Author Topic: NASA class MMOD as primary threat to commercial crew vehicles  (Read 17352 times)

Offline M_Puckett

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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.

You would think the skin of the ISS would show quite a record of MMOD damage after all these years in orbit.

Offline Hobbes-22

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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....

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. The most lethal job in the US is logging, 127.8 fatalities per 100,000 or 0.1%/yr. Some people do more dangerous things for fun: BASE jumping had a 1/60 fatality rate in 2006. That's the only sport that gets above 1%.
That puts spaceflight close to the top of a very narrow pyramid. 1% is a percentage worth putting effort into lowering it.

Offline docmordrid

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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....

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.
DM

Offline b0objunior

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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....

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.

What's your minimum risk then?

Offline Robotbeat

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The more expensive a project is, the more man-hours it requires. The more man-hours, the more chance of an industrial accident, like workers falling from scaffolding, being crushed by slung loads, etc.

So there's a very, very good /worker safety/ argument that increasing safety for just the astronauts is not worth it if it causes the project to balloon in cost.
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Online yokem55

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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?

Offline guckyfan

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So if MMOD is so dangerous. How are Boeing and SpaceX expected to mitigate? Build armor against tank breaking ammunitions?

Offline Cherokee43v6

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Quote
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. 

And the time you are discussing is so small it will be measured in minutes.  Not worth considering.

In that case, Sully's in charge and you're going to be in the Hudson.
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Offline baldusi

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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.
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.

Online jgoldader

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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.

You would think the skin of the ISS would show quite a record of MMOD damage after all these years in orbit.

It surely does.  I was at the NASM last week and saw WFPC2 on display.  The outer panel of WFPC2, which was exposed to space on HST for over 10 years, had many impact sites cored out for analysis.

As noted above, MMOD is no laughing matter.  While no people have died in space from it, IIRC several satellites are believed to have been damaged or rendered inoperable due to MMOD. The odds of a LOM/C/V cannot be reduced to zero.  You minimize the risk, and then either you fly, or you stand down if you are unable or unwilling to accept the risk.
Recovering astronomer

Offline whitelancer64

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.
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.

What I described is part of Failure Mode and Effects Analysis (FMEA), specifically the Risk Priority Number (RPN) analysis. It allows for an analysis to determine what failure mode has the highest risk and needs to be given priority to reduce or eliminate it. FMEA been around for a long time, and has had several different methods for risk assessment associated with it. An RPN analysis is certainly not the only way to assess risk, just the one I've been taught and use most often. I'm not surprised you asses risk in a different way.

My point was to demonstrate that even though there has never been a catastrophic failure of a spacecraft or a space station due to a MMOD strike, the overall risk that such a failure could happen is still high.
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Offline whitelancer64

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.

You would think the skin of the ISS would show quite a record of MMOD damage after all these years in orbit.
It does. There are dozens of small strikes and probably thousands of small ones. Likely millions of tiny ones. There are pits in the Cupola window, holes through the solar arrays and radiator panels, and lots of examples of damage to the micrometeorite blankets.

Example, one of the get-ahead tasks performed on the last spacewalk was photographing the MMOD blankets on the Alpha Magnetic Spectrometer to see if they need to be replaced.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
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Offline NaN

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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?

There is far more orbital debris today than there was at the dawn of spaceflight, giving most of the history limited utility. As with many things spaceflight, there is a small sample size leaving a lot of room for interpretation via statistics.

I remember back in the Shuttle era they had unreasonably low goals for LOV/C (1-in-1000 IIRC), which was addressed after the first bright spotlight shone on it, ultimately resulting in the 'about 1-in-90' number cited in the article. And yet, Ares somehow jumped back to a mythical 1-in-1000 number before getting reined in.

It is hard to ignore the tension between political needs (hard to sell 1-in-90 for funding) and engineering reality. The current MMOD model is noted as 'speculative', 'quite robust', 'perhaps too robust'. So making pessimistic assumptions at the top allows closing the gap later via revision of the model. And in the meantime, MMOD makes a much safer primary factor than what may be reported, in some places, as vehicle 'flaws' being primary factors. Yes, I am getting rather cynical...

Online abaddon

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Again, 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?
Quote
The 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.

Offline NaN

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So if MMOD is so dangerous. How are Boeing and SpaceX expected to mitigate? Build armor against tank breaking ammunitions?


They wouldn't be expected to stop every possible impact, of course, but would mitigate with methods including armor. Dragon has trunk armor protecting the TPS, and I remember a statement from SpaceX (or Musk, perhaps) regarding how large a hole could be punched in the PV without excessive loss of pressure. Depending on the model, they could for example choose to mitigate up to "x" energy of impact over "y" percentage of the vehicle surface to reduce total risk.
The risk goal is "programmatic" risk, which I interpret to mean that ISS safe haven could also mitigate this risk.
Larger/brighter orbital debris would be tracked, which should place an upper limit on how severe an impact would be expected to be survivable.

Offline whitelancer64

Again, 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?
Quote
The 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.

Tell that to the zero failed flights from o-ring burn through prior to Challenger. Or the zero failed flights due to foam strike on the heat shield tiles prior to Columbia.

Even though there were no failures prior to it happening, the risk was always there!! I've read that the RPN for 0-ring burn through on the SRBs was about 800 - a VERY high number for an RPN - and management decided the risk was acceptable.

I have to keep repeating it because dismissing a potential failure mode with a risk as high as MMOD strike as though there is no risk is wrong.
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Offline SWGlassPit

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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?

All of these things are absolutely taken into account. Velocity, size, mass, and directionality are all present in the risk assessment calculations.

So if MMOD is so dangerous. How are Boeing and SpaceX expected to mitigate? Build armor against tank breaking ammunitions?

Yes.  Shields are used to defend the most vulnerable hardware.  Other hardware is designed to survive being struck.

Offline Robotbeat

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MMOD is obviously a risk. But I doubt it's greater than risks we haven't fully characterized.
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To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline whitelancer64

MMOD is obviously a risk. But I doubt it's greater than risks we haven't fully characterized.
Such as?
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Offline Robotbeat

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MMOD is obviously a risk. But I doubt it's greater than risks we haven't fully characterized.
Such as?
Unknown unknowns. I should have been more specific: things not even really considered, not just "not fully characterized."
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

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