United Launch Alliance has closed its Delta rocket assembly line in Alabama after the 389th and last Delta rocket rolled out of the factory for the journey to its launch base in Florida, clearing real estate in ULA’s sprawling manufacturing center for the next-generation Vulcan launch vehicle.

The major pieces of the final Delta 4-Heavy rocket arrived at Cape Canaveral in May after a journey from ULA’s factory in Decatur, Alabama, on the company’s rocket transport vessel, dubbed the R/S RocketShip. They will begin final launch preparations in Florida for the last flight of the Delta rocket program, an historic milestone mission scheduled for early 2024.

The three first stage boosters and upper stage for the final Delta 4-Heavy rocket were trucked from Port Canaveral through the gate to the military-run spaceport in mid-May, while ULA engineers were preparing the second-to-last Delta 4-Heavy rocket for liftoff with a classified satellite for the National Reconnaissance Office, the U.S. government’s spy satellite agency.

That mission is slated to lift off early Wednesday from Pad 37B at Cape Canaveral on a mission codenamed NROL-68. The final Delta 4 launch in 2024 goes by the designation NROL-70.

ULA, a 50-50 joint venture between Boeing and Lockheed Martin, is retiring the Delta family of rockets in favor of the new-generation Vulcan launch vehicle, which is scheduled to make its first test flight later this year from Cape Canaveral. The Vulcan rocket will also replace ULA’s Atlas 5 launcher, which will fly 19 more times before retirement later in the 2020s.

Gary Wentz, ULA’s vice president for government and commercial programs, said factory workers in Alabama wrapped up assembly and testing of the final Delta 4-Heavy rocket earlier this year, soon before the company shipped the rocket hardware down the Tennessee, Ohio, and Mississippi Rivers into the Gulf of Mexico for the trip to Florida’s Space Coast.

“We’ve completed all the belts for hardware there in Decatur,” Wentz said in an interview Tuesday with Spaceflight Now. “We’re in the process of transitioning the factory to support higher rate production of the Vulcan hardware.

“That was a huge accomplishment for the Decatur team to be able to complete the last Delta 4, get it shipped down to the Cape, and now it’s in the hands of our launch ops team,” Wentz said. “As soon as we launch L-68, we’ve already started doing some of the horizontal processing of L-70, and our plans to prep it to support the customer’s launch next year.”

The last two Delta rocket missions will use the most capable version of the Delta, the Delta 4-Heavy, made by combining three large booster cores together to create a triple-body rocket. The three Aerojet Rocketdyne RS-68A engines, burning super-cold hydrogen fuel, will generate 2.1 million pounds of thrust at full power.

The launch set for Wednesday will be the 15th flight of a Delta 4-Heavy rocket, which debuted in 2004, and the 44th flight of the Delta 4 family since 2002. The primary customer for ULA’s Delta 4 rocket has been the U.S. military and the NRO.

The Delta 4 rocket program followed the Delta 2 rocket, a workhorse for NASA, the U.S. military, and commercial satellite operators in the 1990s and 2000s.

The launch of the NRO’s NROL-68 mission on the second-to-last Delta rocket was delayed from April to allow time for ULA engineers to swap out a leaky hydrogen valve on the Delta 4’s upper stage. The troubleshooting required the NRO’s spy satellite to be removed from the rocket, resulting in about a two-month delay.

Wentz said engineers found a “really small, hard to see” piece of particulate on the sealing surface of the valve, which is used to pressurize the upper stage fuel tank. The debris caused the valve to leak.

ULA has procured spare parts to be able to respond to replace faulty components on the final two Delta 4 rockets, even though the launch vehicle is no longer in production. Parts obsolescence is a common concern for aerospace vehicles nearing retirement.

“We were able to secure, in our final order of parts, some additional critical spares, and we’ve kept those on hand,” Wentz said. “In a case like this with the valves, we were able to take the valves back and were able to rework those and put them back in stock as a spare part. So that’s been our strategy all along is identifying the critical components, particularly the ones that aren’t used in either Atlas or Vulcan.”

