Friday, July 8, 2011

The Last Space Shuttle Launch. July 8, 2011, STS-135 Atlantis. Why does the countdown take so long?

Watching a space shuttle launch with small children can be really frustrating!
The countdown clock has all of these built-in holds that seem to last FOREVER.
For instance, Rex was so excited this morning about the launch.  There is an astronaut aboard named Rex Walheim today, too (from left to right in the photo: Commander Chris Ferguson, Pilot Doug Hurley, Mission Specialists Sandy Magnus and Rex Walheim).

So when the countdown clock...the big one...the one they show on TV, got to 20 minutes, my son freaked out!  I tried to explain that when the clock got to T-minus 9 minutes they were going to stop the clock for 41 minutes, but that explanation fell on deaf ears.  Besides, I couldn't answer the question, "Well, why would they do that?".  So, I looked it up.

First of all, there are two countdown clocks (from

Why Are There Two Different Countdown Clocks?
Time Until Launch:
simple countdown clock This clock is a simple, real-time countdown to the scheduled launch time. It will be reset as soon as possible in the event of a delay. After launch, the clock will run forward, recording mission elapsed time (MET).

Official Countdown Clock:
official countdown clock This is a virtual replica of the official countdown clock at the launch site at NASA's Kennedy Space Center. It begins T-43 hours and counting mark before launch and mirrors the real clock by pausing during the various built-in holds during the countdown. For an explanation of the built-in holds, read "Countdown 101".

On the page called "Countdown 101", it explains all the built-in holds.  Right now, as I write this, we are at "T-minus 9 minutes and holding".  It is July 8, 2011 and the time is 10:49 a.m. EST.  So, this is the final sequence of events that is going on:

T-9 minutes and holding
This is the final built-in hold, and varies in length depending on the mission.
  • Final launch window determination
  • Activate flight recorders
  • Final "go/no-go" launch polls conducted by NASA Test Director, Mission Management Team and launch director
T-9 minutes and counting
  • Start automatic ground launch sequencer
  • Retract orbiter access arm (T-7 minutes, 30 seconds)
  • Start auxiliary power units (T-5 minutes, 0 seconds)
  • Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds)
  • Start orbiter aerosurface profile test, followed by main engine gimbal profile test (T-3 minutes, 55 seconds)
  • Retract gaseous oxygen vent arm, or "beanie cap" (T-2 minutes, 55 seconds)
  • Crew members close and lock their visors (T-2 minutes, 0 seconds)
  • Orbiter transfers from ground to internal power (T-50 seconds)
  • Ground launch sequencer is go for auto sequence start (T-31 seconds)
  • Activate launch pad sound suppression system (T-16 seconds)
  • Activate main engine hydrogen burnoff system (T-10 seconds)
  • Main engine start (T-6.6 seconds)
  • Solid rocket booster ignition and liftoff!


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