(James Oberg) wrote:

We're also seeing that the NASA analysis of tile threat was correct -- foam
hitting tiles didn't hurt and was unlikely ever to do so.

The spillover of this experience to foam-hitting-RCC was the flaw, as I see
it, and here (I'm writing up a longer piece) the culture failed.

But tiles? Bang on 'em, scrape them, knock some off -- they take a lickin'
and they kept on tickin', and seduced space officials into thinking that RCC
would act the same way -- and they NEVER, EVER did tests to check out this
convenient analogy.

Wait a minute. Your interpretation of the managment thinking may be correct.
It probably was the RCC this time. But regarding the tiles the question
of early boundary-layer transition came up last months. I`m unfamiliar
with hypersonic transition. But in subsonic there is a range of
Reynolds nummbers inside them the occurence of transition is only
predictable by statistics. It seems the shuttle in early re-entry
is in such a range:


http://www.newsmax.com/archives/arti...7/194414.shtml
The first known aerodynamic shift occurred on mission STS-28 in 1989 and
was studied carefully by Gibson, an aeronautical engineer who helped
investigate the Challenger disaster and redesign shuttle's solid rocket
boosters. The Challenger exploded on liftoff in 1986, killing its
seven-member crew. Gibson was also commander during four shuttle missions
and piloted a fifth mission.

Gibson told UPI he found the surface of Columbia's wings was two-to-four
times rougher than the wings of the three other shuttles -- Atlantis,
Discovery and Endeavour -- and that Columbia's left wing was 50 percent
rougher than its right. He suspected the roughness caused the 1989 shift
and another in 1995.

NASA engineers did not pay much attention to Gibson's concern in 1989,
he said, finding another cause for the shift. Other experts told UPI,
however, that such roughness could trigger a premature aerodynamic shift,
leading to additional heating and drag.


The transition (here called shift) is not only related to the mean
roughness but could be triggered by very small flow obstacles in
specific distance to each other. Like inducing an oscilation by some
wavelenght. The transition occurs as an accumulation of small vortices
over some lenght to the whole turbulent layer. In the case of the shuttle
a single damaged tile will never be a problem. But what if a streamline
crosses several damaged (super-rough) places? It seems whether the
transition occurs is not only a matter of how many damages we have but
their location to each other too. It could cause an burn through in an
un-damaged downstream area:

http://www.chron.com/cs/CDA/story.hts/space/1790143
Foam or ice damage to the wing can cause turbulent airflow earlier than
normal in a shuttle's re-entry to Earth's atmosphere, wrote Dennis
Bushnell of NASA's Langley Center. This can produce friction at the
hottest point in the shuttle's flight.

"If the flow is turbulent at peak heating, the heat shield would/could
burn through the wheel well doors (even with undamaged tiles)," he
wrote, based on his research for the shuttle's original flight
certification in 1980.



http://www.orlandosentinel.com/news/...cssboundary170
31703mar17,0,669319.story
A Columbia flight in 1995 underwent the earliest-known boundary-layer
transition at Mach 19, or roughly 13,000 mph. That's approximately the
speed the orbiter was traveling Feb. 1 as it began to break up.

That was one of four times Columbia is known to have experienced early
transition. During another instance, in 1989, a few of its protective
silica tiles partially melted on the dive through the atmosphere.



The following seems me to refer to a later super-sonic flight regime
where heat is less the issue then the now higher stagnation pressu

http://www.floridatoday.com/columbia...ry2A44731A.htm
"One tile here and one tile there does not affect anything; you get
some local turbulence," said Greg Sakala, a former NASA shuttle
program engineer. "You get in too deep . . . it does not take much
to get a loss of laminar flow, which could create a lot of turbulence.
Then you get what they call burbling, which would start ripping tiles
off behind like that zipper effect they keep talking about. Then the
skin heats up."

"You might not cause burn through, but you could cause enough turbulence
to create drag that overcomes the shuttle's ability to compensate,"
Sakala said.


To sum up this has not much to do with Columbia. But the danger of tile
damage in further missions should not be downplayed.


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