Hey, I think you might be on to something. I had assumed that we wanted to eliminate slippage because it would make the tower too high, but if you try the model with a coefficient of friction of ZERO (i.e. no friction whatever) the motion is still OK. I think the reason for this is that when the block is horizontal the forces that are acting can only rotate it. As the block rotates the force along the line of slip gradually increases, but to begin with it is quite small and produces very little acceleration on the huge mass of the block. By the time the angle has increased sufficiently to generate any serious force, the block has already gained considerable rotational speed and is on its way towards the hole anyway. It seems the height of the tower does not need to be increased by much to compensate. So although there is more slippage, it is not the huge problem that I had imagined. However, because of the lack of friction, there is a much greater side load on the tower; almost 20 tons if the frictional force is zero. We will need some pretty strong bracing to withstand that!

One further idea that will need much more consideration is that we design the tower to collapse or withdraw before the point of slip occurs so that the stone then rotates and drops in free fall. Wow!