Space Shuttle launch countdown


The launch of Space Shuttle missions was governed by a countdown. Two clocks were maintained, the unofficial, but sometimes mentioned, L clock represented the time remaining before the scheduled launch in real time, and the more often referred to T clock included several built-in holds where additional verifications were made. Built-in holds were extended if mission parameters allowed for additional checks or to correct issues. Launches of the Space Shuttle to the International Space Station did not allow for holds to be extended due to the launch window which was limited to no more than 10 minutes due to the 90 minute orbit period of the station and speed of Earth's rotation, which puts the launch pads east of the station on its next orbit.

Milestones

This built-in hold typically lasted four hours, but was extended if PRSD offload is required. During this hold:
This built-in hold varied between 13 and 14 hours.
This built-in hold typically lasted two hours, or one hour for a 24- or 48-hour scrub. Countdown may be resumed at this point after a 24‑hour scrub/turnaround.
This built-in hold typically lasted two-and-a-half hours.
This built-in hold typically lasted 10 minutes.
This was the final built-in hold, and varied in length depending on the mission.