Book 6: Damage and Dangers

Taking It and Dishing It Out in GURPS

Introduction

This Book is full of improvements for GURPS combat mechanics that only work well at human scales. It reworks other dangers as well: falls, collisions, cold, and so on. Your character's size can make a big difference in whether she survives these hazards or not!

Another topic covered is that eternal online debate: are GURPS' Fatigue rules broken, and if so, how do you fix them? Following that is a discussion of realistic sustenance requirements and life spans for nonhuman designs, possibly a first for a game supplement! And plenty more.

Size and Damage

No special rules are needed to cover the basics. A Giant does lots of damage based on its ST, and an Ellyl does little. Dragon teeth are big and kitten teeth are small. Apply the appropriate damage to the target's HP and go from there.

ST and Damage

Combat ST vs Load ST

Under GULLIVER rules, damage from punches, hand weapons, bites and any other direct application of ST is figured from Combat ST, not Load ST. (Load ST can be used to hurt indirectly, such as by picking up and tossing a foe.)

All-Out Attack damage

GURPS allows characters to add +2 points of damage with an All-Out Attack. But a straight +2 means +2 whether it's a slap from a ST 6 weakling or a battleaxe smash by a ST 17 bruiser.

For a more realistic effect, add +33% damage, rounding up, instead of +2 points. This will please the high-ST players in your game, and provides appropriate results for characters of any size.

Handling Tiny Damage

Despite appearances, GURPS isn't actually saying that an attacker of ST 4 or less does no damage. It's saying that the attack does damage so puny it's of no concern to the standard human. Of course, to a Cidi a "mere" ST 4 blow is pretty hefty, and should be gamed as such.

An ideal system would "scale" damage neatly with ST. The simplest reworking would have ST 20 deal twice the damage of ST 10, ST 30 three times the damage of ST 10, and so on. Bringing weapons into the fray, a ST 40 Giant with an appropriately giant shortsword would do 4 times the damage of a ST 10 human with a normal shortsword. For low ST scores, damage gets appropriately small: ST 2 dishes out one-fifth the damage of ST 10, which can be rounded as appropriate (i.e., ignored if too small), or applied as fractional HP loss to small creatures for whom those fractions matter.

That's all intuitive and neat. Unfortunately, the GURPS damage progression doesn't scale so neatly (see the Appendix for one that does), but it's often close enough at the high end to be of use if you're not too picky.

At the low end, Vehicles 2E p. 97 offers a suggestion for fractional dice of damage:

• Less than 0.1 die: esssentially no damage.
• 0.1 to 0.2 die: Roll 1d-4.
• 0.21 to 0.4 die: Roll 1d-3.
• 0.41 to 0.6 die: Roll 1d-2
• 0.61 to 0.8 die: Roll 1d-1
• 0.8 die or higher: Round up to 1d.

The above makes for a rough solution, but it works well enough. The GM will have to make judgment calls on creatures with really low ST, until a neater ST damage progression and an "any-size-goes" melee weapon design system come along. Or head for the Scaling the Game rules in Book 8, which cover turning the tiny stats of puny combatants into more typical, easy-to-use stats.

Handling Split HP

That's split HP, not HT. Split HT means you have HT 12/28, which is HT 12 and HP 28. No big deal; HT and HP are separate things anyway.

But creatures like Centaurs can require two separate HP scores, one representing a "big half" and the other the "small half". Here are some ideas for playing this:

Pick one of the two body "halves" as the HP "base". Its HP are your character's base HP. Damage to that body "half" has normal effect.

Damage to the non-base "half" is applied to the same HP pool, but with a multiplier equal to (base HP / non-base HP).

Example: Your Centaur has HP 24 on bottom, HP 12 on top. Say you pick the horse half as the base, to give him 24 HP overall; damage to the horse half works normally. Damage to the human half, though, is multiplied by (24 / 12 = 2) before being applied to HP 24.

Alternately, pick the human half as the base, call your design HP 12, treat damage to the human half normally, and multiply damage to the horse half by (12 / 24 = 1/2).

Using GULLIVER rules for the treatment of HP, it won't matter at all which you choose; the effects will be the same. In general, it's easier to go with the larger HP, as you'll multiply rather than divide when the other half is hit. (And obviously, it's all easier to game when the larger HP is a neat integer multiple of the smaller. Design your creature that way.)

You're on your own in winging other effects: for example, will a tranquilizer dart take effect more quickly if you target a Centaur's human half, or is the creature an undifferentiated, horse-size mass for purposes of resisting? You be the judge.

See Book 3 for notes on hybrid creatures and buying split HP.

