Musings about slam damage in GURPS

In one of my online games, my character has an air elemental ally in  the form of a living tornado. The ally's primary attack is a low damage, high knockback aura of whirling winds. As the air elemental is a better combatant than my main character, I've been using the air  elemental in combat a lot and have noticed some weirdness in the standard GURPS slam rules.

The GURPS rules for knockback first appeared in Man to Man, or maybe Melee before that, and haven't changed as the rest of the system evolved around them. The rules were probably acceptable for Man to Man, when all combatants were humans standing on two feet, but they're  inadequate for modern GURPS, with quadrapeds and snakes and vehicles as combatants, and foes ranging in size from SM-6 pixies to SM+4 or more vehicles.

Stating the Problem

Resistance to knockback should be based on mass and friction: heavier objects are necessarily harder to shove around, while someone lying down has more of their body in contact with the ground and more static friction than someone standing on their feet. The standard rules use ST or HP as a proxy for mass, and don't take into account into account friction or position at all.

Using HP as a proxy for mass has two weird effects. First, things of the same weight will have different HP depending on whether they're living, unliving, of homogeneous. A 125 lb kobold has 10 HP, but so does his 2 lb shortsword. It's a little strange that the same force that sends the kobold stumbling for 3 yards only sends his shortsword the same distance: you'd expect the shortsword to go much, much farther. Second, since HP is relative to the cube root of mass, big items have less resistance to knockback, relative to their mass, than smaller items. As an illustration, a 45-ton T-72 main battle tank has 176 HP, while a 4-ton Hummer jeep has 72 HP. A mighty superhero who hits a T-72 hard enough to push it 2 yards will push the Hummer 4 yards with the same blow, instead of the 10-20 yards you'd normally expect.

Solving the Problem

All this bothers me, and I'd like to come up a fairly simple rule for knockback that handles small things and large things, scales fairly well with weight, and accounts for friction and position at least a
bit.

The first thing is to find a decent proxy for mass. HP obviously isn't quite right, but going with straight mass will probably interact poorly with other GURPS assumptions. Basic Lift is proportional to the square of HP, and thus to weight raised to the 2/3rds power, and it's something precalculated for most human scale combatants, which in the end are the things I'm most concerned about. It's straightforward to add a linear divisor to the BL of unliving or homogeneous characters or items to account for their extra HP compared to living things of the same mass.

So my solution is that knockback resistance should be based on BL:

• BL/2 for living
• BL/8 for unliving
• BL/32 for homogeneous

for items where the weight is known, but BL isn't, that works out to:

• wt^(2/3)/10

And the Object Hit Points Table on p 558 would get an additional column:

Object Weight	Knockback Resistance
1/64 lbs	1/40
1/8 lbs	1/10
1/2 lbs	1/4
1 lb	1/2
2 lbs	1/2
3 lbs	1
5 lbs	1
8 lbs	2
11 lbs	2
16 lbs	2
27 lbs	4
43 lbs	5
64 lbs	6
91 lbs	8
125 lbs	10
216 lbs	14
343 lbs	20
512 lbs	26
729 lbs	32
1000 lbs	40

This actually increases knockback resistance for the GURPS standard HP 10, 150 lb man, going from 8 to 10. That's actually somewhat problematic.

Another problem

Melee damage in GURPS is hard to relate to real world quantities, except for one value: an all-out two-handed push from an average man does 1dx2 knockback. That value averages to 7, not quite enough to push back another average man on average.

That's a kind of disappointing result. In play, it means that most pushes are failures, so people don't use them and the rules might as well not be written. It also fails the reality check: if I push someone as hard as I can, they're going to move away from me. I'd like to make pushes a little easier, so they're used sometimes.

Solving the Other Problem

Earlier, I mentioned that posture should affect knockback: a standing man should be easy to push, while a crawling man would be harder, and a prone man fairly difficult to move. Adding a multiplier for posture would address that.

This is pretty simple:

• x1/2 for people who standing on two legs, flying, levitating, or for objects held in one hand
• x1 for people or creatures standing on four legs (or their arms and legs), or for objects with 4 points of contact with the ground or objects held in two hands
• x2 for prone people or objects lying on the ground or tracked vehicles

Putting It All Together

Now the GURPS standard HP 10 man has a knockback resistance of 5 when standing, and will almost always be pushed away if another average man gives him a two-handed push. Lying prone, the same man has a knockback resistance of 20, and won't be moved by any attack he can survive.

