The launch countdown for the Space Launch System (SLS) rocket and the Artemis I mission is underway at the Kennedy Space Center in Florida ahead of a planned liftoff within a two-hour launch window that opens at 8:33 AM EDT (12:33 UTC) on Monday, August 29.
The launch count comes after more than a decade of development, capped by successful Green Run and Wet Dress Rehearsal tests — both of which provided good lessons learned and changes in procedures for launch day.
The countdown is the first for launch operations at LC-39B since Ares I-X’s only mission on October 28, 2009, and marks the first in a series of three NASA-led missions to return humanity to the surface of the Moon for the first time since December 1972.
The 46-hour 40-minute countdown will see the Exploration Ground Systems (EGS) launch team take the SLS vehicle through final configurations for launch, including the all-important fueling process to give SLS the needed propellants for its four RS-25 Core Stage engines and its single RL10B-2 engine on the Interim Cryogenic Propulsion Stage (ICPS).
The twin five-segment Solid Rocket Boosters (SRBs) and the European Service Module (ESM) had their propellants (Polybutadiene acrylonitrile-Ammonium perchlorate composite propellant for the SRBs and mixed oxides of nitrogen & monomethylhydrazine for the ESM) loaded before the SRB segments were shipped from Utah and before the ESM and the Orion capsule were brought to the Vehicle Assembly Building (VAB) for stacking.
The start of the count commences with a five-hour operation to fill the water tower at LC-39B for use as sound suppression during engine start and liftoff. The tower will be topped off just before terminal count (which begins at T-10 minutes) in the traditional fashion of pumping water into it until water starts to cascade from an over-fill line — indicating the tank is full.
Simultaneously, teams will start bringing online the liquid oxygen and liquid hydrogen ground systems ahead of final fuel loading preparations.
At Launch minus 42 hours (L-42 hours), the 30-minute process of powering up the Orion spacecraft will begin; however, there is an option to perform this operation at the very start of the countdown instead.
At L-38 hours 30 minutes, teams will bring their attention to the ICPS and its initial power-up. After verifying functionality and systems, the ICPS will be powered back down until later in the count.
While this is taking place, final launch preparations for the four RS-25 engines will begin.
At L-34 hours, teams will pick up the process of powering up the Core Stage, an operation that should last approximately 40 minutes.
At L-31 hours, the Core Stage Composite Overwrap Pressure Vessels (COPVs) will be pressurized to flight levels, followed by charging of the Orion and Core Stage flight batteries. Final ICPS power-up for flight will take place at L-18 hours 30 minutes.
All non-essential personnel will be required to leave LC-39B between L-12 and L-10 hours. The launch team will then bring the Ground Launch Sequencer online, a two-hour process beginning at L-11 hours 15 minutes.
The critical transition from air to gaseous nitrogen purge will then follow at L-10 hours 45 minutes.
After this, a two-and-a-half-hour built-in-hold will then take place from L-9 hours 40 minutes to L-7 hours 10 minutes. This hold has been extended one extra hour from the Wet Dress Rehearsal counts due to gaseous nitrogen changeover lessons learned.
The launch team, led by Charlie Blackwell-Thompson, will then conduct a final pre-tanking weather briefing at L-9 hours 40 minutes and will issue a “go” or “no go” decision for fueling around L-8 hours 40 minutes — though this decision could come earlier if the meeting does not require the full hour earmarked.
At L-8 hours 15 minutes, while still in the planned hold, fueling operations of SLS for launch will pick up with liquid oxygen transfer line chilldown, a process that slowly thermally cools the transfer and propulsion system lines so they are not cryo-shocked at the start of fueling.
The count will then resume at L-7 hours 40 minutes for the fueling process on launch day.
Controlling the count
Serving as the brain underneath all the operations during the count is the Command & Control System Software in the Launch Control Center.
As related by John Ramsey, lead for the Command & Control System Software in an interview with NASASpaceflight, this is the system upon which all the other sub-system level software programs live.
The Command & Control System Software is “responsible for the testing and processing of the spacecraft at the pad and responsible, ultimately, for the launch down to T0 when the vehicle leaves the pad,” said Ramsey.
“The master control operator — you’ll hear that console position on launch day — operates the Launch Control System, and they’re doing all the housekeeping and monitoring of the system, making sure it’s healthy, and they’re executing the things to advance the different stages of configuration that the system needs to be in to support the outcome.”
In this manner, the Command & Control System Software is the inner workings, “the guts,” as Ramsey described it, of launch control. Systems like the Ground Launch Sequencer are applications that “live in our system.”
This base code for Artemis I was built and then modified with lessons learned from the Green Run campaigns at Stennis in 2021; however, the four Wet Dress Rehearsal runs and the lessons learned there were all at the application level and not at the Command & Control System Software level.
For countdown operations, Ramsey noted that “if something comes up and it’s not something we already have documented or have a design waiver for, [there’s a back room team] there to support the master control operator.”
In this case, the back room would analyze the situation, advise the master control operator on the issue and forward paths and potential solutions. Meanwhile, the master control operator would focus on how the issue interfaces with other aspects (and console positions) of the overall launch count processes.
Moreover, from a design perspective, the team will be monitoring the system to make sure it functions as expected and that it has expected behavior.
“There are certain things that are irregular but expected, and when we see those, we want to confirm that that signature is what we’re expecting to see or what we’ve seen before.”
“As we get deeper into the count, we’ll be looking at specific performance data. We’ll be doing retrievals from our recorded enclave data — the processed data — in the system and making sure that the patterns we’re expecting to see are there, and that the rates we’re expecting to see are similar to what we saw in Wet Dress and the countdown sequence.”
If all systems and software are in agreement and all launch team positions agree, SLS will liftoff Monday, August 29 on its historic flight to send Orion and the European Service Module on a 42-day mission to a Distant Retrograde Orbit of the Moon.
As of publication, the 45th weather squadron of the Space Force is predicting a 70% chance of acceptable weather conditions on Monday, with the primary concerns being the cumulus cloud and surface electric fields rules (aka natural and triggered lightning) as well as flight through precipitation.
(Lead photo: SLS on LC-39B ahead of launch. Credit: Jack Beyer for NSF)