design ratings and re-usability
it's typical for military equipment to have a design rating that is based on the thing NOT being replaced very often. In the case of a nuclear reactor, let's say, the metals will become damaged by radiation over time. But the limits are set so that you can use the thing for 30 years and not have to replace it completely during the life of the ship. Similarly, a rocket engine is probably running at certain limits according to design margins and maximum stresses based on expected material failure modes, yotta yotta yotta. Additionally, CYCLIC stress is a major factor. Think "cracks that grow". At higher power levels you're more likely to have a crack that grows to an "unrepairable" size, but without catastrohic failure, at the higher power level. Lower power levels would let you "thermal cycle" it a lot more without getting a catastrophic failure (i.e. re-use the thing).
Even though that's just a projection of what I've seen before in military equipment, it's probably close to reality. Material strength and failure modes and how to prevent catastrophic failure, and operating limits based on minimum detectable design flaws and design margins, along with periodic X ray of materials and various other kinds of maintenance. It's how you do it, yeah. But for ONE-TIME use... if you stress it up to the breaking point, you're getting the most possible use out of it!
(but for warships, you also have WAR TIME limits, which are a *bit* different)
Biting the hand that feeds IT
