What physics model do spinfusor disks use?

Solution 1:

None of the above, you're just not accounting for a mechanic in the game called inheritance:

Projectile Inheritance (or simply inheritance) is a mechanic in Tribes: Ascend which causes projectiles you fire to "inherit" some of your current velocity. T:A currently has 50% inheritance on all projectile weapons. This means that if you fire a Spinfusor perpendicular to yourself while traveling at 200 km/h, the shot's end-point will be a trajectory taking into consideration its firing speed, aiming angle, and 100 km/h worth of inheritance in the direction you were moving.