ULA investigates explosion, delays Vulcan rocket debut launch until end of this year
United Launch Alliance's upcoming Vulcan Centaur rocket will have to wait just a bit longer for its debut flight after teams concluded an investigation into an explosion that occurred during testing earlier this year.
Company CEO Tory Bruno told reporters Thursday the Vulcan rocket, which will replace the current Atlas V and Delta IV Heavy fleet, won't be able to launch its debut mission with a lunar lander from Cape Canaveral Space Force Station until the fourth quarter of this year. The delay was triggered by the explosion of a Centaur upper stage during testing in March.
While testing the rocket's second stage at NASA's Marshall Space Flight Center in Alabama, a small hydrogen leak cropped up, which led to a crack forming in Centaur's 18-foot diameter tank. That, coupled with a rise in pressure, resulted in an explosion that teams have been investigating.
Since then, engineers have come up with a solution to strengthen the tank and continue the testing campaign. The second stage was on its 15th test in March, or about a quarter of the way through simulations designed to stress the hardware through different mission profiles to Earth orbit and beyond.
What caused the problem with Centaur V?
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During the investigation, ULA engineers determined the cause of the leak was due in part to the thinness of Centaur V's stainless steel sheets.
"This very large, 40-foot long, 18-foot diameter tank is built from very, very thin stainless steel sheets, thinner than a dime," Bruno said. "In fact, the thickest part of that structure is probably similar to really high-quality cardstock."
Particularly, the issue occurred in a domed portion of the tank where the sheets are cut, curved, and welded together.
A combination of lower-than-expected strength of the new laser welding process and a particular stress point where sheets of different thicknesses met up led to the hydrogen leak and change in pressure.
Bruno stressed to reporters that the issues were something that likely wouldn't have happened outside of the testing campaign. When it did happen, ULA was about halfway through the 15th consecutive test on the same Centaur V test article – far more stress than any other Centaur V would ever be expected to endure.
"The two things together – higher load and somewhat lower strength of the welds – are what caused the crack to begin," Bruno said.
How is the problem being fixed?
ULA decided that while unlikely to cause problems with a one-time-use, flight-ready version of a Centaur V stage, it was a significant enough problem with a simple fix.
"The corrective action is a pretty low-tech thing," Bruno said. "It's as simple as we wished (the stainless steel) was just a little bit thicker at the top of the dome, and the corrective action is to make it a little bit thicker."
Teams will strengthen the domes by adding another layer of super thin steel. It adds about 300 pounds of weight, which Bruno said it negligible since the upper stage can deliver 60,000 pounds of hardware to low-Earth orbit.
In line with that decision, ULA chose to remove the Centaur V upper stage of the first flight-ready Vulcan rocket, which completed a successful "flight readiness firing" of its main engines at the Cape's Launch Complex 41 last month.
"It is quite possible that the (Centaur V) vehicle that we had on top of the booster when we did the flight readiness firing could fly successfully because it doesn't have all of that long exposures and multiple cycles," Bruno said.
"Quite possible really isn't good enough. Other people might have thought seriously about flying that vehicle in the condition it was, but we're not other people." Bruno said. "We're going to take the time and apply the corrective action so that we know this won't be an issue in that flight."
It's since been shipped back to ULA's production facility in Decatur, Alabama, while the first-stage booster has been returned to storage at the Cape.
All told, Bruno expects that qualification testing of a Centaur V test article with the fixes in place should be wrapped up six to eight weeks ahead of Vulcan's first flight before the end of this year.
"By the way, we never stopped the (production) line on Centaur through all of this," Bruno said. "There's another three behind that in the factory moving along at pace right now."
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Contact Jamie Groh at JGroh@floridatoday.com and follow her on Twitter at @AlteredJamie.