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Q: Space Shuttle re-entry speeds ( Answered 4 out of 5 stars,   6 Comments )
Subject: Space Shuttle re-entry speeds
Category: Science > Technology
Asked by: publish1000-ga
List Price: $26.00
Posted: 01 Nov 2005 08:07 PST
Expires: 01 Dec 2005 08:07 PST
Question ID: 587531
Why do they have to come back to earth so fast?  Can't they just slow
down in orbit or even stop and point the nose down and gently glide
back to earth? I remember a test pilot that went up in a balloon to
the edge of space and free fell back to earth and he never burned up> 
Then the shuttle would not need all those heat tiles.
Subject: Re: Space Shuttle re-entry speeds
Answered By: tutuzdad-ga on 01 Nov 2005 09:59 PST
Rated:4 out of 5 stars
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

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



?Coming home: steps of a shuttle's re-entry?




Google ://


Space shuttle




split-S maneuver
publish1000-ga rated this answer:4 out of 5 stars
Thank you for the quick response.

Subject: Re: Space Shuttle re-entry speeds
From: kottekoe-ga on 01 Nov 2005 19:41 PST
A shorter way to say this:

Orbital velocity is ~17,000 miles per hour. To get out of orbit you
can burn a little bit of fuel to lower your orbit enough to encounter
the drag of the atmosphere and then use that drag to slow you from
17,000 mph to zero. Alternatively, in principle, you could burn a lot
of fuel to slow your velocity to zero before hitting the atmosphere
and then glide back at low speed. To do the latter would require
approximately the same amount of fuel that it takes to get into orbit
in the first place. Imagine the enormous size of rocket that would be
required to put the entire space shuttle with fully loaded external
tank and solid rocket boosters into orbit. Not a sensible way to go!
Subject: Re: Space Shuttle re-entry speeds
From: publish1000-ga on 03 Nov 2005 17:53 PST
Ok, I see what you are getting at.  BUT, what if you insert your craft
into orbit in the direction the earth below you is turning, and
achieve the same speed as the earth turns, seems to me you could hover
over florida and point your nose down and push off at about 25 miles
an hour.  Your craft would pick up speed gently and just glide in
circles till you reach the ground.  Somewhere I have heard of shuttles
doing 24000 miles an hour, up there in space.  My whole work place
thanks you for your input.  I don't know about this.  You know a lot
more that I do.    What do you think.... David
Subject: Re: Space Shuttle re-entry speeds
From: kottekoe-ga on 03 Nov 2005 21:51 PST

The only way to be in orbit and be stationary with respect to a spot
on the earth's surface is to be in geosynchronous orbit, at an
altitude of 22,000 miles. This is far beyond the capability of the
shuttle, which can only reach an altitude of about 200 miles. Even if
you could get that high, you couldn't hover over Florida, only over a
spot on the equator. If you were in a cricular orbit far above the
atmosphere, pointed the craft down, and turned on the rocket to give
yourself a small downward velocity, you would only succeed in slightly
perturbing the orbit into a ellipse with your altitude decreasing for
a while, then increasing, reaching a higher maximum altitude, then
decreasing again until it crossed the original orbit exactly one orbit
later. This is easier to explain with a diagram and a bit confusing if
you haven't thought about orbital mechanics before.
Subject: Re: Space Shuttle re-entry speeds
From: iang-ga on 04 Nov 2005 10:34 PST
There's a java applet at
that shows how the orbit of a satellite depends on its velocity.

There's another one at which does
the same thing, but in a slightly different way -  more fun, but less

Ian G.
Subject: Re: Space Shuttle re-entry speeds
From: publish1000-ga on 04 Nov 2005 16:22 PST
My friends and I want to thank each of you for helping out with my question.
You have answered the question on orbital speeds that we have kicked
around for years, thank you very much.  Now we can think about women. 
Subject: Re: Space Shuttle re-entry speeds
From: tutuzdad-ga on 04 Nov 2005 16:36 PST
We've answered those question here too.  ;)

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