I guess it's not too surprising I've been thinking about giant robots in my own universe, and I've actually put together quite a few details over the years. You may recall reading about them a little bit in the novel excerpts I posted, and the All-Terrain Suit does feature in that book. I have plans to make at least one video game about them, too.
So here's the major categories of description. If I've missed anything, let me know, and I'll add it.
Mechas have a huge range of size, from the "barely bigger than a human" Marauder suits of Roughnecks: Starship Troopers Chronicles to "throwing galaxies around like they're rocks" Super Galactic Gurren Lagaan, and ranging everywhere in between.
I'll say up front I've done a lot of research on the engineering issues that would prevent giant robots from being viable in the modern era. Thankfully, this is science fiction, and I think I figured out solutions to the majority of the issues.
Anyway, among the various franchises that range from fairly realistic to "LOL physics", I think Front Mission is among the most realistic. The wanzers aren't crazy-huge, nor crazy-fast (unless they're using their mecha-roller skates), and can't fly.
The All-Terrain Suit is roughly the same size, averaging 25-35 feet (8-12 meters) in height. It's crewed by a single trained human (details on the interface below). As far as the external appearance goes, I've seen a huge range of mecha designs, from nothing but curves to all sharp angles, with exposed skeletons to full-enclosed, and every imaginable distinctive head design you can think of. To my surprise though, I've never seen ones built to resemble historical suits of armor. Even ones that incorporate SOME elements, like Gundams, only have one or two elements and otherwise still look distinctly sci-fi. Escaflowne was a medieval world with steampunk-ish giant robots, and they still looked fairly sci-fi.
The All-Terrain Suit does indeed look like a 30-foot suit of armor, but with modern weaponry and signs of in-built electronics. The eyes of the "helmet" can be seen to have cameras and other sensory equipment, there's usually a backpack with extra ammo or other devices, and the joints between the armor plates are protected by advanced flexible polymers to keep cosmic dust and other small objects out of those parts.
Just as there were a huge variety of plate armor styles, the ATS has undergone countless iterations in the centuries of its service. Some are lighter, meant more for scout roles and speed, others and heavier and bristling with weapons. I haven't devised too many specific variants as yet, given the sheer range of time present, but the basics of the appearance remain constant, although there are undoubtedly some variants that, while they still look like giant suits of plate armor, aren't directly based on any pre-industrial designs.
One of the biggest issues facing the development of giant robots today is getting them to move at a workable pace, maintaining their balance while doing so, making them out of materials that can withstand the weight, and having a solid power source for all this equipment that won't run dry in just an hour or two.
Obviously, with a few more centuries of work, I think we can take care of all these problems. One of the primary technologies of the Solar Alliance is their graviton manipulation technology. Gravitons are the sub-atomic particles believed to be responsible for gravity. Although the means to manipulate them are beyond our science currently, I think it might be possible to figure them out and manipulate them as easily as we do water today. Artificial gravity is easy to do on spaceships, and forms the basis of propulsion for pretty much all vehicles used by humanity, including the All-Terrain Suit. As part of its all-purpose nature, the ATS is capable of fighting on land, in the sky, and in space, and switch between any of the three without need for additional preparation or modification.
Through the use a powerful gravitic drive, the ATS is able to maneuver easily through any environment, changing speeds and directions instantly with no adverse effects on the pilot, giving it an incredible amount of agility. It can also bear weights far in excess of material tolerance because the mass of those weights are greatly reduced. Basically gravity means almost nothing to an ATS. Also of note is that all Solar Alliance vehicles, contrary to most sci-fi vessels, don't have giant blazing engines or dozens of thruster rockets all over them. Since they propel themselves by creating localized gravity fields that pull them in whatever direction they want, the need for such visual elements are unnecessary. Indeed, this principle forms the basis of their FTL travel as well, though the warp drives required are too big to fit on anything smaller than a full-blown starship.
Powering all this is another standard piece of Solar Alliance technology: the thalite core. Another form of tech that's been perfected over centuries of development, the thalite core is a scalable power source that powers everything from fighters to every class of starship. It's actually made of two distinct systems. At its heart is a fairly standard sci-fi anti-matter reactor, providing the raw energy for various systems via controlled matter annihilation. The second major part is a series of collectors designed to absorb various types of exotic particles and energy waves present throughout the universe to augment the energy generation of the reactor. Additional collectors maintain stores of cosmic dust and hydrogen for conversion into the raw matter/anti-matter mix, meaning that a thalite core can theoretically run forever. The matter/energy conversion process also goes the other way, as fighters and ATS' will have micro-replicators built into them for use with their weapon systems, hence why the fighters in my novel seem to have an endless supply of missiles.