The Atlas 5 and Delta 4 rockets currently flown by ULA show little resemblance to their forebears, but the names are steeped in history. Rockets bearing the Delta name began launching in 1960, and 387 Delta rockets have flown to date, most recently a Delta 4-Heavy launch from Vandenberg Space Force Base in California in September. That was the final Delta launch from the West Coast spaceport.

The Delta 4 rocket was developed by Boeing in the late 1990s and early 2000s, at the same time Lockheed Martin was bringing its Atlas 5 rocket into service. Boeing and Lockheed Martin merged their rocket programs in 2006 to form ULA, which was the sole provider of launch services for the military’s most expensive national security satellites until SpaceX broke into the market.

The military certified SpaceX’s Falcon 9 rocket for national security missions in 2015, and SpaceX and ULA split multibillion-dollar contracts in 2020 for a series of military satellite launches. While SpaceX will employ its Falcon 9 and Falcon Heavy rockets already in service, ULA will use its new Vulcan Centaur rocket to carry out its military launch commitments.

ULA says the Vulcan rocket is less expensive than the Atlas and Delta rockets, and it uses engines built in the United States, replacing the Russian engines that power the Atlas 5 rocket. The Delta 4 rocket also uses all U.S.-made engines, but it is more expensive than the Atlas 5. In its most powerful configuration, the Vulcan Centaur will outlift the Delta 4-Heavy, without needing to use three first stage boosters to do the job.

The schedule for the first flight of ULA’s new Vulcan rocket is uncertain as engineers wrap up their investigation into an explosive incident on the rocket’s Centaur upper stage during a structural test earlier this year in Alabama. Earlier this year, ULA hoped to launch the first Vulcan rocket in May, with an eye toward certifying the new vehicle for military satellite launches by the end of 2023.

It now appears doubtful the Vulcan rocket will be certified for national security space missions by the end of the year. The Space Force’s certification requires two successful Vulcan test launches.

But ULA has more than 70 Vulcan rocket missions in its backlog, primarily for the Pentagon and for Amazon’s Kuiper broadband network, a potential future rival to SpaceX’s Starlink internet constellation. In response to the deep backlog, ULA is expanding the footprint of its 1.6 million-square-foot factory in Alabama, which was originally built by Boeing for the Delta 4 program, before ULA shifted Atlas rocket production there from Colorado. The factory is now transitioning to focus fully on Vulcan.

“For a while, obviously, we had Delta 4 and Atlas flowing through that factory,” Wentz said. “The most significant and obvious parts are in the final assembly area for Delta 4, which coincidentally, we’ve been processing Vulcans through. So Vulcan final assembly will flow right in there, and we’ll be able to increase our rate in that area.”

The last remnant of the Delta 4 program at the Alabama factory is the third and final upper stage ULA is building for NASA’s Space Launch System rocket to carry astronauts back to the moon. That upper stage is derived from the Delta 4-Heavy design, and will power the Artemis 3 mission into space in a few years. Then NASA will switch to a more powerful upper stage for future SLS moon rockets.

Despite a strong backlog of missions, delays have kept ULA’s launch pads silent for the first half of 2023. The Delta 4-Heavy launch Wednesday morning from Cape Canaveral will be the company’s first mission of the year, while rival SpaceX has launched 41 flights in 2023 with its Falcon rocket family.

The two-month delay in the Delta 4 launch was one reason for the slow start to the year. The first flight of astronauts on Boeing’s Starliner crew capsule, which will launch on ULA’s Atlas 5 rocket, has also been delayed from April until late this year, at the earliest, to allow Boeing to resolve several technical issues with the spacecraft.

And the Vulcan rocket’s first launch is also in a state of schedule uncertainty as ULA determines that it needs to do to overcome the Centaur fiery anomaly in March, which destroyed the upper stage’s structural test article after an unexpected leak of flammable hydrogen fuel.

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Follow Stephen Clark on Twitter: @StephenClark1.

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• Source: https://spaceflightnow.com/2023/06/20/ulas-delta-rocket-assembly-line-falls-silent/