BMD: Signature Size

A custom-built solution to the scaling problem

One or two paragraphs will suffice to explain away size differences in most TTRPGs. But in BMD, an object’s Size—formally called Signature Size in case the meaning is ambiguous—ties into many gameplay effects.

One of the persistent challenges that wargames face is questions of scaling¹. In BMD’s case, the scaling is not so much about the size of the opposed forces but in how to represent battles with infantry, orbital support, and everything in between all participating on a single battlefield. Signature Size provides an easy way to move between all scales of force while increasing the communicability of various aspects of the game.

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Navigating All Scales

The Signature Size of an object is more of a category than a measure, and these categories are used in BMD to support play at every meaningful scale. Before we can break into interpretations, we need to have ready examples. Here are small, average, and large examples of different Size categories that are most relevant in gameplay:

• Size 0: credit card, human hand, Desert Eagle 45

• Size 1: laptop computer, microwave, rifle

• Size 2: wolf, human, motorcycle

• Size 3: golf-cart, rhinoceros, Hummer H2

• Size 4: full-size APC or MBT, helicopter, fighter jet

• Size 5: submarine, house, cargo plane

• Size 6: aerospace hangar, reactor building of a nuclear plant, aircraft carrier

The categories continue, but examples become more sparse:

• Size 7: large sports stadium, city block, spacefleet battleship

• Size 8: large suburb, golf course

• Size 9: a city

• Size 10: a region

• Size 11: a continent

• Size 12: a planet

• Size 13: a star

• Size 14: a solar system

Now we’ve got the idea.

Scale of vehicle Sizes

It’s worth noting before we continue that Size plays into the storage and storage space aspect of the game. For example, a container made to fit Size 4 can fit one Size 4 object and three Size 3 objects. A container made to fit Size 2 can fit one Size 2 object and three Size 1 objects. This system is possibly overly simplistic for storage, but it’s being watched for issues.

Apart from the simplistic case of storage, Size has an enormous impact on gameplay—particularly on weapon usage and fitness. But before we look into that, let’s briefly review how to determine the Size of an object in gameplay terms.

The Right Category

The intent behind Size categories is that the next category up offers a significant leap in a combination of dimensional reach (silhouette), extent and breadth (volume), and mass—though none of these three requirements are a hard rule.

The small and large examples exist nearer the “boundary” than the average example, but there are not strict boundaries. Comparisons between the small of one size with the large of the previous size can possibly invite confusion, but these are extreme examples meant to illustrate the range of objects that belong in that category. In principle—because objects can be made to arbitrary dimensions—the boundaries of each category can be filled with examples and counter-examples.

However, these issues are relatively minor because most important objects and features clearly belong to the different categories. Exceptional cases being handled by adjudication will have little or no impact on gameplay.

Silhouette Comparison

For a more intuitive way to envision what belongs in higher Size categories, compare their silhouettes. Consider the case of trying to classify the Size of a laptop computer. Carl thinks it should be Size 0, while Bob thinks it should be Size 1. Choosing Size 0 to measure against, we ask two questions:

1. Is a laptop easier to shoot than a human hand?

2. Can a hand fit “inside” a laptop?

Both of these questions are really silhouette comparisons.

Imagine throwing a dart at a laptop vs. a human hand. Most objects have a “small” dimension—for example, the sides of a laptop and the side projection of an open human hand—but we will compare objects along non-extreme dimensions to avoid confusion. A laptop is a big rectangle when closed—or two rectangles when open. A human hand is a rectangular palm plus some extra fringe from fingers/thumb. The laptop is a substantially easier target to hit.

Can the hand fit “inside” the laptop? If we project a laptop onto a desk, we would find that usually three or four hands on average will fit easily inside the projection. The fact that it’s comfortably more than two is a strong indicator that a laptop is larger than a hand.

Exhibit A: laptop vs. “human” hand

As long as we follow the given examples, this kind of examination will quickly answer this question of which category an object belongs to.

Accuracy and Massive Damage

The most frequent and exciting gameplay consequence of Size is on Accuracy tests and Damage.

