Author Topic: ISS Commercial Resupply Services 2 (CRS2) 2017-2030  (Read 478551 times)

Offline jongoff

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #920 on: 11/07/2015 08:51 pm »
All of the capabilities in the table below are required.  Note that the only  provider who can meet all requirements is SNC (based on best information available at this time).

SpaceX or Boeing could tick the "pressurized disposal" box just by saying they can pack it in their vehicle, land it, and take it to the nearest landfill.

The reason NASA wants at least some disposal flights that burn up on return is that a decent amount of that cargo they consider to be biohazard materials and crap like that that would be an expensive pain to deal with from a regulator standpoint. At least that's the scoop I got from a friend who was involved in CRS-1. Now, I'm sure that if the cost of pressurized disposal ends up being too high, they could waive that requirement and deal with the terrestrial disposal requirements. But they do have a reason for not just bringing it back and "taking it to the nearest landfill."

~Jon

Offline joek

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #921 on: 11/07/2015 09:16 pm »
How is disposal on a dump or incineration facility on earth less desirable than disposal in a container burning up in the high atmosphere?

Or: Why would disposal by SpaceX on the ground not be equal to disposal by Orbital Sciences Cygnus burning up in the high atmosphere?

Cost.

Offline ChrisWilson68

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #922 on: 11/07/2015 10:00 pm »
If you want a more accurate analysis, we could use (n+1)/(k+2) on both launch vehicle and spacecraft independently to derive the probability of failure of at least one system on the return to flight.

Sure, you can make up whatever nonsensical formulas you like.

Or you could take a class in statistics and learn about prior probabilities and how to do calculations of updated probabilities based on prior probability distribution assumptions plus empirical data.

There's no way to make an accurate assessment of probability of some future event based on historical data without some sort of assumption about prior probabilities.

You're also not taking into account that after a failure an attempt is made to fix the root cause, so there's a smaller chance of failure after the fix.
« Last Edit: 11/07/2015 10:00 pm by ChrisWilson68 »

Offline TrevorMonty

Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #923 on: 11/07/2015 10:12 pm »
There are other advantages to having a disposal vehicle like Cygnus. Running experiments like the one where they plan start a fire inside Cygnus.

Offline watermod

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #924 on: 11/07/2015 10:18 pm »
There are other advantages to having a disposal vehicle like Cygnus. Running experiments like the one where they plan start a fire inside Cygnus.

Always wondered why not giant room sized garbage bags with some disposable thruster device to guide them down to a fiery death?

Offline ChrisWilson68

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #925 on: 11/07/2015 10:26 pm »
There are other advantages to having a disposal vehicle like Cygnus. Running experiments like the one where they plan start a fire inside Cygnus.

Always wondered why not giant room sized garbage bags with some disposable thruster device to guide them down to a fiery death?

I think you just described Cygnus. :-)

Offline sdsds

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #926 on: 11/08/2015 03:55 am »
after a failure an attempt is made to fix the root cause, so there's a smaller chance of failure after the fix.

Prove that?

Why is an implementation of the fixed design less likely to fail than the implementation of the original?
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Offline ChrisWilson68

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #927 on: 11/08/2015 04:51 am »
after a failure an attempt is made to fix the root cause, so there's a smaller chance of failure after the fix.

Prove that?

Why is an implementation of the fixed design less likely to fail than the implementation of the original?

There are two reasons:

1) You're not redoing the whole design, just the part of the design that failed.

2) You're putting much more resources into analyzing that new design.

For any complex system, there's an enormous number of design decisions.  The chances of any one of them being a fatal design flaw is tiny, or else the system would never work.  The only reason there's a non-tiny chance of failure of the whole system is that there are so many of those choices, any one of which could cause a failure.  When you keep all the other choices the same and replace the choice that is known to have caused a failure, the chances of it being bad are again very small.  And since it failed all the resources of the company are focused on the replacement design.  Many more people spend much more time analyzing it.

Suppose, for example, you have 1,000 design decisions and the chances of a fatal flaw being made in the design of each and not being caught is 1/10,000.  Then you have a bit under a 10% chance that at least once fatal design flaw will get through.  But if you find that flaw and replace it, if the replacement again has a 1/10,000 chance of being a fatal flaw that isn't caught, your chances of the replacement also being bad are 1/10,000.

Of course, that's an oversimplification and some decisions are more dangerous than others, but the principle still holds as long as you're keeping a portion of the design fixed and changing only the portion that failed.

Offline sdsds

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #928 on: 11/08/2015 09:59 am »
Thanks, that example is really helpful in seeing this from your perspective.

I don't see it that way, though. Are you by any chance familiar with what's called the "Swiss Cheese" model?
https://en.wikipedia.org/wiki/Swiss_cheese_model

If you replace one slice of cheese, even if it has fewer holes, isn't there a good possibility that you're opening a new passageway for hazards to get through?
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Offline ChrisWilson68

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #929 on: 11/08/2015 11:46 am »
Thanks, that example is really helpful in seeing this from your perspective.

