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OUTER SPACE PROPULSION
Name: albertkeg
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1993 - 1999
Question:
Hello, my name is Kevin E. Gaasch. I was wondering; if outer space
is made up of nothing, then how does a space vessel get it's propulsion? My
guess is that the substance created by the thrusters propels the ship. I would
appreciate any answers you could give me.
Replies:
Your guess is correct! When we watch the Shuttle blast off, it really
does look as if the exhaust gases are pushing against the Earth to force the
rocket upward. But that isn't what happens. The rocket would move off in
the direction opposite the exhaust-gas velocity even in outer space, away
from any planetary surface to "push" on. Here's why. When you blow up a
balloon and let it go without tying it, the air escapes and makes the ball-
oon fly around. Let's think about why this happens. First, suppose the
balloon is blown up and tied. The air inside pushes outward on the balloon,
and the balloon pushes back on the air. But the balloon doesn't go anywhere
because the air inside pushes with the same force in all directions, so all
of the forces on the different parts of the inner surface of the balloon
cancel each other. Thus, there's no net force on the balloon due to the
inner air pressure. Now think of the untied balloon. In this case, the net
force on the balloon due to the air inside is not zero, because the force
exerted on the part of the inner surface opposite the neck of the balloon
is not balanced (canceled): the air at the neck rushes out instead of push-
ing on the balloon. Thus, the balloon accelerates in a direction opposite
the neck of the balloon. The situation with a rocket is very similar. The
burning fuel expands inside the fuel chamber and pushes on the inside of
the chamber; since it can escape out the exhaust tubes, the force on the
rocket is unbalanced and the rocket accelerates in the direction opposite
to the direction in which the exhaust gas moves.
RC Winther
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Update: June 2012
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