Discovery fueling question

They tanked up Discovery yesterday, but had to scrub the launch and
de-fueled it.
Today they are tanking it up again, although they say that there is only
about a 30% chance that the weather will cooperate.
If it's another "no-go" today then they may either try it again
tomorrow, or hold off for Tuesday the 4th.*
My question is this: Since they thought that the loss of the PAL ramp
foam on the last flight may have been exacerbated due to the multiple
fuelings due to the faulty LH2 sensor causing the foam to contract at a
slower rate than the tank's metallic structure and debond from its
surface, is this tanking up and de-tanking a good idea to do repeatedly?
Might it not make more sense to just wait for a day when the weather
forecast looks quite favorable for a launch rather than subject the foam
to the thermal stress over and over again on iffy weather days?

* You know, when we do the big fireworks displays with the exploding
rockets... :-\

Pat

On Sun, 02 Jul 2006 07:18:53 -0500, Pat Flannery
wrote:

They tanked up Discovery yesterday, but had to scrub the launch and
de-fueled it.
Today they are tanking it up again, although they say that there is only
about a 30% chance that the weather will cooperate.
If it's another "no-go" today then they may either try it again
tomorrow, or hold off for Tuesday the 4th.*
My question is this: Since they thought that the loss of the PAL ramp
foam on the last flight may have been exacerbated due to the multiple
fuelings due to the faulty LH2 sensor causing the foam to contract at a
slower rate than the tank's metallic structure and debond from its
surface, is this tanking up and de-tanking a good idea to do repeatedly?
Might it not make more sense to just wait for a day when the weather
forecast looks quite favorable for a launch rather than subject the foam
to the thermal stress over and over again on iffy weather days?

* You know, when we do the big fireworks displays with the exploding
rockets... :-\

Pat

Couldn't they just leave it full for a few days? If a little
evaporated, they could top it off as needed.

Pat Flannery wrote:

* You know, when we do the big fireworks displays with the exploding
rockets... :-\

The current launch campaign has been marked by senior NASA folks saying
and doing things that, should a not-all-that-improbable (5% or so)
unfortunate event eventuate, I would not not want to have on my record.
Including launching on Fireworks Day.

Even if the seven astronauts have to safe-haven at ISS and somehow get
home, that wouldn't be a positive mark on the managers' record given
what they've said.

But then I'm not them.

wrote:

The current launch campaign has been marked by senior NASA folks saying
and doing things that, should a not-all-that-improbable (5% or so)
unfortunate event eventuate, I would not not want to have on my record.
Including launching on Fireworks Day.



Yeah...I wouldn't do that... I don't know why, but I wouldn't do it on
the 4th.

Pat

On 2 Jul 2006 17:19:30 -0700, in a place far, far away,
" made the phosphor on my
monitor glow in such a way as to indicate that:


Pat Flannery wrote:

* You know, when we do the big fireworks displays with the exploding
rockets... :-\

The current launch campaign has been marked by senior NASA folks saying
and doing things that, should a not-all-that-improbable (5% or so)

Where do you get that number?

I continue to think that the probability of a mishap is lower than
it's ever been, and it's never been more than one percent or so.

Rand Simberg wrote:
On 2 Jul 2006 17:19:30 -0700, in a place far, far away,
" made the phosphor on my
monitor glow in such a way as to indicate that:


The current launch campaign has been marked by senior NASA folks saying
and doing things that, should a not-all-that-improbable (5% or so)

Where do you get that number?

It comes from the interpretation of "probable" as meaning =50% chance
of happening over the next (and last) 16 flights if the per-flight
risk of ramp shedding remains what it is on this flight. Which, it is
hoped, it won't, but we're talking about this flight. 0.95^16 = 0.44,
0.96^16 = 0.52, so the ice ramp alone gets you to the neighborhood of
5% on this flight. Add to that the chance of misadventure from all
other causes, and 5% seems not all that improbable -- IMHO, of course.

This does assume that the NASA people who put the ramp problem in the
probable/catastrophic box are to be taken seriously.

