Ask A Scientist© Astronomy Archive

Question: If outer space is made up of nothing, then how does a space 
vessel get its propulsion?  My guess is that the substance created by the 
thrusters propels the ship.  I would appreciate any answers you could give me.
------------------------------------------------
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 is not 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 is why.  When you blow up 
a balloon and let it go without tying it, the air escapes and makes the 
balloon fly around.  Let us 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 does not 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 is 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 pushing 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.
Ronald Winther
=========================================================