Speaking of tanks:
Slawkenberg doesn't have the STC for the Leman Russ. Ciaphas Cain knows this, and also knows he can't possibly justify not filling that hole in the USA's Table Of Equipment when the Imperial Guard's tanks are central to its doctrine. Or at least that he could only put it off for so long.
So he orders the development of a new tank, with the very latest equipment and every possible survivability enhancement. Cain might not know much about tank design, but he does know that STC designs can't be beat for field-ruggedness and logistical overhead and so figures that the new tanks would be a subtle drain on resources, the fancier and fewer the better.
The request is passed to the BORG. Tesilon-Kappa has better things to do than reinvent the tank and picks a young and dynamic tech-priestess to head the project. After reviewing the requirements (vague) and her own knowledge of tank doctrine (nonexistent) she calls up a old friend, an Engi-seer working with the USA, who provides his own thoughts on the matter and also directs her to his contacts in the officer corp. A board of half Mechanicus, half officers of the USA, is assembled.
That done, she starts by reviewing the technologies available. The 120mm Battle Cannon has been the standard for millennia, and it's well within BORG's ability to reproduce, so no worries there. Same with the engine, the Lascannon, and while the sponson bolters might be a problem the stubbers won't be.
The problem is the armor. Plassteel is a nanostructured steel alloy, and its manufacture is a long, involved, and delicate process with lots of subtle nuances to trip up the unwary. While the production of any one plassteel alloy, once developed, is simple enough to automate, the task of developing a new alloy is orders of magnitude more difficult than it is for conventional steel. Slawkenberg industry is capable of rolled homogeneous A2400, the standard structural plassteel, but the tougher and denser A2354, an osmium armor plassteel, is liscensed only to those Forgeworlds that produce armored fighting vehicles. Reverse-engineering of captured Sentinels has been underway ever the Karamazov Incursion, and the BORG expect to have an equivalent armor alloy sometime in the next decade. Unfortunately, the USA needs a tank sometime this decade.
The idea is raised to just use the A2400 alloy in the manufacture of Leman Russes-it works for the Chimera, which have been using A2400 hulls for millennea. It's almost immediately shot down: the Chimera is lighter and cheaper because it uses thinner thicknesses of this lighter alloy, but it's not proof against the heavy anti-tank weapons like Meltas and Plasma Guns the Leman Russ tends to run into. A Leman Russ with equivalently effective thicknesses of A2400 armor would be ten tonnes heavier, the sponsons would have to be reworked to accommodate the increased hull thickness which may introduce new vulnerabilities, and it would be 5 kilometers per hour slower off-road (or so a tech priest estimates). The officers in particular view the increased weight and bulk as a non-starter. The Devourer heavy dropship is designed around the Imperial standard 'square' armor company of 14 Leman Russes and two Chimera command vehicles, and the A2400 Leman Russ is just slightly too bulky to fit the full complement. It would be possible to use the A2400 Leman Russ for planetary defense, but the USA is convinced that offensive action is paramount to preempt the massive response the Imperium is no doubt preparing.
How is the tank meant to survive the battlefield, without armor ? The Leman Russ's armor and sponsons allows it to close the distance and support infantry in attacking fortified positions, but without A2354 this would be suicidal-any man with a Locke-pattern missile launcher can peel a Chimera open from half a klick out, with a little luck, and even the doubled thickness of the Leman Russ won't cope very well, without A2354.
Perhaps the anti-gravity platforms, one officer pitches out of left field, a flying tank could be agile enough to defeat a Locke missile while being more resistant to flak than a Vulture. The argument goes back and forth, mostly on capabilities versus the cost of outfitting multiple air-armor regiments. Eventually the General shows up and notes that while a flying tank would make for hellacious armored cavalry it's probably not what the Liberator and the General Staff have in mind and the cost would be prohibitive anyway.
Back to the original question: how to make the tank survivable? Perhaps the first officer has the right idea-a change of doctrine is needed. In order for a tank to be killed it must be spotted, shot at, and penetrated in that order. Tanks are always going to be difficult to conceal, the General notes. But perhaps not as difficult as the Leman Russ, the tech-priest notes, whose hull easily stands twice as tall as a man. It has to be that tall, of course, to mount the sponsons.
Does it need the sponsons? Well, the General notes, they're very useful in the overrun attacks the Imperial Guard uses the Leman Russ in-but that brings the tank closer to the very Meltas they can't armor against, a younger officer points out. Perhaps a doctrinal change-a focus on long range direct fires. Certain others dicker over whether this is properly Khorne-like behavior. The General points out that marksmanship is a martial skill like any other, and furthermore that they had no sacred duty to charge in 'and die like idiots' in his words-that's what Imperials do.