All About Blowthrough

"Blowthrough" is the maximum damage you take from a bullet or impaling wound. After inflicting some number of basic hits, the weapon passes through the body, doing no more damage. (See BS p. 109.)

How much damage?

A correction to the rulebook: blowthrough damage for any target should be set at some multiple of HP, not HT. For a standard torso hit, damage greater than straight HP blows through the torso and is lost.

A flaw remains: the rules increase blowthrough for characters who bought Extra HP as innate toughness, letting a weapon inflict more damage before passing through. To fix this, ignore Extra/Reduced HP when computing blowthrough. HP adjustments for size, on the other hand, definitely matter!

Example: Grog the Big has base HT 12 , HP 15 (he's a mutant with redundant organs, tough to put down), boosted to HP 30 for his Size +2. When the bullets start flying, base blowthrough damage on HP 24 (HP 12 times Linear Scale), not HP 30.

But this still penalizes high-HT characters – a HT 14 human suffers 14 points of damage before blowthrough, while Joe Average takes only 10.

Improved rule: An even better solution:

Set blowthrough damage at 10 points, scaled for size (i.e., 10 points x Linear Scale).

Example: Grog takes 20 points of damage before blowthrough, and no additional points for his high HT and base HP.

Using this rule, weakly PCs with low HP for their size will take a relatively high level of damage from blowthrough, which of course makes sense. Tough PCs with lots of HT and HP for their size will better survive piercings. This is probably the fairest solution for all characters, and is simple too.

Blowthrough modifiers

The deadliness of having having a hole rammed through you depends largely on where that hole was made! There's a x3 blowthrough multiplier for the vitals or head (but not the brain; rather, there's no maximum damage for injuries there, and blowthrough isn't used). A bullet or rapier in the chest will inflict up to 10 x 3 = 30 points of damage to the average person, before passing through.

Muliply blowthrough by x1/2 for limbs, x1/3 for hands or feet.

Beam, fireball, and lightning attacks double blowthrough for the torso, head, or vitals.

Creatures with odd shapes or compositions may further modify blowthrough levels; see blowthrough notes in Book 3.

Bullet size and damage

Then there's the matter of the size of that hole: is it a narrow, nearly bloodless channel, or a gaping cavern that lets the wind through?

Be sure to modify blowthrough for bullet type and size: x1.5 damage for slugs of .40 caliber or larger, x1.5 damage for hollow-point bullets, etc. Bullet damage in GURPS tends to be high, and thus is often limited by blowthrough, not damage dice; the multipliers are important in ensuring that a massive .44 round punches a more lethal hole than a .22 round, just as it should. 

Damage and Shock Meta-system

Damage and Shock in GURPS

Problems with HP levels

GURPS' rules for damage and injury are centered on human scale, which causes trouble at more extreme ends. It's odd to have both a HT 10/1000 Elder Octopoid and a HT 10/1 rat make the same death rolls at -10 HP and every -5 HP thereafter. And how do you work the "slowdown from injury at 3 HP" rule with the rat when it starts with less than 3 HP?

CII p. 152 offers help, distinguishing HT from HP. But the rules introduce another table, get tricky with regard to when to use HT and when to use HP for determining damage "levels", and still don't scale too well: a hulking HT 10/10,000 bioship rolls for death at -30 HP, the same as a HT 10/30 Giant.

Problems with HT rolls

Similarly, GURPS suggests certain HT rolls that don't work well for characters other than the generic ST 10 human (and sometimes not even then). Consider the "crushing attack to the vitals" rule, which forces a HT roll to avoid knockout regardless of damage:

Player 1: "My Pixie kicks your Ogre in the chest."

Player 2: "Ha! I don't even feel it! It tickles!"

GM: "Roll your HT, Ogre. Whoops, tough luck – you're out like a light."

The results are silly even if you substitute a human for the Ogre.

Too many rules

Hits on specific body targets in GURPS can require special HT rolls to avoid knockout, or stun, or sometimes both. High Pain Threshold helps resist some of those, but the bonus changes when resisting Pain spell effects. Stunners and nerve guns carry their own unique HT rolls (but with no DX/IQ penalty at all from a stunner should it fail to stun). There are special rules for dropping weapons due to pain from the Spasm spell and whips, but not from the Pain spell or a plain old whack on the hand with a stick. Eye gouges and face-pull attacks offer their own mechanics. All with no rules in sight to explain how a Pain spell might work on mice and Giants.

