The crash of a Proton-M at Baikonur, Kazakhstan, on 2 July was traced to incorrect installation of angular rate sensors, according to Russian space agency Roscosmos.
Investigators going over the wreckage immediately discovered that three of the rocket's six angular sensors, crucial for maintaining correct trajectory, were installed upside-down.
A 0.4s early lift-off was a contributing, but not deciding, factor, Roscosmos found, and was probably due to an early cable disconnect from the rocket's launch support tower.
According to Flightglobal's Ascend SpaceTrak database, the Proton failure on 2 July is the fifth in the last four years. Likewise, over the last 10 Proton flights there have been three failures. Of all heavy-lift rockets operating today, only Russia's Proton-M and Zenit have had any failures in their most recent 10 missions, with a partial failure on SpaceX's Falcon 9 that resulted in primary mission success but the loss of a secondary payload.
The recent Proton failures were largely traced to part defects or factory quality control problems, though all are due to problems with the upper stage. Following each accident, investigation boards are formed by both Russian manufacturer Khrunichev and US-based International Launch Services (ILS), which markets the Proton for commercial launches. Facing a reputation problem, ILS and Khrunichev have repeatedly reassessed manufacturing and launch programmes.
"There's no doubt that some of the failures in recent years that have occurred were quality-related," said ILS president Phil Slack in May, following Proton's return to flight after losing two of 11 payloads during 2012 launches. "Improving quality is without a doubt our number one focus area with ILS and [Proton builder] Khrunichev, mission success is what we value most."
At least three launches scheduled before the end of the year are probably on hold. A new return to flight date is unknown.
Neither Khrunichev nor ILS immediately responded to questions.
The 2 July launch went awry immediately after departing the launch pad. Video shows the rocket tipping in one direction, and then the other before inverting and crashing into the ground. Nobody was injured in the crash but a nearby village was reportedly evacuated due to toxic gas fumes.
The three Glonass precision navigation satellites the rocket carried were destroyed.