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Doggydog123579 t1_j15sg77 wrote

Reply to comment by toodroot in The European Vega-C rocket was lost shortly after lift-off from French Guiana on Tuesday with two Airbus satellites on board by DoremusJessup

How are you supposed to compare reliability Other then comparing total missions to failures then?

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toodroot t1_j15sun4 wrote

These 3 rockets are reliable enough that you need a lot more launches to accurately measure their reliability. It's a statistics thing.

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Doggydog123579 t1_j15u5ee wrote

You can't claim they are reliable enough without actually launching them though. The shuttle managed near the same performance but it was not a reliable rocket.

So other then using the data we have, how are we supposed to compare them?

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toodroot t1_j15ullq wrote

You mis-read what I meant with my comment. Another way of saying the same thing: They are so reliable that it's impossible to accurately say which one is most reliable.

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Doggydog123579 t1_j16si0g wrote

Ok, let me start over. > They are so reliable that it's impossible

This part cant be proven without more flights. The Space Shuttle was supposed to be super reliable and had 2 failures in 135 flights, which is comparable to Ariane 5 or Falcon 9. However we know the shuttle wasnt actually reliable, but this was only learned after the fact. You cant argue reliablity with any credability unless you have a large enough data set, which we do not.

So, the only data we do have is the rockets flight record and any discoveries made during it. The current data supports the Falcon 9 and Ariane 5 being reliable, but its not currently enough to argue they actually are that reliable.

So, other then using what we already have, how are you supposed to compare launch vehicles reliability?

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toodroot t1_j16t6fp wrote

You're arguing about something I didn't say or mean. The entire sentence is what I meant. I'm not calling any rocket "reliable" in an absolute sense.

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fabulousmarco t1_j16gifp wrote

>So other then using the data we have, how are we supposed to compare them?

We can't. We can qualitatively say that all three are essentially 100% reliable launchers, you can't pick a winner with this data because the very few failures they had may very well have been statistical anomalies. Their failure rate is below the margin of error.

Now suppose you multiply everything by 10. For Ariane5 imagine we'd had 20/1150 failures instead of 2/115. And for F9 imagine we'd had 10/1550 failures instead of 1/155. That would give enough margin to safely determine that the difference between the two is statistically significant, i.e. not due to statistical anomalies but rather to an actual better performance for F9.

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Doggydog123579 t1_j16ss03 wrote

I don't disagree, my issue was with his claim they are super reliable while also saying we don't have enough data to say they are statistically better. The Shuttle was supposed to be super reliable and look what happened to it.

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toodroot t1_j1740hi wrote

u/fabulousmarco appears to have read what I actually meant. I did not say the thing you're having an issue with.

If you want to talk about Shuttle, the fact that the SRBs were recovered several times with eroded O-rings before the Challenger "accident" kind of blows any statistical analysis out of the water.

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Doggydog123579 t1_j1788cj wrote

The problem appears to be you misreading what I meant, though it doesn't help i was hastily typing it out on my phone. I fully understand what you mean and never even actually said one rocket was more reliable. My original post was me pointing out he forgot a failure, and me then pointing out Falcon 9s perfect record if I arbitrarily specify Block 5.

The argument is happening because you haven't answered my question. If the rockets are reliable enough we need thousands of launches to get the required data set, how are you determining they are reliable enough to require said data set to compare?

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toodroot t1_j17jvoo wrote

u/fabulousmarco answered your question.

Here's the wikipedia article on the overall topic.

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Doggydog123579 t1_j17kzph wrote

Put another way, How do you know they are similar enough that a sample size of 1-200 isnt enough to determine which is more reliable.

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Doggydog123579 t1_j17k8rb wrote

You dodged again. Im not asking about statistical probabilities. You said Falcon 9 and Ariane 5 are reliable enough we cant compare them without a thousand flights. How do you know that.

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