Improving things

Shock in GURPS is more messy and complex than it should be. It's simpler to replace all the special rolls and rules with a comprehensive, works-everywhere system:

1. Instead of special rules for stunning and shock effects, treat shock like damage: measure it in points in all situations, and apply the effects of those points appropriately.
2. Toss out the HP tables, and make the effect of points of damage and shock proportional to HP.
3. Use HP to determine the amount of damage or shock that causes any given effect, and HT as the roll to resist those effects when such a roll is called for.

Those simple guidelines can cover pretty much everything. Stun wand vs hippopotamus? No problem!

Effects of Damage

Computation of damage points needs no explanation here. Modifiers for attack type and hit location are per GURPS (use CII p. 53).

The following effects are the result of actual damage, not shock. What matters for effect purposes is damage points relative to HP. Damage levels are extrapolations from the effects of damage on standard "HP=HT" humans. The suggestions are intuitive, scale perfectly, and bring the same proportionate effects to humans, mice, elephants, and bioships.

Generally, Strong/Weak Will does not affect HT rolls to resist effects.

Slowdown from injuries

Halve Move and Dodge when HP are reduced to less than x1/3.

This mirrors GURPS' slowdown rule.

Knockdown

You fall down on damage of over HP x1/2 and a failed HT roll.

This mirrors GURPS' knockdown rule, and is separate from falling down after knockback from a collision.

Crippled limbs

An arm or leg is crippled on damage in one blow of over HP x1/2, a hand or foot on damage over HP x1/3. Excess damage is lost.

Instead of the automatic stunning rule, compute shock points normally for the wound, but double for the added trauma of crippling.

Except for the change to shock, this is the same as the GURPS rule.

Amputations

Yuck. A single cutting blow inflicting twice the damage needed to cripple a limb (i.e., over HP damage for an arm or leg, over HP x2/3 damage for a hand or foot) amputates it on a failed HT roll. Amputation is automatic if damage is twice that amount.

Actual damage taken, though, is equal to the amount needed to cripple the limb; excess damage is lost.

Double shock for the trauma of crippling/amputation.

This is a new rule.

Decapitation

Crippling damage for a neck is the same as a leg or arm.

A crippling blow "cripples" the neck and paralyzes the body (attempt recovery roll normally). An amputating blow decapitates, or fatally breaks the neck if crushing.

This is a revision of the GURPS decapitation rule.

Knockout

Roll vs HT each turn while at HP 0 or less to avoid unconsciousness.

Death

Roll vs HT at -HP and every -HP x1/2 thereafter. Automatic death is at -HP x5.

Recovering from Damage

It makes sense to let wounds heal in proportion to size: a night of rest and a HT roll heal HP/10 damage (which will be 1 point for most PCs). See Effects of Shock for hints on rounding this number.

As an option, allow success by 10 or more points on this HT roll to heal twice that much damage (2 points for most PCs), success by 20 points heal three times as much, etc. This allows some pretty speedy healing for high-HT characters, especially those with the Rapid Healing advantage.

The Regeneration ability should use the same HP/10 base for increments of healing.

Regrowth: Realistically, growing back lost body parts is a quicker task for tiny creatures. Divide the speed of Regrowth by the square root of your Linear Scale to get this effect.

Measuring Shock

Let shock be measured in points in all situations, not just from weapon blows. Base shock points equal damage points, after modifiers. A cutting blow inflicting a final 6 points of damage inflicts 6 points of base shock.

Shock can further differ with weapon type, hit location, and other factors. Below are additional modifiers to base shock:

Body target:

• Crushing attacks to the vitals (including kidneys): x1.5
• Crushing attack to the nose, jaw: x2
• Any attack to the brain: x2 (above and beyond the x4 damage multiplier!)
• Any other attack to head: x1.5
• Any attack to the eyes: x3
• Any attack to the male groin: x2 (at least...)
• Attack cripples or amputates an appendage: x2. This is mentioned in the crippling/amputation rules above.
• Attack aggravates an existing wound: Add damage from existing wound to new shock.

Weapon type:

• Whip: x1.5, regardless of hit location. (Optionally, use x2 for limbs, just for fun.)
• Fire, electricity: x2

Target condition:

• Target surprised/not in combat: x2 (no adrenaline rush!)
• "Bracing" yourself for pain: x1/2 (requires Concentrate maneuver and Will roll each turn)
• Low Pain Threshold: x2
• High Pain Threshold: x0 (or see revised trait in Book 3)

Other attacks

Handle shock from stun weapons with these same rules. Decide how many points or dice of shock the weapon inflicts, and apply normally. A weak stunner might inflict 2d shock, a strong stunner 4d. Some stun weapons inflict actual damage, such as 1d damage from a Type II Nerve Pistol; you can compute shock using a damage multiplier, such as x4.