A 7-ton Hummer jeep with HP 72 has a knockback resistance of 180, and a 45-ton T-72 tank with treads has a knockback resistance of 1850. If both vehicles are levitated before being knocked around, their knockback resistances drop to 90 and 462, respectively.

Mecha Against the Giants Sessions 11, 12, and 13

So as part of the Great Summer Blog Hiatus, I forgot to write session summaries for Mecha Against the Giants. We have played a bit, and we finally got to the big scene I've been waiting for, so let me summarize.

Beware the Dead

When we left off, +Theodore Briggs had just poked a couple of dead dwarf corpses  that got up and objected to him. There was a brief exchange of combat, but the pilots backed off quickly. I guess the gold wasn't that important after all. The dead dwarves didn't want to be disturbed, and  they were okay with not being disturbed. After the pilots backed off, the dead dwarves laid back down on the floor.

The Ancient Fortress

The pilots then turned to the next bend, and found an ancient dwarven  fortress carved into the cave walls. A massive pile of rotten troglodyte corpses, filled with crossbow bolts and carved up by heavy chopping weapons, lay in drifts before the fortress. On the walls above, a couple of improbably armored dwarves (DR 18!) stood in watch, pointing their empty crossbows at the pilots.

There was a brief discussion of what to do, and I think a bit of inconclusive fire. The dwarf's helmets were thick enough to bounce carbine fire, and had thick wire grids across the eye slits that made the usual tactic of aimed shots to the eyes impossible. Stymied, the pilots pulled back and the players debated.

+Kevin Smyth  reasoned that they had access to dwarven armor from the dead troglodyte hero, and  that everyone pretty much looks the same if they're wearing 70+ pounds of steel plates. So he went  back, put the armor on, and knee-walked up to the fortress door, pretending to be a  dwarf. This worked, to an extent, in that he got to the fortress door without being attacked. Sadly, his Acting skill was not good enough to hoodwink the golems up close, and he discovered the flaw in his plan: the armor was heavy and bulky enough to impair his combat abilities. He was still faster than the golems, so he retreated while everyone else put down some covering fire.

The golems chased them out of the cavern again. Everyone passed the point of pursuit, restocked, and reloaded. The new plan was pretty straightforward: get in close and disable the dwarves somehow.

A Fortunate Mistake

At this point, Kevin noticed he had a Move and Attack technique on his character sheet, and started using his Move 7 to get behind the dwarves and kneecap them. Normally, I don't allow uncapped melee Move and Attack, because it's too unbalanced and confusing, but I figured I must have   changed my mind due to the escapist nature of this game. After the end of that session, I reread my notes and realized I'd let him buy it off for Ritter combat, not for personal combat. Sadly, the damage was done.

Between several of the dwarf golems getting hit with mud to the eyes, a couple of bolas that knocked them over, and Kevin kneecapping them, the dwarf golems stopped being combat effective fairly quickly. They were fairly easy to render combat ineffective, but hard to destroy completely, so the fight was becoming a bit tedious. Eventually, I just handwaved the end of the fight: there were another 4 golems coming as reinforcements, but they weren't going to do anything so who cared?

Alternate Solutions

At one point, the players asked me how I had intended to solve this particular combat challenge. I admitted that I really didn't have a solution in mind: sometimes, as the GM, it's interesting to put an improbably difficult fight that has hard counters to the PC's usual tactics and see what comes out of it.

After a few moment's thought, I did admit I would have solved it a  different way than they did. The 25mm rifles that their mecha carried could easily be dismounted from the mecha and put on a cart. It would have been hard but possible to manhandle the cart through the mine (the 25mm rifle weighs about 250 lbs). They could have used a pick axe to mine through the back wall of the fortress, which would have aroused the golems' ire but left the golems coming through a narrow tunnel in the cone of fire of the rifle. The golems were tough, but 25mm APEX rounds would have taken care of them very easily.

When I said all that, there was just a moment of collective silence from the players. That was a level of bloodymindedness that I don't think they were anticipating.

Things That Worked, Things To Change

Overall, the high-tech dungeon crawl was a lot of fun, so much so that it's going to be the focus of our next campaign. A couple of things didn't work out as well as I'd hoped. If I were going to do it again, I'd give the dwarven axe-armor golems ammo for their crossbows: without it, they just weren't effective enough against the PCs. And I'd definitely go back and reread the rules for everything, like the trolls that I messed up. Of course, I always tell myself to reread the rules closely and I rarely do.

