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#41
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reliability and survivability
To me, man-rating boils down to one thing: increasing LV reliability
to an acceptable number of nines. The trick is deciding how many nines one can afford or is willing to live with. I would think a harder trick is trying to figure out whether you can believe the number you have calculated for reliability. In the 1990's they increased shuttle reliability (on paper) from 98.9% to 99.7% (or whatever the exact numbers are, I'm referring to some studies at least one of which was done by SAIC and showed one in 248 probability of an accident). But given the small sample sizes, we have no way of knowing whether these estimates were truth or fiction. Given the vagueries of the process of calculating a reliability, it's pretty easy to start with the answer you are hoping to get, and work backwards to the assumptions which will produce it. One illustration of the difficulties is that that both shuttle accidents to date were caused not by a random, anticipated event, but by something which was known, just not fully appreciated. |
#42
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reliability and survivability
"Rand Simberg" wrote in message ... The problem is the word "might." I don't think that you (or anyone) really knows what that cost was. As I said, and perhaps someone can correct me if I'm wrong, but I think that at least with regard to EELV, the payload community (primarily the Air Force) and the insurance industry thought they were already getting 99%. And you might be right. I pulled that 95% out of thin air, and it was likely a bad choice. The actual rate might be much higher, but I believe it can't be any higher than two nines. Getting past that two nines is where the expense mounts quickly, IMHO. -Kim- |
#43
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reliability and survivability
On Wed, 17 Sep 2003 03:21:02 GMT, in a place far, far away, "Kim
Keller" made the phosphor on my monitor glow in such a way as to indicate that: "Rand Simberg" wrote in message .. . The problem is the word "might." I don't think that you (or anyone) really knows what that cost was. As I said, and perhaps someone can correct me if I'm wrong, but I think that at least with regard to EELV, the payload community (primarily the Air Force) and the insurance industry thought they were already getting 99%. And you might be right. I pulled that 95% out of thin air, and it was likely a bad choice. The actual rate might be much higher, but I believe it can't be any higher than two nines. Getting past that two nines is where the expense mounts quickly, IMHO. I think that we concur. And I suspect that getting past that two nines is beyond the state of the art with current expenables... -- simberg.interglobal.org * 310 372-7963 (CA) 307 739-1296 (Jackson Hole) interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org "Extraordinary launch vehicles require extraordinary markets..." Swap the first . and @ and throw out the ".trash" to email me. Here's my email address for autospammers: |
#44
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reliability and survivability
Jim Kingdon wrote
In the 1990's they increased shuttle reliability (on paper) from 98.9% to 99.7% (or whatever the exact numbers are, I'm referring to some studies at least one of which was done by SAIC and showed one in 248 probability of an accident). Checking previous references to this report, the number was originally given as 1 in ~140, with 1 in 230 and 1 in 76 being the 5% confidence bounds. (I think there were later reports that did move the central figure up by a factor or two or so as a consequence of Shuttle improvements.) It's interesting that the as-flown Pa is now running not much outside the lower end of the range. One illustration of the difficulties is that that both shuttle accidents to date were caused not by a random, anticipated event, but by something which was known, just not fully appreciated. Yes, a feature and limitation, though not a fault if properly understood by the users, of the PRA method is that it depends on identifying relevant fault trees and estimating the probabilities of the events along each tree. |
#45
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reliability and survivability
"Rand Simberg" wrote in message ... On 16 Sep 2003 11:15:05 GMT, in a place far, far away, "Kim Keller" made the phosphor on my monitor glow in such a way as to indicate that: "Rand Simberg" wrote in message .. . Then again, given the value of satellites, and the insurance rates, why didn't they design this way to begin with? If you're talking about just unmanned launchers, then the answer would be that the builder strove for an economic balance between launch cost and launcher reliability. The satellite industry seems to tolerate a .95 reliability rate or thereabouts. Would they be willing to spend the much greater launch cost on a booster that has .99 reliability? That's the compromise the industry currently looks for, because that extra .04 seems to be exponentially more costly. I don't think that "tolerate" is the right word. The idea behind new launchers such as A-5 and the EELVs was not just reduced costs, but increased reliability. I doubt if the insurance industry expects, or is happy with five losses out of a hundred, and I suspect that their disappointment is going to be reflected in their rates, which should encourage the builders to do something about the problem, regardless of what the payload is. Considering how much these vehicles cost now, if it raises it "exponentially" to make them reliable, then we can't afford to put people up on them. While I can not speak specifically for the Delta-4, I can say with some knowledge that, for the Atlas V, the idea of what the insurance industry wanted to see was not a big factor. When the Atlas V EELV program was first developed, it was with the idea that the vehicles would be used for government missions, not commercial ones. That was the big driver. There were only going to be three basic configurations: 301, 401, and Heavy. The first time that anyone thought of marketing it for a commercial mission was when they decided to go after Teledesic. That was what drove them to finally expand the number of configurations to include SRBs. Up until that point, they had never even thought of adding solids! By that time though, the basic design of the core was set, and it was too late (or too costly) to change the core in such a way that they might have been able to have up to 6 solids; that's why they have to settle for 5. Oops...sorry to stray here...merely meant to show that the commercial market, and the associated insurance industry considerations of extra reliability, etc, were not big factors in the original design of the Atlas V. |
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