Handle magical stun/shock spells in the same way: 1d shock from a Pain spell, for example. You might even let a mage "grow" a Pain spell to 3d size, like a fireball.

Effects of Shock

The following effects are the result of shock, not actual damage. What matters for effect purposes is shock points relative to HP.

Let Strong/Weak Will add to HT rolls to resist effects.

IQ and DX penalties

Apply -1 per (HP/10) points of shock, for one turn. This mirrors GURPS' regular shock rule.

Rounding: A suggestion for rounding: use the normal -1 penalty per point of shock for creatures with HP 8 to 14. Use -1 per 2 points of shock at HP 15 to 24 (which confers a nice combat benefit to a high-HP character), -1 per 3 points at HP 25 to 34, etc.

For low-HP creatures, use -2 IQ/DX per point of shock at HP 4 to 7, -3 per point at HP 3, -5 per point at HP 2, and -10 (!) per point at HP 1. Those numbers make sense: a HP 1 rat hit for a point of shock has taken the same massive shock as a human eating 10 points.

Stunning, general

You are stunned automatically on shock of over HP x1/2.

This mirrors GURPS' stunning rule

For a more interesting rule, let stunning occur on a failed HT roll on shock of over HP x1/3, or automatically on shock of over HP x2/3.

Stunning, blow to brain

Apply a x2 shock multiplier to brain hits, and use the general stunning rule above.

This replaces the GURPS rule.

Stunning, limb

An arm or leg is stunned on shock of over HP x1/2, a hand or foot on over HP x1/3. The character is not necessarily stunned, but items held by the stunned limb are dropped. (Allow an HT roll to maintain grip if shock is just enough to stun.)

Excess shock points are not lost, and can cause overall stunning.

This is a new rule, and replaces GURPS' special rule for whips and dropped items.

Knockout, general

Roll vs HT to avoid knockout on shock of over HP. Knockout is automatic on shock of over HP x2.

This is a new rule.

Knockout, blow to brain

Apply a x2 shock multiplier to brain hits and use the general knockout rule above.

Other

Extreme pain applied continually may have psychological effects. Details are up to you, but it would take an incredible Will to resist constant pain of over HP points while remaining conscious (the intended effect of torture devices!), without breaking down in some manner. The Fright Check rules might be useful in determining effects.

Recovering from Shock

Knockout

Use normal GURPS rules for recovery.

Stunning

Unless specified otherwise, recovery from stunning follows the normal GURPS rule: roll vs HT each turn to recover. The same applies to a "stunned" limb.

IQ/DX penalty

The GURPS rule is that shock lasts throughout that turn only; it's gone on your next turn. (The shock effects of certain weapons, such as high-tech stunners, may last longer.)

That's all easy to play, and works fine in the game. It's possible to get a much more detailed effect, though:

Advanced rule: Roll vs HT to recover from the IQ/DX penalty. You reduce the penalty only by the amount by which the roll succeeds. For example, if you're at -3 IQ/DX from shock, roll HT on your next turn. Success by 1 or 2 removes that many points of penalty. Success by 3 or more removes it completely.

Failure on the HT roll means the penalty persists throughout the turn – during which you might pick up even more penalty points from further shock! Roll again next turn...

The rule is easy to play and has a great realistic feel – but it does add die rolls and makes combat deadlier for all involved. Try some sample fights before springing the change on everyone.

Wrapping Up

Examples applying the damage and shock rules:

Examples: A HP 1 rat would make death rolls at -1 HP, and every additional point of damage thereafter would force two death rolls. Automatic death occurs at HP -5.

A HP 120 dragon would suffer a -1 DX and IQ "shock" penalty for every 12 points of damage taken. At 30 HP, he'd slow down from injuries, and after passing out at 0 HP, he'd be in danger of death if HP dropped to -120. Further death rolls are required every additional -60 HP, with automatic death at HP -600.

Play with specifics as you like. For example, you might decide that a whip does extra shock only for purposes of DX/IQ penalties and stunning, and not knockout. (That'd be a good feature of any torture instrument.) You can always opt to ignore any single effect (knockout, stun, etc.) for a given circumstance, especially when exotic weapons and magical effects are involved.

And if that HP 120 dragon looks too impossible to kill, see the Appendix for a solution.

Exposure Hazards

Body size plays a role in the effects of hazards like freezing cold or a sea of flame. Consider a 2" mouse exposed to flame: with his Area Scale he has only x1/1000 the exposure of you to the flame. Sounds like he's far better off than you'd be – until you consider that that flame is acting on perhaps x1/30,000 the body mass! It's that "cube-square law" in action: he should be char broiled but quick.

