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NASA is counting down to the 'wet dress rehearsal' for its 'Mega Moon Rocket'

What the heck is a "wet dress rehearsal" and what does it have to do with flying to space?

If you've never heard that term before, NASA is about to enlighten you, as it undertakes a so-called wet dress rehearsal (WDR) with a rocket bound for the moon. During a WDR, a series of pre-launch tests load a rocket with liquid, supercooled fuel; verify launch systems; and practice different countdown scenarios in preparation for liftoff. It's a process that the Artemis I mission's Orion spacecraft and Space Launch System (SLS) rocket are about to undergo while parked at Launch Pad 39B at Kennedy Space Center in Cape Canaveral, Florida, beginning on Friday (April 1) and continuing through Sunday (April 3).

After that, if all goes well, NASA officials are expected to set an official launch date for Artemis I, representatives announced at a briefing on Tuesday (March 29).

The WDR's "call to stations" will take place on April 1 at 5:00 p.m. EDT and the targeted end of the test window will be on April 3 at about 2:40 p.m., NASA representatives announced at the briefing.

Related: NASA's new moon rocket spotted from space rolling to the launch pad (photos)

On March 17, Orion and the SLS rode to the launch pad on an enormous "supertank" vehicle: NASA's Crawler-Transporter 2 (CT-2), a 131-foot-long (40 meters) and 114-foot-wide (35 m) behemoth that weighs approximately 6.6 million pounds (3 million kilograms, or the weight of about 15 Statues of Liberty) and was built more than 50 years ago to transport Apollo rockets bound for the moon.

Since NASA's last moon missions in the late 1960s and early 1970s, CT-2 (and its partner, CT-1) have undergone modifications and improvements to prepare them to carry even heavier payloads — such as Artemis' massive SLS rocket — to and from the Kennedy launch pads, and the crawlers are expected to be in use "for many years to come," according to NASA.

"Like watching a ballet"

At the start of the rehearsal, communications systems in the Orion spacecraft will be powered up and tested, Artemis launch director Charlie Blackwell-Thompson said at the briefing.

"We have an opportunity in there for the flight control team to do some commanding to the vehicle," Blackwell-Thompson said. "Then we start doing our pad configuration," such as positioning the launch pad's flame deflector and removing handrails. Technicians will then load the tanks of the rocket's core stage and upper stage with propellant: liquid hydrogen chilled to minus 450 degrees Fahrenheit (minus 268 degrees Celsius) and liquid oxygen at minus 273 F (minus 169 C), Tom Whitmeyer, NASA associate administrator for common exploration systems development, said at the briefing.

"It's like watching a ballet," Whitmeyer said. "You've got pressure, volume and temperature, and you're working all those parameters to have a successful tanking operation."

Then the countdown tests begin. Controllers will count down to T-minus 1 minute and 30 seconds, and then pause to demonstrate the system's ability to hold for up to 3 minutes. They'll then resume counting, reaching 33 seconds before launch. At 33 seconds, the count will pause again, then rewind back to 10 minutes before launch. A second terminal countdown will reach about 10 seconds before launch, and then end. This enables launch controllers to test a scenario in which a launch may need to be cancelled — or "scrubbed" — due to a technical or weather-related issue.

NASA officials will then examine the results of the WDR tests and review the data before setting an official launch date for Artemis I. They will evaluate performance of all WDR tasks and inspect the rocket and spacecraft to see if the tests revealed anything unusual or unexpected that needs to be addressed, Whitmeyer said.

After the WDR is completed and the rocket's fuel tanks have been drained, SLS and Orion will be rolled back to the Vehicle Assembly Building (VAB) for additional testing of the flight safety systems prior to launch, Blackwell-Thompson said at the briefing.

"Once all of that is complete and the vehicle is in the final configuration for launch, then we'll roll back out to the pad for our launch operations," Blackwell-Thompson said. The flow of operations when the rocket is finally rolled out for launch will be very similar to what was done during the rehearsal, she added.