Re:SpaceX Raptor: Methane fueled full-flow staged combustion...
by ncmncm on Sunday May 26, 2019 @12:31PM
(#20015740)
That would make more sense if, in fact, engines failed or didn't fail as a group, but in fact they tend to fail individually. This can make the difference between success and failure for the mission, when other engines of the same type fill in for the failed unit.
But in defense of counting the number of missions, each mission has similar "external" characteristics: ambient temperature, sunlight level, burn duration, wind shear, humidity, etc. A combination of these environmental characteristics could cause a failure (Challenger failure was caused by excess cold). Since the environmental characteristics are the same for the mission, the number of distinct missions an engine has performed is a decent proxy for robust performance throughout the space of environmental conditions.