An investigation into quality control issues in the Russian space industry has discovered that nearly every engine currently stockpiled for use in Proton rockets is defective, the RIA Novosti news agency reported March 30, citing Igor Arbuzov, head of state rocket engine manufacturer Energomash.

71 engines, mostly used to power the second and third stages of the Proton rocket, require complete overhauls to remove defects. Arbuzov did not specify what was wrong with the engines. In January, Interfax reported on an investigation into high-quality metals swapped by a plant manager for cheaper alternatives.

“Most of the work will be done in 2017, but we understand that some portion will inevitably slip into 2018,” Arbuzov said. “Our main goal is to avoid disrupting the government space program’s launch schedule, or the schedules of the Defense Ministry and commercial customers.”

Proton, a Soviet-designed rocket that has flown since the late 1960s, was once considered to be the most reliable rocket in the world. Since 1967, the design has been launched 400 times, and at one point was used to launch some 30% of commercial satellites into space.

But over the past decade, Proton’s reliability and that of the Russian space industry as a whole has been thrown into sharp question amid a series of spectacular launch failures. The problem goes beyond engines, pointing to a general quality control crisis across multiple factories and rocket designs.