Timo Nieminen wrote in message ...
On Thu, 10 Jul 2003, George Kinley wrote:

if there is no Atmosphere, where do rockets that go in Space get thrust from

Stand on a skateboard on a smooth flat surface, with a bag of rocks. Throw
the rocks in one direction, and you'll go in the other direction (except
for the inconvenient effects of friction - use bigger rocks and you should
see an effect). The rocket works exactly the same way. Throw mass away in
one direction, and you go in the other direction. "Conservation of
momentum" is what it's all about.

The direction of the rockets acceleration is irrespective of the
direction of mass explusion. I could build a rocket that expels its
mass upwards and it would still accelerate upwards. It is the
explosion in the combustion chamber that pushes out in ALL directions.
The upwards push makes contact with the top of the combustion chamber
transferring a net upwards momentum to the rocket. However, the
downward push from the explosion does not make contact with any
structure from the rocket and escapes - so the net momentum is
upwards. It should be noted that the actual upwards acceleration has
nothing to do with the output direction of the expelled mass (I could
very well route the mass from the explosion pushing downards via the
top or side, although this is both extremely difficult and very
inefficient).

Also note that you don't even need mass to escape from a rocket.
Inertial propulsion is not prohibited in physics if you think about
it. All that needs to be done is to make the upwards push a greater
impulse than the downwards push - the rocket would essential jerk its
way upwards - after all there is no "conservation of displacement"
with such an inertial system.