NASA kicked off a two-day dress rehearsal countdown for its giant Space Launch System moon rocket on Friday, a major milestone that will culminate Sunday when engineers load it up with more than 750,000 gallons of supercold rocket fuel to test launch-day procedures.
The goal is to make sure the 322-foot-tall rocket and the ground systems needed to fuel and safely launch it will be ready for the Artemis program’s maiden flight when an Orion crew capsule will be fired off on an unpiloted voyage beyond the moon and back.
“Primarily this is about loading the vehicle,” said Charlie Blackwell-Thompson, NASA’s first female flight director. “It is utilizing the ground support equipment, the services and the commodities out at launch complex 39B, along with the command-and-control system here in the LCC (launch control center), getting through those loading operations and then getting into the terminal portion of the count.”
Mounted atop a towering mobile launch platform, the rocket was hauled from NASA’s iconic Vehicle Assembly Building to launch pad 39B on March 18, cheered on by thousands of Kennedy Space Center workers and their family members.
Equipped with two extended solid-fuel boosters and a core stage powered by four modified space shuttle main engines, the SLS rocket will tip the scales at 5.75 million pounds and generate a ground-shaking 8.8 million pounds of thrust at liftoff, making it the most powerful rocket ever built for NASA.
The long-awaited “wet dress rehearsal,” or WDR, countdown, the rocket’s final major test before launch, began at 5 p.m. EDT Friday, at the launch-minus 45-hour 40-minute mark, with a traditional “call to stations” in Firing Room 1.
While NASA normally carries out countdowns in the open, allowing reporters to listen in to firing room chatter and to follow along with detailed timelines, the agency is limiting realtime updates on the dress rehearsal to its social media channels and blog posts, ostensibly to make sure export control restrictions are not violated.
“Export control is a significant consideration for our program,” said Tom Whitmeyer, NASA deputy associate administrator for exploration system development. “This is the first flight of the rocket. Typically, what (adversaries are) looking for is timing and sequencing data, flow rates, temperatures, how long it takes to do certain tasks.”
“That’s considered to be important information by other countries, and so we have to be very careful when we share data, particularly for the first time,” Whitmeyer added.
Whitmeyer declined to say what might make an SLS countdown any different from that of a space shuttle, which was carried out in the open, using virtually identical engines, the same propellants and similar tanks.
In any case, NASA provided an approved countdown overview with general guidance on major events.
The initial hours of the countdown called for battery charging, engine preps, ground power system configuration, sound suppression water system loading and other tasks familiar from shuttle operations.
The Orion crew capsule will be powered up, followed by the first and second stages. Then, during a 90-minute hold starting at 6 a.m. Sunday, NASA’s mission management team and Blackwell-Thompson will meet to assess the weather and readiness to press on with propellant loading.
If all goes well, the countdown will resume around 7:30 a.m. with a “go” for fueling. Just after 8 a.m., 196,000 gallons of liquid oxygen, at minus 297 degrees Fahrenheit, will begin flowing into the core stage, followed by 537,000 gallons of liquid hydrogen at minus 423 degrees.
About two hours later, the team will begin filling the SLS rocket’s Interim Cryogenic Propulsion Stage, or ICPS, with another 22,000 gallons of cryogenic oxygen and hydrogen fuel. The ICPS is a modified upper stage originally developed for United Launch Alliance’s Delta 4 Heavy rocket.
As the countdown continues, engineers will review data, carry out leak tests and other checks to make sure they can control the flow of propellants as required.
The SLS terminal countdown begins after a final 30-minute hold ends at the launch-minus 10-minute mark. In an actual launch, the countdown would proceed through main engine start, booster ignition and liftoff, but for the dress rehearsal, two test runs are planned.
In the first run, the countdown will proceed from the 10-minute mark to the 33-second mark when a cutoff command will be issued by the ground launch sequencer computer. The countdown then will be recycled back to L-minus 10 minutes and after a one-hour hold, it will resume, ticking down to just inside 10 seconds before another cutoff command.
At that point, engineers will begin “safing” the rocket, draining the propellant tanks and beginning work to ready the SLS and its mobile launch stand for rollback to the Vehicle Assembly Building.
Engineers then will address any problems that might have developed or been identified during the countdown test.
NASA has not yet set a target launch date. While a May launch is possible in theory, it appears more likely launch will slip into the June timeframe. But that will depend on the results of the countdown test.
“I could kind of give you some guesses right now, but I don’t think it really makes much sense,” Whitmeyer said. “We’re literally going to have a wet dress rehearsal on Sunday. We’ll come back and talk to you Monday, and I think we’ll be in a much better position to give you some good information about what the path looks (like) at that point.”
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