In a discussion on Twitter, someone said we couldn't use Starship to get
back to the moon in 2024, because who knew when it would be "human
rated"? I pointed out that it will never be human rated (even ignoring
the fact that there is no certifying authority for that). It's absurd to
talk about "human rating" a fully reusable vehicle (other than in terms
of keeping acceptable accelerations). By definition, it has to be
reliable enough to not lose hulls often, which means it should be fine
for humans, at least in the context of spaceflight (the reason we ended
the Shuttle program wasn't lack of safety -- it was because we couldn't
afford to keep losing orbiters).
I can't wait for the day we stop using this stupid archaic phrase.
On 2019-07-13 17:27, Henry Spencer wrote:
On Sat, 13 Jul 2019, William Claybaugh wrote:
Apollo was estimated at the time as having a one in five probability of loss of crew.
The actual flight history argues -- somewhat weakly -- that the risk
for a lunar flight was about one in ten.
There were 9 such flights (Apollos 8, 10, and 11-17), one of which
(13) was a failure. While no crews were lost, the Apollo 13
oxygen-tank burst came *extremely* close to being fatal; it would have
been fatal had it happened on the trip home, or had any of several
other factors been a little worse. Moreover, the same flight (!) came
very close to a catastrophic launch failure due to the pump-amplified
second-stage center-engine Pogo (which is not well known because the
tank burst overshadowed it and MSFC understandably kept a low profile
about it -- 13's Flight Evaluation Report has a whole chapter about
it, but with such bland matter-of-fact wording that you have to read
and understand the numbers to realize what a close call it was).
There were a few other lesser incidents on other flights that were a
bit worrisome.
That adds up to maybe 1 in 9, which you might as well call "about one
in ten" because there's room to argue about the addition, and the
sample size is too small to be very precise anyway. (If one case
turning out differently would greatly change your estimate, then your
estimate can't be very precise.) It would be a bit surprising if the
real odds were as bad as one in five or as good as one in twenty, but
there isn't enough data to reject such a suggestion very convincingly.
There *is* enough to inspire real doubt that the official
crew-survival spec (99%) was met.
A decade ago the then Constellation architecture for earth-Moon human
transport came in at about one in ten.
The initial flight of the Space Shuttle was retrospectively estimated at one in four; the last flight at one in 63.
Plausible numbers.
However, a caution: all this is not saying that human spaceflight is
*inherently* very dangerous. It's saying that human spaceflight done
the traditional way is inherently very dangerous. Which isn't too
surprising, given that all of those crews were (or would have been)
flying on what should be considered *early test flights* of complex
new vehicles, all also involving major hardware that was flying for
the very first time.
We *know* how to do much better. Nobody puts valuable payloads, or
crews other than test pilots, on the first flight of a new aircraft
design *or* the first flight of a new production article of an
established design. Proper flight-test programs for new designs
involve hundreds of flights, not two or three, including systematic
envelope expansion and tests of contingency cases like engine failure,
and every single new production vehicle is test-flown before it's
used.
Doing the equivalent for spaceflight should be possible. It would
require vehicles that are designed rather differently, and development
programs that are designed and run very differently. (Notably, the
ability to fly some semblance of a proper flight-test program, at an
affordable cost in a reasonable time, needs to be a non-negotiable
design requirement. This does preclude (a) major expendable
components and (b) vehicles that have to be essentially rebuilt after
every flight.)
"Spaceflight is difficult -- today. It is expensive -- today. And
the level of risk remains high -- today. But it need not remain so
forever. We must resist the idea that space is inherently difficult,
expensive, and risky. Aviation once seemed so, too. Today, aviation
is efficient and safe. Space can get there -- if we accept that it
can improve, and realize that it will require hard work, investment,
and experimentation." -- Joseph C. Anselmo (AW&ST Editor-In-Chief),
3/10 Nov. 2014 issue.
Henry
