How are you measuring "safe enough" in this context? My understanding is that the Shuttle didn't have programmatic probabilistic requirements for mission safety. Ie, they didn't use overall reliability estimates and instead relied on engineering judgement (which the Challenger report showed varied widely depending on who you interviewed). It's hard to make a claim on whether it was safe enough without explicitly defining the safety goal.
Early 1970s documents were given safety requirements of 1:20 flights being successful. [1] That's the closest I've found to an actual requirement, and the shuttle record falls well within that requirement. I do think such an early spec is not a good basis for final performance, though.
Shuttle started at 1:10 and ended with 1:90 chance of loss of crew. That was bad and got it canceled as soon as the station was built, and supposedly ISS was finished only because international obligations. The last mission to extend Hubble's life was almost canceled because the NASA admin thought it was too dangerous.
NASA wanted 1:270 (so just "3 times safer than shuttle at the end of the shuttle program") from commercial crew (i.e. SpaceX Crew Dragon and Boeing Starliner).
Current US law for commercial space works on consent: passengers sign a waiver saying they know it's dangerous, and then the company doesn't have to promise safety the way an airline does. The company is on the hook for debris and innocent bystanaders on the ground though.
SpaceX plan to fly Starship a lot and gain so much experience they can find and fix all problems and reach airline safety. At least private jet level of safety, at a start.
Right, those are historical numbers, not design specs. I was asking for the design spec. Ie, in order to say whether it was “safe enough” the historical numbers either exceed the requirement spec (or not), like what exists for CCP.
There are multiple layers of risk consent, from engineering, safety, medical, and crew, for NASA. Of those, I think crew are the least likely to say no because they tend to come from a very risk tolerant population.
>It's hard to make a claim on whether it was safe enough without explicitly defining the safety goal.
I don't think it's that hard. "We're not sure exactly how safe it is, and do not have any actual requirement that it meet any specific level of safety" is clearly not safe enough for manned flight. (At least not outside of war-time or other such times we're willing to take on extra risks)
Would you get on an elevator that has a weight limit listed as "hasn't fallen yet", "has only fallen that one time", or "did fall that one time, but not since the redesign". Would you trust millions of dollars of human training and several lives to such an elevator?
The question was what is “safe enough” threshold. If you say 3.9% isn’t, you need to define what is. The better way to count isn’t “what percent of crew” but “What percent of crew-events” because astronauts were often on multiple missions. That number is 1.64% (14 fatalities over 852 crew events) Maybe that meets your threshold, maybe not.
The point was that wasn’t explicitly defined up front…they seemed to have fallen into the same trap you did of “we’ll know it when we see it”. CCP has a loss of crew threshold requirement of 0.37%.
Fine. If you want an acceptable design parameter for expected deaths: 0%.
We do not design civilian, optional, exploratory space missions with a threshold for acceptable fatalities. There is no number of dead astronauts that we can accept as a design goal.
Unforeseeable fatalities can happen, but death rate should not be a predetermined design parameter.
How are you measuring "safe enough" in this context? My understanding is that the Shuttle didn't have programmatic probabilistic requirements for mission safety. Ie, they didn't use overall reliability estimates and instead relied on engineering judgement (which the Challenger report showed varied widely depending on who you interviewed). It's hard to make a claim on whether it was safe enough without explicitly defining the safety goal.
Early 1970s documents were given safety requirements of 1:20 flights being successful. [1] That's the closest I've found to an actual requirement, and the shuttle record falls well within that requirement. I do think such an early spec is not a good basis for final performance, though.
[1] page 326: https://ntrs.nasa.gov/api/citations/19700005884/downloads/19...
Shuttle started at 1:10 and ended with 1:90 chance of loss of crew. That was bad and got it canceled as soon as the station was built, and supposedly ISS was finished only because international obligations. The last mission to extend Hubble's life was almost canceled because the NASA admin thought it was too dangerous.
NASA wanted 1:270 (so just "3 times safer than shuttle at the end of the shuttle program") from commercial crew (i.e. SpaceX Crew Dragon and Boeing Starliner).
Current US law for commercial space works on consent: passengers sign a waiver saying they know it's dangerous, and then the company doesn't have to promise safety the way an airline does. The company is on the hook for debris and innocent bystanaders on the ground though.
SpaceX plan to fly Starship a lot and gain so much experience they can find and fix all problems and reach airline safety. At least private jet level of safety, at a start.
There are multiple layers of risk consent, from engineering, safety, medical, and crew, for NASA. Of those, I think crew are the least likely to say no because they tend to come from a very risk tolerant population.
Whatever official spec that did or did not exist is irrelevant.
I don't think it's that hard. "We're not sure exactly how safe it is, and do not have any actual requirement that it meet any specific level of safety" is clearly not safe enough for manned flight. (At least not outside of war-time or other such times we're willing to take on extra risks)
Would you get on an elevator that has a weight limit listed as "hasn't fallen yet", "has only fallen that one time", or "did fall that one time, but not since the redesign". Would you trust millions of dollars of human training and several lives to such an elevator?
I think we can say that it wasn’t safe.
The point was that wasn’t explicitly defined up front…they seemed to have fallen into the same trap you did of “we’ll know it when we see it”. CCP has a loss of crew threshold requirement of 0.37%.
We do not design civilian, optional, exploratory space missions with a threshold for acceptable fatalities. There is no number of dead astronauts that we can accept as a design goal.
Unforeseeable fatalities can happen, but death rate should not be a predetermined design parameter.