The sponsons come off. The hull shrinks by almost a meter of height, as the inside no longer needs to be entirely walkable, then a little bit in width and length as the sponson ammunition space is eliminated. Another half meter is shaven off as a critical eye is turned on the hull-mounted Lascannon-if the tank is going to be fighting from range, then it's probably fighting hull-down, where the hull mount is going to be blocked. The tech-priestess ponders ditching it entirely, but the officers want to keep it: while it might be a bit redundant beside the Battle Cannon, it's capable, reliable and doesn't require ammunition. The Lascannon is moved into the turret, coaxial alongside the Battle Cannon.
Further space savings are hard to find. There's no replacement for the HL230 V12 forthcoming-it's powerful, reliable, reasonably efficient and takes anything combustible-but it is tall and the hull has to accommodate that. Fuel tanks are shuffled around, not without argument-they're a decent last line of defense against anything that penetrates the tank, especially from below, from mines, where the armor has to be thin. The priestess's attention is drawn to the tracks-do they need to wrap around the hull like that? Yes, the General says, so it can cross trenches and anti-tank ditches-if the treads don't go all the way around the tank would nose into a ditch and get stuck there. Does it need to cross trenches, the tech-priestess argues, if it's going to hang back from the front of the attack anyways? Yes, it would, otherwise the platoon would have to wait for a bridgelayer or for the infantry to fill in every little ditch in the way. The tracks stay where they are.
Attention is drawn to the turret. Feedback from former crews is clear that they would prefer the turret be bigger-the Leman Russ turret was too small already for gunner, commander and loader and now they're gone and stuck a Lascannon in coaxial to the Battle Cannon. Well, the tech-priestess considers, do they need the loader? Autoloaders are well within BORG's technical ability, although it might take a bit of fiddling. The USA side notes that the loader also provides a fourth hand to help around, but maybe with the sponsons gone the maintance load might not be too bad. They decide that worse comes to worse, extra personnel can be brought along in Scout Cars or Chimeras. A contest for a Battle Cannon autoloader design is issued and the board moves on.
Decisions over the sights and periscopes goes with speed. The Leman Russ's optical gunner sight is servicable, but the focus on long range direct fires demands better and the board has access to options orthodox Martians wouldn't or can't consider. It's very quickly decided to put in a cogitator-driven sight with las rangefinder, while the prior optical sight is retained as a backup. Slightly more controversial is the use of a remote optical sight for the commander, but the high magifications offered and in-built prey sight eventually win the USA side over. Besides, the Liberator is demanding the best of the best.
Hatch placement, on the other hand, drags on. It's probably not possible to save the tank if it gets hit, but it is possible to save the crews-and the machine spirits, the tech-priestess exclaims, the cogitator's memory banks can be made to be rapidly disconnected and carried with the crew. The General considers the latter idea to be a bit silly, but relents when the Mechanicus side puts their collective feet, wheels and treads down. Placing hatches that are large and easy to escape through without compromising the armor is easier said than done. Mockups go up and are torn down. The frustration is palatable.
The tech-priestess asks: Does the crew need to be in the same compartment as the gun, ammunition, engines, etc? No, she decides, after reviewing the latest automation technology, and forwards the idea to place the crew in an armored compartment in the hull, with the gun operated by remote control. That way the crew would survive where the tank wouldn't, and all the hatches can be in the same place, on the top of the hull where they're protected by the front glacis. The decreased access for field maintenance is unfortunate, but can probably be solved by judicious placement of the exterior hatches which-because they're not protecting the crew compartment-can be less well armored. The General isn't so sure-he points out that field maintenance is essential and that it's best done inside the protection of the tank-if they have to go outside they'd have to pitch tents against the elements, increase the size of dugouts, and so on. Would the shrinking of the hull make up for this? Maybe-the tank is already shorter than before, so a wider, shallower dugout might not be too much of an ask. The argument goes back and forth before the Mechanicus side yields. The idea is filed away.
Some more testing turns up hatch placements that are acceptable, or at least least bad. With that, the hull design is finalized enough to proceed with prototyping.
Time passes. A first three prototype hulls have been fabricated, two with dummy turrets to test the weight distribution, and have been run all over the new tank courses
Several autoloader design drafts have reached the board. Most come from third parties in the Mechanicus with little experience with artillery, and have been discarded immediately as being too mechanically complex, or requiring components not manufacturable on Slawkenberg, or not completely controlling the shell from rack to breech. The list is quickly parred down to three.
