MJOLNIR armor

The different MJOLNIR generations and their development.

Development
Each "Mark"-designated generation is its own iterative platform, and each of them evolved considerably over the years, both generation-wide and individually. The Marks were paradigm shifts that completely overhauled both the suit's hardware and software, and for the most part required their own unique production lines and procurement chains. In addition to the best-known changes, there were numerous tune-ups and adjustments to every Mark-designated platform, both internal and external. However, due to the realities of military logistics and funding, as well as select benefits unique to given platforms, different models sometimes remained in use in parallel by different Spartan units for some time.

Some components, such as helmets, shoulder pads and various UA plating components, are inter-compatible between platforms, but previous-generation helmets in particular may require electronics upgrades to function with newer systems.

Mark IV
The Mark IV was initially only known as Project MJOLNIR, as Dr. Halsey saw the suit as distinct from the Materials Group's earlier HRUNTING exoskeletons. Still, as with the name of the SPARTAN-II program itself, she acknowledged her armor's lineage to its considerably inferior (and almost entirely technologically unrelated) predecessor, which would still spawn its own offshoot program over the course of the coming years.

The first-phase Mark IV was first upgraded before the suit was even widely issued, adding a refractive iridescent coating to disperse directed-energy strikes. This was the first in line of UNSC-wide improvements refocusing armor from protecting primarily against ballistic attacks to plasma and particle beam damage. Still, the first version of the coating was fairly rudimentary, due to the little data available on Covenant weapon damage at the time, and did little to defend from all but glancing blows. Further iterations would improve the coating based on data gathered in the field, and more underlayers would be added, including a superconducting mesh designed to distribute incoming energy to a wider area as well as an outer thermal regulating layer.

Base MJOLNIR suit with hastily-added refractive coating. While a quantum leap in technology for its time, the Mark IV was quickly found to have numerous teething troubles and issues beyond the designers' control: the suit was fine-tuned for fighting human insurrectionists with ballistic weaponry, not aliens with energy weapons.
 * Phase 1 (2525)

Rapid field-testing and iteration project of individual suits over the course of two years, culminating in the second-phase overhaul of the suit.
 * Projects COBALT and CRIMSON (2526-2528)

The result of the rapid-iteration and field-testing Project COBALT, the Phase-2 Mark IV was a generational upgrade that brought the suit fully up to speed with data and experiences gathered from engagements with the Covenant, including the effects of their weaponry. It had improved refractive coating, thermal regulation layer, heat sink and superconducting EM mesh, integrated vacuum sealant foam dispensers, and armor plating optimized for countering plasma strikes, along with numerous upgrades to its internal electronics and firmware.
 * Phase 2 (2528)

The Phase-2 Mark IV was introduced in 2528 and issued to most of the Spartan-IIs over that year. It was the largest single upgrade of the suit between 2525 and 2535.

In 2535, Halsey was forced to segment Project: MJOLNIR into neat generational models for budgetary reasons, as opposed to her prior approach of applying constant - and often costly - upgrades and overhauls to the suits.
 * Phase 3 (2535)

The 2535 generational upgrade of the suit included enhanced blast-resistant plating and padding insets, overhauled techsuit and exo-frame design with further fall damage mitigation, and increased armor coverage to defend against DEW attacks. Various upgrades to the suit's plating architecture to respond to widely-reported ergonomics issues. The Phase-4 suits also contained "proto-shielding", an electromagnetic impeller system projected by the plating designed to mitigate the effects of plasma weapons by disrupting plasma bolts' containment fields before impact. The suit's design was also streamlined for increased variability, field repairs, upgrades and individual specialization, partly due to the Spartans being increasingly spread out across dozens of light-years and often operating on their own.
 * Phase 4 (2540)

Less of a singular phase and more like a collection of discrete field-iteration programs, some of which saw the first field-tests with external shielding modules.
 * Legacy-hardware shield testing (2547-2549)

Mark V
The Mark V was an iterative platform that came in two major phases, the first introducing its shielding and the second adding in the AI housing substrate.

Project BULWARK not only brought the aging internal design architecture up to date with advances made in materials science and power transmission during the war, but also introduced energy shielding as an integrated fixture of the suit; while experimental variants of the Mark IV had been fielded previously with external shield units, these were error-prone, imprecise and cumbersome. The first-phase Mark V was first fielded in 2550 as a kind of extended field-testing project, though Halsey, ever the perfectionist, was hesitant to dub it "Mark V" until the full AI support could be added; nevertheless, this pseudo-Mark V -- in fact more like a Mark IV/Mark V transitional phase -- would receive the designation "Mark V [B]", or BULWARK, in official procurement documentation. The AI support was the bit that delayed the Mark V's production considerably: the integrated shield system was finished long before the AI substrate layer and interface, which would remain the armor's most expensive component. This is also why BULWARK-derived systems would remain in mainstream use for some time even after the war, with AI-supporting suits reserved only for Spartans who strictly needed them.
 * Phase 1 (Project BULWARK) (2550)

