Back when I was still high on the excitement from my trip to Florida to watch the final launch of Atlantis I wrote a bit about the pre-launch sequence. The coolest part of the ground sequence, I think, is the bit described in the last two paragraphs. I was re-inspired to write about it when I watched a cool video that follows the solid rocket boosters up and down. It gives a great view of my favourite part of the launch.
Basically, the last ten seconds is where shit starts to get super rad. The hydrogen burn-off ignitors light first to make sure any pre-launch or post-abort exhaust burns below the shuttle (hydrogen is very light so it would flow up the side of the shuttle otherwise). That is when I start grinning. With 6.6 seconds to go the three main engines start and take a few seconds to come to a full burn. If full thrust doesn’t happen in six seconds, that is the last chance to do a pad abort. At this point the shuttle is bolted to the orange tank, the orange tank is bolted to the solid rocket boosters and the solid rocket boosters are bolted to the ground. Everything is nicely balanced. Except that the three main engines just started dumping over 5500 kN of thrust. This causes the whole assembly to rock backward for three seconds and then it starts rocking forward again. Just as it returns to the balance point we hit time zero – pyrotechnic charges blow off the eight bolts clamping the solid rocket boosters to the ground – and we have liftoff.
An elegant symphony of physics, I think. Watch for the shuttle to rock back and forth just before liftoff in this super rad video that follows the life of the (reusable) solid rocket boosters during a typical launch.