Don’t stop me now: countdown to launch

Update: on 3 April the pre-launch test countdown was stopped due to fans needed to provide positive pressure to enclosed areas in the mobile launcher and keep hazardous gases out were not working correctly. The issues have been resolved and countdown resumed on 4 April, including the tanking process.

Today NASA will start a so-called Wet Dress Rehearsal for the new Artemis rocket that will launch Orion and the European Service Module to the Moon. In essence, much like a stage dress-rehearsal this rehearsal will be the real thing to ensure all elements of a rocket launch are ready without the actual launch. The rehearsal will take up to two days.

As the SLS rocket is at the launchpad after last month’s roll-out, technicians will go through the process of getting it ready for launch. This includes pumping the tanks full of fuel, start a launch countdown, show that the countdown clock can be restarted, and then drain the tanks, all to practice the procedures that will be used for launch.

European Service Module atop the rocket

During this time the European Service Module is already at the top of the mega-Moon rocket and is an integral part of the Artemis I mission, it will also be acting its part in the wet dress rehearsal.

Before liftoff the European Service Module will be powered on, its thermal control system will be put online, the propulsion systems initialised and main engine movement verified, as well as the solar array driving electronics activated. A series or checks are done to verify that everything is operating as expected and no sensor is sending suspicious measurements.

If everything is as it should be after the subsystems are verified, the spacecraft module will be put in its launch configuration by setting the propulsion valves in the position needed for flight, setting the heaters for the thermal environment of launch, and removing the inhibits that protect against accidentally activating the solar wing rotations while still on ground. Finally, 10 minutes before liftoff, the European Service Module team give the thumbs up for launch and the last part of the launch sequence will be executed.

T-9 seconds

So, at what point do you stop a dress rehearsal? The launch countdown will go as far as 9.3 seconds before liftoff, over two sets of countdown routines. The first countdown will go to 1 minute 30 before liftoff, then pause for up to three minutes to continue to 33 seconds before launch and pause again.

The procedure will then restart at 10 minutes before launch and continue down to 9.3 seconds, when the practice launch countdown will be cancelled, and the tanks will be drained of fuel.

These pauses and returns to earlier moments in the countdown allow the ground systems to gain experience in the procedures and handling of equipment. Although for this rehearsal the pauses and rewinds are scripted for practice, they can occur on actual launch day for any number of reasons such as weather or sensor alerts.