Playbook: Zero-G Dodge

GURPS rarely reduces a character's active defences due to environmental factors. My guess is that this is more of a game balance choice than a realism one, since Dodge is typically already low for non-superpowered characters and dropping it down even a single point represents a huge decrease in survivability, especially against ranged weapons where Dodge is the only available defence. I've recently been planning for a sci-fi campaign in a setting with no artificial gravity and decided I wanted a set of rules for dodging attacks in free fall, to help drive home the feeling of a different environment. So I came up with three different mechanics, which you can pick at your leisure.

Quick and Dirty

Each turn a character wants to dodge in free fall, roll their Free Fall skill, at default if they don't have any points invested in the skill. If they succeed, they Dodge normally this turn. If they fail, Dodge is halved for this turn. If they critically fail, they cannot dodge at all!

This is the simplest and the easiest to remember of these rolls as it involves no calculation other than halving your regular Dodge score, although it does increase the number of rolls you do per combat turn. It's also the harshest and most variable, since it relies on dice rolls, and if your Free Fall skill is low you're going to take a lot more hits with a halved dodge score. 

Dexterity Substitute

Calculate a separate Basic Speed: add either your DX or Free Fall skill, whichever is lowest, to your HT, divide by 4 and drop all fractions. If you have no points in the Free Fall skill, it defaults to DX-5. Add 3 to this number to find your Free Fall Dodge. Any bonuses that apply to all active defences (such as Combat Reflexes) or specifically to dodge defences (such as Enhanced Dodge) apply to Free Fall Dodge as well. For example, a character with DX 10, HT 10 and Free Fall-9 has Dodge-8 (10+10/4 = 5 and 5+3 = 8) but Free Fall Dodge-7 (9+10/4 = 4.75, dropped to 4, and 4+3 = 7). If they had Combat Reflexes, these would increase to Dodge-9 and Free Fall Dodge-8. If the same character had no points in Free Fall, they would have Free Fall Dodge-6 (5+10/4 = 3.75, dropped to 3, and 3+3 = 6). When dodging in free fall or zero gravity, roll only your Free Fall Dodge, never your normal Dodge.

This is probably the more complicated option since you have to work out Basic Speed every time either DX or Free Fall changes but given that Free Fall is a DX/A skill, your dodge score in free fall will always be either the same as your regular dodge, or a single point lower, regardless of what your DX and Free Fall are. This method also preserves the effect HT has on Dodge, which is important for less speed-focused characters.

Alternative Active Defence

Characters have a new active defence called Free Fall Dodge, which is equal to 3 + Free Fall ÷ 2, dropping all fractions. Any bonuses that apply to all active defences (such as Combat Reflexes) or dodging (such as Enhanced Dodge) apply to Free Fall Dodge as well. Free Fall defaults to DX-5, so if a character has no points in the Free Fall skill, Free Fall Dodge defaults to (DX-5) ÷ 2 + 3. When dodging in free fall or zero gravity, use either your normal Dodge, or your Free Fall Dodge, whichever is lowest. 

This is fairly similar to the above in that it requires calculations that need to be updated any time your DX or Free Fall skill change. The calculation is easier, however, involving fewer steps. It doesn't preserve the effect HT has on normal Dodge, which may be tough for less speed-focused characters.

Verdict

I personally prefer the Dexterity Substitute rule, primarily because it isn't as harsh as the others but also continues to allow HT to influence Dodge. Defaulting on Free Fall does cause a bigger hit to Free Fall Dodge but with Dexterity Substitute, factoring in HT softens the blow, making it less harsh than Alternative Active Defence.