with two small, rotating MG turrets flanking the main gun turret, giving an impression of a "land battleship".

Development

The origin of the Cruiser tank concept could be traced way back to the build-up of the Shevali civil war in 715, the 11-years conflict was fought between Nordenfeld backed Royal Shevali Army loyal to King Michael II and Republican revolutionaries wishing to topple the monarch and set up a democratic republic in Sheval, supported by the United Terran Republics and various international volunteer groups.

Three days after the civil war broke out on 5th December 715, Emperor Alexander I of Nordenfeld announced the Imperial Council's support for the Shevali Royalist under the pretence of "solidarity of monarchs", although its ultimate goal was twofold: stopping UTR's influence on the western hemisphere and testing Imperial Land Army's combat doctrine and tactics in the first major war against UTR-aligned force since the Partition war in the early 6th century.

However, wary of provoking a direct confrontation with the UTR, only a limited number of high-ranking ILA and IAF personnel were officially sent as advisors to the RSA general headquarters. However, a large number of highly-trained troops also joined the effort disguised as international volunteers under the command of their "official" compatriots.

In addition to the skilled manpower, ILA also sent a number of war materials including trucks, locomotives, and some of their new Medium Tank Mk.II to be tested on the new battleground.

Experience from the opening years of the Shevali civil war shows that while Mk.II's armour was sufficient to withstand any anti-tank weapon possessed by the Republican force, its speed and cross-country ability was woefully inadequate in the highly mobile modern war, especially compared to Republican M2 medium tanks that were supplied by United Terran Republics. Moreover, the tank must be constantly transported from one battle to another by trains or heavy trailers due to its excessive weight, which makes it vulnerable in transit and adds burden to an already overloaded logistic chain.

It is clear that the Royal Shevali Army, and by extension Imperial Land Army, required a new tank design to replace the Medium Tank Mk.II. In 718, a joint design committee was established between the RSA and ILA advisors to study a new tank design based on experiences gathered from the battlefield. To mask ILA direct involvement in the project, the new tank was to be designated as the Indigenous Shevali Tank Model 1 or ISTM-1 for short. The tank would be built locally by Shevali engineering firms using materials and tools available at the time.

The main problem encountered by the committee was the limited power of contemporary engines, those with sufficient power were either too heavy, or not reliable enough for frontline service. Therefore, the only way to increase the speed required for flanking and exploitation of the enemy rear line was only by sacrificing armour protection.

To make the best use of the limited armour, the tank needs to be made as small as possible. A smaller silhouette might appear as an advantage, but limited space also means less firepower could be carried by the new tank, not to mention less space available for the crew, fuel and ammunition.

Its role to exploit the breach in enemy line also necessitated that the tank must be able to operate independently from the towed anti-tank platoon that normally followed the armoured spearhead, requiring whatever firepower fitted to the tank to possess an adequate armour-piercing capability to deal with enemy armour on its own.

This set of design constraints were what led to the Infantry-Cruiser tank dichotomy in the Imperial Land Army armoured doctrine, one optimized for breakthrough and infantry support while the other for flanking on open grounds and exploitation of enemy rear line. The divide between Infantry and Cruiser tanks would persist until better automotive and armour technology were available for a tank design to carry a balanced combination of mobility, protection and firepower, thus creating what would be known as the Universal tank concept.


Design

Due to the weight restriction, the design of Cruiser Mk.I was incredibly simple and compact. The majority of the tank was fabricated of RHA steel plates riveted or bolted to a mild steel framework, the hull was divided into a fighting compartment on the front and engine compartment on the back, separated by a mild steel firewall. The armour protections ranged from 25 mm on the lower and upper glacis to 9 mm on the rear transmission cover.

The ILA advisors were content with the Imperial Motors ASV960 V6 petrol engine used in Medium Mk.II for its decent power rating and good reliability. However, since there were no equivalent V6 petrol engines currently in production in Sheval, the committee decided to use two Inline-4 engines as substitutes. The two engines were placed side-by-side and joined together by a shared output shaft, resulting in a U-engine that was slightly longer and more complicated but could readily be manufactured by the less sophisticated Shevali industry.

The whole contraption generates 170 brake horsepower (126 kW) quite reliably and even provides some degree of redundancy as the tank could be driven, at a measly speed of 5 km/h, by using only one-half of the engine.

The engine drove, through multiple dry-plate clutches, a five-speed transmission at the rear of the vehicle. The transmission output shaft was connected directly to the bevel gearbox that divided the power to separate epicyclic reduction gear for each drive sprocket.

The fighting compartment housed a crew of four. The driver was situated in the front left-hand side of the hull, controlling the tank by means of twin steering levers. The bow gunner sat to the right of the driver and armed with a single 7.5 mm rifle-calibre machine gun on a rotating mount, no gun sight was provided for the hull machine gun but the bow gunner can observe the fall of the tracer through a single fixed periscope on the hull roof.

