The timeline below covers major countdown activities planned during the NASA’s wet dress rehearsal for the Artemis 1 mission. The wet dress rehearsal will include loading of more than 700,000 gallons of cryogenic liquid hydrogen and liquid oxygen into the Space Launch System moon rocket on launch pad 39B at the Kennedy Space Center in Florida.
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EDITOR’S NOTE: The times below are listed using the “T” time on the countdown clock, which includes build-in holds.
• T-06:40:00: The countdown enters a 90-minute built-in hold; Mission management team meets to give “go” for tanking
• T-06:40:00: The countdown clock resumes after the build-in hold; Core stage liquid oxygen chilldown begins
• T-05:55:00: Core stage liquid oxygen slow fill begins
• T-05:40:00: Core stage liquid oxygen fast fill begins
• T-05:35:00: Core stage liquid hydrogen chilldown begins
• T-05:30:00: Core stage liquid hydrogen slow fill begins
• T-05:10:00: Core stage liquid hydrogen fast fill begins
• T-04:00:00: Core stage liquid hydrogen topping begins
• T-03:55:00: ICPS liquid hydrogen chilldown begins; Core stage liquid hydrogen in replenish mode
• T-03:50:00: Orion communications system activation begins
• T-03:30:00: ICPS liquid hydrogen fast fill begins
• T-02:55:00: Core stage liquid oxygen topping begins
• T-02:50:00: Core stage liquid oxygen in replenish mode; ICPS liquid oxygen chilldown begins
• T-02:45:00: ICPS liquid hydrogen validation and leak test begins
• T-02:30:00: ICPS liquid hydrogen topping begins; ICPS/SLS telemetry data verified with MCC and SLS support center
• T-02:10:00: ICPS liquid hydrogen in replenish mode; ICPS liquid oxygen validation and leak test begins
• T-01:50:00: ICPS liquid oxygen topping begins
• T-01:40:00: ICPS liquid oxygen in replenish mode
• T-01:30:00: Core stage liquid hydrogen/liquid oxygen stop flow and recover test begins
• T-00:10:00: Final 30-minute built-in hold begins; Final NASA Test Director briefing; Final Launch Director poll
• T-00:10:00: Countdown resumes
• T-00:06:00: Orion ascent pyros armed; Orion transitions to internal battery power
• T-00:05:57: Core stage liquid hydrogen replenish terminates
• T-00:04:00: Core stage auxiliary power unit starts; Core stage liquid oxygen replenish terminates; Solid rocket boosters armed for ignition
• T-00:03:30: ICPS liquid oxygen replenish terminates
• T-00:03:07: RS-25 main engine purge sequence 4
• T-00:01:56: ICPS transitions to internal battery power
• T-00:01:30: Core stage transitions to internal battery power; Countdown pauses for approximately 3 minutes to demonstrate “launch ready” hold
• T-00:01:20: ICPS enters terminal countdown mode
• T-00:00:50: ICPS liquid hydrogen replenish terminates
• T-00:00:33: Ground Launch Sequencer sends cutoff command; Countdown recycles back to T-minus 10 minutes
EDITOR’S NOTE: The countdown will recycle back to T-minus 10 minutes to allow the launch team to rehearse their ability to hold the countdown and set up for a second launch attempt on the same day.
• T-00:10:00: Final NASA Test Director briefing; Final Launch Director poll
• T-00:10:00: Countdown resumes
• T-00:06:00: Orion ascent pyros armed; Orion transitions to internal battery power
• T-00:05:57: Core stage liquid hydrogen replenish terminates
• T-00:04:00: Core stage auxiliary power unit starts; Core stage liquid oxygen replenish terminates; Solid rocket boosters armed for ignition
• T-00:03:30: ICPS liquid oxygen replenish terminates
• T-00:01:56: ICPS transitions to internal battery power
• T-00:01:30: Core stage transitions to internal battery power; Countdown pauses for approximately 3 minutes to demonstrate “launch ready” hold
• T-00:01:20: ICPS enters terminal countdown mode
• T-00:00:50: ICPS liquid hydrogen replenish terminates
• T-00:00:33: Ground launch sequencer sends “go for automated launch sequencer” command
• T-00:00:30: Core stage flight computer transitions to automated launch sequencer
• T-00:00:28: Solid rocket booster hydraulic power unit starts
• T-00:00:19: Solid rocket booster thrust vector control gimbal profile
• T-00:00:10: Ground launch sequencer sends “go for core stage engine start” command
• T-00:00:09.34: Ground launch sequencer sends cutoff command
The countdown will stop at T-minus 9.34 seconds, just before ignition of the SLS core stage’s RS-25 main engines. Pre-programmed software settings in the Ground Launch Sequencer computer will ensure the countdown holds at this point, and an engineer in the Launch Control Center will be ready to send a manual cutoff command if necessary.
Once the countdown is stopped, the launch team will begin draining liquid hydrogen and liquid oxygen from the rocket and secure the launch vehicle.
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