#mazinkaiser — Public Fediverse posts

Mecha design; Monocoque vs Frame vs Unibody construction

When discussing mecha design, how the mecha are generally constructed is often overlooked. There are multiple reasons, mostly because fiction doesn’t need to adhere to reality. Fictional materials and construction methods are often hand waved off in lieu of more pressing and important story elements, like how a combination sequence functions and how these parts into new shapes.

However, considering how a mecha is constructed in-universe allows unique world building and storytelling possibilities otherwise unavailable. This can come through as a single artisan crafting all the parts necessary to build a work of art, or if a mecha is build on an assembly line among thousands of others. While these may be just background information to many, they nevertheless directly affect the mecha when it goes through maintenance, or even how it functions.

In this post, I’m going to touch on three basic ways to consider how your mecha is built. Take these as very broad generalizations, as each manufacturing method has books worth of details to get into.

I’m also going to talk about Gundam somewhat extensively towards the end as it showcases some more examples how the construction changes within the setting.

Monocoque is a way of construction, where the outer shell, or the skin, bears the structural load. This means there is nothing supporting the mecha inside, as there is no need for an internal frame. Monocoque construction in general is great when need for a fast and lightweight machine is needed. The structural rigidity should also translate into well-handling mecha, especially when aerodynamic shapes are easily integrated into the design. The light weight also comes from the lack of any sort of internal weight from support frames or similar, making this sort of construction ideal for high-performance applications, like boosting.

On the downsize, monocoque construction is the most expensive option out of the three choices here. Designing a monocoque mecha would necessitate far more calculations on how each section would bear the loads, which necessitates highly accurate and precise construction. The more precise the manufacturing has to be, the longer it will take and the more it will cost. Even when when considering fictional materials, whether or not they’d be alloys or composites, their cost would be high. Assembly itself would require advanced engineering.

This would also translate into repairs being equally as expensive, as damaged skin compromises the whole structure. For example, a gaping hole in a mecha’s leg could lead into catastrophic collapse as a point of failure. Simply welding that hole shut might not be enough, as that would change how the leg bears the load of the rest of the body. Welding an additional plate as a patch job might do it, but on the long run monocoque design would require changing whole body parts on the mecha to maintain the original structural integrity.

The lack of modularity would also rear its ugly head, unless the mecha would have specific hardpoints or some kind of extenders built to it to accept additional elements to it. However, there is only so much the skin can take, making monocoque probably the weakest option for modularity. Should probably also mention that monocoque mecha would be loud to the pilot, as there’d be no vibration isolation between the body and frame, unless the cockpit was somehow floating in suspension in the middle. Vibrations include everything between sound and shaking, e.g. with cars vibrations isolation makes the cabin quieter as the wheel rumbling from the ground doesn’t vibrate through the frame to the cabin.

Manufacturing monoqocue parts would require molds. For example, layers of composite plies at few mm thick are set in a mold, then cured under pressure and heat for several hours in order to cure them. Of course, casting metal in high-accuracy mold would be another option, if you have a way to remove complex parts out. Manufacturing the parts would be as important in the fiction as the repair, maintenance and modifications, as everything starts how the mecha was built in the first place.

Another kind of example for a monocoque mold from https://www.researchgate.net/figure/Jilin-University-Formula-Student-Monocoque-in-2021-and-its-two-pieces-of-female-mold_fig2_369379104

The best real-world example of monocoque construction would be the F-1 racing cars. They weight relatively little compared to the sheer speed and control they have. The monocoque nature makes these frames very rigid, and due to being composite materials weight somewhere around twenty percent less than equivalent aluminium frames, and are very aerodynamic due to lack of flexing. These are cars are design and built for maximum performance, at the cost of manufacturing and repairs. Material and labour costs are high and require specialised techniques and materials.

Another one would be the F-22 Raptor. With the Raptor, composite monocoque body cut down the weight and allowed better stealth to be implemented. Each Raptor still costs about 135 million euros, but as an air superiority fighter it seems to be doings its job well enough.

Opposing monocoque construction is Frame, or Body-on-Frame construction. In this approach, an internal frame bears the load. This frame is a distinct structure within the mecha, to which components are attached to. In real world term, the “body” is a separate component that is attached to the frame, providing enclosure.

Frame construction is first and foremost easier and cheaper to manufacture as the frame and body are two distinct components, which also makes repairs much more easier and efficient. This approach is flexible by its nature, allowing easy modifications and custom builds without any specialised knowledge or manufacturing methods. With some engineering, vibrations and noise can be isolated between the frame and the body.

