Dear publish1000-ga;
You posed a great question. The answer is really much simpler than it
first appears.
The US Space Shuttle uses a rocket propulsion system to get into its
final orbit in its approach to Earth. Other than that the Shuttle is
essentially falling and during re-entry the aircraft is actually a
glider. The reentry speed is largely dictated by the Earth
gravitational pull and given the physics of such a reentry the speed
is typically approximately 25,405 feet per second (17,322 statute
miles per hour). The Shuttle has relatively small rockets that can be
fired to get the aircraft in a position to maximize the drag that aids
in slowing the vehicle down enough so the pilot can select an
appropriate trajectory and target (called the deorbit maneuver). Even
with this at their disposal it only decreases the Shuttle?s velocity
approximately 300 feet per second (205 mph) for reentry. This in turn
allows the crew to select the best angle of approach and dissipate the
reentry speed to a safe level to ideally ensure that the vehicle does
not ?burn up?. This maneuver is called a hypersonic split-S maneuver,
or a kind of zigzag flight pattern that generates additional drag in
the same way that a skier going down a hill executes the maneuver to
decrease his speed.
At 17,500 miles per hour there simply isn?t enough time, given the
distance of the craft from the Earth and the Earth?s gravitational
pull to execute enough split-S maneuvers to slow the vehicle down more
than they already do. To further simply the explanation, a downhill
skier is essentially experiencing a controlled fall. If, for example,
a skier is careening down a one mile long slope at 500 miles per hour,
assuming he doesn?t kill himself in the process, is able to pull off
the maneuver, he might have enough time at that speed to execute one
split-S before he reaches the end of his glide path. This maneuver
will slow his descent a certain percentage but it is impossible to
slow down more because there simply isn?t enough time from beginning
to end to perform the maneuvers enough times to bring him to a halt or
slow him down significantly to make his decent a leisurely glide.
What?s more, objects in space are not stationary. They may ?appear? to
be stationary but like a boat under the influence of a stream, objects
in space are orbiting or falling (due to gravitational pull) or they
are propelling under their own power. The shuttle drifts in space in
the same way that a boat drifts in the stream. In space there is no
physical drag factor so brakes and the like, in the conventional
sense, cannot function and would achieve nothing. The only drag in
space is the drag created by oppositional forces. In other words, for
the boat to give the appearance that it has stopped, it would need to
exert an equal amount of propulsion against the force of the stream.
In essence, the boat would have to speed upstream under enough power
to cause it to remain in one place, but not more lest it travel
upstream. While it appears motionless it is effectively moving forward
just enough to maintain its seemingly stationary position.
For the Shuttle to do this against the force of the earth?s
gravitational pull, it would have to have en enormous store of fuel
that exceeds, under current technology, the weight restrictions that
would allow us to feasibly get the vehicle in space to begin with.
Even if, theoretically of course, we were to create some type of
refueling station in space that the Shuttle could dock with just prior
to it?s return and take on additional fuel to create enough drag
through opposing propulsion, the concept would still be unfeasible.
The amount of fuel it would take to pull this off successfully would
be prohibitive because it would like trying to put enough fuel in a
small boat to propel it hard enough to stabilize it and cause it to
maintain a seemingly stationary position midway up the face of Niagara
Falls. It simply could not carry enough fuel or exert enough thrust to
do that.
In the end, the best we can do under current technological constraints
is to slow the Shuttle enough to control the fall until it reaches the
Earth?s lower atmosphere where it can fly like a normal airplane and
the low atmospheric laws of physics take over. By then of course the
fall is already technically over and the rest of the journey is
flight.
I hope you find that my answer exceeds your expectations. If you have
any questions about my research please post a clarification request
prior to rating the answer. Otherwise I welcome your rating and your
final comments and I look forward to working with you again in the
near future. Thank you for bringing your question to us.
Best regards;
Tutuzdad-ga ? Google Answers Researcher
INFORMATION SOURCES
SPEED REGIMES
http://www.grc.nasa.gov/WWW/K-12/airplane/hihyper.html
CBC NEWS
?Coming home: steps of a shuttle's re-entry?
http://www.cbc.ca/news/features/shuttle_columbia/columbia_reentry.html
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION (NASA)
http://spaceflight.nasa.gov/shuttle/reference/shutref/sts/requirements.html
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