Simple Question:
Gear damage depends upon two factors: vertical speed and vessel mass.
What's the formula? I would expect something rather complex, but I hope I will be surprised.
You were right in your assumption, vertical speed and total mass.
From the XR Flight Operations Manual:
Fully Loaded Mass w/14-day LOX and full crew complement (no cargo): 35,748 kg
Maximum Payload Mass: 10,795 kg (max recommended)
Max touchdown descent rate, max load (full fuel, full cargo): 2.8 m/s
[math]\mathrm 2.8\cdot (35748+10795) = 130320.4 \;kg\cdot m\cdot s^{-1}[/math]
The 35748 kg FLM includes 16080kg (empty mass) + 14200kg (Main+RCS) + 3350kg (SCRAM) + 1024kg (14 crew) + 268kg (APU) + 364kg (LOX) + 462kg (???) {
I have no idea where those extra 462 kg are from. A fully loaded default XR2 with no cargo, actually weighs 35286 kg}
Max touchdown descent rate, typical load (33% fuel, 75% cargo):4.2 m/s
[math]\mathrm 4.2\cdot (16080+(14200*0.33)+1024+268+364+462+(10795*0.75)) = 130117.05 \;kg\cdot m\cdot s^{-1}[/math]{
Here I am making the assumption that all the SCRAM fuel has been used}
For an autopilot, I'd go for [math]\mathrm M_{total}\cdot (-vel_y) < 13e4 \;kg\cdot m\cdot s^{-1}[/math]
Notice that an empty XR2 with only the pilot, 1 kg of LOX and 5kg of APU can land with a vertical speed:
[math] \mathrm -vel_y = \frac{13e4}{16080+78+1+5} = 8.04\; m\cdot s^{-1}[/math]
IIRC that value is correct. I have landed empty Ravenstars with vertical speeds of about 7.5 m/s without the gear collapsing.