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Comment by cromwellian | Hacker News Reader

According to (https://www.rocket.com/sites/default/files/images/media/news...), the engines being flown have flown like 6-12 times, for 4000-7000 seconds of firing. Engine 2045 flew 12 missions, 14 restarts, and 7016 seconds.

No Merlin has yet achieved that, and we don't know how much refurbishment they need relative to the RS-25 either. But no RS-25 has ever failed during flight, whereas Merlins have failed multiple times, and Raptors at this point seem to fail quite often.

Now, I'm a SpaceX fanboy so I'm rooting for Raptor and Starship/Super Heavy, but I think it's unfair to say that the RS-25 hasn't proven itself.

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> But no RS-25 has ever failed during flight

There was near miss though:

>By about T-3 seconds, all engines were up and operating at 100% of rated power level. Exactly when it happened is not clear, but on the right engine, the gold plated pin from LOX post 32 in row 13 came shooting out. [...] Now two scary things could have happened. First, the LOX post, which was pinned for a reason, could have failed allowing LOX to pour into the engine cavity where the hot hydrogen was introduced. [...] Failure of the LOX post was considered a CRIT 1 failure [...] How close we were to disaster has never been determined. [...] Second, the nozzle extension could have failed. [...] Someone had calculated that if 5 adjacent cooling tubes split or were otherwise ruptured, there would not be enough local area cooling and a burn through would occur, causing a cascading failure of the nozzle and ... a CRIT 1 failure. [...] The bullet shaped LOX post pin hit the side of the right engine nozzle extension about two thirds of the way to the end with great force. Just by sheer luck, three nozzle tubes were breached.

https://waynehale.wordpress.com/2014/10/26/sts-93-we-dont-ne... https://www.youtube.com/watch?v=u6rJpDPxYGU

You've brought up the key piece here, the scope of refurbishment. If engines become a Ship of Theseus problem, then it compromises the idea that age proves a particular engine serial number reliability. I think there's a difference between the design overall being reliable (which I agree RS25 has shown), and lifetime improving a unit's reliability.

For comparison on refurb, a Falcon9 first stage with 9 engines is refurbished for $250k[1], while refurbishing 16 RS25 engines (and building a new engine controller) cost $572M[2]. Shuttle and its components weren't reused, they were rebuilt, and this (probably unfair) cost comparison shows it.

[1] https://www.elonx.net/how-much-does-it-cost-to-launch-a-reus...

[2] https://oig.nasa.gov/docs/IG-20-012.pdf

> and Raptors at this point seem to fail quite often

I know this isn't actually a dig at Raptors, but I would expect that Raptors are flying at a point in the development cycle where most engines were still only used on test stands.

It does seem like a serious drawback to making improvements rapidly that the post-firing engine is unavailable for examination, of course... :)

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