October 22, 2021

— A massive rocket topped with a spacecraft bound for the moon is now standing in the Vehicle Assembly Building (VAB) at NASA's Kennedy Space Center in Florida for the first time in 49 years.

The space agency completed stacking the components of its first Space Launch System (SLS) rocket just after midnight (EDT) on Thursday (Oct. 21). The integration of the core stage, twin solid rocket boosters, a propulsion stage and the Orion spacecraft marked a major milestone towards the launch of the uncrewed Artemis I mission to loop around moon as soon as February 2022.

"It's taller than the Statue of Liberty," Tom Whitmeyer, NASA's deputy associate administrator for exploration systems development, said in a call with reporters on Friday. "I like to think of it as the Statue of Liberty because it is a very engineering-complicated piece of equipment and it is very inclusive and represents everybody."

"The rocket itself was built using people from every state in the United States of America and it also involved a partnership with the Europeans. We're very proud of that," said Whitmeyer. "Like the Statue of Liberty, I think it really represents what this country is about."

Towering 322 feet tall (98 m) — just 41 feet shorter than NASA's last moon rocket, the Apollo-era Saturn V — the SLS will now undergo a series of verification tests in the VAB's High Bay 3 ahead of being rolled out to Complex 39B for what NASA calls a "wet" dress rehearsal. This final test will run the rocket and its launch team through the loading of propellant into the SLS and conducting a countdown to just before ignition at about T-10 seconds.

Assuming a successful rehearsal, the stack will then be rolled back into the VAB for its last checks and a target date for launch will be set.

Artemis I Orion lift and mate. Click to enlarge and view video in pop-up window. (NASA)

Piece by piece

Stacking operations began almost a year ago in November 2020 with the two solid rocket boosters that will provide most of the thrust at launch.

Ten solid rocket motor segments — five for each booster — were brought by train from Northrop Grumman's Utah manufacturing facility in June 2020. Beginning in Kennedy's rotation, processing and surge facility, the aft motor segments were put atop aft skirts and exit cones before being moved to the VAB to be mounted on the Artemis mobile launcher.

Assembly of the 177-foot-tall (54 m) boosters was completed in March, clearing a way for the SLS core stage to come next.

Following a "Green Run" series of tests that concluded in a full-length static fire of the SLS's four RS-25 rocket engines, the core stage was shipped by barge to Kennedy in April. The 212-foot-tall by 27.6-foot-diameter (65 by 8.4 m) stage — the largest rocket stage ever assembled — was hoisted by crane up and over the boosters and then lowered down between them in June.

The backbone of the SLS, the Boeing-built core stage supports the weight of the upper stage and crew capsule, as well as carrying the thrust of its four engines and two five-segment solid rocket boosters, the latter attached at the engine and intertank sections.

The launch vehicle stage adapter, a cone-shaped piece connecting the core stage and interim cryogenic propulsion stage (ICPS), was stacked soon after the core stage was put into place. The ICPS, which will provide the thrust to send the Orion spacecraft out to the moon, was then mounted atop the adapter in July.

A month later, a structural test article for the Orion stage adapter was added to the rocket to allow umbilical tests to proceed while the Orion spacecraft and its launch abort system continued being prepared elsewhere at Kennedy.

"In August, we started powering up the rocket, first one element at a time and then as an integrated launch vehicle," said Mike Bolger, NASA's Exploration Ground Systems program manager at Kennedy Space Center. "Then in September, we performed an umbilical retract test where we ran through a launch countdown and proved our capability to retract all of the umbilicals at T-minus zero, ensuring that we will have the appropriate clearance on launch day."

"Right after that, we attached shakers and sensors and conducted integrated modal tests and shared that data with the program. Both of those tests were really successful," said Bolger.

The Orion stage adapter mass simulator was later removed and replaced with the actual stage adapter on Oct. 13. The Orion spacecraft was then lifted and mated with the SLS to complete the full stack on Thursday.

"Looking ahead, we're in the process of reconnecting all of the umbilicals and getting ready for the integrated verification tests that ensure the flight hardware and the ground interfaces are working," Bolger said. "We'll start with Orion power up this weekend and we'll follow up during the month of November, powering up the core stage and the second stage again, and then the entire integrated launch vehicle."

"As we get into the first week of December, we'll conduct a communications end-to-end test where we will verify all the communication systems are operating on the ground and in the flight phase. Then teams in Firing Room One in the LCC [Launch Control Center] will perform a countdown sequence test before the fully stacked SLS is rolled out to Pad 39B for the wet dress rehearsal," he said.

Back to the moon

The last time that a moon-bound rocket stood stacked and ready to roll out to the launch pad from the VAB was on Aug. 28, 1972, when the Saturn V assembled for NASA's Apollo 17 mission departed the voluminous facility. Stacking of the 363-foot-tall rocket was completed four days earlier.

Apollo 17 lifted off on NASA's sixth moon landing mission on Dec. 7, 1972.

The launch date for the Artemis I mission will be determined by the results of the wet dress rehearsal, but the first available period opens on Feb. 12, 2022 and extends to Feb. 27. A launch on Feb. 12 would occur within a 21-minute window starting at 5:56 p.m. EST (2256 GMT).

"We have a roughly 15-day launch period in February," said Mike Sarafin, Artemis I mission manager at NASA Headquarters. "Half of those [days] are what we call 'long class missions,' the duration is about six weeks, where we'll spend a little bit of extra time in the distant retrograde orbit to set up landing lighting conditions in the splashdown zone. Then, the second half of that window is what we call 'short class missions,' which is about four weeks in duration."

Additional launch periods are from March 12 to March 27 and April 8 to April 23.

During the flight, the SLS will shed parts in the reverse order they were stacked. Two minutes into flight, the two solid rocket boosters will separate and fall back to the Atlantic Ocean. The core stage will be jettisoned about six minutes later after reaching more than 100 miles in altitude (160 km).

Once in orbit at about an hour and a half into the Artemis I mission, the ICPS and its single RL-10 engine will provide the nearly 25,000 pounds of thrust to send the Orion spacecraft on a precise trajectory to the moon. After separating, the ICPS will deploy 13 cubesats mounted in the stage adapter.

The Orion capsule will spend about three weeks looping around the moon once at a distance farther into space than any previous crew-rated spacecraft has traveled before. Powered by a service module provided by the European Space Agency (ESA), the Lockheed Martin-built Orion will then return to Earth for a splashdown and recovery in the Pacific Ocean.

Artemis I will mark the first integrated test of the Orion spacecraft and Space Launch System ahead of crewed flights to the moon. Under the Artemis program, NASA aims to land the first woman and first person of color on the lunar surface and establish a sustainable presence on the moon.