Comment by adgjlsfhk1
6 days ago
for human spaceflight we want a lot more than "likely" (>50%). The standard is usually "extremely likely" (~1/100 to 1/1000 chance of failure)
6 days ago
for human spaceflight we want a lot more than "likely" (>50%). The standard is usually "extremely likely" (~1/100 to 1/1000 chance of failure)
Maybe. What was the probability of Loss of Crew during Apollo? There were 9 crewed missions and 1 almost killed its crew (I will omit Apollo 1 for now). I could argue that Apollo had a 1 in 20 chance of killing a crew. Indeed, that was one reason given for cancelling the program.
The first Shuttle launch probably had a 1 in 4 chance of killing its crew. It was the first launch of an extremely complicated system and they sent it with a crew of two. Can you imagine NASA doing that today?
In a news conference last week, a NASA program manager estimated the Loss of Mission chance for Artemis II at between 1 in 2 and 1 in 50. They said, historically, a new rocket has a 1 in 2 chance of failure, but they learned much from Artemis I, so it's probably better than that. [Of course, that's Loss of Mission instead of Loss of Crew.]
My guess is NASA and the astronauts are comfortable with a 1 in 100 chance of Loss of Crew.
> There were 9 crewed missions and 1 almost killed its crew (I will omit Apollo 1 for now). I could argue that Apollo had a 1 in 20 chance of killing a crew.
That's not how risk analysis works.
Let's say every Apollo mission had gone flawlessly, and no one even came close to dying. Would you then say that the risk of death for future missions would be zero? No, of course not.
I thought I'd look this up. If you've had 9 successful attempts, assuming nothing has changed between them and no other prior knowledge about success probability, then Laplace’s Rule of Succession says the probability of the next mission being a success is about 83.3%, i.e. there is a 1 in 6 chance of failing next time.
3 replies →
If you only look at this data it would be the most reasonable guess.
Of course one should also analyze the technical sytems involved and then it is clear that 0% failure is not reasonable.
1 reply →
> I could argue that Apollo had a 1 in 20 chance of killing a crew.
NASA computed the chance of “landing a man on the Moon and returning him safely to Earth” as less than 1 in 20. I would think a lot more than 1 in 20 of those failures would result in killing crew members.
https://ntrs.nasa.gov/api/citations/20190002249/downloads/20...:
“Appreciating and deemphasizing risk in Apollo
Joseph Shea, the Apollo program manager, chaired the initial Apollo systems architecting team. The “calculation was made by its architecting team, assuming all elements from propulsion to rendezvous and life support were done as well or better than ever before, that 30 astronauts would be lost before 3 were returned safely to the Earth. Even to do that well, launch vehicle failure rates would have to be half those ever achieved and with untried propulsion systems.”
The high risk of the moon landing was understood by the astronauts. Apollo 11's Command Module pilot Mike Collins described it as a “fragile daisy chain of events.” Collins and Neil Armstrong, the first man to step on the moon, rated their chances of survival at 50-50.
The awareness of risk let to intense focus on reducing risk. “The only possible explanation for the astonishing success – no losses in space and on time – was that every participant at every level in every area far exceeded the norm of human capabilities.”
However, this appreciation of the risk was not considered appropriate for the public. During Apollo, NASA conducted a full Probabilistic Risk Assessment (PRA) to assess the likelihood of success in “landing a man on the Moon and returning him safely to Earth.” The PRA indicated the chance of success was “less than 5 percent.” The NASA Administrator felt that if the results were made public, “the numbers could do irreparable harm.” The PRA effort was cancelled and NASA stayed away from numerical risk assessment as a result.
1/100 is absolutely terrible. Shuttle had 1.5% failure rate. Bonkers.
[edit]
For comparison, commercial aviation has something like 1 in 5.8m or 6x 9's of reliability.
There have been 412 manned space flights, ever, by anyone, anywhere.
A large commercial airport handles many times that number of flights every single day. Worldwide there are a hundred times more flights per day than the number of manned space flights in history.
I suspect every model of commercial plane has flown far more flights than all the human rated rockets put together.
It's not terrible for space flight. Flying a rocket to the moon and commercial aviation are obviously very different things.