The physics engine of Halo 1 is an in-house implementation which simulates the movement of dynamic objects like units, items, projectiles, and point_physics dynamics systems like particle_system and weather_particle_system. Unlike Halo 2 and later titles, Halo 1 does not use any middleware like Havok.
Different object interactions are simulated in different ways. For example, vehicles use physics to drive themselves, collide with scenery and the BSP, and collide with other vehicles, but their model_collision_geometry is used in collisions with projectiles, bipeds, and particles. The scenario_structure_bsp collision BSP is tested differently for vehicles and projectiles than it is for players, making phantom BSP more apparent.