Hazards for use with these rules include acid, flame, heat, cold, sunburn, poisonous gas or atmospheres, and electricity. On the other hand, hazards like altitude, snow blindness, and rapid decompression won't be particularly affected by size or shape.

Simple Rules

The simple way to get this effect is to make up appropriate damage for odd-sized creatures exposed to hazards like heat or cold. Small PCs have to take less absolute damage than normal-sized ones, but more damage relative to their small HP. A large creature should take more absolute damage than a human, but less relative to its large HP. Wing the numbers to this effect.

Detailed Rules

Base damage for hazards is usually expressed as damage taken by a human. You'll want to modify this base as per the simple rules. Below are guidelines.

Two damage modifiers are at work:

• a damage modifier for the surface area you present to the hazard, i.e., the size of the "dose" you get; and
• a damage modifier for "cube-square" effects, i.e., the amount of mass relative to that surface area.

The advanced rules look at these separately, but if you have (roughly) human proportions, the net result is this:

Multiply base damage by the square root of your Linear Scale.

If you're a Giant with Linear Scale x4, take double normal damage from environmental hazards. (Not to worry, you should have four times human HP.) If you're a two-incher with Linear Scale x1/36, take one-sixth normal damage. (Too bad you only have one-thirty-sixth human HP.)

This gives you the effects described under Simple rules, and is reasonable for a creature of any size, without unusual surface area or density.

Advanced Rules

No change from the above, other than to unveil the geekery behind it:

First, adjust damage for the smaller area you present to the attack: a small creature gets a smaller "dose" of exposure than you do. Area Scale is the key, but to mesh this with the way HP are scaled, use the square root of Area Scale. Unless you have an odd surface area, that'll be the same as Linear Scale.

HP scale with Linear Scale too, so that leaves big and small creatures taking the same proportionate HP damage. Now consider that "cube-square" problem:

Take your ratio of area to mass, using the inverse of your MAR (i.e., 1/MAR). The result will be greater than 1 for Leprechauns, less than 1 for Giants.

Take the square root of the result, and multiply damage by that.

The net result for non-exotic creatures will be that in the detailed rules: absolute damage multiplied by the square root of your Linear Scale. But by doing the full calculations you can get exact results for creatures with exotic densities or unusual surface areas.

Miscellaneous Notes

Jet attacks

The rules for jet attacks in Book 5 are these exposure hazard rules, with the fixed-size nature of the attack handled appropriately. The jet carries a fixed level of damage; damage reduction for small size depends on target size relative to jet width, not target size relative to human size.

It's up to you, though, whether cube-square considerations should affect damage from a cone of fire, or whether that's not worth worrying about. If you go for the detail, use the calculations from the Advanced rules above to properly combine cube-square effects with the Book 5 jet rules:

Example: A foot-wide jet of flame does 1d damage. A 3" PC takes an initial x1/4 damage for reduced exposure; if he has a area-to-mass ratio 20 times a human's, multiply his damage by x4 (= roughly the square root of 20). Net effect: he takes a full die of damage, even though most of the jet washed over him – and that's a lot of damage for his HP. Small things cook up crispy!

A Giant, meanwhile, takes a human-sized 1d to start with – that's as big as the jet gets, even though the Giant's bigger. But that damage will be cut if you add cube-square effects; the Giant has lots of protective mass relative to the surface area burned.

Non-damage effects

Rules for exposure don't always measure effects in damage. Handle appropriately: for example, you can modify the effective rads taken from radiation in the same manner as damage.

Other exposure might involve fatigue or HT rolls, not damage. Non-extreme heat or cold are examples. How can you apply exposure effects?

If the exposure causes fatigue, modify fatigue in the same manner as damage.

If HT rolls are involved, use a penalty equal to half your Size: -4 for a Size -8 creature, +3 for a Size +6 character etc. (Intense GMs can use more exact surface-to-mass ratios for odd creatures: if you're Size -4 but with a high density that leaves you with the same area-to-mass ratio as a Size -2 humanoid, your HT penalty is only half of that -2, or -1.)

Remember that cube-square effects cut both ways: a tiny PC baking in the sun can also cool down quickly in the shade. A warm room will heat him quickly on a cold day.

More on freezing

When resisting freezing, insulation helps. Add +1 to HT rolls for Overweight, +2 for Fat, and +3 for Obese.

Cold has another special effect: numbness, especially in the extremities. GMs may wish to lower DX by 1 for every one-third Fatigue or HP lost to cold. (Realistically, smaller body parts suffer even more. Manual dexterity is particularly quick to go in cold weather; apply additional penalties as you see fit.)

