Fascinating programme. I had read previously about the importance of the back-spin, in that the Magnus effect on the store as it sank would have generated a force that caused it to hug the wall of the dam so that the water-tamping would do the damage when it exploded. But I hadn't realised how important the backspin was for the bouncing performance.
I'll look at my recording again but I think the commentary defined the benefit of the back-spin then as the aerodynamic lift it generated on the store (again, Magnus effect) as it travelled through the air. I think there might be another reason. We all know that pebbles skip better on water the faster you throw them. Well, a back-spinning store hitting the water has a contact speed between the two that is much higher than the simple forward speed of the mine - it's that plus the perimeter spinning speed. So the bouncing dynamics are those of a store hitting the water at a higher speed, and hence it bounces better and longer. That, plus the angular momentum of the spinning store keeping it orientated correctly make the whole concept of 'Upkeep' a triple work of genius.
I'll get back to the pictures of Miss York now. Thank you BEagle.
