Friday, June 23, 2006
Evaluating Risk
Here at Andromeda, we understand risk. Why, just the other day, as we were putting on our flip-flops, we noticed the center toe masts had signs of wear. Naturally, we performed the required wind tunnel tests and reviewed data on the materials, as well as historical data showing failure frequency, severity and cause. Once that was all done, we were late for work, so we ran out the door. As we dashed across the street through heavy traffic, the completely unpredictable happened: the flip-flop broke and fell off! We turned around to get it, just as a fuel tanker was careening towards us and an unfortunately timed "Baby Carriage Parade" formed on the street corner. Hopefully, you are not like the police, going on and on, all about "who's to blame," blah, blah, blah, but you recognize the only important question is: Did we thoroughly study the risk? Yes, we did.
Similarly, the risk of foam liberation from the ice/frost ramps on the external tank has been thoroughly studied. Looking at 11 flights, Lockheed-Martin found a mere 12 foam losses believed to be due to delamination. NASA showed the "top ten" at the post-FRR press conference. The top ten ranged in mass from 0.022 pounds to 0.121 pounds. Happily, only one exceeded the "maximum expected debris limit" of 0.08 pounds. Hooray!
Now, there may be some silly people out there who ask, in their silly way, what "maximum expected debris limit" has to do with anything. Oh, you big sillies! It has nothing to do with anything. But talking about it shows only one instance in excess of it, so that's good. Of course, there's always the kooky, wild-eyed crazies, like the Marshall Space Flight Center Safety Engineering Review Panel, who insist on bringing "safety requirements" into this. Safety requirements? Who needs em? But just to humor those big dopes, what is the the safety requirement limit?
"On the order of 0.004 pounds"? Hmm, well, that would mean there have been a lot of foam loss events that exceed safety requirements, and it is certainly possible, then, in theory, that there could be such an event when Discovery is launched. But what type of loss event is reasonably possible?
You may ask, who should you believe? Lockheed-Martin who, as the manufacturer of the external tank, is completely unbiased and has studied only 11 tanks, or this "Marshall Space Flibberty Hoohaw" and their nonsensical 'three sigma' babble?
Well, if that is what you ask, then you just have not been following along. The only question is, did you study it, and did you discuss it? The only two questions.
Similarly, the risk of foam liberation from the ice/frost ramps on the external tank has been thoroughly studied. Looking at 11 flights, Lockheed-Martin found a mere 12 foam losses believed to be due to delamination. NASA showed the "top ten" at the post-FRR press conference. The top ten ranged in mass from 0.022 pounds to 0.121 pounds. Happily, only one exceeded the "maximum expected debris limit" of 0.08 pounds. Hooray!
Now, there may be some silly people out there who ask, in their silly way, what "maximum expected debris limit" has to do with anything. Oh, you big sillies! It has nothing to do with anything. But talking about it shows only one instance in excess of it, so that's good. Of course, there's always the kooky, wild-eyed crazies, like the Marshall Space Flight Center Safety Engineering Review Panel, who insist on bringing "safety requirements" into this. Safety requirements? Who needs em? But just to humor those big dopes, what is the the safety requirement limit?
The safety requirement is believed to be more than an order of magnitude smaller than .08 lbm. (on the order of .004 lbm), but has not been fully defined and allocated down to the ET.
"On the order of 0.004 pounds"? Hmm, well, that would mean there have been a lot of foam loss events that exceed safety requirements, and it is certainly possible, then, in theory, that there could be such an event when Discovery is launched. But what type of loss event is reasonably possible?
MSFC Engineering has performed a similar assessment and has identified a potential loss of up to .2276 lbm (3 sigma) and more variability in the flight history data. MSFC Engineering has considered other tanks (not just SLWTs) in their analysis and has not concurred with the Lockheed Martin findings/approach.
You may ask, who should you believe? Lockheed-Martin who, as the manufacturer of the external tank, is completely unbiased and has studied only 11 tanks, or this "Marshall Space Flibberty Hoohaw" and their nonsensical 'three sigma' babble?
Well, if that is what you ask, then you just have not been following along. The only question is, did you study it, and did you discuss it? The only two questions.