Note that the GURPS rules for succumbing to heat and cold require rolls every half-hour to resist loss of one Fatigue or HP. As temperatures get more extreme, the chance of missing this HT roll increases, but the degree of effect itself doesn't change at all – a wet, naked PC in a raging Antarctic blizzard will still lose only 1 Fatigue per half hour. That's too slow.

A fix for this: lose 1 Fatigue or HP for every full or partial 3 points by which the HT roll was missed. The more extreme the temperature, the quicker you go. Space the loss of points: a loss of 6 Fatigue for the half hour means 1 fatigue every 5 minutes.

Advanced rule for water: Water conducts heat away from a body far more quickly than air does. Swimmers will feel cold in water of a temperature that would be comfortable as air temperature; they'll suffer hypothermia in a surrounding temperature that would be quite survivable on land.

Simulating this is pretty easy. First, set a single, optimal environmental temperature within the creature's temperature "comfort zone". For a human, this might be 70°.

Now take the difference between water temperature and that central temperature of the immersed creature, and triple that for purposes of effect. For example, if your optimal temperature is 70° and you're in 60° water, treat that as a 10° x 3 = 30° difference – it's as if you're spending time in 40° air, which is getting cold for a human.

This rule applies equally to aquatic creatures; it's just a tough part of the environment they live in, and explains why many fish are so sensitive to temperature differences – in water, a small difference is a big difference. (The rule does not affect the "stiffen up" temperature of a Cold-Blooded creature; that's based on absolute temperature.)

See Book 3 for more on setting a character's temperature tolerances.

Collisions

Realistic handling of collisions should start with all that physics class stuff about masses and velocities, and boil those down into easy rules that still get the basics right. PCs aren't billiard balls on frictionless surfaces, though, so it's OK to take liberties Newton would not have approved of. The rules below do so, and, for better or worse, attempt to express steps in English instead of mathematics.

Existing GURPS rules for collisions include the Basic Set falling rules and Vehicles' collision rules. With average damage rolls, the typical HP 10 human takes a deadly 45 points of damage from a 50 mph (30 yard) fall using Basic Set rules, but walks away with only 8 or 9 points of damage in a 50 mph Vehicles collision. GULLIVER rules will hand out a middling 30 or so points in either situation (though the exact amount will vary with the details of the collision).

Damage numbers won't necessarily behave properly when applied to vehicular collisions using Vehicles stats. Vehicles scales HP in a way very different from GULLIVER and GURPS scaling for creature HP; this is an unresolved "bug" in GURPS. See Book 8 for ideas on integrating Vehicles with GULLIVER.

For simplicity, these rules assume nice inelastic collisions: things don't "bounce" interestingly. That makes post-collision speeds fairly easy to compute. For more elastic collisions, hit the physics books.

Speed of Impact

Start with combined speed of impact: add velocities for a head-on collision, subtract the slower from the faster for a rear-end collision.

Example: Green is coming from the left at 40 mph, Yellow from the east at 50 mph. Their combined speed is 90 mph.

If Yellow hit Green from the rear instead, the speed of collision would be only 10 mph.

For collisions at angles, guess at things or get a physics book. In a right-angle collision, damage comes only from the object which does the striking – i.e., the one that broadsides the other. (If they strike each other equally, average their two velocities as the combined speed.)

Speed Change

What happens next? Velocities change. Take the mass of either object: what's its percentage of the combined mass of both objects? That's the percentage of combined speed that the other object takes as speed change.

This is usually easy to eyeball. If the objects are of equal mass, each takes half of combined speed as speed change. If Blue is nine times as massive as Red, then Blue is 90% of the combined mass and Red is 10%. Blue's speed change is 10% of combined speed, Red's is 90% of combined speed.

Each object's speed change happens in the direction the other object was moving.

Example: Green (1000 lbs.) is coming from the left at 40 mph, Yellow (500 lbs.) from the east at 50 mph (combined 90 mph). Green's proportion of total mass is two-thirds; Yellow's is one-third.

Green's speed change is one-third combined speed, or 30 mph; Yellow's is two-thirds combined speed, or 60 mph.

Green's speed changes 30 mph back to the left, so becomes 40 - 30 = 10 mph. Yellow's speed changes 60 mph back to the right, so becomes 50 - 60 = -10 mph (i.e., 10 mph in its backward direction).

Example: A 10-ton semi-truck coming at 60 mph collides with your PC. No need to mess with math: from respective masses, the truck's speed change will be almost 0, while your speed change will be almost the full 60 mph. That's your new speed, in the direction of the truck (and firmly embedded in the grill).

Note that the results would be the same if you flew at 60 mph into the stationary truck: you'd suffer essentially 60 mph of speed change, in the direction of the truck's travel. (It's stationary, so you stop dead.)

