[Sethomas]

Well, if you're going that way, then your entire superstructure is flawed. If the ground is exerting force on the spaceship, then the tangent molecules (the only ones relevant) become part of the spaceship's frame of reference, so relativistic effects go out the window. So, deriving an equation is pointless, as you'd have to treat the frictional surface as a fluid for a realistic representation of how the tangential surface becomes unified with the spaceship while the ground below it doesn't. I doubt you're that smart. I for one wouldn't want that for an assignment.