The second or final phase of the Mark V added a functioning AI processing memory-crystal layer, which remains the single most expensive addition to the MJOLNIR platform. It contained numerous lesser upgrades, yet omitted just as many that Halsey did not have time to incorporate before Admiral Stanforth and the rest of FLEETCOM forced her to finish her work on the suit and initiate Operation: RED FLAG. In many ways, the Phase 2 Mark V was an all-new suit, though one still built on the Phase 1's hardware - something that had already been surpassed by the Mark VI team on Earth, but did not unfortunately make it for RED FLAG.
 * Phase 2 (2552)

AQUILA was an offshoot program designed with the intent of creating a leaner and more cost-effective MJOLNIR variant, produced a battlesuit platform built on the core architecture of the Phase 1-era Mark V, omitting the prohibitively expensive AI substrate layer while incorporating some of the optimization upgrades of the Mark VI. Still, it includes the option for an external armored data matrix as a plug-in module, though the system remains more cumbersome than the integrated one in the Marks V and VI. The AQUILA suit was issued mostly to SPARTAN-IIIs and SPARTAN-IIs operating without integrated AI, though it would eventually be phased out in favor of the Phase 3 Mark VI as the crystal-substrate layer technology became cheaper to manufacture.
 * Project AQUILA (2554)

Mark VI
Largely developed in parallel with the Mark V, with development starting in Seongnam and Chiron in 2551, the Mark VI did not add major features like its predecessor had, largely focusing on optimizing the technologies developed for the Mark V and integrating them more thoroughly into a coherent whole. While advanced for its time, the Mark V's shield system was still relatively wasteful and crude in light of new advances made over the past two years. As well, the AI crystal layer had been hastily added to a suit that still partly relied on legacy technology, causing issues with power management in particular. The introduction of the second-phase Mark VI several years later would partly remedy these issues, though the armor would remain prodigiously expensive for some time due to the complex manufacturing process required for the unique symbiosis of exotic technologies.

While mostly developed in parallel with the Mark V, the final iterations to the Mark VI's design were made with an exceedingly hectic pace in the weeks since the fall of Reach. Internally, the Mark VI differs quite little from the Mark V, outside some alternate materials choices and technical solutions, essentially being the product of a competing Materials Group team with only external input from Dr. Halsey. Because Halsey was extremely ambitious about the upgrades made to each MJOLNIR generation, she would have been against differentiating the Mark VI from the Mk. V suit in the first place; there probably are fewer functional differences between the Mark V and Mark VI than a Mark IV suit from 2535 and one from 2549. Regardless, Halsey wasn't there when Navy procurement bureaucrats decided to call the suit Mark VI, some time before the Gettysburg survivors returned to Earth. Indeed, the final upgrades to the upcoming Mark VI suits were made based on data from said survivors' experiences on Reach, Halo and the Unyielding Hierophant.

In terms of design and architecture, the Mark VI was the first MJOLNIR suit designed to be truly viable for mass production, with a streamlined manufacturing process as well as increased built-in modularity, based on the example of the Phase-2 Mark IV. Consequently, specifications of the suit were partially divulged to a handful of defense contractors tasked with developing use case-specific variants.

As one notable change to the Mark V, the Mark VI relies more on its shield than the physical armor, using a lighter, more streamlined plating architecture and slimmer techsuit to allow for increased mobility, but this too can be a detriment depending on who you ask. But the VI's shield and power system are unquestionably better, if damage-prone; the shield-projecting quantum mirrors in particular can be damaged quite easily.

The Mark VI's passive stealth system, EVA features and core sensor package have also been improved from those of the Mark V.

One of the most notable differences between Marks V and VI is that the former (particularly the [B] variant) is more robust and easier to repair and upgrade in the field; whereas the Mark VI is a fine-tuned high-tech instrument that requires frequent maintenance by a dedicated crew. In terms of electronics, the Mark VI opts for a contact-based rapid data interface that can transfer an entire AI in a mere second, but while an ambitious show of technological prowess, this can compromise the suit's cybersecurity much easier than the Mark V, which relies on more robust and reliable physical data transfer and has better hardened electronics.

The Mark VI's power distribution used Covenant-derived metamaterials as well as a fine-tuned quantum mirror array to optimize the energy shields' shape in much more detail as well as reducing power consumption. This required a complete overhaul of the suit's internal architecture, which enabled several further changes to the AI processing layer. However, the system was more also delicate than that of the Mark V, and required more frequent maintenance.

New plating structure and layer construction, optimized shielding and power management suite.
 * Phase 1 (2552)

Cybersecurity upgrades, etc.
 * Phase 2 (2557)

Refinements to the manufacturing processes and an overhaul of plating materials and composition to modern specs.
 * Phase 3 (2564?)

Mark VII
Introduced in the 2570s, the MJOLNIR Mark VII was developed specifically for the next generation of Spartans. Features include Covenant-derived lightweight nanolaminate alloys for armor plating, advanced energy shield shaping, and an active stealth package.