Frustrating. Looks like when the Mobility setting is changed, it needs a refresh before you see the new collision type. The collision type for the floor is WorldDynamic when Mobility is set to Movable and WorldStatic when set to Static. The collision type of the rocket was custom and set to ignore WorldDynamic and block WorldStatic. When the floor is WorldStatic (Mobility Static) the rocket is set to block WorldStatic and hence the rocket collides. When the floor is WorldDynamic, the rocket passes though because the rocket is set to ignore WorldDynamic.