The investigation went deeper than previous ones, since the previous hadn’t actually resulted in any improvement, and what they found was surprising to me. It wasn’t just quality control issues in manufacturing, as had previously been suspected; there are honest-to-gosh design defects in both the Briz-M upper stage for the Proton, and the upper stage of the Soyuz 2-1a.
The Soyuz 2-1a is a new model of the venerable R-7 family. Being quite new, it’s perhaps unsurprising there’s a problem, but what’s interesting is that the design flaw only shows up with the Progress as payload. (Presumably, it could also happen with Soyuz, which shares its service module entirely with Progress.) This explains why it never had a problem on any of its other flights, but the defect must be fixed before it can carry Soyuz or Progress into orbit.
The Proton defect is a bit more worrisome, because it’s been in the system for decades without anyone noticing — despite multiple failures with similar characteristics and multiple investigations. A small vernier engine used for steering the vehicle during third stage flight has a design defect which makes it extremely easy to unbalance. And when it becomes unbalanced, it experiences increasingly violent vibrations until it undergoes what rocket scientists dryly refer to as an “unscheduled disassembly”. The good news is that existing engines can be reworked with a different rotor shaft in their turbopumps to prevent the problem happening again. Also, it’s back to flight now, having been cleared for all non-Progress/Soyuz flights, placing a military satellite (possibly a Kobalt or Persona spy satellite) into orbit:
So, good on you, Russia, for finding these defects! Hopefully the design change and rework will do the trick, and keep Proton flying safely. Alternately, I’d be happy with them replacing it; Proton’s one of the last flying launch vehicles to use hydrazine and nitrogen tetroxide in the first stage. Nasty stuff to release into the atmosphere.