The turret housed the gunner and commander, who also doubled as the loader. The turret was fully traversable with an electrical traverse mechanism and a manual backup, it mounted a single 37 mm L/48 high-velocity cannon and a coaxial 7.5 mm machine gun on the Combination gun mount Mk.I. Both the gunner and commander were provided with their own 7 mm thick hatch and rotating periscope on the turret roof.

The weight of the tank was supported by four leaf-spring bogie suspensions, two on each side with each bogie pair connected by a torsion bar that ran the width of the hull floor. Each bogie contained four rubber-tired road wheels, no shock absorber was provided for the suspension.

The upper run of the tracks was supported by three rubber-tired return rollers, The front idlers are mounted on an eccentric shaft that could be manually rotated from outside of the tank to adjust the track tension.

Service

The pilot model for Cruiser Mk.I rolled off the production line in March 719 and after brief testing, it was approved for mass production by Waalraven & Smith Heavy Industries. In total, 115 Cruiser Mk.I was produced throughout the Shevali civil war.

Unlike the Medium tank Mk.II, the new Cruiser tanks were distributed to cavalry units instead of being part of infantry units. These vehicles were formed into the 1st, 3rd and 4th Armoured Cavalry Battalions and first saw combat on 5 July when ten Mk.Is of the 1st battalion blocked five UTR-supplied M2 medium tanks trying to break out from Royalist encirclement. During the engagement, the Royalist forces destroyed four Republican tanks and capture another in exchange for the loss of three Mk.Is.

Later in the war, the Cruiser Mk.I was often used in a small group to raid Republican supply centres and artillery batteries, like one particular case on the night of 18 December when a company of twelve Cruiser Mk.I, under the command of Captain William A. Partridge, advanced 15 km behind enemy lines and inflicted significant damage to Republican field artillery batteries. During the raid, the company destroyed at least ten 80 mm field guns, three artillery tractors and ignited their ammunition dump in the process. On the way back to friendly lines they encountered five more L2 tankettes of the Republican forces, which Mk.Is' 37 mm guns made short work of with the loss of only one Mk.I. For the night's action, Captain Partridge was awarded the Shevali War Cross and the Star of the Nordenfeld Empire.

Despite its praiseworthy performance, Shevali crews that manned the tank often complained that its thin armour barely withstands 15 mm anti-tank rifles and could be penetrated by 25 mm AP shots. Moreover, a direct hit from HE shell could potentially send loose rivets and bolts flying across the fighting compartment, causing crew injuries and equipment damages. At the end of Shevali civil war, 52 of 115 Cruiser Mk.I had been disabled by various means, although the majority were repaired and returned to service.

Further Development

Following the Shevali civil war, twelve Cruiser Mk.I were brought to Nordenfeld for demonstration to the Imperial Armoured Corps and the Imperial General Staff. The tank impressed the general staff officials so much that they ordered further development for use by the Imperial Armoured Corps

The design of Cruiser Mk.I was improved further by using more advanced technologies available to Nordenfeld heavy industries. Several engines were proposed to replace the improvised Shevali U-engine, including the original ASV960 V6, ASI980 Inline-6 and the newly developed ACV1100 V8 compression-ignition engine. In the end, ACV1100 V8 was chosen for its high torque output and better fuel economy

The hull and turret plates were welded together to improve structural integrity and reduce the incidence of spalling by loose bolts and rivets. Thanks to increased engine power, the armour could be increased to a more useful 35 mm on the lower and upper glacis.

The Combination gun mount Mk.I that combined the main gun, machine gun and the telescopic gunsight into a single coaxial mount was retained and became standard configuration on every tank produced in the future. It was however slightly modified to fit ILA's own 40 mm L/47 gun G2A1 and the 7.62 mm G720 machine gun, and redesignated as the Combination gun mount Mk.II.

The improved tank was designated as the Cruiser tank Mk.II and would kickstart the long and winding development of Nordenfeld Cruiser tanks until the last of its kind, Cruiser tank Mk.XIII Caracal was replaced by the Universal tank Mk.I Cuirassier in 774

Specification

Type : Cruiser Tank
Place of origin : Nordenfeld Empire / Kingdom of Sheval
In service : 719 - 750 (Royal Shevali Army)
Manufacturer : Waalraven & Smith Heavy Industries
Produced : 719 - 721
Number built : 115
Weight : 13 short tons (11.79 tonnes 11.60 long tons)
Length : 5.6 meters (18 ft 4.4 in)(hull)
Width : 2.3 meters (7 ft 6.5 in)
Height : 2.0 meters (6 ft 6.7 in)
Crew : 4
Main Armament : 37 mm L/48 High-velocity gun
Secondary Armament : 7.5 mm rifle-calibre machine gun
Engine : 2x Waalraven Condor 4-Cylinder, SI engine coupled to a single output shaft, 170 bhp (126 kW) total
Power/weight : 13.07 bhp/st (10.67 kW/tonne)
Transmission : 5-speed Watson gearbox
Suspension : Leaf spring bogie suspension
Speed : 40 km/h (25mph)