BTW
To pick up on Ivor's question (Hi!)
Timed rockets go off at a set time rather than when they strike something (although they would explode on contact if they did hit something before their time was up.
I think the game set them to 5 seconds. If I remember rightly, the default keyboard command for using these was Ctrl + 3.
One of the stock Luftwaffe missions has you going against a formation of B-17s with timed rockets - great fun when you drop one right in the middle! Not so much fun to be on the receiving end though, eh?

~S~
Pads