Frame construction also isn’t as susceptible to localised damage as monocoque. That gaping hole on a leg could be just the outer armor being damaged while the internal frame is intact. That armour damage can be remedied via some plates attached to it, or just swapping out the armor plating.

Roofs, in general, are an example of frame construction. https://www.nachi.org/gallery/framing-1/roof-framing-2

Frame construction, however, is less efficient as monoqocue due to the additional weight, unless the mecha is mostly just the frames with a cockpit. Because of the overall weight, the center of gravity is also higher, especially the bigger the shoulder armours are or with a bigger backpack. The overall structure is also less rigid due to separate elements being put together rather being one whole body, leading to less overall poorer performance. In a same sized construction between monocoque and body-on-frame, monocoque has more internal space for fuel or pilot’s cockpit.

To illustrate Frame construction vs unibody, cars make the best example. Here, you can see the cab being placed on top of the frame, while in unibody there is no separate body; it’s all one piece.Monocoque is a subset of https://philkotse.com/safe-driving/crumple-zones-and-car-chassis-design-how-it-keeps-you-alive-2746

Manufacturing frame construction is expensive due to needing additional components, but designing and manufacturing drops the costs down quite a lot. It would be easier to design a frame-built mecha to be strong and durable, especially for rough terrains with heavier loads than monocoque. The beams and the rails the frames are constructed from usually are much stronger than the body, which could be just stamped out.

To throw a real-world example about body-on-frame could be any pick-up car. Here, I’m going to use Toyota Land Cruiser, as it handles pretty well in off-road environments and is easy to repair with basic tools. Sure, it weights over 2 500kg and isn’t exactly stable at high speeds, but it is used because its reliable and sturdy car in any road its taken to. Can take a beating too.

The M1 Abrams would be the military example. Supposedly its easy to repair and upgrade even on the field. Much like the Land Cruiser, the Abrams has a massive weight almost 62 000kg, and can take combat stress on top of that. Its separate chassis should, in principle, make those repairs that much easier. Sure, it eats fuel about 400l per 100km, but it needs that for its powerful engines to push forwards. Its intended design is for extreme durability and adaptability on the battlefield, something monocoque just can’t do.

Unibody construction is sometimes mistaken as a synonym with frame construction, but it’d better to think it as a slight combination of the previous two. The skin may be the main load-bearer, but there also an internal framework that’s providing further support. The stress the machine goes under is shared between these two elements. The confusion can come from all monocoque constructions being unibody as a single-unit construction, but not all unibodies are monocoque due to internal frame. It’d be good to think monocoque as a strict subset of unibody.

As such, combining the outer shell with the internal structure is unibody’s selling point. Modern city cars are usually build like this, using sheet metals with reinforced sections on an internal frame of sorts. This carries the fuel efficiency with it as well as better handling due to the rigidity the construction provides. Space within the structure is also more available than with frame construction. Stability is also increased due to lower center of gravity. Compared to the previous two, unibody is also surprisingly cost-effective to mass produce due to fewer overall components needed to manufacture the whole thing.

On the flip side, the structure isn’t as durable under heavy stress. Repairs are naturally more complex and costly due to the integrated structure, sometimes necessitating switching whole sections out as it might be cheaper to manufacture a new part than repair the damaged one. While there can be modularity, the mecha has to be designed this in mind as any hardpoint must be part of the integrated frame. Still, the heaviest options should always go to the body-on-frame unit, as unibody is inherently weaker in this regard. Then of course, all those vibrations become harder to isolate when body and frame are the same.

The actual construction of a unibody parts would be a combination of monocoque and body-on-frame methods, applied together.

Real world example car would be the classic Honda Civic. Lightweight and economic car, good to drive due to stiff chassis but rather noisy. It’s a car you don’t really use for towing something and shouldn’t go off-road with either. Civic’s chassis also tends to take beatings pretty easy, but they are intended for the general mass-markets anyway. They’re good for what they do, which is nothing too exciting. Maybe that’s the best a car can be, if we’re honest.

There’s not much widespread military application for unibody construction per se. However, as we discussed how monocoque is a subset of unibody, sections of a craft can use an additional support structure while others are monocoque. The F-35 Lightning II is often called unibody, but has monocoque elements to it. The main fuselage skin is monocoque, constructed using materials like aluminium and carbon fiber composites. The wings have minimal internal spars to provide support, but the main design is monocoque to prevent bending and torsion. These choices cut down the weight of the F-35.

