(see the bottom panel) 
Continues on from this link |
Someone at the top is
finally waking up to reality, just like they did after Challenger. Then,
they realised that the ELV program was almost non-existent and took steps to
rectify it. Now, they find that only three Orbiters remain to look after the
ISS/HST and other great investments. Aviation is not isolated from the rest
of history in repeating itself every so often. The politicians see to that!
OVERTALK's sacrificial strip is an interesting concept. I feel,
however, that preventing bits falling off the ET during launch might have
first priority, as the TPS has worked satisfactorily on 111 occasions and
has been upgraded in the light of operational experience.
On the question of ET ice, I remain doubtful. It was not a cold day and
those who were around the stack prior to launch say that there was no
visible ice present on the tank. Also, OT, the ET coating is
weatherproof and, theoretically cannot soak up water. I understand
that there were no signs of cracked foam on launch day. However, the thermal
cycles you mention are valid. What if micro-cracks evolved
that were not easily visible but let in some moisture?
Ice has always been an issue on the ET, but large chunks of
foam falling off has not, until STS-112/107. There may be other issues here,
perhaps the earlier redesign of the foam to eliminate CFCs, for example. I
hate to raise it, but QC has to be considered too. As I said on an earlier
post, without the ice, one 2.5lb mass (or two!) striking at just the wrong
angle, at that speed, is quite a whack.
With all the descriptives flying around about the RCC sections (my favoured
theory) I thought it might help to post a diagram which illustrates the RCC
attachment system. This is from the Curry/Johnson
presentation (pdf 900kb) at the Future Space Shuttle Conference 1999.
The enlargement also shows the later mod and a T-seal. To my mind, the
(flexible) T-seals are one feature that would not enjoy being whacked and
could let in a lot of plasma, initially in a metered manner.
 |
Enlargement
As mentioned elsewhere, if plasma gets past, or through, an RCC panel, how
long would it take to reach past the secondary protection and on to the
aluminium attach points? Where it goes thence is largely academic - but
surely a "bad day."
Dr SYN
It may be the case that the ET foam is water-proof but it would only take a minor imperfection to allow water to infiltrate and affect the adhesive over 39 days of rainy freeze/thaw/re-freeze (mechanism for that described below). A large section of iced foam might detach not only because of degraded adhesive but because of the weight of water behind it. But there is a further factor - when you're talking about a cryogenic fuel-load.
"It was not a cold day and those who were around the stack prior to launch say that there was no visible ice present on the tank." Any poorly adherent foam section would look like any other but once the External Tank is fuelled with liquid hydrogen (at minus 250deg F), I understand that it is one large icicle. But you have to further consider that any water infiltrating behind the foam is not just ice, it is super-cooled and will contract quickly and mightily. That almighty contraction would suck in the surrounding foam - forming a circumferential crack (albeit one that may not be visible and may be initially only in the adhesive substrate - but that's where it counts). That crack then delineates the piece that will/could later detach. At max Q I would guess that there are areas of lower pressure around the ET (i.e. where the airflow sucks). Once thermodynamic heating reduces the adhesive quality of the ice itself behind that flawed section of increased weight foam (like hand-warming an ice-tray) that flawed section is free to detach (but it's still an icicle in stalactite form).
Even though the external tank's cladding may be tested waterproof where it's made, transportation, erection and attachment stresses on the empty vehicle may well compromise the water-proofing of the foam cladding on that flimsy, empty (and therefore flexible) tank. The solution may be to simply give it a good ScotchGuarding spray top-to-bottom once it is in the launch position.
But as further insurance, a sacrificial rubbery L.E. wedge (aka false leading-edge) on the Orbiter's wing would easily deflect any such stalactite and burn away early on re-entry. I will be surprised if they don't go for this as a fix.
OVERTALK from this link
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Wing breached by Super-hot Plasma |
An
Interpretation of the Starfire Columbia Photo |
|
Important Diagrams and Photos |
AW&ST Article about the Leading Edge
Theory |
|
"Obviously, it would be very important to understand what those pieces are, particularly the ones that started falling off at the very beginning" because they would shed light on the earliest stages of the breakup, he said.
However, Hallock said the pieces did not seem to be very big, judging from the light reflected off them.
"For us to find something that far back along the path, I think it's going to have to be a pretty substantial piece of the shuttle itself," he said.
"That's a lot of area to be looking ... We have the Grand Canyon area and all of the areas of Southern California, the mountainous area and stuff like this, that even if we could home in on some of these things, it's going to be very difficult to find it. But we sure would like to see it."
In their second news conference in as many weeks, the board members also said they are not convinced that a piece of debris that hit the shuttle's left wing shortly after liftoff on January 16 was insulating foam from the external fuel tank.
It was possible the debris was ice or a much heavier coating material beneath the foam, they said.
Hallock said a suspected breach in Columbia's left wing had to have been bigger than a pinhole, in order to allow the superheated gases surrounding the ship to penetrate the hull. Yes, as the RCC section's shattered pieces were oxidized and broke away as red-hot segments, more and more of the wing's flat leading edge surface (aluminum good to only 175deg F) would have been exposed to the super-heated plasma.
The board said it would hold its first public hearing next week to listen to non-NASA experts who have theories about what destroyed the shuttle. A location has yet to be chosen for the February 27 event.
The board has been criticised by some US lawmakers for being too closely tied to NASA.
Board chairman Harold Gehman Jr, a retired Navy admiral, said today: "We
will invite experts who are not associated with any US government program
who have theories or hypothesis, who have written to us or provided research
documents, to express to us their opinions."
The Associated Press
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