NASA’s Artemis 2 wet dress rehearsal countdown clock is ticking away towards a simulated launch window which will open today at 9 p.m. EST tonight.
NASA provided an udate this morning which says:
Technicians are currently replacing the ambient air in the SLS and Orion crew module with gaseous nitrogen, which is an inert gas that does not support combustion. By replacing air – which contains highly-combustible oxygen – with nitrogen, engineers can remove the oxidizer needed to sustain fire and dramatically reduce flammability risks in a launch environment filled with high-energy systems and propellants.”
“The step also includes an inerting purge that removes oxygen and prevents contaminants like moisture or particulates from entering sensitive systems on SLS and Orion. This keeps propulsion and life-support hardware clean and stable and creates a non-reactive environment that protects hardware and minimizes chemical reactions during countdown and ascent.”
While gaseous nitrogen is excellent for fire prevention, it is hazardous for humans to breathe because it displaces oxygen, so all personnel have cleared the area during operations.
A key moment this morning was the call for tanking operations. NASA’s Artemis launch director, Charlie Blackwell-Thompson conferred with and polled the management team after a weather briefing. That “go” for tanking had been expected around 10:45 a.m. EST. Reports from the Kennedy Space Center say that the “go” was given and the countdown clock which had been in pre-planned hold, has resumed. At this time the clock is ticking down toward the simulated launch window which will open at 9 p.m. EST.
Prior to today’s activities the countdown clock had started on Sat., January 31 at 8:13 p.m. EST which was 48 hours, 40 minutes before the opening of a simulated launch window. NASA said the wet dress rehearsal “is expected to go until approximately 1 a.m. Feb. 3.”
The countdown sequence
Background from NASA:
The countdown contains “L minus” and “T minus” times. “L minus” indicates how far away we are from liftoff in hours and minutes. “T minus” time is a sequence of events that are built into the countdown. Pauses in the countdown, or “holds,” are built into the countdown to allow the launch team to target a precise launch window, and to provide a cushion of time for certain tasks and procedures without impacting the overall schedule. During planned holds in the countdown process, the countdown clock is intentionally stopped and the T- time also stops. The L- time, however, continues to advance.
L-49 hours 15 minutes and counting
- L-49H, 15M: The launch team arrives on their stations and the countdown begins
- L-48H40M: The countdown clock begins
- L-47H30M – L-38H30M: Fill the water tank for the sound suppression system
- L-48H45M – L-39H45M: Liquid Oxygen (LOX)/Liquid Hydrogen (LH2) system
preparations for vehicle loading - L-39H30M – L-38H45M: The core stage is powered up
- L-40H30M – L-39H: The interim cryogenic propulsion stage (ICPS) is powered
up - L-38H45M – L-34H30M: Final preparations of the four RS-25 engines
L-34 hours 30 minutes and counting
- L-33H45M – L-33H10M: The ICPS is powered down
- L-32H30M – L-28H30M: Charge Orion flight batteries to 100%
- L-30H30M – L-23H30M: Charge core stage flight batteries
- L-19H15M – L-17H45M: The ICPS is powered-up for launch
- L-19H30M – L-16H: Orion crew suit regulator leak checks
L-15 hours and counting
- L-14H30M – L-13H: All non-essential personnel leave Launch Complex 39B
- L-12H45M – 11H15M: Ground Launch Sequencer (GLS) activation
- L-13H15M – L-11H05M: Air-to-gaseous nitrogen (GN2) changeover and vehicle
cavity inerting
L-11 hours, 40 minutes and counting
- L-11H35M – L-9H20M: 2-hour 15-minute built in countdown hold begins
- L-11H40M – L-10H30M: Launch team conducts a weather and tanking briefing
