Third Stage Design Problem Cause of Most Recent Proton Failure

schwit1 writes: The Russian investigation into the latest Proton rocket failure has concluded that the failure was caused by a design failure in the rocket's third stage. The steering third stage engine failed due to excessive vibration as a result of an imbalance in a rotor of a pump unit. While it is always possible for new design issues to be discovered, I wonder why this problem hadn't been noticed in the decades prior to 2010, when the Proton began to have repeated failures.

Comments

The real reason.

[Anonymous Coward]

The Proton series once had a world record for the longest uninterrupted series of succesful launches.

The real reason behind the switch from Proton-K to Proton-M was that the M one had a digital guidance computer, and they were able to find peope to program them, while the K modification had analog computer that had to be rebuilt for every payload.

--Russian vodka engineer

Slapped by The Tail of the Failure Distribution

[Irate Engineer]

The reasons why this flaw was not identified previously was because it was a low probability occurrence. The shaft was just barely adequate to survive most of the launches, but sometimes it failed before engine cutoff. Since the debris is hard to access, gathering evidence that this was indeed the culprit was very difficult, especially when they didn't know what to look for. The engineers got some hints from previous failures that caused them to put vibration sensors in an area of the rocket that allowed them to identify the current failure mechanism.

This is a problem in rocket design where you have two opposing constraints - they need a pump that works reliably all the way to orbit, but since the rocket is disposable and extra mass reduces payload, overbuilding the pump is not ideal either. This pushes one toward a design that is just barely good enough and no better. It turns out that they wanted a pump that would work for 99.9% of the flights, but they got one that worked 86% of the flights instead.

This was actually a pretty challenging problem in engineering forensics. I hope this fixes their issue. The Proton is a pretty solid rocket otherwise.