WASHINGTON — United Launch Alliance is now planning a first launch of its Vulcan Centaur rocket in the fourth quarter after the company completes modifications to and testing of the upper stage.

In a call with reporters July 13, Tory Bruno, president and chief executive of ULA, said the changes to the Centaur upper stage stemmed from an investigation into a test mishap in March, where hydrogen leaked from a Centaur test article and ignited, damaging both the stage and the test rig. The company announced June 24 that it would delay the launch to make “minor reinforcements” to the Centaur.

The mishap took place in the 15th of a series of tests of what ULA calls the Centaur 5, a larger version of the Centaur 3 upper stage used on the Atlas launch vehicle. The hydrogen leak developed halfway through the test and grew over the course of four and a half minutes. The hydrogen collected in an enclosed space above the stage and found an ignition source before the test could be stopped.

The investigation found that the leak originated in the forward dome of the tank, which is made of very thin stainless steel, near a door at the top of the structure. A very detailed finite element model of that part of the tank revealed a “stress riser,” or intensification of loads, because of the complicated geometry around that part of the dome. That had been missed in earlier, coarser analysis of the tank.

In addition, the Centaur 5 uses a new laser welding technology, rather than arcwelding on earlier Centaurs, for seams in the tank dome. The strength of the laser welds is less than what had been expected from earlier tests.

“The two things together — higher loads, somewhat lower strength in the welds — are what caused the crack to begin,” Bruno said. The amount of testing of that specific Centaur may have also contributed to the crack, he added.

To correct the problem, ULA will add a layer of stainless steel around the door on the top of the dome and strips along the welds extending about 60 centimeters from it. “It’s not a very sophisticated or high-tech or high-risk action,” he said. “We simply need it to be just a little bit thicker.”

The additional material will add about 135 kilograms of mass to the Centaur initially, but that increase will be cut in half on later ones. That will reduce the payload performance by the same amount, but Bruno said none of the upcoming Vulcan missions would be affected by that change given the requirements of those missions and vehicle margins.

ULA is already incorporating those changes in a Centaur that was originally going to be used on the third Vulcan mission. That will instead be used for the inaugural Vulcan launch, Cert-1, while the Centaur that had already been shipped to Cape Canaveral and tested for that launch will be returned to ULA’s Decatur, Alabama, factory for modifications.

The Centaur intended for the second launch will be converted into a test article to carry out the remaining tests needed to qualify it for Cert-1, while another Centaur will carry out a fuller range of tests needed to qualify the vehicle for all potential missions.

“In terms of schedule, that means that we expect to fly in the fourth quarter of this year,” Bruno said. ULA is working with the primary customer of Cert-1, Astrobotic, to identify the windows in the fourth quarter that would be available for launching that company’s Peregrine lunar lander. ULA previously said only a few days were available each month for launching Peregrine.

Cert-2, which would carry Sierra Space’s Dream Chaser spaceplane on a mission to the International Space Station, would launch a few months after Cert-1 if that inaugural launch goes as planned.

The success of both missions would certify it for national security missions under its National Security Space Launch Phase 2 contract. The first such launch could take place as soon as the second quarter of 2024. Asked if that first national security launch would still be a mission designated USSF-106, carrying the Navigation Technology Satellite-3 spacecraft, Bruno said there had been no changes to mission assignments.

BE-4 test failure “really isn’t news”

Bruno also addressed in the call a recent failure of a BE-4 engine intended for the Cert-2 mission during acceptance testing at a Blue Origin facility in West Texas. CNBC reported July 11 that the engine exploded 10 seconds into a static-fire test in late June.

Both in comments on Twitter and in the call, Bruno played down the significance of the failure. The BE-4 engine design is already qualified, he said, and the failure took place during an acceptance test, or ATP, to confirm the workmanship of that specific engine.

“ATP failures across the rocket are not uncommon,” he said. That is particularly true at the beginning of an engine program where automated thresholds for parameters like temperature and pressure that would trigger a shutdown are still being tuned.

This particular engine, he said, did not pass its first acceptance test, and was back on the test stand after changes when it exploded. “In this case, it didn’t pass it in a dramatic fashion,” he said, likely because the test thresholds were set too high. “It wasn’t able to automatically shut the engine down before the engine had a burnthrough.”

“This doesn’t indict the qualification at all,” he said, noting that BE-4 engines have more than 26,000 seconds of cumulative runtime. “We’re very confident in the design and the workmanship of the assets that have passed acceptance. This is not unexpected.”

He added that the production rate of the BE-4 is high enough that the test failure will not delay upcoming Vulcan launches. Blue Origin and other suppliers, along with ULA itself, are working to increase production to meet a rate of 25 vehicles a year by mid-2025, an initiative he called “25 in ’25.”

“I’m flattered by the attention we have now that a routine acceptance test was colorfully discussed on social media, but it really isn’t news,” he concluded.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...