So here goes with my first article in a planned series on ‘Mech design. In these, I’ll be focusing rather exclusively on Inner Sphere technologies, since we all already know how potent Clan tech is. For the first entry in the series, I thought I’d start with the bones and work my way out from there: Internal Structure types.

Inner Sphere ‘Mechs have access to four types of Internal Structure (not counting Standard), those being Endo Steel (2475), Composite (3061), Endo-Composite (3067), and Reinforced Structure (3057). Internal Structure choice is one of the largest weight-saving decisions that can be made in ‘Mech design, second only to engine selection.

Endo Steel and Endo-Composite are real straight-forward: any time your build needs tonnage more than it needs slots, use Endo or Endo-Composite. Their only down-side comes at C-bills, since Endo Steel is 4x the cost and Endo-Composite is 8x. Your best-case scenario for either is saving about ¹/₃ of a ton per slot on a 100-ton ‘Mech.

Because of how Endo and Endo-Composite are calculated, some tonnages are less efficient than others. If you’re building a ‘Mech, or considering one, that uses Endo Steel you should build at a tonnage divisible by 10 (10, 20, 30, etc.). When considering Endo-Composite, the best tonnages are ones divisible by 20 (20, 40, 60, etc.).

Composite Structure provides the same benefits as Endo-Steel, but instead of using up crit slots, it comes at the cost of taking double damage to Internal Structure. Which is why people I know HATE it. But Composite is where Internal Structure gets interesting, and its applications aren’t as apparent as you’d expect.

People don’t like Composite because it, seemingly, makes ‘Mechs brittle and vulnerable; once all SI in a location is gone, that location is destroyed. So fewer Structure Points sounds like a bad thing, but what is Structure really? Internal Structure points are Armor but worse, because when you lose a point of Armor you don’t have to make a crit roll. So if you could trade 1 point of SI for 1 or more points of Armor, wouldn’t you?

Looking at it from this perspective, we can do some math that tells us how many effective SI we lose for each ton we save. If that value is 16 or under, then adding a ton of Standard Armor is a direct upgrade. This happens at (in order of most to least benefit) 100, 80, 70, and 90 tons. If your ‘Mech is mounting Light Ferro-Fibrous armor (16.96/ton), this is also true for 95, 85, 75, 20, 50, and 40 ton ‘Mechs. Regular Ferro-Fibrous (17.92/ton) makes the cut at 30, 60, and 65 tons while Heavy Ferro-Fibrous still benefits 55, 45, and 35 ton mechs. 25-ton ‘Mechs never see this benefit from Composite Structure because it costs them 21.5 SI per ton saved, which is brutal.

For examples of ‘Mechs that have made great use of Composite structure, check out the Hercules HRC-LS-9003 (70 tons), any of the Zeus-X models (80 tons), or the King Crab KGC-011 (100 tons). On the other end of that is the Brigand LDT-XPR5 (25 tons), which takes a huge gamble on using Composite Structure in the hopes that being able to jump 8 hexes per turn will make up for the substantial loss in durability.

I’m not too sure about the Brigand LDT-XPR5’s odds myself, but I wanted to bring it up because it’s a clever design and it shows how the numbers and advice I give in these articles isn’t an absolute thing. The XPR5’s designer probably had a good idea of just how big the loss in toughness would be by using Composite, but they used the crit slots it saved to mount Improved Jump Jets and Double Heat Sinks. Getting that ‘Mech up to a +4 TMM per turn (not counting terrain or the Nimble Jumper bonus) shows there was consideration given to its otherwise crippling vulnerability. At nearly 750 BV, I still think it’s a terrible ‘Mech made of paper. But if you’re going to make poor life choices, they should be interesting, and the XPR5 is definitely that.

After my previous take gushing over Composite Structure, it should come at no surprise that I love Reinforced Structure for many of the same reasons: it trades tonnage for defense. Where Composite frees up tonnage you can expend on additional armor, Reinforced doubles how much damage your Internal Structure can take, which lets you tank damage with your ‘Mech’s bones!

But hold on. Just a minute ago I said Internal Structure was just worse armor and advocated trading Structure for Armor because it meant fewer crits! So why do I also like Reinforced Structure? Reinforced lets you maximize durability by giving you the equivalent of more than maximum Armor whereas Composite lets you save a bit of tonnage you can turn into armor. On top of that, Reinforced Structure applies a -1 to any roll on the Critical Hit Table. That -1 means never taking 3 crits or losing a limb from any one hit, and overall it means taking an average of around 40% fewer critical hits (36.11% instead of 61.11%).

Once again, the quirks of how all of BattleTech’s stats are calculated means that some tonnages are better candidates for Reinforced Structure than others. The measuring stick here is any time you would gain an average of more than 16 points of Structure by going to Reinforced, it’s worth it (to me). That means if you’re looking at ‘Mechs in the 65 to 100 ton range, you can do better things for survivability than Reinforced Structure. Where this benefits you the most is on the other end of that, from 20 to 60 tons. Three tonnages outshine even those, providing more Structure per ton than Light Ferro-Fibrous provides in Armor: 25 (17.20), 30 (17.00), and 60 (17.17).

If you’re looking for canon examples, I can name two that I really like. The Mongoose MON-96, which mounts a bunch of ER Lasers, and the Paladin PAL-3, which mounts a Medium X-Pulse Laser (my favorite ever weapon). Okay, admittedly, it does also have a UAC/20 which I guess is important. Anyway, the Paladin takes an interesting risk: it mounts an XXL engine, banking on the critical hit resistance to keep the engine running. In fairness, I’ve never put a Paladin on the table and only found it because of the research I did for this article. But look at it! It’s pretty cool, isn’t it? Ignore the explosive side torso.

Now, if you want a really bad example of a ‘Mech that mounts Reinforced Structure? Look at the HGN-641-X-2. It’s not that bad, is it? Until you realize you can use Standard structure, a regular Fusion engine, and max out the armor values instead while still having a half ton left over. Worse, doing it my way cuts the cost of the ‘Mech from around 22 million C-bills to just under 12 million. This is an example of a ‘Mech costing itself a ton of resources to do the same thing it could have done without those expenses, only worse.

I don’t know, Composite and Reinforced Structures are both way better than people realize. Especially when used to their strengths. So think about how you might use them in your own builds! Let me know what I got wrong or right. What do you think of the example ‘Mechs I chose?