Design Aleph is a bustle autoloader, using a swiveling arm on a track to draw ammunition from the bustle, swing it around, and put it in the breach. This would require the least modification to the turret design, as it closely approximates the motions of a human loader, and can load from any gun angle, however it has the highest cogitator complexity of the finalist designs.
Design Beta is a much simpler bustle autoloader, using a hydraulic ram and extending ramp-tube to ram the shell into the breach. The shells would need to be facing forwards instead of backwards, which would complicate handling shells inside the turret, but if the bustle rack is extended it probably won't be necessary to replenish from hull storage very often. The gun needs to be leveled to load, but with remote optics and cogitator-driven sights this isn't too much of an issue as the gun can be returned automatically.
Design Gamma takes a different tack-the bustle ammo rack is removed, instead the autoloader draws from a carousel in the bottom of the turret basket. This requires the most modification to the design and more mechanical complexity than Beta, but gives the autoloader access to all of the tank's ammunition storage.
After much deliberation, Designs Beta and Gamma are selected for prototyping. Aleph is interesting enough to file away, but the tank is already heavy on the cogitators and Slawkenberg's rapidly expanding electronics industry is still swamped by the government's need for data-looms and the demand for personal data-slates, a luxury that had been long out of the common Slawkenberger's hands. It's possible to get priority for military production, but the benefits over the other two are deemed marginal for a tank that is most likely fighting at range anyway.
Prototypes No. 2 and No. 3 are fitted with the Beta and Gamma turrets after an initial round of turret prototypes. Gamma runs into multiple issues centered around its field maintainability, as it has a lot of moving parts, particularly the large, armored carousel, in some very awkward places to service. In the interests of time, Beta, which proved itself reliable and more easy to maintain, is selected as the final design.
With that, the tank is essentially complete. A further two prototypes are produced as the board homes in on the last teething issues, and two years after the initial order, the design is ready to be presented to the Liberator.
Ciaphas Cain, of course, is delighted-the Cain Battle Tank(because of course the cretins are calling it that) is less armored than the Leman Russ, goes only a little faster, and only has half the weapons, while costing nearly as much or even a little more. The autoloader is slower than a human loader and addes more mechanical complexity to maintain on a crew one hand less. He enthusiastically signs off on Mahlone's request for 1500 tanks to equip the USA's first four armor regiments, the 30st through the 34th Armoured. The first Cain rolls off the assembly line five months later, and is delivered along with ten others to the newly formed 32nd for familiarization. The new tank is very different from the Leman Russ or the endless Chimera variants that everyone is familiar with, and officers soon recognize that it demands an entirely new doctrine, which the 32nd is still experimenting with when it's called up for the Hellavu campaign.
In a twisted way, the new tank doctrine is a reflection of Ciaphas's true personality. The Cain is very much a defensive tank, not an offensive tank like the Leman Russ, and the 32st primarily fights from dugouts, or hull-down behind any convenient hill or wall, exploiting their superior gunnery to the fullest. They avoid charging into direct confrontations with Imperial armor, instead methodically isolating and defeating tanks in detail. This earns a certain reputation for cowardice and low cunning among Imperial officers, but in actuality the 32nd's small-unit tactics require a great deal of restraint and tactical thinking from their mainly Khornate crews, which serves them very well well later in their careers.
The tank's technological advantages encourages this approach. The cogitator optics pay their way several times over-Cain crews very often shoot first, and where they shoot first they shoot last. The autoloader is slower than a human loader-a veteran loader who isn't exhausted from several days of fighting, something which tells in the grinding attritional warfare that Hellavu devolves into.
Tank losses are still very high, but Slawkenberg industry keeps up with demand and the losses of tank crews are lower than ever. This has a compounding effect as tankers are more likely to survive to achieve veterancy, and to instruct new recruits-assisted by the recovery of the tank's machine spirits, whose data-recordings augmenting many an after-action report. In fact, tank crews became attached to their tank's machine spirits and swear up and down that their sights become more accurate and their vox systems less effected by jamming as the tank becomes more 'experienced'.
By the end of the campaign, Cains are trading one-to-one or better with Leman Russes manned by increasingly green and exhausted conscripts, the 32nd is a mean, lean fighting machine, and the USA's tank doctrine is set. And Ciaphas Cain is-very privately-a little flabbergasted.
EDIT: 11/11/2023. Big Retcon with regards to Leman Russ armor. This should now be more canon-compliant.