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I do understand that static fire helps discovering malfunctions, as explained in this answer. A launch exerts an important stress on rockets' engines (high temperature and pressure, vibrations,...) and, except for SpaceX's launchers and the space shuttle, there is no need to make the engine reliable enough to survive more than one launch (one ignition, a few minutes of full thrust).

One static fire represents a significant part of the number of ignitions it will handle in its entire life. The same reasoning goes for the number of seconds it will be running.

My reasoning is that a static fire puts a strain on the engine, and thus may decrease its life expectancy significantly. Am I right? Does the static fire reduce the engine's life expectancy and thus its reliability?

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  • $\begingroup$ An engine being reliable enough to survive one launch over a few minutes should be able to survive some additional seconds. Some reserve of life expectancy is needed for reliability. You can't predict life expectancy very precisely for every single engine. One will fail early, another one will fail much later. $\endgroup$ – Uwe Jul 25 at 10:50
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Probably highly relevant here is the concept of the failure curve where parts will generally fail either at the start or end of life. Some engine designs may be such that they do not have much of an early failure tail but shallow (unpredictable) end of life tail, these would not be test fired. The Apollo ascent engine might qualify.

For a more conventional engine with a lot of moving parts and potential for latent fabrication problems the aim would be to design it such that can be test fired into the flat part of the curve, and does not reach the up curving tail over the remaining flight time, hopefully backed up by a large pool of test data from engines fired for longer than expected and then autopsied with engineering changes as required.

So yes, a test fire does use up some engine life but if engineered correctly it makes the engines that actually go into the rocket more likely to perform as required.

And if you found out that your production process was so reliable that no/few engines failed the test fire it would be valid to make them lighter/run them harder and not test them before flight.

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