- L-10H20M: Launch team decides if they are “go” or “no-go” to begin tanking the
rocket - L-10H10M – L-9H50M: Core stage LOX transfer line chilldown
- L-10H10M – L-9H25M: Core stage LH2 chilldown
- L-10H20M – L-9H: Orion cold soak
L-10 hours and counting - L-9H50M – L-9H10M: Core stage LOX main propulsion system chilldown
- L-9H25M – L-9H: Core stage LH2 slow fill start
- L-9H20M: Resume T-Clock from T-8H10M
- L-9H10M – L-8H55M: Core stage LOX slow fill
- L-9H – L-7H40M: Core stage LH2 fast fill
- L-8H55M – L-6H10M: Core stage LOX fast fill
- L-8H45M – L-8H10M: ICPS LH2 chilldown
- L-8H10M – L-7H25M: ICPS LH2 fast fill start
- L-7H45M – L-6H: ICPS LOX main propulsion system chilldown
- L-7H40M – L-7H30M: Core stage LH2 topping
- L-7H30M – terminal count: Core stage LH2 replenish
- L-7H25M – L-7H05M: ICPS LH2 vent and relief test
- L-7H05M – L-6H55M: ICPS LH2 tank topping start
- L-6H50M – terminal count: ICPS LH2 replenish
- L-6H10M – L-5H40M: Orion communications system activated (RF to mission
control) - L-6H10M – L-5H40M: Core stage LOX topping
L-6 hours and counting
- L-6H – L-5H15M: ICPS LOX fast fill
- L-5H40M – terminal count: Core stage LOX replenish
- L-5H15M – L-5H: ICPS LOX vent and relief test
- L-5H – L-4H40M: ICPS LOX topping
- L-5H40M: Stage pad rescue
- L-5H40M: Closeout crew assemble
- L-4H40M – terminal count: ICPS LOX replenish
- L-4H40M: All stages replenish
- L-4H40M: Start 40-minute built in hold
- L-4H40M-L-4H25M: Closeout crew to white room
- L-4H30M – L-4H20M: Crew Module hatch preps and closure
- L-4H20M – L-3H20M: Counterbalance mechanism hatch sealpress decay
checks - L-3H20M – L-2H40M: Crew Module Hatch service panel install/closeouts
- L-2H40M – L2H20M: Launch Abort System (LAS) Hatch closure for flight
- L-1H10M: Launch Director brief – Flight vehicle/TPS Scan results with CICE
- L-1H45M – L-1H40M: Closeout crew departs Launch Complex 39B
L-40 minutes and holding
- L-40M: Built in 30-minute countdown hold begins
L-25 minutes and holding
- L-25M: Transition team to Orion to Earth communication loop following final NTD
briefing - L-16M: The launch director polls the team to ensure they are “go” for launch
T-10 minutes and counting
- T-10M: Ground Launch Sequencer (GLS) initiates terminal count
- T-8M: Crew Access Arm retract
- T-6M: GLS go for core stage tank pressurization
- T-6M: Orion set to internal power
- T-5M57S: Core stage LH2 terminate replenish
- T-4M: GLS is go for core stage auxiliary power unit (APU) start
- T-4M: Core Stage APU starts
- T-4M: Core stage LOX terminate replenish
- T-3M30S: ICPS LOX terminate replenish
- T-3M10S: GLS is go for purge sequence 4
- T-2M02S: ICPS switches to internal battery power
- T-2M: Booster switches to internal batter power
- T-1M30S: Hold for three minutes to verify core stage certification hold time
- T-1M30S: Core stage switches to internal power
- T-1M20S: ICPS enters terminal countdown mode
- T-50S: ICPS LH2 terminate replenish
- T-33S: GLS sends “go for automated launch sequencer” command
- T-33S: GLS Cutoff/Recycle
Inside the terminal countdown, teams have a few options to hold the count if needed.
- The launch team can hold at 6 minutes for the duration of the launch window,
less the 6 minutes needed to launch, without having to recycle back to 10
minutes. - If teams need to stop the clock between T-6 minutes and T-1 minute, 30
seconds, they can hold for up to 3 minutes and resume the clock to launch. If
they require more than 3 minutes of hold time, the countdown recycles back to T-
10. - If the clock stops after T-1 minute and 30 seconds, but before the automated
launch sequencer takes over, then teams can recycle back to T-10 to try again,
provided there is adequate launch window remaining. - On launch day, after handover to the automated launch sequencer, any issue
that would stop the countdown would lead to concluding the launch attempt for
that day.