Shuttle: Onwards and downwards
- From: "GWhyte" <gwhyte3003@xxxxxxxxxx>
- Date: Thu, 11 Aug 2005 06:54:07 -0400
Shuttle
Onwards and downwards
Aug 4th 2005
>From The Economist print edition
http://www.economist.com/science/displayStory.cfm?story_id=4246425
A near miss for the shuttle-and red faces at NASA
Reuters
ON TUESDAY July 26th, the space shuttle Discovery launched from the Kennedy
Space Centre in Florida to much national jubilation. The delight was
short-lived. It was soon discovered that a large piece of foam had fallen
off her external tank. This looked ominous: it was a large chunk of falling
foam that damaged the shuttle Columbia in 2003 and caused it to burn up when
re-entering the Earth's atmosphere, killing the entire crew.
After several years of refitting at a cost of around $1.5 billion, NASA,
America's Space Agency, had hoped that it would not face another such risk.
But the situation may not be as bad as it first appeared. Discovery's chunk
of foam is smaller than the piece that helped to destroy Columbia. It was
travelling more slowly and going in a completely different direction. And,
what is more, this large chunk did not hit the shuttle. As a result, NASA
believes that Discovery should land safely.
It was never NASA's intention to eliminate entirely the shedding of
foam-nor, indeed, is it thought possible to eliminate foam shedding.
Instead, the agency focused on reducing the size of the bits that fell off
to the point of insignificance. It also took steps to prevent debris from
hitting the shuttle. Indeed, NASA managed to slash the number of times the
shuttle was struck by falling foam from an average of 150 on previous
launches to just two dozen this time and Discovery's launch was relatively
clean. NASA administrator Mike Griffin says Discovery is "the cleanest bird
we've had on orbit in recent memory. So we think she is safe to bring home."
Yet the problem of that one largish chunk, weighing around a pound, remains.
Why did it come off? A bit of background is needed. The fuel tank is coated
with a polyurethane-like foam, about an inch thick, which insulates the cold
liquid propellants inside and prevents ice from forming on the outside of
the tank. Falling ice, too, could damage the shuttle on launch.
Some 90% of the foam is applied by robots. This tends to be less likely to
flake off, as there are fewer voids or imperfections. On geometrically
tricky areas, however, the foam is applied by hand. These areas are most
prone to shedding. When modernising the tank, NASA had focused on them. For
example, one problematic area was at the place where the shuttle is attached
to the external tank. Here foam was removed completely and heaters were
installed instead to prevent ice building up.
Although the so-called "PAL ramp" area-from which the large chunk of foam
was lost-was considered for a new improved spraying process, NASA decided
the shuttle was safe to fly without this modification. One of the reasons
for this decision was that foam from this area had not come off since 1983.
Another is that tests on the hand-applied foam revealed no voids or
imperfections hidden underneath the surface. NASA was open about its
decision and shuttle commander Eileen Collins was aware of the decision.
NASA has said it will not fly any more shuttles until it has fixed this
particular problem. At present, it is too early to say whether this will be
a difficult problem to fix and thus whether the shuttle will remain grounded
for long.
However, the team working on it is already said to understand what is
causing the problem. Furthermore, a number of previously researched fixes
are also available. These include redesigning the area to eliminate the
ramps, changing the foam composition or application process, and greatly
reducing the volume of foam.
In many ways, this flight has shown up both the strengths and weaknesses of
NASA's approach. The new arrays of cameras and sensors will provide data
that will be used to make the shuttle safer. But it has also given everyone
a lot more information to fret about-from damage caused by relatively small
foam chunks hitting Discovery to bits of filler sticking out from the
shuttle's undersurface.
These bits of filler were a worry because NASA scientists calculated that,
by disturbing the smooth flow of air, the extruded filler could increase the
heat on re-entry of tiles downstream of it by between 10% and 30%. So the
agency decided this week to order an unprecedented space walk to fix the
problem. It was completed successfully on Wednesday.
The shuttle remains an ageing, expensive and difficult-to-maintain vehicle
that needs replacing. And yet, given the complexity of building reusable
space vehicles, its record has not been that bad. The shuttle programme has
managed over 110 flights in which foam chunks, extruding filler or other
problems have not brought an orbiter down. There were, however, two flights
in which, tragically, faults did cause disaster and the loss of 14 lives.
The shuttle Challenger disintegrated soon after take-off in 1986, killing
the entire crew. The other was Columbia. Today, with the redesigned shuttle
and less damage from debris, it should be safer-even if, in some areas, foam
is still coming away in larger chunks than was hoped.
Still, the pressure is very much on NASA. This latest mishap has breached
the agency's own protective shielding. If NASA cannot hunt down and
eliminate this problem, then it is difficult to see how it could continue
with the shuttle programme-whatever its obligations to the international
partners in the space station and even if Discovery has a picture-perfect,
and safe, landing this Sunday.
.
- References:
- Writing on the wall for US space shuttle
- From: GWhyte
- Writing on the wall for US space shuttle
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