Loss: A little energy is lost in the process. Don't worry about it, except to quickly round numbers or overrule odd effects. The above stationary truck might receive 1 mph of speed change from the above calculations, but friction with the ground will absorb that much change. Too bad, if you were sacrificing Captain Speedo's life in the hopes of nudging that evil truck over a cliff.

Dice-based speed change

Book 5 uses these collision rules for combat slams, with one addition: an optional ST-based Contest for determination of speed change. It's the same system, with a random factor added.

You can use either the "mechanistic" collision rules here, or Book 5's Contest, to determine speed change in any collision – animate or inanimate, powered or unpowered. In general, though, the Book 5 Contest "feels right" when one or both sides in the collision is an animated slammer. The mechanistic rules for speed change feel right when both sides are inanimate objects.

Collision Damage

Next, compute damage. Each side computes a damage score based on its own speed change and mass – but the higher of these damages applies to both objects.

After that, modify the damage each side suffers for the hardness of the other side. Finally, reduce damage suffered by each side for its own ability to absorb damage.

That's a few steps, but it plays quickly for typical objects like people, while remaining flexible enough to handle a wide variety of situations.

Speed

Compute damage for each side as 1d per 10 mph of its own speed change. Don't apply damage yet, though.

Mass

With apologies to calculator haters, multiply speed-based damage dice for each side as follows:

multiply damage by the square root of (own mass in lbs. / 150)

That's roughly the square root of Volume Scale for many characters, and is 1 for the average human. Once you've computed it for a character, jot it down on his form for easy recycling. You might want to recalculate if the character gains or loses a large amount of carried items, but don't bother recalculating because he picks up a battleaxe.

Note that for each side, you're multiplying damage for its own mass. Small things start and stop on a dime; big things suffer horribly doing the same. This may differ from collision rules found in every other RPG, but that makes it the only right rules set out there. Imagine the scene: a toy Tonka truck has far fewer HP than a real truck, but throw both into a cliffside at 50 mph, and it's the Peterbilt that comes off far worse.

Applying damage

Take the damage dice scores calculated so far for each side, and apply the higher to both sides. In general, this will be the damage computed for the smaller object.

Example: When a motorbike weighing 600 lbs. (with rider) crunches into a tank at 40 mph and stops dead, the bike suffers 40 mph of speed change and all attendant damage. The much heavier tank's computed speed change will be nearly zero – as will its computed damage, despite the high multiplier for the tank's mass.

Both vehicles suffer the higher of the two damages, which is the impact of a slamming motorbike. That's (40 mph speed change x the square root of (600/150) = 8d).

After that, there are a couple more factors to account for:

Hardness

Multiply the damage dice suffered by each object for the hardness of the other object:

A sharp point is generally very hard, and further inflicts impaling damage!

Example: In the above example, both objects are hard steel. Each side multiplies damage dice received by 2.

Roll

You've now got the damage dice each side takes; go ahead and roll separately for each.

Damage absorption

Characters have an ability to absorb a certain amount of speed change: low-speed impacts simply don't hurt at all. That's largely because the impact from a collision with a large object is spread all over your person. You can simulate this by reducing rolled damage by some number based on your surface area – or the square root of your surface area, keeping in line with the way HP are scaled.

In fact, HP itself works great here. The rule:

Subtract a portion of HP from collision damage suffered so far:

half HP for widely spread (full-body) impact

one-fourth HP for localized impact

no subtraction for very localized impact.

There's your final damage.

Examples of a widely spread impact would be a collision with the big, flat end of a bus. Localized impact include slams, or a car hitting a person. A very localized attack is a collision striking a small area, such as the area hit in a weapon strike.

Don't forget that DR also protects against collisions. Some armors might protect fully against localized impacts but less against full-body impacts, as suggested by GURPS' falling rules.

Example: Rolling 16d damage, the above motorbike is in trouble. But subtract its DR and half its HP (for widely spread impact) from damage suffered in the collision.

The tank, struck against a much smaller portion of its size, should receive a lesser HP subtraction, for localized or very localized impact. But its high DR makes the problem moot.

Falling down

Getting knocked about by a collision may make you fall down. Make a balance roll, or see the knockback rules in Book 5 for detail.

Splat

What if something collides with you while you're against a solid wall? You don't go anywhere. Take damage based on speed and mass of the colliding object, not your own speed and mass, adjusting as appropriate for hardness.

Overrun

The bang of collision can just be a start; it's a whole new world of damage when a large vehicle overruns a small one. As suggested by Vehicles p. 159, this may happen when the colliding vehicle is 3 Size levels larger than the other.