Alternatively, I haven't fully decided, instead of relying on anti-matter reactions, the thalite core uses micro-singularities instead (i.e. tiny black holes), with the collectors still performing the same task and funneling matter into the singularity as it "evaporates". I haven't crunched the numbers on which would generate more power, but the singularity type seems like it'd be more in line with the graviton manipulation technology.
Either way, power can be routed to systems on the fly, and multiple high-drain functions can be sustained for a while until the power system's capacitors (or equivalent) need time to recharge. If the drain is low enough compared to the core's output, then those functions could be sustained indefinitely. There's one scene in the novel where fighters are launching continual streams of shield-breaking missiles against a Great Enemy vessel, but the barrages are sustained for so long many of them need to shut down after a while and pray they don't get picked off while they drift.
Piloting an ATS is vastly more complex than most mechas. I always found it weird a pilot can hop into something so complex and move it about as if it were a perfect extension of their body with just a pair of joysticks and a butt-load of buttons.
The primary means of controlling one is an extensive neural interface. The pilot is suspended in a complex harness, and essentially hangs there, though to them it doesn't feel like they're hanging, almost more floating, though super-localized gravity fields provide instant and accurate feedback to what the suit feels (i.e. the ground or, to a lesser extent, impacts), as well as protect the pilot from any form of inertial feedback. Numerous wires and muscle sensors attach to both a skin-tight suit and directly into neural jacks on the pilot, creating an accurate response to the pilot's movement. Because a pilot performs the same physical actions their suit does, they actually need to be in peak physical condition, though genetic conditioning and select cybernetic implants will enhance their endurance and reactions far beyond the norms.
Perhaps the most important part of the interface is the "crown", a partial helmet laced with wires and connection ports, which connects the pilot directly to the suit they pilot. Of course, there are a number of neural implants which help the pilot process the massive stream of data. However, the training and conditioning for this still takes a full year to complete, as too rapid of integration throws the pilot right into shock and/or brain-death. A lesser form of these interface implants, the Adjutant Chip, is commonly used throughout the Solar Alliance to aid in complex tasks, and Grand Admiral du Saniel has one in the novel.
Not all information is relayed via neural interface, and the pilot can choose to have their cockpit project a variety of information, from simply showing their surroundings to displaying various systems information. Sufficed to say, a pilot is always fully aware of the status of their suit, their surroundings in all directions, and the function of their weapons, leading to a vastly superior means of control than seen in most mecha franchises.
Naturally all this fancy technology needs protection from the dangers of the universe and anything the pilots may need to fight. The most rudimentary defense is plain old armor, in the form of sculpted plates protecting the bulk of the internal components. Such means don't necessarily work for the flexible joints, however, and most ATS designs will incorporate thinner/smaller plates and/or the aforementioned flexible material. Obviously the level of protection is lower, but the added flexibility can easily make up for it.
And, of course, given the ATS being manufactured on dozens of planets across centuries of service, the style of the armor can vary greatly. The majority of designs resemble historical suits of armor, although there are numerous designs that resemble plate armor but take no basis in pre-industrial designs. They may incorporate a variety of aesthetic embellishments, such as spikes or layered plates. A rarer variant is the use of small armor plates layered like the scales of a reptile, essentially replicating the historical "scale-mail" style of armor. The smaller plates can be used in conjunction to afford flexibility and protect the vulnerable joints, and are far easier to replace, although the smaller (and typically thinner) plates cannot absorb the repeated impacts of weapons fire.
Another contrast to contemporary materials is what the armor of the All-Terrain Suit is made of. The most common material used is tritanium, an advanced alloy capable of withstanding the full range of extreme temperatures of space, as well as being extremely impact resistant. Indeed, this is the same material most starships make their outer hulls out of. Other materials may sometimes be used in varying combinations with tritanium, such as adamantium or impervium. An ATS may also carry a tower shield in one hand which provides additional protection from incoming fire or close-range blows. Smaller shields may be mounted to the shoulders, providing some protection while leaving both hands free.
