Space shuttle launch countdown

Liftoff of STS-110 in 2002

Launch of a space shuttle mission is governed by a countdown. 2 clocks are maintained, the unofficial, but sometimes mentioned, L (launch) clock represents the time remaining before the scheduled launch in real time and the more often referred to T (test) clock includes several built-in holds where additional verifications are made. Built-in holds may be extended if mission parameters allow for additional checks or to correct issues. Launches of the space shuttle to the international space station do not allow for holds to be extended due to the launch window which is limited to no more than 10 minutes due to the 90 minute orbit period of the station and speed of Earth's rotation (913.6 miles per hour (1,470.3 km/h) at the Kennedy Space Center[1]), which puts the launch pads 1,000 miles (1,600 km) east of the station on its next orbit.[2][3][4][5]

Milestones

T-43 hours and counting

T-27 hours and holding

T-27 hours and counting

T-19 hours and holding

This built-in hold typically lasts four hours, but may be extended if PRSD offload is required. During this hold:

T-19 hours and counting

T-11 hours and holding

This built-in hold varies between 13 and 14 hours.

T-11 hours and counting

T-6 hours and holding

This built-in hold typically lasts two hours, or one hour for a 24- or 48-hour scrub. Countdown may be resumed at this point after a 24hour scrub/turnaround.

T-6 hours and counting

T-3 hours and holding

This built-in hold typically lasts two-and-a-half hours.

T-3 hours and counting

T-20 minutes and holding

This built-in hold typically lasts 10 minutes.

T-20 minutes and counting

T-9 minutes and holding

This is the final built-in hold, and varies in length depending on the mission.

T-9 minutes and counting

T-0

References

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