United Launch Alliance has identified the root cause of a failure that destroyed the upper stage of its Vulcan rocket in late March. According to the company's chief executive, Tory Bruno, the Centaur V upper stage failed due to higher-than-anticipated stress near the top of the liquid hydrogen propellant tank and slightly weaker welding.

Bruno outlined the nature of the failure and steps the company is taking to remediate it during a teleconference with space reporters on Thursday. He said United Launch Alliance is working toward flying the heavy lift Vulcan rocket on its debut mission during the fourth quarter of this year.

Tank failure

The Centaur V upper stage was destroyed during pressure testing at Marshall Space Flight Center in Alabama on March 29. Bruno said this was the 15th test in a series of 45 tests to qualify the Centaur stage for all potential mission profiles. However, about halfway through the test the hydrogen tank started leaking, and over the course of four and a half minutes the leak expanded.

During this time hydrogen leaked into a confined area of the test stand, a relatively enclosed space. After it reached a flammable concentration and found an ignition source, the hydrogen caught fire. This seriously damaged the test stand as well as the hydrogen tank and liquid oxygen tank—loaded with liquid nitrogen for this test.

Pretty quickly, the failure investigation team identified that the leak occurred in the forward dome, or top section, of the hydrogen tank. Analyzing pieces of the tank using "fractography," Bruno said investigators were able to pinpoint where the crack in the stainless-steel tank originated, near the top of the dome.

To understand the nature of the failure, Bruno said the company ran a high-fidelity model of the loads and stresses on the hydrogen dome in this location and found there were unexpectedly higher loads there. Additionally, the team analyzed the strength of the welds nearby and found they were not as high as previously assessed.

"The two things together, higher loads and somewhat lower strength in the weld, are what caused the crack to begin," he said. "The other thing I would ask you to appreciate, since we're being completely transparent here, is how we were already 15 tests in, which is considerably more testing and exposure to many more pressure cycles and lots and lots of more time with the structure sitting under pressure than would ever happen in any single flight."

Corrective actions

To address the failure, Bruno said the fix is relatively straightforward. The area near the failure will be strengthened with an additional ring of stainless steel and strips of metal. These corrective actions will add about 140 kg to the mass of the upper stage, which he characterized as a negligible loss of payload capacity on a vehicle projected to lift 27 metric tons to low-Earth orbit.

The company is preparing a new dome with these fixes, and this Centaur V tank will undergo a handful of additional pressure tests to verify its predicted behavior. In addition, the Centaur V upper stage that was to be used on the debut flight of the Vulcan rocket—known as the Cert-1 mission, as it is the first of two certification flights for the US Space Force—has been shipped from the launch site back to the company's factory in Decatur, Alabama. It will undergo similar modifications.

Bruno said performing the final qualification tests for the Centaur V anomaly and modifying the flight version of the tank are the final two steps needed before Vulcan can launch. He said he was pleased with the performance of the rocket's first stage during a recent "flight readiness firing" test, when the rocket's BE-4 engines ignited for a few seconds.

"We were really, really pleased with the flight readiness firing," Bruno said. "Everything was phenomenal. And there were no, I'll say, no actions required on any of the hardware, any of the procedures."

Schedule

Bruno said he anticipates a period of four to six weeks between the completion of the qualification tests and the launch of Vulcan. This mission will carry a lunar lander for Astrobotic, as well as two test satellites for Amazon's Project Kuiper megaconstellation. In addition to the rocket's readiness, the Astrobotic mission launch date will also be determined by lighting conditions on the Moon for the lander, so there are only a few acceptable launch dates per month.

The second flight of Vulcan will launch the Dream Chaser spacecraft for Sierra Space. CNBC reported this week that one of the two BE-4 engines intended to fly on this Vulcan core exploded during acceptance testing in June at Blue Origin's facilities in West Texas. Bruno said it was fairly routine for rocket parts to fail this acceptance testing, especially in the early stages of development. He said Blue Origin understands the cause of the problem and has additional engines under construction.

Assuming this second certification mission occurs during the first quarter of 2024, United Launch Alliance and the US Space Force would need a few months to review data from these first two launches. After that, the Vulcan rocket would be certified for national security launches. Bruno said the company is targeting the second quarter of 2024 for this initial Space Force launch.

As is often the case in space missions, that timeline seems rather optimistic. But it does appear as though United Launch Alliance has a clear path toward finally launching its Vulcan rocket and getting the vehicle ready for a lengthy manifest that includes both missions for the military as well as dozens of flights for Amazon's Kuiper constellation.