PARIS — Russia’s Proton rocket is scheduled to return to commercial service in late August or early September following the conclusion of a government investigation into a May failure that destroyed a large Mexican telecommunications satellite and shook once again the market’s confidence in the rocket.

The customer on the next flight will be mobile satellite services provider Inmarsat of London, whose third and final Global Xpress Ka-band mobile broadband satellite has been awaiting a launch for months. The delay forced Inmarsat to delay the service’s full commercial start as many customers were awaiting an unbroken global service area – which requires three satellites – before committing to it.

International Launch Services (ILS) of Reston, Virginia, which commercializes the Proton vehicle, on Aug. 3 said unsurprisingly that its own failure review agreed with the Russian government inquiry.

“[T]he most probable cause of the failure was a result of a higher than expected vibration environment most likely caused by the combination of a marginal mechanical joint used to mount the Stage III steering engine turbo pump and a steering engine turbopump rotor material that had marginal strength under maximum operating environments,” ILS said in a statement.

“This led to a premature shutdown of the turbopump and loss of [third stage] control authority and subsequently to the failure of the mission during 3rd stage operation approximately 497 seconds after liftoff.”

In response to a SpaceNews request for a clearer summary of the failure review board’s conclusions, ILS said:

“A bolted connection attaching the steering engine turbopump to the main engine frame lacked the necessary holding strength – due to a sub-optimal design – to properly support the turbopump during operation.

“The lack of adequate holding strength resulted in the turbopump experiencing a high and detrimental vibration environment that led to breakage of a fuel line supplying fuel to the turbopump, and ultimately the premature shutdown of the turbopump.”

Proton is one of the world’s most heavily used vehicles, for both commercial launches and for the Russian federal space program. The ILS statement left unclear why a design flaw in the structure keeping the turbopump in place had not been uncovered on previous missions.

ILS did not immediately respond to SpaceNews questions on this point. But industry officials said the issue was heavily discussed during the Russian government commission’s review and by ILS’s secondary investigation.

These officials said the design flaw – similar to a flaw found in 2014 in the Russian Soyuz rocket’s Fregat upper stage – was in that it permitted too much leeway in the way the hardware was assembled.

This resulted in “a potentially marginal installation that was still within design documentation requirements,” one official said. “Most likely, a small, undetectable change has occurred in materials and/or assembly processes within the last couple of years that has taken this sub-optimal design and made it even more marginal against normal operating environments.”

This official said the May launch, of the large Centenario mobile communications satellite for the Mexican government, caused no unusual conditions that could by themselves accounted for the failure.

“No special stresses over and above operating environments was required to trigger the sequence of events leading to the failure,” the official said. “This is strictly a design issue that has been addressed by a design change that removes the potential for variability in the installation process and creates a more robust and balanced support system for the turbopump.”

The failure resulted in an insurance claim of $300 million for the loss of the satellite and $90 million for the launch vehicle. While this is likely to weigh heavily on insurance underwriters’ results for 2015, industry officials said they expect no sudden spike in insurance premiums given the amount of money now available to underwrite space launches.

Commercial Proton customers, starting with Inmarsat, that are awaiting launch have already secured their insurance policies. The latest Proton failure – the fourth in four years – is unlikely to be viewed as a major new event that would allow underwriters to push up rates for these near-term Proton customers, officials said.

Satellite fleet operator Eutelsat of Paris told investors July 30 that the new ILS schedule calls for launching two telecommunications satellites with Eutelsat payloads on two Proton vehicles in November.

Proton is scheduled to launch a Euro-Russian Mars mission in early 2016, a launch that European Space Agency officials have said has a priority over commercial customers in the Proton launch queue.

Peter B. de Selding was the Paris bureau chief for SpaceNews.