If you like, use Book 5's trampling rules for all overrun situations. This sets trampling damage dice as the square root of (weight in lbs.) / 20. However, the resulting numbers don't seem to do justice to the effects of a milk truck backing over a scooter; vehicles' rigid structures seem especially vulnerable to crushing weights. Double trampling damage when one vehicle is overrun by another.

The conditions for overrun: you must collide with your target and be the same Size or larger if your vehicle has legs, or 3 Size levels larger if you have wheels or other ground propulsion. (What's important is tires roughly taller than your target; appropriately-sized Monster Truck tires help!)

Handle special situations appropriately. A skilled dirt bike rider can "overrun" a parked car. A bike knocked flat in a collision is a Size level smaller for overrun purposes. And so on.

Powered Collisions

This section borders on advanced rules, so ignore if you like – but it does look at an interesting detail. What if an object powers its way into a target – engine roaring, jets thrusting, legs straining forward? It's pushing throughout the moment of collision, instead of just coasting passively in!

Turn motive power into a higher effective mass for collision purposes. If the object has a Load ST score, let effective mass be mass + (Load ST x 10 lbs.).

If the object has a vehicular thrust rating or kW engine power rating, the job is tougher – there are no clear guidelines at all for equating these with creature ST. Try this: Add a vehicle's engine kW rating x 10 to effective mass if it's accelerating forward. If it has a thrust rating instead, add thrust to effective mass if in water, thrust x 0.1 if in air.

These are completely arbitrary numbers that may be a magnitude off from more proper conversions, but in the meantime they do something.

Advanced rules

Torque: Multiply the bonus to effective mass by 2 if the vehicle is traveling at less than 1/4 top speed, and multiply by 1/2 if the vehicle is traveling at over 1/2 top speed. This represents the greater acceleration of power systems at low speeds.

Traction: The extra effective mass from power only works if you can effectively push forward. For ground objects, the extra mass bonus from power will be the lower of what you've computed so far, and:

• Perfect traction (magical or ultra-tech "never-slip" ability): no limit
• Excellent traction (spikes; racing tires on asphalt): object weight
• Good (studs): object weight x 2/3
• Standard: (regular tires on asphalt): object weight x 1/2
• Poor (regular tires in rain): object weight x 1/3
• Very Poor (regular tires on snow, mud): object weight x 1/5
• Near None (regular tires on ice): object weight x 1/10

The use of object weight instead of mass is intentional.

Vehicles and traction: Take the Off-Road Speed multiplier calculated from a vehicle's Contact Area and Ground Pressure, from Vehicles p. 130. Multiply the extra mass bonus from power in a collision by that same multiplier. The effect is that a low ground pressure – i.e., a lot of contact with the ground – provides the traction that lets your engine add oomph to a broadside. A high ground pressure represents little traction to keep you plowing forward.

Power and damage

In powered collisions, use effective masses not only to compute speed changes but to compute mass-based damage multipliers as well. Objects that accelerate into each other will hurt each other more than those that coast to a crash.

Example: If the above 600-lb. motorbike has its 30 kW engine at full throttle when slamming that tank, it'll hit harder than it would by coasting into the collision.

Let's throw in the advanced traction rules, too. Assume standard traction. Compute the bike's effective mass bonus as the lower of (30 kW x 10) = 300, and (600 lbs. x 1/2) = 300 – it's 300 either way. The bike slams with an effective mass of 600 + 300 = 900 lbs. That won't make any real difference in terms of pushing the giant tank, but will increase damage for all involved.

Wrapping Up

The collision system meshes with other aspects of GURPS. Vehicles also scales damage linearly with speed (if not necessarily arriving at the same final numbers as these rules). Even damage from thrown weapons somewhat scales with speed in GURPS: speed presumably scales with ST, and damage more or less scales with ST.

The damage muliplier for mass agrees roughly with Vehicles' collision damage modifiers for weight, which scale with vehicle weight to the two-thirds power, based on vehicular HP. The difference between that and GULLIVER's square root-based scaling isn't large.

GULLIVER turns to the collision rules a couple more times for falling and combat purposes, but stops short of suggesting a unified meta-system handling all things kinetic, from a mace upside the head to a dive into a dry pool. Yes, it could be done, but it's a task for the energetic.

Extra credit question

The rules scale damage with speed – but you're thinking damage should scale in some way with kinetic energy, or the square of speed times mass, right?

By computing damage from speed times the square root of mass, you essentially are using the kinetic energy equation. You're just taking the square root of it all, which is fine. It's an artificial adjustment that lets things play nicely with the scaling used for the artificial units known as Hit Points.