Nozzle erosion blamed for Vega C launch failure

Europe plans to return the Vega C rocket to flight by the end of the year after concluding an eroded nozzle component caused the failure of its previous launch last December. The European Space Agency announced March 3 it had completed an independent investigation into the failed Dec. 20 launch of the Vega C on the VV21 mission, which experienced a loss of thrust from its Zefiro-40 solid-fuel second stage. The failure resulted in the loss of two Pléiades Neo imaging satellites for Airbus Defence and Space. The investigation concluded that a component in the motor called a throat insert, made of carbon-carbon material designed to withstand high temperatures, suffered “thermo-mechanical over-erosion” during the launch. That insert regulates the flow of exhaust through the nozzle, and as it eroded the chamber pressure dropped, causing thrust to decrease. By 207 seconds after liftoff, or a little more than a minute after the second stage ignited, acceleration of the vehicle became “quasi-null” and put the rocket on a ballistic trajectory, said Pierre-Yves Tissier, chief technical officer of Arianespace and co-chair of the independent investigation, in a briefing about the report. A fault tree analysis led investigators to rule out other causes of the failure. The erosion of the insert, he said, was linked to higher porosity of the carbon-carbon material, confirmed in other testing of the material but not detected earlier. “The acceptance criteria established for this material were not able to detect such a weakness,” he said. Avio procured the throat insert from a Ukrainian company, Yuzhnoye. Giulio Ranzo, chief executive of Avio, said the company went with Yuzhnoye during the design phase of Vega C in 2015 to 2017 after concluding that other European suppliers could not provide the material in required quantities “on the schedule compatible with the development program” of the rocket.

A Vega C lifted off Dec. 20, only to suffer a failure with its second stage about two and half minutes later. Credit: Arianespace webcast

The flaw, he said, was not directly linked to disruptions caused by Russia’s invasion of Ukraine a year ago because they were manufactured before the war. He suggested that lockdowns linked to the pandemic might have been a factor, though.

The erosion was not seen in the Zefiro-40 motor used on the inaugural Vega C launch last July, or in two earlier qualification firings. “The acceptance criteria were not the right ones,” said Giovanni Colangelo, ESA inspector general and the other co-chair of the investigation. In those earlier firings of the motor, the materials exceeded requirements.

“The one for VV21 was exactly in line with specification, so were not as good as the previous one, so that is the reason why there was a failure,” he said.

The investigation concluded that the Yuzhnoye material can no longer be used on the Zefiro-40 motor. It will be replaced with a carbon-carbon throat insert from ArianeGroup, which already produces similar components for motors used on other Vega stages.

Ranzo said that Avio had already purchased several such throat inserts from ArianeGroup last year as a hedge against potential supply chain disruptions caused by the war in Ukraine. “We have secured the next several flights by having procured this strategic stock several months ago,” he said, adding that Avio was looking at longer-term options for the component from ArianeGroup or others.

ESA said that additional analysis and testing of the Zefiro-40 is planned for the coming months, allowing a return to flight of the Vega C tentatively scheduled for late this year. The payload for that mission will be the Sentinel-1C radar imaging satellite.

“Sentinel-1C is indeed a very precious payload,” acknowledged Josef Aschbacher, ESA director general. That satellite will replace Sentinel-1B, whose radar payload failed last year. “We are putting measures in place which are very robust and we have confidence this will all succeed.”

The Zefiro-40 problem does not affect the original Vega rocket, which uses a different motor in its second stage that does not have a throat insert from Yuzhnoye. ESA plans a launch of that rocket, one of two remaining of that version, before the end of the summer.

Stéphane Israël, chief executive of Arianespace, said that Vega launch will carry two primary payloads and several smallsat rideshares. “We will give more information on these passengers in a few weeks,” he said.

In addition to fixing the problem with the Zefiro-40, there will be a broader assessment of the vehicle’s supply chain to look for other potential quality issues. “We will try to improve substantially the monitoring of the performance and supply chain management activities,” Ranzo said.

Both ESA and Arianespace leadership expressed some frustration with the Vega program, noting that the VV21 failure was the third in eight flights of the Vega and Vega C. They are also dealing with delays with the development of the Ariane 6 that have pushed its first launch back to at least late 2023, as well as the loss of the Soyuz rocket after Russia’s invasion of Ukraine.

“We are in a crisis,” Aschbacher said. “For me, this is really a moment where we need to reflect deeply how to regain independent access to space for Europe.”