Thursday's back-to-back last-second scrubs of an Ariane 5 rocket launch were caused by the vehicle's first stage pressurization system, an issue similar to the problem that triggered an unusual Arianespace quality review earlier this year, the company's top executive said Friday.

the Ariane 5 rocket during Thursday's final countdown. Credit: Arianespace

"Of course, when we have a delay it's a bit frustrating," said Jean-Yves Le Gall, Arianespace's chairman and CEO. "Everything is done to guarantee our customers the very best quality."

Thursday's countdown was targeting liftoff at 2141 GMT (5:41 p.m. EDT) before the clocks were halted at Minus-17 seconds. Engineers recycled the countdown clock and tried again an hour later after extending the launch window, a rare decision to give teams more time to overcome the issue.

But the second attempt fell victim to another abort at Minus-16 seconds, this time ordered by Le Gall himself.

"We had a very quick review because I wanted to have the full confidence that we were proceeding well," Le Gall told Spaceflight Now from Kourou. "So I stopped the countdown myself in the final seconds."

Le Gall said there was a suspicious parameter in the helium subsystem responsible for keeping the first stage propellant tanks pressurized. The Ariane 5's first stage engine burns liquid hydrogen and liquid oxygen propellants.

"We have a value in the pressurization system that was just at the limit of what we could accept," Le Gall said in an interview Friday. "We adjusted the limit to demonstrate that this was acceptable. This has been done and now we are working to resume the preparations for the countdown and launch tomorrow night."

The Ariane 5 rocket will hoist an Arab communications satellite and a South Korean spacecraft to test communications equipment, observe weather and study the oceans. The next try to blast off is slated for Saturday night.

Arianespace already had a razor's focus on the first stage pressurization system, which was the culprit in scrubs of the last two Ariane launches in December and April. A pressurization issue was also responsible for the scrub of the Ariane 5's first launch attempt Wednesday.

"It's the same system, but it is not, in fact, an anomaly this time," Le Gall said. "It's a consequence of the fact that the part that is used now has been manufactured with a lot of care. The values of the pressures were right at the limit, which was not the case before."

Arianespace ordered a review of its quality control procedures in April after a helium pressurization regulator caused the abort of the countdown of a previous Ariane 5 rocket launch. The company announced parallel inquiries focusing on the troublesome helium subsystem and overall quality control practices to ensure other parts of the vehicle are in good shape.

Engineers finished the technical investigation, replaced the faulty regulator with a new unit, and approved the launch of an Ariane 5 booster in May. That flight blasted off on time and successfully orbited two communications satellites.

But the top-level quality control review still continues, and the board is scheduled to present the results of the inquiry July 5, according to Le Gall.