The rockets would be grounded for some three-and-a-half months, Russian Deputy Prime Minister Dmitry Rogozin said on Saturday, while visiting the Voronezh plant that supplies engines for Proton-M systems.

The move comes after the results of a thorough checkup followed a failed launch in December. The failure was caused by a breakup of the third-stage engine, an official commission reported, adding that an "assembly fault" was among possible causes.

According to a report in the daily "Kommersant," a test showed that a key heat-resistant part has been swapped during the assembly.

"Instead of using materials containing precious metals (…) less heat-resistant components were used, which are normally used for other engine models," the report said. Authorities, including the FSB security service, were now trying to determine how this swap managed to slip through various quality controls.

Speaking in Voronezh on Saturday, Rogozin also suggested that employees may have tampered with the production process.

"All those responsible for switching document and technology would be harshly punished," he said. "Three Proton-M rockets will be dissembled and their second and third-degree engines replaced."

He also tweeted several pictures from his tour of the plant in southwest Russia.

Russia is currently the only country capable of sending manned space flights to the International Space Station. However, their space program has been hit by several failed launches of unmanned rockets, including Proton-M, which is most commonly used for transporting military and civilian satellites.

Last week, the head of the Voronezh factory resigned "for reasons of unsatisfactory work and product quality."

dj/sms (AFP, AP)