However, under the monocoque skin there is a reinforced metal framework. Its spread inside the fighter much like a car’s unibody construction, and provides the mounting points for weapons systems. It also adds further rigidity to the monocoque skin. In truth, the F-22 has this too, but let’s put that aside for the sake of having recognizable examples. The F-22 in general relies heavily on monocoque design.

How can these examples be put into action in fiction? We can approach this in terms of how the robots are depicted in media, as not even information books want to dwell too deeply in to the reality of the constructions. In fact, there is a dire lack of mecha media that would be solely about the mecha, how they’re built, what goes into their maintenance with little to no characterisation on the people.

To use the provided examples as some kind of reference, perhaps the first monocoque mecha I can think of is the VF-1 Valkyrie from Macross. The reason is very simple; using a real-world analogue to turn into a giant robot. The reason why VFs in Macross require monocoque construction (with some minimal internal supports thrown in there) is to maintain rigidity during flight and transformation. The aerodynamic shell is a must to optimize for performance over toughness. A VF can be cast and nimble, but it can’t shrug off similar damage a Zaku II can. Furthermore, as some VFs have those energy-conversion armours, a monocoque design would be a better conduit for energy transfers, where frame build would require more connections.

I would hazard a guess that Cybuster is also mainly a monocoque design, probably along with most other magical robots. When magic is powering your robot, and you can shape magical metals to whatever form you can, there’s surprisingly little reason to go for anything less than high-performance, lightweight designs. Cybuster itself doesn’t really do modular weaponry, only carrying what’s it designed to have outside Original Generation settings. Of course, there are settings with mass-produced magical robots, so balancing between the two should be taken into consideration when world-building.

Transforming and combining mecha in general probably have monocoque construction, if their outer shell is rigid. This rigidness and prevents any of the parts from flexing during the sequences and ensures alignment between parts is perfect. An internal frame would put significant stress on the joints during transformations and combinations, so having the skin take most of stress away as the components integrate while making the overall mecha as light as possible is plausible. Think how accurate Linear Gao has to be when it goes through GaiGar’s body to form the shoulders and upper arms. There’s no room for it to flex, rigidity is the kind.

Mazinger Z could be described as body-on frame mecha. It is a highly durable super robot that can take a beating from punctured torso and legs to missing bits altogether without losing much functionality or disastrous breakdowns near the damaged regions. The modular weapons systems would indicate numerous hard points under the Super Alloy Z armouring, especially when we consider the Rocket Punch to be a weapon that launches and returns on a regular basis. The analogous description of heavy built, heavy duty mecha fits it and its successors like the Grungust pretty well.

However, it could also be argued that Mazinger Z is more akin to unibody construction, as per Mazinger Bible. The outer armour seems to function as much as an integral structure to which internal components are directly attached. Nevertheless, the internal frame is there in both cases due numerous different weapons and options Mazinger Z can have. Repairing Z could be as easy as switching out some of its armour plates, or as complex as needing to manufacture a whole arm if needed.

Getter Robo is definitely a unibody construction, where the internal frame works with the outer shell to transform and take new shapes. Due to the nature how Getters transform and combine, the internal frame has to take most of the load off the skin, but the skin probably is partially reinforced by the Getter itself, if not for any other reason but because of the sheer will of the pilots. Gunbuster is similar this manner, designed for performance with tons on space inside for some internal structure.

Armored Troopers are equally definitely a frame-on-body mecha, a very definition of rugged and reliable. The design approach for ATs like Scopedogs were very utilitarian to begin with, and despite weighting six and a half ton, they’re speedy machines intended for warfare on multiple theaters of combat. A separate chassis supports its armored body, allowing similar durability, modularity, and ease of repair as with the M1 Abrams.

The RX-78-2 Gundam throws a monkey wrench in the cogs though. In canon, Movable Frames were introduced with the RX-178 Gundam Mk-II, preventing any Mobile Suit prior it being a frame construction.

Movable Frame is treated as an outright upgrade in every term, allowing separation of the inner frame, or the MS’ skeleton from the thrusters, sensors and whatnot external elements like armour and propellant tanks while the cockpit, generators, actuators and other more vital parts were integrated into it. This made maintenance easier while dropping overall weight and increasing maneuverability alongside increasing MS’ fluidity of movement.

