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Comment by anonymouz | Hacker News Reader

While this is indeed an awesome feature to have, it doesn't by itself mean much. The total reliability of course also depends on the probability that one engine will fail, a rocket that needs all its engines may still be more reliable than one that is capable of of flying with two failed engines, but regularly has to do so.

What makes the airliner seem safe is that it can fly with a failed engine and it very rarely loses an engine.

I guess we will have to wait for some more flights till we have decent statistics on the reliability of the Falcon 9. Of course, I'm hoping that it will turn out to be an awesome rocket!

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For whatever it may be worth, it seems right now the engine failure rate for Falcon 9 is 1 in 27.

Is there a similar rate known for regular jet engines?

No idea. I am sure it is astronomically small though.

A lot of Western aviation qualifying authorities specify a maximum allowable mean time between failure (MTBF) of 1 per 100000 flight hours. I imagine most rocket engine designs would be more like one per hour! That said, they only have to operate for about ten minutes though before they are disposed of, in the case of expendable launch vehicles.

In Falcon 9's case, and totally from memory and probably incorrect and very approximate, we've had 4 launches, each has had 9 Merlin C engines on the first stage, the first stage burn time is 170s or about 3 minutes. There have been two failures - a non destructive engine shutdown on the previous ISS mission on the pad at T-very_little, and the rather more messy rapid disassembly failure of something near the engine or the engine itself on the most recent launch.

So, for this first stage engine the MTBF (remember this is all a bit silly and inaccurate and just for ballpark comparisons with the airline industry) the MTBF has been:

9(engines/launch)4(launches)0.05(3 min burns in hours)*0.5(2 failures) = 0.9, i.e. a MTBF of 55 minutes for Merlin 1C lower stage, vs most regulators requiring mtbf > 100000 hours on commercial jet airliner engines.

So there's a bit of a way to go yet!

Ah yes, good stuff. I forgot about the aborted launch, and the initial Falcon 9 launch without Dragon. My 1/27 figure, in addition to being overly simplistic, is wrong.

I agree that there is plenty of room for improvement, but you can't launch a rocket with commercial jet engines.

I don't know if it makes sense to normalize for thrust or not, but it seems like thrust should be included somehow.

They're not saying, "commercial jet engines are more reliable; therefore we should use them". The issue is that the Merlin 1C engine simply hasn't had enough flights to build up its denominator.

It doesn't make sense to normalize for thrust here. This is just failure rate. There's nothing wrong with having a comparatively high failure rate when the sample size is several orders of magnitude smaller: the statistical significance is just weaker; we acknowledge that and move on.

Here's a 1993 paper giving MTBF failures for the space shuttle's main engine. With some improvements that the author of the paper suggests, it looks like MTBF is about one per 254 flights at 100% power and one in 139 flights at 104% power . (It drops to one in 20 flights at 109% power! It's surprised the failure rate has such a strong dependency on power output --- the relationship suggests that failures have something to do with thermal stress.)

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1993001...

It's interesting to compare that MTBF with the fact that there are about ten million airliner flights per year in the US. If we arbitrarily assume that the average airliner flight is two hours, that's a substantial number of failed engines per year, so obviously the redundancy on these craft really does work.

Incidentally, I went searching for airliner engine MTBF statistics and your comment was already near the top of Google's results.

Edit: erroneously said five million flights per year because I was looking at 2012 statistics for only the first six months. of the year. Oops.

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