Every weapon is designed with some particular target in mind. We don’t use scalpels to cut down trees nor do we use chainsaws to perform surgery. For now, we will refer to a weapon as Class X if it is designed to strike at Size X targets.

Infantry Scale and Accuracy

Most infantry weapons are Class 2—designed to acquire and destroy Size 2 targets. Humans (Terrans) are Size 2, and most of their alien opposition figures will be Size 2, with the notable exception of the Dhross at Size 3 (rhino- or horse-sized).

Consider a Terran soldier firing a typical rifle at a lizardman in optimal range. Terran soldiers have Accuracy 4. We use the D10 in BMD, and most scores act as a bonus to a roll where a success is marked by a 10 or higher. This means the Terran soldier will need to roll 6 or better (6+) to successfully hit the lizardman (because 6 + 4 = 10).

What if the soldier was firing at a console (about the size of a laptop)? That’s a Class 2 weapon firing at a Size 1 target. If we take [Target – Weapon Class] or [1 – 2], we get a –1 penalty to Accuracy and will need a 7+ to hit. What if the soldier was firing at a keycard (Size 0)? By the same logic, he will need 8+ to hit.

If the soldier is firing at a personal vehicle like a sedan or pickup truck (Size 3), he will receive a +1 bonus to his Accuracy and will need a 5+ to hit. Firing at a tank, he will only need 4+ to hit.

Ground Vehicle Threats

Since most (military) ground vehicles are Size 4, drivers must be worried about enemies carrying Class 4 weapons. These weapons are recoilless rifles with armor-piercing rounds, guided rockets with shaped charges, high-powered burst lasers, and so on.

Infantry threats include recoilless rifles and man-carried launchers, with modern-day examples consisting of the Carl Gustaf, NLAW, Javelin etc. But Size 4 vehicles are a natural platform for Class 4 weapons, meaning that vehicles are always a threat to other vehicles.

Consider a Terran tank trying to fire on a unit of lizardman infantry. If using the typical Class 4 primary weapon—such as a giant 100mm autocannon—the targeting system designed to hit other vehicles will receive its natural –2 penalty for a Class 4 weapon against a Size 2 target. If the gun operator has Accuracy 4, he is going to need the familiar 6+ to hit another large vehicle—but he will need 8+ to hit the infantry unit.

Massive Damage

What if the tank gunner gets lucky and does hit the infantry unit? An over-Classed hit—one where the Class of the weapon is higher than the Size of the target—will deal double the rolled Damage for each level of difference. For example, if our Class 4 autocannon lands a hit on the lizardman infantry unit and rolls a final tally of 8 Damage, our gunner multiplies that tally by 4 (2×2) to deliver 32 Damage to the unit. That is a devastating hit to any infantry squad, likely to wipe out more than half of them!

A good way of dealing with infantry for vehicles is to have an escort. But a cheaper way is to equip with a lower Class weapon that will not have as much of an Accuracy penalty. A machine gun or similar SAW (like an MG 50) is a Class 3 weapon, for example.

Outclassed Damage

Consider the opposite direction: what if the lizardmen attack the tank? A strong attack with Class 2 weapons will produce rolled Damage in the vicinity of 12, owing to their increased ease on Accuracy tests. But when a weapon is outclassed by the target’s Size, the target Blocks 2 Damage for each level of difference; lizardmen lose a total of 4 Damage when attacking the tank with regular rifles.

Again, the solution to this problem is to bring up-Classed weapons to deal with up-Sized opponents.

Extreme Scale Differences

What happens when an infantry squad (Class 2 rifles) attacks an assault lander (Size 5)? They have an easy time hitting with a +3 bonus to Accuracy, but they will have 6 Damage from every attack Blocked. A lander with a Class 5 weapon (unusual but possible!) would have trouble hitting the infantry with a –3 penalty to Accuracy, but would deal 8 times its rolled Damage.

Size Glues the Scales Together

Because the final tally of Damage is scaled between Sizes and weapon Classes, each Class of weapon can have roughly similar numbers. We don’t need to roll 170 dice when we want to use a battleship’s main gun—we can stick with about 20 and let the Massive Damage rule take care of the rest!