A morvable frame

In real-world terms, Movable Frame would actually increase MS’ weight. Building an inner frame and then adding armour plating and whatnot on top of it is simply more material than having a monocoque approach. You can achieve great fluidity of motion without frame construction, and we could argue the fluidity would be even better with unibody construction due to higher rigidity and less possible flexing due to numerous different parts. Also, because the cockpits are part of the Movable Frame rather than being separate of it, e.g. suspended via shock absorbers , chances are one of the benefits from body-on-frame construction was lost and the pilot feels every nook and cranny the MS walks on.

The Movable Frame was most likely invented to sell the Gundam plastic model kits. Perhaps the way manufacturing is easier or makes the model building more interesting, when you have an inner skeleton to which parts are attached to. Nevertheless, the whole inside-frame carried over to vast majority of other Gundam series, but it makes no canonical sense with some Universal Century Gundam kits.

Because the Movable Frame is specifically mentioned to be the first frame construction MS, that should mean all MS before UC 0086 had to be either monocoque or unibody structures. Perhaps the weight of the MS is some indication. The Gundam’s empty weight is 43.4 tons, while the Zaku II 58.1 tons. Just based on this, I’d wager the Zaku II has unibody construction with a minimal internal frame to support the outer armour. The Zaku II was developed in an era where beam weaponry wasn’t yet miniaturized for Mobile Suit scale, hence its main enemies would’ve used physical projectiles and missiles. The Gundam however is almost fifteen metric tons lighter, which would indicate that it was of monocoque design to cut down weight and maximize performance. The Luna Titanium Alloy made it stupidly impervious against physical projectiles, while its main weapon, the Beam Rifle, cuts through any physical armour like a red-hot knife cuts butter.

Not depicted here; Zeon pilots praying to God their death with be swift and painless

Incidentally, the RGM-79 GM most likely has a monocoque structure as well. Its even lighter than the RX-78-2 it was developed from at 41,2 tons. This mostly comes from the Titanium Alloy used for the GMs, but I’d wager also from further developed manufacturing methods and simplified designs. The GM should, but all means, be an absolute performance monster in the mid to late One Year War, outperforming any other generic MS on the field. However, in games its stats and performance is almost always worse than that of the RX-78-2 Gundam. Probably to sell its role as a cannon fodder MP unit. Of course, the Gundam managed to last as long as it did because Amuro Ray was a powerful Newtype, and during the One Year War saw enough action to become an Ace pilot few times over. The GM did saw numerous variants and modifications down the line, which would indicate it has at least some unibody construction elements integrated to it.

Having a lighter MS is absolutely necessary in post-OYW Universal Century due to the beam weaponry based warfare. We could assume that part of the reason why Movable Frame made MS lighter was because armour was simply cut down in mass. New materials and less of it to lower the overall mass of the MS, as physical projectiles were less and less relevant outside missiles. Almost every MS would use a some variation of beam weaponry, making dodging far more important than dodging.

Incidentally, in the alternative UC timeline seen in GQuuuuuuX, the gMS-Ω GQuuuuuuX design has either unibody or monocoque construction. Ikuto Yamashita showcased this in his design, which can be seen in the elbow joint being fully integrated into the arms’ plating. This is very similar to real life Fanuc industrial robots, which are fully monocoque designs. A good example how using one of these approaches influences the design when taken seriously.

In no manner consider the above examples as definitive. As said, these are assumptions made through the examples. Outside Gundam, I’m considering their on-screen depiction more over whatever source material we have at hand.

If I were to sum all this up, I’d give recommend thinking these three approaches as reference methods how you can design the visuals of your mecha while considering its construction method. We can think these as separate if we want to, reflecting the role or nature of the mecha in overall terms; a monocoque design could be something unique and beautiful with monstrous performance, while frame constructed mecha would play the part of a rough and heavy workhorse. Unibody might work best as a mass produced model, not as easily modified or repaired, but easy to replace with a new one once the manufacturing pipeline is up.

Such simplification would be selling the manufacturing methods a bit short though, as a mecha could just as well incorporate all three in some fashion. To have a sturdy inner frame works well in places where toughness is needed and accuracy is secondary, where we know modularity and parts changing is required. Similarly, other sections could be monocoque to save in weight while allowing maximum space within, like the cockpit. Following realistic construction methods can create hypertextuality with real-world applications and designs, but equally so breaking them can yield very alien and out there designs where needed.

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