During the initial trials, the LOX pump part of turbopump of F1 engine had some problem with its vanes. They were damaging after approximately 110 seconds of operation. What was the problem and how was it overcome?
- shock loads due to high acceleration of the turbopump shaft
- rubbing between critical seals and other moving parts
- fatigue in the impeller section
Directly quoting from this link:
The turbine to drive the separate propellant pumps was an impressive piece of machinery itself-it developed 410 000 watts (55 000 brake horsepower). Designers located the turbine on the fuel-pump end of the turbopump. In this position, the units of the turbopump with the most extreme temperature differences (816°C [1 500°F] for the turbine and - 184°C [-300°F] for the oxidizer pump) were separated. Hot gases for the turbopump turbine originated in the gas generator and entered the turbine at 77 kilograms per second.47 A series of failures, 11 in all, dogged the development of the turbopumps for the F-1 engine. Two incidents were traced to structural failures of the LOX pump impeller, which called for redesign of the unit with increased strength. Explosions occurred in the other nine instances, with five during engine tests and four during component tests of the turbopump. The explosions developed from a variety of causes, such as shock loads due to high acceleration of the turbopump shaft, rubbing between critical seals and other moving parts, fatigue in the impeller section, and other problems. With some new design work and manufacturing techniques, these conditions disappeared, and investigators proceeded to cope with other problems that continued to crop up, such as the engine turbine. For the engine turbine manifold, Rocketdyne chose a new material known as René 41. This material was quite new to the manufacturers of rocket engines, and the welding process produced cracks adjacent to the weld in the heat-affected zone created by the welding pass. As a result, the company devoted considerable time and effort to ascertaining proper welding conditions and to training welders on the production lines. With the proper welding requirements finally established, Rocketdyne adopted an automatic welding procedure to complete the "fix" on this situation.48
The four destructive LOX pump failures had happened at 110, 110.5, 107.7 and 109 sec; this looked statistically significant, but after much study the team wrote it off to a freak coincidence. Rocketdyne beefed up the impeller vanes, made changes to eliminate fretting and set a 3,500 sec life-limit on impellers used for ground testing. Flight engines never got up above 800 sec. Although Rocketdyne solved the LOX pump problem, the people familiar with the engine would cringe when 110 seconds went by and, then, breathe again.