It's a good question because no one in a position to know feels comfortable enough to say. Immediately after the Thursday story broke about rolling the SLS back to pad 39B, videos started showing up on YouTube essentially saying that because flight managers said they weren't going to do another full Wet Dress Rehearsal that they were endangering the crew.
From my viewpoint, the crew is never going to be out of danger, and re-running every test that they've run so far isn't going to make the SLS safer, it's going to help wear it out. My perspective is that every time they fill and drain those cryogenic tanks, the tanks go through a series of thermal contraction and expansion changes due to the temperature changes, and those reduce the life of the tanks. They could wear the tanks out without ever launching. Testing more times doesn't make the probability of success better, it makes the probability worse.
I think Space.com has a good summary with an article entitled, "How risky is the Artemis 2 astronaut launch to the moon? NASA would rather not say". There's simply too many unknowns. Remember the quote from Donald Rumsfeld, Secretary of Defense under George W. Bush in 2002?
Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tends to be the difficult ones.
I think I'd change those last few words to the "the latter category tends to be the ones that cause the trouble."
The fundamental problem is that there has been one flight of the SLS so far. It was unmanned so there are systems that are being used for the first time in flight and it's arguable whether it's really reasonable to use SLS I as an example SLS flight. There's simply not enough data to properly quantify the risks involved.
"I wouldn't actually put a number on it," Lori Glaze, acting associate administrator for NASA's Exploration Systems Development Mission Directorate, said during a briefing on Thursday (March 12) following completion of the Artemis 2 flight readiness review.
Reporters repeatedly pressed Glaze and John Honeycutt, chair of the Artemis 2 mission management team, for numbers during that briefing. And a few did come up.
For example, Honeycutt noted that new rockets have historically launched successfully on their debut flights about 50% of the time. ...
Human spaceflight programs that are launching regularly could probably expect a failure rate of about 2% — 1 in 50 — on their second or third liftoffs, Honeycutt added. But Artemis' cadence isn't exactly regular, given that there will be about a 3.5-year gap between the first and second missions if Artemis 2 does indeed get off the pad in early April.
Honeycutt went on to say, "That basically means we're probably not 1 in 50 on the mission going exactly like we want to, but we're probably not 1 in 2 like we were on the first flight." (1 in 50 is the mission failure rate).
For context, a 210-day commercial crew mission to the International Space Station is rated at a 1 in 200 (0.5%), and during the Apollo days, the risk of crew loss was 1 in 10 (10%). In the early days of the Space Shuttle program, they believed the chance of crew loss was 1 in 100, but they later realized that for the early shuttle flights it was actually 1 in 10.
A persistent problem with doing this calculation for manned spaceflight is the small numbers available to work with.
"We have pursued loss of mission, loss of crew-type number assessments, but I'm not sure we understand what they mean," Honeycutt said.
To his credit, Honeycutt realized that he was working with a model that said the most dangerous aspect of the mission would be micrometeors and orbital debris (MMOD) as the biggest single risk, saying "Really? Is that the biggest risk to the mission — MMOD?"
Honeycutt seemed to realize that such admissions, valid and honest as they are, would probably spawn stories like the one you're reading now. "Well, this oughta make for some good reading over the next few days," he said with a smile, drawing laughter from the journalists in the room.
The four crewmembers of NASA's Artemis 2 moon mission. Left to right: The Canadian Space Agency's Jeremy Hansen and NASA's Christina Koch, Victor Glover and Reid Wiseman. (Image credit: NASA/Aubrey Gemignani)
No comments:
Post a Comment