New Campaign Ramblings: Castle of Horrors and the Caves Beneath

My online group has been hit or miss this summer, missing quite a few gaming opportunities. We're still cranking through Chaos Scar and Mecha Against the Giants (MatG) if slowly. So I'm writing up some thoughts for our next campaign that won't happen for months at the earliest.

At the end of our last session, +Kevin Smyth mentioned that he had really enjoyed the recently completed dungeon crawl portion of MatG, and that he'd really like to do more with the "modern day people do fantasy dungeon crawling." That sparked general agreement and a fair bit of discussion.

Solving a Problem

+Emily Smirle tossed a bit of a quandary in: she likes the idea of modern day dungeon crawling, but she really, really likes playing tall, ugly semi-monsters like ogres, minotaurs, and trolls. Sadly, ogres and trolls aren't an obvious fit for a modern day campaign. I could have taken the attitude that she wasn't going to get indulged, but it seemed easy enough to solve.

My first, spur of the moment proposal was the District 9 solution: ugly but not particularly bright extra terrestrial refugees arrive on Earth, bereft on their advanced technology. That solved Emily's desire for big, ugly people, while still preserving the modern day technology that the rest of us wanted. The idea was batted around a bit, and quickly evolved into Shadowrun style goblinization: in 2012, some people started shrinking or growing or calcifying or whatever. It's really just a background detail, but it justifies a group of people that are culturally American humans but physically different.

Where to go, what to do

My next and most important challenge as the GM was to come up with a dungeon. As usual in these situations, I dug through my collection of adventure PDFs, and surprisingly did not pick up Keep on the Borderland. I knew I wanted a fairly large dungeon, something that if not a mega-dungeon, at least a kilo-dungeon. I also wanted something with a fair number of traps and physical challenges, as essential puzzles and skill sinks, and to provide more interesting terrain for the fights.

The problem with most published, large dungeons is that they're poorly designed for use as mega-dungeons. They tend to be fairly linear, with a single entrance at the top level, a single entrance to the next level, and so on down through the levels. Often, the opposition is too organized: there's a massive army in the dungeon that should be expected to respond to a small group of adventurers in overwhelming numbers. I'm thinking of Dragon Mountain, for instance, as a terrible example of this.

One published module that doesn't have these faults is Castle Ravenloft. It's a fairly big dungeon: 88 major numbered rooms, with dozens of sub-rooms. It's very non-linear, with several entrances into the first level and then multiple ways to go up and down through the eight or nine different floors of the titular castle. Aside from Strahd, who is surprisingly easy to excise from the place, the monsters are disorganized and isolated, and the large swaths of more or less empty rooms can easily be filled with new, potentially feuding groups.

The more I thought about Castle Ravenloft, the more I liked it. My biggest concern was that other people in the group might know it well, but a quick email discussion showed that to be a groundless worry.

I also considered Dragons of Despair, because I love that sunken city, and Dragons of Hope, because Skullcap dungeon is another interesting multi-level dungeon, but both had organizational flaws. 80% of Dragons of Desolation is an annoying rail-road, but the floating tomb is a very non-linear, multi-level dungeon, if a little too small for what I had in mind. I read through a couple of other obscure early D&D adventures (I4 Oasis of the White Palm and X8 Drums on Fire Mountain) and rejected them, but I probably would have chosen one of them if Castle Ravenloft wasn't available.

Expanding the dungeons

While reading through Dragons of Despair, I noticed the chapel entrance to the sunken city has almost the same shape as the Ravenloft chapel. That sparked the idea that they could be same building, and the caverns of Xak Tsorath are under Castle Ravenloft. Looking at it a bit more, I realized it wouldn't quite work that way, but the concept was strong.

So I think I'm going to put a bunch of the almost-ran adventures as expansion areas under Castle Ravenloft. There's a set of spiral stairs somewhere in Castle Ravenloft (possibly extensions of canonical stairs K83 or K20A) that lead down to Xak Tsorath Hall of Ancestors, and a well on the grounds that lead straight into the Xak Tsorath cavern. Inside the cavern floats the floating tomb from Dragons of Desolation, and Skullcap and entrances to the dungeons from I4 and X8 are on the cavern floor. That gives the players a bunch of different dungeons to try, all reasonably accessible, but lets them focus on Castle Ravenloft to start.

Why not create my own?

I could have created my own kilo-dungeon from scratch. That'd be a lot of work, and requires a certain level of inspiration that I'm not feeling at this time. Cobbling together a bunch of published dungeons and converting them to GURPS is a manageable level of work for me.