Wednesday, May 12, 2010

Space Shuttle Countdown


Regular HOLDS are important in Shuttle countdowns, allowing planned "pauses" for tasks and procedures as NASA teams attempt to pinpoint precise launch windows. Holds of varying lengths always occur at T-MINUS 27, T-19, T-11, T-6, and T-3 hours, as well as T-20 and T-9 minutes.

While every situation can be highly variable, and scrubbed at any point with a NO GO, these are the general milestones in each Space Shuttle countdown…

T-43 Hours and COUNTING
The clocks is activated when the Shuttle Test Director (STD position currently held by Jerry Spaulding) makes the first Call to Stations. Tasks include:
- Final vehicle and facility close-outs for launch
- Review flight software stored in memory units and display systems
- Load backup flight system software into orbiter computers
- Remove middeck and flight deck platforms
- Activate and test navigational systems
- Complete preparation to load power reactant storage and distribution system

T-27 Hours and HOLDING
The first built-in hold typically lasts 4 hours, whereupon the launch pad is cleared of all non-essential personnel and the rotating service structure around the Shuttle is slowly rolled back. Ground crews then load cryogenic propellants.

T-27 Hours and COUNTING
- Propellants go from power reactant storage & distribution (PRSD) system to fuel cell storage tanks.

T-19 Hours and HOLDING
Another four hour hold, where the STS mid-body is de-mated from the umbilical unit, and an external tank nose cone purge is carried out. They also vacuum the crew module. That’s right… house-cleaning!

T-19 Hours and COUNTING
- Begin preparations of the three main engines for main propellant tanking and flight
- Fill-up of launch pad sound suppression system water tank
- Closing out of tail service masts on the mobile launcher platform

T-11 Hours and HOLDING
The third hold can be up to 14 hours long, and includes engineering briefings, weather review, a launch pad inspection and the activation of the orbiter’s communications systems.

T-11 Hours and COUNTING
- Activation of the orbiter’s fuel cells
- “Blast Danger Area” is cleared of all non-essential personnel
- Orbiter is purged of air and filled with nitrogen mix

T-6 Hours and HOLDING
This hold typically lasts two hours, whereby the Mission Management Team (MMT) reviews the weather and the launch team must verify that all launch commit criteria has been met. If they have, the external tank is loaded with 500,000 gallons propellant and the launch pad is then cleared of ALL personnel.

T-6 Hours and COUNTING
- External tank filled with flight load of liquid hydrogen and oxygen.

T-3 Hours and HOLDING
Another two hour hold where the Final Inspection Team conducts analysis of the orbiter and launch tower, and if all is well, another weather review ensues. The Closeout Crew configures the crew module and the Astronaut Support Person also enters to perform communications checks.

T-3 Hours and COUNTING
- Upon arrival at the launch pad, astronauts enter via the “White Room”
- Air-to-ground voice checks with Launch Control (Kennedy Space Center, Florida)
- Air-to-ground voice checks with Mission Control (Johnson Space Center, Texas)
- Hatch is closed and checked for leaks, then Closeout Crew departs

T-20 Minutes and HOLDING
This built-in hold usually lasts a mere 10 minutes, in which the NASA Test Director conducts final launch team briefings.

T-20 Minutes and COUNTING
- Transition of the orbiter's onboard computers to launch configuration
- Fuel cell thermal conditioning
- Closing of orbiter cabin vent valves
- Transition of backup flight system to launch configuration

T-9 Minutes and HOLDING
This is the final built-in hold, and varies in length depending on the mission. The ground crews determine the parameters of the final launch window, and activate the flight recorders. The NASA Test Director, Mission Management Team and launch director conduct final GO vs. NO-GO launch polls.

T-9 Minutes and COUNTING
- Automatic ground launch sequencer starts.

T-7 minutes, 30 seconds: Retraction of orbiter access arm
T-5 minutes: Auxiliary power units start
T-3 minutes, 55 seconds: Aerosurface and engine gimbal profile tests
T-2 minutes, 55 seconds: Retraction of beanie cap (i.e. gaseous oxygen vent arm)
T-2 minutes: Crew members close and lock their helmet visors

T-50 seconds: Orbiter transfers from ground to internal power
T-31 seconds: Ground launch sequencer starts
T-16 seconds: Launch pad sound suppression system activated
T-10 seconds: Main engine hydrogen burnoff system activated
T-6.6 seconds: Main engine starts

Solid rocket booster ignition and liftoff!

Godspeed, Atlantis...