There is a part where they show the explosive bolts that holds the shuttle onto the pad. You can see them detonate with a little puff and then the whole thing lifts off

I had never seen that before.
I also noticed, in the part where the shuttle is viewed from the top of the tower, that once the 3 main SSME's ignite the whole vehicle actually bends forward due to the thrust. That must surely put some serious strain on the SRB joints!
I think I read somewhere once that even if the bolts didn't detonate it would just break them once the SRB's are fired up
Here is was:
Quote:
The SRB bolt is 28 in (710 mm) long and is 3.5 in (89 mm) in diameter. The frangible nut is captured in a blast container. In the event of a hold down failure the thrust from SRB ignition is enough to break the bolts, freeing the vehicle.
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(See
http://en.wikipedia.org/wiki/Space_S...Rocket_Booster)