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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.
The real reason behind the switch from Proton-K to Proton-M was that the M one had a digital guidance computer, that could've been programmed by a rookie engineer, while Proton K relied on analog circuits that had to be rebuilt for every trajectory/payload combination.
--Russian vodka engineer
I think too, not sure, but with the third stage usually you're fully ballistic when it's running. (don't need to fight gravity)
Absolutely right. Play Kerbal Space Program for a few hours, and you really feel this. All your upper-stage engine needs to do is give you enough horizontal velocity to stay in orbit. So you want it to be as efficient as possible, and as weak as possible (since a weak engine is lighter), while still allowing it to finish its burn before you fall back into the atmosphere.
The first stage is quite different. As you said, the first 1g of acceleration it gives you is wasted fighting gravity, so you want the thrust-to-weight to be as high as possible to minimise this fractional loss. On the other hand, go too fast too soon, and you're losing energy to drag. You don't want to go really fast until you're above most of the atmosphere. The mathematical formulation of this is called "Goddard's problem", and the optimum solution is something like: accelerate flat-out until you reach the speed where atmospheric drag becomes significant, then cut your thrust back, and gradually ramp your acceleration back up to max again as the air thins out.