A quick overview of the last steps to launch taken from NASA’s Artemis site.
The launch 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 launch countdown, and can be paused. Pauses in the countdown, or “holds,” allow the launch team to target a precise launch window, and to provide a cushion of time for tasks. 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.
| Event | Countdown time |
| The launch team arrives on their stations and the countdown begins | L-46 hours, 40 minutes |
| Fill the water tank for the sound suppression system | L-46 hours – L-41 hours |
| Liquid Oxygen (LO2)/Liquid Hydrogen (LH2) System Preparations for Vehicle Loading | L-46 hours – L-37 hours |
| The Orion spacecraft is powered up if not already powered at Call to Stations | L-42 hours – L-41 hours 30 minutes |
| The interim cryogenic propulsion stage is powered up | L-38 hours 30 minutes – L-35 hours 30 minutes |
| The core stage is powered up | L-34 hours – L-33 hours 20 minutes |
| Final preparations of the four RS-25 engines | L-36 hours 20 minutes – L-31 hours |
| Core stage composite overwrapped pressure vessel, Pressurization to Flight Pressure | L-31 hours – L-22 hours |
| Charge Orion Flight Batteries to 100% | L-30 hours – L-26 hours |
| Charge core stage Flight Batteries | L-27 hours – L-21 hours |
| The ICPS is Powered-up for launch | L-18 hours 30 minutes – L-15 hours 30 minutes |
| All non-essential personnel leave Launch Complex 39B | L-12 hours – L-10 hours |
| Ground Launch Sequencer Activation | L-11 hours 15 minutes – 9 hours 15 minutes |
| Air-to-gaseous nitrogen Changeover for vehicle cavity inerting | L-10 hours 45 minutes – Launch |
| Built in countdown hold begins | L-9 hours 40 minutes – L-7 hours 10 minutes |
| Launch team conducts a weather and tanking briefing | L-9 hours 40 minutes – L-8 hours 50 minutes |
| Launch team decides if they are “go” or “no-go” to begin tanking the rocket | L-8 hours 40 minutes |
| Core Stage LO2 transfer line chilldown | L-8 hours 15 minutes – L-8 hours |
| Core stage LO2 main propulsion system chilldown | L-8 hours – L-7 hours 20 minutes |
| Core stage LO2 slow fill | L-7 hours 20 minutes – L-7 hours 5 minutes |
| Core Stage LO2 fast fill | L-7 hours 5 minutes – L-4 hours 15 minutes |
| Core Stage LH2 chilldown | L-7 hours 15 minutes – L-7 hours 5 minutes |
| Core Stage LH2 slow fill start | L-7 hours 5 minutes – L-6 hours 15 minutes |
| Core Stage LH2 fast fill | L-6 hours 15 minutes – L-5 hours 5 minutes |
| Core Stage LH2 topping | L-5 hours 5 minutes – L-5 hours |
| Core Stage LH2 replenish | L-5 hours – Launch |
| ICPS LH2 ground support equipment and tank chilldown | L-4 hours 45 minutes – L-4 hours 30 minutes |
| ICPS LH2 fast fill start | L-4 hours 30 minutes – L-3 hours 30 minutes |
| Orion communications system activated | L-4 hours 20 minutes – L-3 hours 45 minutes |
| Core stage LO2 topping | L-4 hours 15 minutes– L-3 hours 55 minutes |
| Core Stage LO2 replenish | L-3 hours 55 minutes – Launch |
| ICPS L02 MPS chilldown | L-3 hours 55 minutes– L-3 hours 45 minutes |
| ICPS L02 fast fill | L-3 hours 45 minutes– L-2 hours 55 minutes |
| ICPS LH2 validation and leak test | L-3 hours 30 minutes – L-3 hours 15 minutes |
| ICPS LH2 tank topping start | L-3 hours 15 minutes – L-2 hours 55 minutes |
| ICPS/Space Launch System telemetry data verified with Mission Control and SLS Engineering Support Center | L-2 hours 55 minutes – L-2 hours 45 minutes |
| ICPS LO2 validation and leak test | L-2 hours 55 minutes – L-2 hours 30 minutes |
| ICPS LH2 replenish | L-2 hours 50 minutes – Launch |
| ICPS LO2 topping | L-2 hours 30 minutes – L-2 hours 10 minutes |
| ICPS LO2 replenish | L-2 hours 10 minutes – Launch |
| Final NASA Test Director briefing is held | L-50 minutes |
| Built in 30-minute countdown hold begins | L-40 minutes |
| The launch director polls the team to ensure they are “go” for launch | L-15 minutes |
| Ground Launch Sequencer initiates terminal count | T-10 minutes |
| GLS go for core stage tank pressurization | T-6 minutes |
| Orion ascent pyros are armed | T-6 minutes |
| Orion set to internal power | T-6 minutes |
| Core Stage LH2 terminate replenish | T-5 minutes 57 seconds |
| GLS is go for flight termination system arm | T-5 minutes |
| GLS is go for LH2 high flow bleed check | T-4 minutes 40 seconds |
| GLS is go for core stage auxiliary power unit start | T-4 minutes |
| Core Stage APU starts | T-4 minutes |
| Core stage L02 terminate replenish | T-4 minutes |
| ICPS LO2 terminate replenish | T-3 minutes 30 seconds |
| GLS is go for purge sequence 4 | T-3 minutes 10 seconds |
| ICPS switches to internal battery power | T-1 minutes 56 seconds |
| Core stage switches to internal power | T-1 minutes 30 seconds |
| ICPS enters terminal countdown mode | T-1 minutes 20 seconds |
| ICPS LH2 terminate replenish | T-50 seconds |
| GLS sends “Go for automated launch sequencer” command | T-33 seconds |
| Core stage flight computer to automated launching sequencer | T-30 seconds |
| Hydrogen burn off igniters initiated | T-12 seconds |
| GLS sends the command for core stage engine start | T-10 seconds |
| RS-25 engines startup | T-6.36 seconds |
| Booster ignition, umbilical separation, and liftoff | T-0 |
Automated Transfer Vehicle page
ATV blog archive
NASA Orion page
NASA Artemis
Airbus Orion page
Discussion: one comment
Super excited, cannot wait until 1333BST for launch (assuming all goes well); I live in Scotland.