Years before an unmanned rocket erupted in a fireball in October, NASA officials knew the metal in its 50-year-old Soviet-made engines could crack, causing fuel to leak and ignite, government documents show.

As early as 2008, a NASA committee warned about the "substantial" risk of using the decades-old engines, and a fire during a 2011 engine test in Mississippi heightened the agency's concern.

The engines had a "fundamental flaw in the materials," said a top manager for NASA's contracted rocket builder, Orbital Sciences, in a 2013 interview with an agency historian. The Soviet engines were built in the 1960s and 1970s in a failed attempt to take cosmonauts to the moon.

"They were never designed to be in storage that long," said the Orbital manager, Ken Eberly, deputy director for the rocket program.

The explosion, just seconds after liftoff from a Virginia launch pad on Oct. 28, destroyed tens of millions of dollars in taxpayer-funded supplies, experiments and equipment, all bound for the International Space Station. The episode has raised questions about NASA's oversight of a new program to hire private contractors to carry cargo and astronauts to orbit, rather than operate the spacecraft itself.

The program aimed to encourage private industry to develop innovative, safe and reliable spacecraft, and ideally save money. But NASA and Orbital officials knew the decades-old engines posed a danger before the agency awarded the company a $1.9-billion deal to launch eight missions.

The company and NASA tried to address the risk by X-raying the engines to find cracks and patching them with welds.

NASA officials knew before the October explosion that the fix had not worked as well as intended. In May, an overhauled engine exploded during a test at NASA's Stennis Space Center in Mississippi.

Under NASA's contract with Orbital, taxpayers shoulder most of the risk of a catastrophe. The company receives as much as 80% of its fee for each launch even if the rocket explodes.

NASA has not said how much the destroyed cargo was worth. The government will also spend up to $20 million to repair damage the explosion caused to the Virginia launch facility, according to legislation approved in December.

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Before explosion, NASA knew aging Soviet engines posed risks