@Mars - The Repulse has more momentum than a 2 Gt warhead, and in low-speed collisions the momentum is in key role when you're guesstimating what happens. Armour plating of FS2 warships is, IMO, primarily designed to withstand energy weapons such as beams and blobs of different kind. The warhead's destructive effect is based on high energy yield, meaning that they basically rise the temperature of the armour until it softens, and that causes damage as well as the shockwaves running through the hull. However, the hull and armour are designed to be as repulsive to this kind of beating as possible.
When a 300 million metric ton spacecraft (my minimum guesstimate of an Orion's mass, does anyone have more accurate knowledge?) hits something, even at 15 m/s, it
really can't be stopped very easily.
AS I said, what would really happen if a two-km destroyer rammed a larger ship is that they would carve their way into each other until the momentum of both ships would be equally distributed to each ship (means that the wreckages would stay joined to each other and continue moving at same speed). Unless they have specially reinforced noses.
In which case the rear end of the ship would continue to move forward when the nose stopped. In Orion's case, the engine section wold start crumbling forwards. In Colossus' case, also the protrusions below the main hull would snap from their attachements and start floating forward, or at least bend forward significantly...
It would be an interesting scene to simulate, actually.
@spartan - F=ma, yes, but it has little to do with this.
It's more interesting to compare momentums, kinetic energy yields and also impulses. Impulse is the change of momentum, and it tells us the average force needed to create mentioned change of momentum in certain period of time. To elaborate,
F = I/dt
where I is change of momentum and dt is change of time - in this case, the time that the collision takes to happen.
Ah well, I'm getting too serious again.
