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Re: Columbia's safety margins misread, expert tells panel




On Wednesday, Mar 26, 2003, at 13:23 US/Central, John A. Magliacane 
wrote:

> --- g6tmk <g6tmk@thersgb.net> wrote:
>
>> I don't believe I have heard anyone explain why it is that foam falls 
>> off
>> the fuel tank during launch.
>
> This is just a GUESS based on some speculation I've heard, but perhaps
> air leaks underneath the foam, becomes liquified as it comes in contact
> with the supercooled external tank, then freezes (which causes the 
> frozen
> air to expand) which in turn causes the foam to break away from the 
> tank
> under the vibration of launch.
>
>
> 73, de John, KD2BD

I don't think liquefied air expands, although if any water vapor 
condenses on the tank it might swell up a bit on freezing, unless it 
condenses directly to frost which it usually does at those 
temperatures.  Water is about the only substance I know of whose volume 
increases when it turns solid .. almost everything else shrinks.

As far as I know, the foam doesn't debond per se, it tends to scour due 
to the aerodynamic forces and sometimes gets weighed down with ice and 
breaks off when the humidity is high.  The early STS flights used ET's 
whose insulation was coated to control the scouring but this was 
omitted on later flights to save weight and allow slightly larger 
payloads, and unless I'm mistaken there are at least three insulation 
configurations for various ranges of payload weights.  STS-107 was 
flying a Spacelab mission and its payload was on the high side, so it 
got one of the lightest ET configurations.

The piece that hit the left wing looked like about the largest piece 
I've ever seen fall off an ET, but I haven't seen much launch film 
showing foam separation/impacts.  I wish someone had been able to film 
the impact from another angle because I would very much have liked to 
see the bottom HRSI surface of the wing before and after -- it's too 
hard to tell what the white stuff is falling below the wing after 
impact from the only angle I've seen ..
           --... ...-- -.. . -. ..... ...- -...
                   Bruce Bostwick N5VB

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