I don't see it that way, though. Are you by any chance familiar with what's called the "Swiss Cheese" model?
https://en.wikipedia.org/wiki/Swiss_cheese_model

If you replace one slice of cheese, even if it has fewer holes, isn't there a good possibility that you're opening a new passageway for hazards to get through?

I can think of many disasters in aerospace where a failed component was redesigned and there was never another disaster involving that component.  I cannot think of a single instance in which a disaster led to a redesign of a part that interacted in some unforeseen way to open a new hole for a different disaster with a new failure mode.

In other words, I think the empirical evidence argues against the Swiss cheese model.

Offline deltaV

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #930 on: 11/08/2015 04:37 pm »
Suppose, for example, you have 1,000 design decisions and the chances of a fatal flaw being made in the design of each and not being caught is 1/10,000.  Then you have a bit under a 10% chance that at least once fatal design flaw will get through.  But if you find that flaw and replace it, if the replacement again has a 1/10,000 chance of being a fatal flaw that isn't caught, your chances of the replacement also being bad are 1/10,000.

The chance of an attempted design fix not working is much greater than 1 in 10,000. There have been order 500 failed orbital launches in the history of rocketry and Orbital's Taurus had two consecutive fairing failures, so from that incident alone the chance of a fix failing is at least 1 in 500.

Offline llanitedave

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #931 on: 11/09/2015 02:02 am »
Thanks, that example is really helpful in seeing this from your perspective.

I don't see it that way, though. Are you by any chance familiar with what's called the "Swiss Cheese" model?
https://en.wikipedia.org/wiki/Swiss_cheese_model

If you replace one slice of cheese, even if it has fewer holes, isn't there a good possibility that you're opening a new passageway for hazards to get through?


If the fix to a failure was just as risky as the unfixed system, then systems would never be fixed.  In which case, failure analysis would never be worthwhile. 


It's hard to argue that the Apollo Command Module post-Apollo 1 was as risky a death trap as Apollo 1 itself was.
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Offline ChrisWilson68

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #932 on: 11/09/2015 04:36 am »
Suppose, for example, you have 1,000 design decisions and the chances of a fatal flaw being made in the design of each and not being caught is 1/10,000.  Then you have a bit under a 10% chance that at least once fatal design flaw will get through.  But if you find that flaw and replace it, if the replacement again has a 1/10,000 chance of being a fatal flaw that isn't caught, your chances of the replacement also being bad are 1/10,000.

The chance of an attempted design fix not working is much greater than 1 in 10,000. There have been order 500 failed orbital launches in the history of rocketry and Orbital's Taurus had two consecutive fairing failures, so from that incident alone the chance of a fix failing is at least 1 in 500.

That's different.  That's a case where the root cause was not properly determined.

It's true that there is some reasonable chance the root cause was not correctly determined.  In that case, the chance of a failure the next time doesn't go down, it's the same as it was before.  Still, since there is a good chance the correct root cause was identified, the chances of a failure after an attempted fix are lower than before the attempted fix.

Offline Nomadd

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #933 on: 11/09/2015 04:38 am »

Falcon 9 - 90%

18 out of 19 is 90%?
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Offline ChrisWilson68

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #934 on: 11/09/2015 05:43 am »

Falcon 9 - 90%

18 out of 19 is 90%?

ncb1397 decided to make up an arbitrary formula for calculating probabilities that has nothing to do with reality: (n+1)/(k+2).  Here n is 18, k is 19, so he gets 19/21, which is approximately 90%.

By ncb1397's formula, if you simulated a 1 in 6 chance of failure by rolling a die and considering it a failure if a "1" came up, then if you rolled six times and it came up "1" once then ncb1397's formula would say the chance of a failure on the next roll is 2/7.

I'd guess ncb1397 came up with this formula because he started with n/k but realized if n == k it would give a 100% chance of success if so far there hasn't been a failure.  Adding 1 to the numerator and 2 to the denominator is a fudge to try to get it to give a non-zero chance of failure even if there hasn't been a failure yet, but a decreasing chance of failure for every trial which does not result in a failure.

While that formula gives some of the desired properties, it's really not a very good estimate.

Really, the way to do a calculation like this is to recognize that you need to take into account your prior probability estimation -- that is, before the rocket flew at all, how likely would you consider it to fail?  Actually, it's a bit more complicated than that because you have to think of it as a probability distribution of probability distributions.  For example, if you have a bag of 10 coins and you know 9 are fair and 1 gives a 75% chance of heads, and you draw one coin and start flipping it, your prior probability assumption is 90% 50/50 and 10% 25/75.  You might believe, for example, that rocket companies fall into two categories: incompetent companies that have a 30% chance of failure on every launch and competent companies that have a 3% chance of failure on every launch and 50% of companies are competent.  Then, each successful launch makes it more likely it's a competent company, and you can calculate pretty easily exactly how likely.

There's no way to compute a probability of failure only from the number of failures so far -- you need to make some assumption about prior probabilities, either explicitly or implicitly without realizing you're making that assumption.  Most people don't understand that, so they think they can calculate "the" probability of a failure in the future based only on the historical data, which is not correct, though the more data you have, the less the answer is sensitive to the prior probability assumptions.