Similarly, we don’t need to introduce the problem of having comically large values of Protection which makes a lot of things completely impervious to unreasonable amounts of Damage. The Outclassed Damage rule implies that, roughly speaking, smaller caliber arms are delivering less overall force/energy.

Stealth and Noise & Motion

Imagine an abandoned library. No one has been there for years. There is no ambient sound; it’s deathly silent. Neither is there ambient movement like the swaying of trees nor the flow of buyers in a bazaar. If we posted a sentry to watch this library, it would take enormous skill to sneak past him.

Now imagine a factory floor—gigantic machines whir, tremble, and spin as huge hovertanks are put together by a hive of robotic activity. Even if we posted multiple sentries, odds are that someone skilled could sneak around beneath notice.

The Noise & Motion system in BMD accounts for the differences in these places—and Size is one of the most impactful factors.

Stealth Actions

The concept of stealth action in BMD is less interactive than in most modern games. Its design much more closely resembles that of AD&D in its fundamental logic; a participant attempts to sneak somewhere and is either successful or not. Passive actors who may notice such attempts are just that—passive.

First, we must understand the effect of Size. Simply, a Size X creature or animated object produces X Noise and X Motion when not idling. Terrans, at Size 2, produce 2 Noise and 2 Motion; they have an NM score of 2|2. Dhross, as much larger creatures at Size 3, produce 3|3 when on the move.

Sized to Sneak

In summary, the objective when sneaking is to reduce NM down to 0. Without going into specifics, for every success on our roll, we can reduce both Noise and Motion by one. Substantial stealth technology can reduce our NM score by 1. Moving carefully (dramatically slower) can also reduce our NM score by 1.

This means that a typical Terran moving carefully, and armed with stealth tech, will automatically succeed at sneaking past typical checkpoints! A Dhross using a similar technique will still have to roll at least 1 stealth success.

Floors

Any given area has a Noise Floor and a Motion Floor. In a dense jungle, we have a substantial amount of ambient sound and movement. Trees and bushes sway; water flows by banks; birds and snakes and vermin skitter all about. Distant animals call; the brook babbles, and the wind is sheared and cut by a thousand leaves.

We might describe this place as having NM Floors of 1|1. It’s quite noisy, and a cat or dog (Size 1) could probably sneak up on us—but we are still likely to notice a human or gorilla or similar. If that human was moving carefully, he could easily sneak up on us!

Some places are aggressively attuned against stealth. Consider sneaking across a tiled floor of an empty room. Even the tiniest footstep would resound off the walls. The slightest hint of motion on the part of the sneak would catch the eye of even the sleepiest sentry. These places can have NM Floors like –1|–1 or even –2|–2!

Note on Passive Stealth

Putting aside sneaking into facilities, Size also factors into detection technology. If we throw enough money and engineering knowledge around, we can equip a stealth system on a tank so that its signature appears to be Size 3 instead of Size 4. This technique has highly effective use cases like stealth hoverbikes that end up looking like Size 1, thus defeating a great deal of detection tech.

Fun at All Sizes

In addition to informing storage, stealth, and the scale of interior spaces—for example, Dhross interiors must be very large whereas Vessamar interiors will feel relatively cramped—the Signature Size concept is a system that unifies the different levels of combat. We don’t need to abstract the artillery when we could just roll its listed weapon dice and let the Massive Damage or Outclassed Damage rules do the resolution work!

Like so many other mechanisms in BMD, Size started as an afterthought addition that ended up gravitating towards the mechanical core of the game to serve a multi-pronged role. The work of fitting all these elements together continues! Now that we’ve seen Size, we can more easily discuss vehicles and weapons in future writeups.

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1. See this in-depth examination of hit-scaling in wargames. BMD will characteristically have combined-arms combat with the number of combatants measured in hundreds (or dozens) rather than in thousands or tens of thousands. The threat of area weapons forces doctrines to adjust downward in combatant density.