[meteorite-list] (OT) Shuttle Investigation: "Foam"

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:36 2004
Message-ID: <3F4C454F.71121E70_at_bhil.com>

Hi, All,

    The investigative report on the re-entry failure of the shuttle
Columbia was released today. The investigation has been remarkably open
to public inquiry and much that is in the report has been available as
it was developed. I imagine many of you have read it.

    What has struck me most is the degree to which the physics of the
"foam" problem has been misunderstood inside NASA. Questioned about the
possible role of the foam fragment that stuck the left wing only a week
after the disaster, the Administrator told Congress (more or less),
"It's as if a foam cooler blew out of a pickup truck on the freeway
ahead of you; you get hit with a piece of styrofoam at 60 mph -- no way
that could cause significant damage."

    OK, the Administrator is not an analytical engineer nor a physicist;
he's only saying what he heard in the briefings, from the experts, from
everybody with a say.

    And that's what's frightening.

    In flight, the Shuttle, the external tank, and its foam insulation
-- all make up what physicists call a frame of reference. These three
objects are all rigidly connected to each other. When a piece of "foam"
breaks off the external tank, the foam is suddenly NOT connected to that
frame of reference any more. It is now subject to the forces of its
situation without any reference to its old framework.

    What forces is the foam subject to? Well, gravity is the most
obvious. Now that the foam is no longer connected to the Shuttle/Tank
framework, it starts to fall. We watch the pictures. The foam drops and
falls away. And that's where the "cooler blown from the pickup" false
analogy gets started.

    Yes, the foam falls. It covers 32 feet the first second. It's going
16 feet per second (about 13 mph) after one second, and so forth, blah,
blah, blah. It falls for 3 to 5 seconds, so it must be going, like, 60
mph, right?

    But, remember, the foam is not connected to its old framework
anymore. AND THAT FRAMEWORK IS ACCELERATING! Throttle control keeps
acceleration down to about 3 gee's, but at 3 gee's, the framework (which
now consists of Shuttle and Tank) is picking up an extra 70 mph every
second. Three seconds after the foam pops off, the Shuttle is closing in
on the foam chunk at a relative velocity of 200 mph! That's in addition
to that 50-60 mph the foam has picked up from gravity.

    Hey, wait, it gets a lot worse. The foam has been cut loose from its
old frame of reference which resisted all aerodynamic forces. Imagine
that you had a "foam gun" that could shoot a 12-inch spherical chunk of
styrofoam straight up at a really high muzzle velocity. Your 2 pound
chunk of styrofoam leaves the muzzle at 2000 mph, travels less than 5
feet, and stops, half-melted (or maybe wholly melted). Why? Because the
air resistance ("drag") at 2000 mph to a 12 inch ball is hundreds of
pounds. Since the chunk weighs only 2 pounds, it is being decelerated at
200 / 2 = 100 gee's of force plus 1 gee of gravity.

    When a physically big, but light weight, chunk of styrofoam pops off
the Shuttle, the aerodynamic forces, even at subsonic speeds, are so
great relative to the weight of the chunk of styrofoam that the
accelerations are in the class of results that you can only normally
achieve with an electromagnetic rail gun.

    So what's really happening is that the chunk of styrofoam is falling
at 1 gee, plus being chased by the Shuttle at 3 gee, plus being
aerodynamically accelerated downward at 20 gee's or 50 gee's or 80
gee's, depending on what portion of the flight we're talking about,
resulting in the chunk of styrofoam moving relative to its old frame of
reference as if accelerated at 24 gee's (or 74 gee's or 104 gee's...)

    In actual practice, the aerodynamic forces seem to be great enough
to accelerate the relative speed of that chunk of styrofoam to a speed
equal to the Shuttle's flight speed in less than a second. The best
generalization is that the Shuttle will be struck by the chunk of
styrofoam at a velocity equal to or greater than its own flight speed.

    The worrisome thing is that this result should have been obvious
from the very beginning of the design to anyone who ever took a high
school (OK, OK, college) physics course in which the instructor
fiendishly concocted a problem with two or three frames of reference in
it (which is to say, every self-respecting physics teacher).

    Does this mean that there was no one in NASA who understood high
school physics? Well, no, it doesn't. (I hope.) But it does seem to mean
that there was (almost) no one who appreciated the physical dimensions
of the problem. (Did no one get the extra credit question right on the
mid-term?)

    First, there were all those earlier foam "pop-off's" that did no
damage. That led people to stop worrying a long time ago. It led them to
tell other people who were newer to stop worrying. And, worst of all,
when told to stop worrying, they did. They took somebody else's word for
it and didn't think it through themselves. Unfortunately, that meant
that everybody just stopped thinking about it.

    If a foam "pop-off" happens very early in a flight, it will be
harmless. If the Shuttle's own acceleration is still at a low gee value,
the Shuttle does not "catch up" with the chunk very fast. If the
Shuttle's own flight speed is also low, the aerodynamic forces produced
by that speed on the chunk are also low. But with every second into the
flight, the potential forces (and the risks) increase by a high
exponential rate (equal to the 4th or 5th POWER of the elapsed time over
a previous time). It is this aspect of the problem that does not seem to
have been appreciated, if at all.

    Nor is it obvious what the "cure" could be.

    I believe that the long term failure to be truly aware of the
physics of the situation has its original root cause in the design
conceptualization of the Shuttle as a Space Transportation System, the
Space Truck, a kind of "space airplane," in other words, a Super Dooper
747. Despite the fact that it has wings and landing gear and an aircraft
layout, is too snooty to carry its own fuel tank around with it, and
needs boosters to kick its ass off the ground, underneath it all ---
IT'S A ROCKET SHIP!

    If you think you're flying a space ship, you look at everything
about this strange new monster with new eyes all the time, every time.
You don't think with your air-experienced "instincts" because you know
no one has the instincts for a space ship. You're forced to analyze
everything from scratch.

    But if you fall into the habit of thinking you're flying an
airplane, bigger, faster, further, yes, but still an airplane, you've
slipped out of the reality groove. You start making analogous
assumptions about the "aircraftness" of a vehicle that isn't really an
aircraft. Everyone "knew" the foam was harmless; it stood to reason...
At least it stood to reason in the foam cooler pickup truck world of
everyday low-energy physics. (Sorry, fellah, but you're in the wrong
world.) In fact, no one had "reasoned" about it.

    Before you know it, you're doing things like flying missions in
which there are only three space suits for a crew of seven. (It's 1915.
Do you fly one of those aero-planes without a parachute? Duh.) You're
managing an long haul airline instead of operating a space port. You are
actually looking backwards instead of trying to see into the future.
You're in trouble.

    OK, enough waste of bandwidth for an OFF-TOPIC post. (I've been
quiet lately; I have a little bandwidth coming.) A topic about which
much more could be said. And anyone who wants to say it or reply can
feel free to, on or off the List.

    Thanks.


Sterling K. Webb
Received on Wed 27 Aug 2003 01:44:48 AM PDT