I continue to think that the probability of a mishap is lower than
it's ever been, and it's never been more than one percent or so.

1% for historical flights isn't ruled out by the record, though 2%
seems a bit more in line with it. But the uncertainties are
significant. And, as noted earlier, it isn't totally clear as to
whether the relevant historical flights are all those since the first
one, or a smaller, more recent set that might have begun, for example,
when changes were made to the foam formulation and application
procedures. Accepting 5% as a risk number for this flight (and at least
the previous two) implies a smaller set of relevant historical flights.

On 3 Jul 2006 09:31:57 -0700, in a place far, far away,
" made the phosphor on my
monitor glow in such a way as to indicate that:

Rand Simberg wrote:
On 2 Jul 2006 17:19:30 -0700, in a place far, far away,
" made the phosphor on my
monitor glow in such a way as to indicate that:


The current launch campaign has been marked by senior NASA folks saying
and doing things that, should a not-all-that-improbable (5% or so)

Where do you get that number?

It comes from the interpretation of "probable" as meaning =50% chance
of happening over the next (and last) 16 flights if the per-flight
risk of ramp shedding remains what it is on this flight. Which, it is
hoped, it won't, but we're talking about this flight. 0.95^16 = 0.44,
0.96^16 = 0.52, so the ice ramp alone gets you to the neighborhood of
5% on this flight. Add to that the chance of misadventure from all
other causes, and 5% seems not all that improbable -- IMHO, of course.

This does assume that the NASA people who put the ramp problem in the
probable/catastrophic box are to be taken seriously.

I suspect they're being overconservative. I also suspect you're
taking them a little too literally, or misparsing their statement.
I'll bet if you asked them, they wouldn't agree that
"not-all-that-improbable" means "probable" in the sense you describe.

I continue to think that the probability of a mishap is lower than
it's ever been, and it's never been more than one percent or so.

1% for historical flights isn't ruled out by the record, though 2%
seems a bit more in line with it. But the uncertainties are
significant. And, as noted earlier, it isn't totally clear as to
whether the relevant historical flights are all those since the first
one, or a smaller, more recent set that might have begun, for example,
when changes were made to the foam formulation and application
procedures. Accepting 5% as a risk number for this flight (and at least
the previous two) implies a smaller set of relevant historical flights.

But it also implies that everything we've done since Columbia to fix
the problem has actually made it *worse*. I suppose that's possible,
but if it's true, they should just give up and shut the program down
now.

" wrote in
oups.com:

Rand Simberg wrote:
On 2 Jul 2006 17:19:30 -0700, in a place far, far away,
" made the phosphor on my
monitor glow in such a way as to indicate that:

The current launch campaign has been marked by senior NASA folks saying
and doing things that, should a not-all-that-improbable (5% or so)

Where do you get that number?

It comes from the interpretation of "probable" as meaning =50% chance
of happening over the next (and last) 16 flights if the per-flight
risk of ramp shedding remains what it is on this flight.

That's how I originally thought NASA defines "probable" as well, but it's
not. NASA does indeed define "probable" as "expected over the life of the
program", and they do indeed define "expected" as "P 0.5", but "life of
the program" is defined as a fixed 100 flights, not the actual number of
flights remaining. 0.5^(1/100) = 0.993, or 0.7% probability of loss, or
about 1 in 140.

This does assume that the NASA people who put the ramp problem in the
probable/catastrophic box are to be taken seriously.

Bryan O'Connor, NASA's chief of Safety and Mission Assurance, is one of
those people, and in this interview he defines "probable" exactly as I have
done:

http://www.spaceflightnow.com/shuttle/sts121/060629preview/part1.html

"The definition of probable/catastrophic in the program's own terminology
is that this thing that we're talking about is likely to cause loss of the
vehicle over the life of the program," O'Connor said in an interview with
CBS News. "And 'likely' doesn't mean 'assuredly.' It's interpreted as a 50-
50 chance that over 100 missions this thing would take out an orbiter."

--
JRF

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