Offline sdsds

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #935 on: 11/09/2015 06:59 am »
you need to make some assumption about prior probabilities

This is a really good point, and shouldn't be overlooked.


Quote
ncb1397 decided to make up an arbitrary formula for calculating probabilities

No he didn't. Ed Kyle has been using this approach on his spacelaunchreport website for years.

Quote
if n == k it would give a 100% chance of success if so far there hasn't been a failure.  Adding 1 to the numerator and 2 to the denominator is a fudge to try to get it to give a non-zero chance of failure even if there hasn't been a failure yet, but a decreasing chance of failure for every trial which does not result in a failure.

It does more than that, it provides the assumed probability distribution. (I think a Bayesian would say, "The prior being used is 1/2.")

----

Although this appears unrelated to CRS-2, the connection is that NASA ///might/// be delaying the decision until after it has data on F9-FT and/or Cygnus on Atlas V. So if I may beg your indulgence (and that of other readers)?

Suppose repeated identical trials of a launch system had a perfect record of 99 successes, and then one failure. The root cause is determined and a fix is designed. Would you rather bet your life on another trial identical to the 99 successes and the one failure, or would you choose to bet your life on the system that had never flown?
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Offline ChrisWilson68

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #936 on: 11/09/2015 08:27 am »
you need to make some assumption about prior probabilities

This is a really good point, and shouldn't be overlooked.


Quote
ncb1397 decided to make up an arbitrary formula for calculating probabilities

No he didn't. Ed Kyle has been using this approach on his spacelaunchreport website for years.

Yeah, but that doesn't make it any less arbitrary.

Quote
if n == k it would give a 100% chance of success if so far there hasn't been a failure.  Adding 1 to the numerator and 2 to the denominator is a fudge to try to get it to give a non-zero chance of failure even if there hasn't been a failure yet, but a decreasing chance of failure for every trial which does not result in a failure.

It does more than that, it provides the assumed probability distribution. (I think a Bayesian would say, "The prior being used is 1/2.")

A prior is not a number.  It's a distribution of numbers.  It has to tell you how likely a rocket is to have a 10% failure rate, now likely it is to have a 1% failure rate, how likely it is to have a 100% failure rate, etc.

For example, if you draw a coin out of a bag and start flipping it and you want to know after a few trials how likely the next is a head, you need to know how many coins in the bag are fair, how many give a 25% chance of heads, how many give a 75% chance of heads, etc.  The prior is your estimate of how many coins in the bag are biased in different ways.

For a new launch vehicles, there's some significant chance the failure rate is 100% -- that is, there's a fatal design flaw that must be fixed and until it is fixed there's a 100% chance of failure.

----

Although this appears unrelated to CRS-2, the connection is that NASA ///might/// be delaying the decision until after it has data on F9-FT and/or Cygnus on Atlas V. So if I may beg your indulgence (and that of other readers)?

Suppose repeated identical trials of a launch system had a perfect record of 99 successes, and then one failure. The root cause is determined and a fix is designed. Would you rather bet your life on another trial identical to the 99 successes and the one failure, or would you choose to bet your life on the system that had never flown?

It depends on how big the fix is.  If the fix is replacing a strut, I'd go with the fixed system.  If the fix is replacing the first stage with an entirely new first stage, I'd go with the old system.

Offline Prober

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #937 on: 11/09/2015 01:55 pm »
this thread has gone way off track :o
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Offline pathfinder_01

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #938 on: 11/09/2015 02:48 pm »

Suppose repeated identical trials of a launch system had a perfect record of 99 successes, and then one failure. The root cause is determined and a fix is designed. Would you rather bet your life on another trial identical to the 99 successes and the one failure, or would you choose to bet your life on the system that had never flown?

The good thing about putting cargo on unmanned commercial rockets is that the rocket can go bang, improvements can be made and all of it without risking said life on said rocket system. In addition in the real world things are never identical. What is important is that the cause of the problem be fixed and thus the system should become safer. i.e. If Challenger had been launched in July, there likely would not be any accident.

What is more problematic are problems that can not be totally fixed(Shuttle and it's foam). Problems(as well as fixes) that can not be tested without putting human life in danger(Shuttle again). Anyway with respect to CRS II, I think that all sides will have had enough time to work out problems.  And an Cargo flight to the ISS makes an great test for Space X's reusability(It is frequent and low risk enough that you could convince someone  to accept an extra cargo run at reduced price. ).

Offline jongoff

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Re: ISS Commercial Resupply Services 2 (CRS2) 2017-2024
« Reply #939 on: 11/09/2015 08:27 pm »
There are other advantages to having a disposal vehicle like Cygnus. Running experiments like the one where they plan start a fire inside Cygnus.

More generally being able to turn it into a simple unmanned free-flyer after leaving ISS. I like the idea of having different vehicles with different combinations of capabilities.

~Jon
« Last Edit: 11/09/2015 08:30 pm by jongoff »

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