Beyond the armor, the ATS typically employs a weaker version of the gravity shields employed by starships. Utilizing an advanced sensor suite to detect incoming projectiles or energy blasts, the shield system projects micro-singularities to intercept would-be attacks, absorbing them into nothingness. While the "shields" can be projected theoretically indefinitely, the system's capacitor coils tend to suffer from overheating (a mix of heat, excess radiation, and absorbed kinetic force), and thus, if employed too rapidly, will overload and require time to "cool down". Therefore, a concentrated attack can overwhelm a shield system and damage the ATS or ship before it has a chance to recover, but, the incoming damage can be mitigated enough, then the craft is functionally invincible.
Of course, what giant robot isn't complete without an array of deadly weaponry with which to destroy everything around it. Well, I suppose Basara from Macross 7, but you know what I mean!
Naturally, fulfilling a variety of battlefield roles, the armaments of the ATS varies greatly. Front-line standard models will carry battle rifles, firing either large-caliber shells or energy blasts. The shells range in caliber from 10mm gatling guns to 150mm high-explosive shells (naturally, the larger-caliber ones will require larger barrels). Recoil isn't an issue, as all solid weapons are fired gauss-style. Nor is ammunition, as the thalite reactor provides enough power for energy weapons or powers a micro-replicator system for the solid weapons. And thanks to gravity manipulation, weight isn't a factor either. An ATS can carry as much as it can fit without worrying about being burdened by the weight at all, and requires no modification to switch between weapon systems, simply needing to pick another weapon up and the links are automatically formed. Thus an ATS can switch between close-range "pistol" type weapons to sub-orbital artillery in a matter of moments.
Secondary systems will often be mounted as well. The shoulders are popular spots for adding heavy weapons, racks of missiles, indirect fire mortars, and various other weapons, or sometimes ECM/ECCM suites. Smaller missile racks are often mounted to the thighs or calves of the suit, as well, providing close-range fire support.
Another common armament is head-mounted machine guns for anti-infantry purposes. These are typically around .50cal in size, but may vary depending on the desired effect. Although the ATS has little to fear from infantry generally, or may not be able to afford the more indiscriminate destruction of the larger weapons, these guns can be good for deterring a determined anti-vehicle assault by infantry.
Although rarer, some come equipped with melee weapons. Yes, I know, it seems like a silly idea, and no, there aren't units of all-melee suits flying around trying to take down their gun-wielding opponents. But carrying at least one melee weapon as a backup is fairly standard. Kinetic shields normally don't work against melee attacks. Sometimes the enemy will be too close, or your guns have all been shot off. Or somehow you've run out of ammo. Stop looking at me like that, it happens!
But yes, the melee weapons utilize tightly projected energy fields to give them lethal edges, and typically come in sword, axe, spear, or halberd form. A few might rely on knives, and some worlds will equip their suits with more esoteric designs.
At last, we come to the final section! All the basics of the ATS have been covered, but there's one additional bit I wanted to cover. There's a rarer class of Suit which far exceeds what regular Suits are capable of. These Suits are typically built with the absolutely latest and cutting edge technology, or even often experimental technology. Although their official designations change regularly, colloquially they're generally referred to as Savior Suits. There's a common saying that a single squad of All-Terrain Suits can alter the course of a battle, while a squad a Savior Suits can alter the fate of a planet.
Essentially the Savior Suits are the Gundams of this universe. As I said, the combat potential of a single Savior Suit can be equal to entire platoons of of regular All-Terrain Suits. They can attain speeds faster than any other human vehicle outside of FTL travel, dodging even near-light-speed attacks, able to engage dozens of targets at once with unerring accuracy, withstand attacks that would annihilate a regular Suit in a single blow, and generally outperform almost anything else on the battlefield.
Such effectiveness is not without cost, however. The genetic and cybernetic alterations made to pilots of Savior Suits are vastly more extensive than normal pilots, and it's not unusual for Savior pilots to become, in many ways, removed from the humanity they protect. Their original personalities will often fade away or be replaced, and many, once wired into their suits, will not leave it until they die or the suit suffers extreme damage. Basic biological needs are fulfilled while connected, and so they see little reason to exit the suits. Companionship, curiosity, social impulses, and other such considerations become subsumed in a singular desire for war. They often go into a sort of hibernation between battles, only half-concious for days, weeks, or even months at a time, until a signal is sent informing them that war calls once again. Whether you agree with such costs or not, their effectiveness cannot be denied. Whether you agree with such costs or not, their effectiveness cannot be denied. And in a war for the very survival of mankind, it is a price most are willing to pay.
So, that pretty much wraps up everything about the All-Terrain Suit in my universe. Did I leave anything out? Any further questions? Did I maybe mess up somewhere? Let me know!