No mission has ever been lost due to MMOD. ISS and MIR both huge, neither had major accident due to MMOD. I bet you could reduce this risk to very low levels if you blocked up the windows.
And 65,000 pedestrians are hit by a car every year just in the US. It's always something, and not even hiding in bed can eliminate risk; gas furnaces and water heaters go boom, you could throw a blood clot, etc.No one gets out of life alive. Get on with it....
Quote from: docmordrid on 08/23/2016 05:09 pmAnd 65,000 pedestrians are hit by a car every year just in the US. It's always something, and not even hiding in bed can eliminate risk; gas furnaces and water heaters go boom, you could throw a blood clot, etc.No one gets out of life alive. Get on with it....That's not a very good comparison. The risk of having a lethal car accident is .01%/yr in the US. Not many civilian activities carry a 1% risk of dying.
Quote from: Hobbes-22 on 08/23/2016 06:53 pmQuote from: docmordrid on 08/23/2016 05:09 pmAnd 65,000 pedestrians are hit by a car every year just in the US. It's always something, and not even hiding in bed can eliminate risk; gas furnaces and water heaters go boom, you could throw a blood clot, etc.No one gets out of life alive. Get on with it....That's not a very good comparison. The risk of having a lethal car accident is .01%/yr in the US. Not many civilian activities carry a 1% risk of dying. Accidents of all types are 5% of US deaths.
QuoteAnd suppose you take a hit on the heat shield after you jettison the service module following the deorbit burn, when no inspection would help. Welp. And the time you are discussing is so small it will be measured in minutes. Not worth considering.
And suppose you take a hit on the heat shield after you jettison the service module following the deorbit burn, when no inspection would help. Welp.
For those who don't know, the way engineers assess risk is with Risk Priority Number spreadsheets. Basically the risk is broken down into 3 parts: the possible Severity of the risk, the Frequency or Occurrence of the risk, and the ability to Detect or Prevent the risk. Each part is given a number between 1 and 10, one being the least and 10 the most. These numbers are then multiplied together, with the final number assessing the risk on a scale of 1-1000. Then risks with the highest numbers are given the highest priority for correction or reduction. We already know that the potential severity of a MMOD strike is that it could cause loss of vehicle or the crew, so that's a 10. We know that MMOD strikes occur on every spaceflight, so that's also a 10. The ability to detect MMOD strikes or prevent them from causing catastrophic failure is the key here. The Shuttle, for example, had multiple coolant loops in its radiators, so if one was damaged it could be shut down. NASA also installed additional layers of shielding over the main coolant loops to prevent or reduce damage in the event of a direct hit. However, even though the risk of damage is reduced, it still hasn't been eliminated, so it will always be higher than 1. I would say it can't be less than 5, which would be a moderate likelihood that current MMOD mitigation will prevent catastrophic damage. So based on the numbers 10, 10, and 5, the overall Risk Priority Number is 500. A high risk, and that's being generous, I'd guess that NASA has assigned an even higher risk level than this.
Quote from: Robotbeat on 08/23/2016 06:36 pmNo mission has ever been lost due to MMOD. ISS and MIR both huge, neither had major accident due to MMOD. I bet you could reduce this risk to very low levels if you blocked up the windows.You would think the skin of the ISS would show quite a record of MMOD damage after all these years in orbit.
Quote from: whitelancer64 on 08/23/2016 06:12 pmFor those who don't know, the way engineers assess risk is with Risk Priority Number spreadsheets. Basically the risk is broken down into 3 parts: the possible Severity of the risk, the Frequency or Occurrence of the risk, and the ability to Detect or Prevent the risk. Each part is given a number between 1 and 10, one being the least and 10 the most. These numbers are then multiplied together, with the final number assessing the risk on a scale of 1-1000. Then risks with the highest numbers are given the highest priority for correction or reduction. We already know that the potential severity of a MMOD strike is that it could cause loss of vehicle or the crew, so that's a 10. We know that MMOD strikes occur on every spaceflight, so that's also a 10. The ability to detect MMOD strikes or prevent them from causing catastrophic failure is the key here. The Shuttle, for example, had multiple coolant loops in its radiators, so if one was damaged it could be shut down. NASA also installed additional layers of shielding over the main coolant loops to prevent or reduce damage in the event of a direct hit. However, even though the risk of damage is reduced, it still hasn't been eliminated, so it will always be higher than 1. I would say it can't be less than 5, which would be a moderate likelihood that current MMOD mitigation will prevent catastrophic damage. So based on the numbers 10, 10, and 5, the overall Risk Priority Number is 500. A high risk, and that's being generous, I'd guess that NASA has assigned an even higher risk level than this.That's not how I was taught probabilities. In this particular case, they are worrying only about LOC. So you need to calculate P(MMOD) x P(LOC|MMOD) and minimize that. The critical part being, obviously, the second term. The way you propose overestimates risks with low LOC probabilities but high frequency.
Given that we have 50+ years of history in LEO, we should have a pretty good statistical model of the breakdown of the distribution of such events. Is this not being taken into the risk calculations?
Again, just because there haven't been catastrophic failures doesn't mean there is no risk.
The overall risk level is still high because a MMOD strike to a critical system could easily cause loss of crew or vehicle.
So if MMOD is so dangerous. How are Boeing and SpaceX expected to mitigate? Build armor against tank breaking ammunitions?
Quote from: whitelancer64 on 08/23/2016 05:34 pmAgain, just because there haven't been catastrophic failures doesn't mean there is no risk.Literally nobody is saying that. Why do you keep repeating it like someone is?QuoteThe overall risk level is still high because a MMOD strike to a critical system could easily cause loss of crew or vehicle.Actually, based on the number of LOV (zero) from MMOD damage on a rather large number of flights, this is provably wrong. Depending on your definition of "high", I guess. Certainly the risk level is high compared to flying in an airplane. Compared to the Shuttle risks not associated with MMOD I'd say they are rather low.
It seems to me that the magnitude of the strike (size*mass*delta-v*burst-quantity) needs to be taken into account. Low delta-v, low size and mass strikes are probably a lot more common then higher magnitude strikes, and are easier to defend against and mitigate. Given that we have 50+ years of history in LEO, we should have a pretty good statistical model of the breakdown of the distribution of such events. Is this not being taken into the risk calculations?
MMOD is obviously a risk. But I doubt it's greater than risks we haven't fully characterized.
Quote from: Robotbeat on 08/23/2016 09:06 pmMMOD is obviously a risk. But I doubt it's greater than risks we haven't fully characterized.Such as?