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.”

Vega C fails on second launch

The second flight of Arianespace’s Vega C failed to reach orbit Dec. 20 after its second stage malfunctioned, destroying two Pléiades Neo imaging satellites. The Vega C rocket lifted off at 8:47 p.m. Eastern from Kourou, French Guiana, carrying the Pléiades Neo 5 and 6 imaging satellites for Airbus. The liftoff took place on schedule and the initial phases of flight appeared to go as planned. However, on-screen telemetry showed that the rocket was deviating from its planned trajectory within four minutes of liftoff, during the burn of the rocket’s Zefiro-40 second stage. Arianespace said in a later statement that the stage malfunctioned 2 minutes and 27 second after liftoff, seconds after ignition of the stage. The flight continued for several minutes, including separation of the second stage and ignition of the third stage, as well as payload fairing separation, even as the stage reached an apogee of 110 kilometers and started to descend. “After the liftoff and the nominal ignition of the P120C, which is the first stage of the Vega, an underpressure has been observed on the Zefiro-40, which is the second stage of the Vega,” Stéphane Israël, chief executive of Arianespace, said on the launch webcast a few minutes later. “After this underpressure, we have observed the deviation of the trajectory and very strong anomalies, so unfortunately we can say that the mission is lost.” He did not provide additional details about the problem. “We will now have to work with all of our partners to better understand why the Zefiro-40 has not worked properly tonight, triggering the failure of the mission,” he said, apologizing to Airbus Defence and Space, the customer for the launch. Arianespace then terminated the launch webcast.

The launch was the second for the Vega C after a successful inaugural launch of the rocket July 13 carrying a set of institutional payloads. This was the first commercial launch of the Vega C. The launch was postponed from late November because of a problem with the pyrotechnics in the payload fairing separation system.

The Vega C is an upgraded version of the Vega rocket with increased payload performance. Among the changes is the introduction of the Zefiro-40 solid-fuel second stage, which replaced the less powerful Zefiro-23 used on the Vega. Avio is the prime contractor for the Vega C.

The Vega suffered two failures in three launches in 2019 and 2020. A 2019 Vega launch of the UAE’s Falcon Eye 1 imaging satellite failed because of a problem with the thermal protection system on part of the rocket’s second stage. A Vega launch in November 2020 failed when its Avum upper stage tumbled immediately after ignition because of what Arianespace later determined to be improperly connected cables.

The failure of the Vega C deals another blow to European efforts to maintain autonomy in launch. The Vega C was one of the cornerstones of that strategy, along with the still-in-development Ariane 6, with the European Union awarding Arianespace a contract Nov. 29 for five Vega C launches of Sentinel satellites. That contract brought the Vega C backlog to 13 launches, along with two remaining launches of the original Vega.

The launch failure also hurts Airbus, which had counted on the launch to add to its constellation of high-resolution imaging satellites. Pléiades Neo 5 and 6 were similar to the previously launched Pléiades Neo 3 and 4 but included laser links for faster transmission of imagery. An unspecified “equipment issue” with Pléiades Neo 3 led Airbus to file a partial insurance claim after its April 2021 launch. Airbus said the launch of Pléiades Neo 5 and 6 would allow it to work around the issues with Pléiades Neo 3 and meet all its customer commitments.