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STS-1: Launch and timing discrepancy

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Author	Topic: STS-1: Launch and timing discrepancy
strangelove New Member Posts: 1 From: Sunrise, FL. USA Registered: Jun 2017	posted 12-23-2019 09:09 PM I haven't been able to find much information on this but STS-1 was about 2 or 3 seconds late getting off the pad at liftoff, correct? During the countdown, main engine start occurred at around T-4 with SRB ignition taking place several seconds later instead of right at T-0. What was the cause of the timing discrepancy?
heng44 Member Posts: 3387 From: Netherlands Registered: Nov 2001	posted 12-28-2019 11:49 AM That was on purpose and built into the countdown procedure. From the countdown summary: Engine ignition comes at T minus 3.8 seconds. The three main engines start at 120 millisecond intervals. They throttle up to 90 percent thrust in three seconds. When they are at 90 percent thrust, the SRB ignition sequence starts. There is approximately 2.64 seconds between T-0 and SRB ignition to allow for the "twang" or forward movement of the stack. SRB ignition comes at T+3 seconds. Then the holddown bolts are blown and the T-0 umbilical explosive bolts are blown under command of the four flight computers. The Mission Elepsed Time resets to zero and the Shuttle lifts off. This was changed on STS-2 when SRB ignition came at T-0.
David C Member Posts: 1015 From: Lausanne Registered: Apr 2012	posted 12-28-2019 11:56 PM What he said. The STS-2 press kit had a slightly different way of phrasing it which may help: STS-1 had two T-0s, one at the estimated main engine 90 percent thrust time and the second at planned SRB ignition.

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