Rockets

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

In article ,
"George Kinley" wrote:

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

They expell mass in one direction, which causes them to go in the other
direction. The atmosphere has very little to do with how rockets work;
it's just a simple law of physics: every action (force) causes an equal
and opposite reaction (counterforce). The rocket "pushes" on the
exhaust gas, causing it to accelerate to a very high velocity; the
exhaust gas pushes back on the rocket, causing it to accelerate in the
opposite direction (but more slowly, since the rocket has a lot more
mass than the exhaust gas -- but that involves a different law).

When you get into high school physics, you'll learn all about Newton's
laws and how things like rockets work. It's really cool stuff, and it's
neat to see you taking an interest in it already -- you'll probably be
way ahead of most of the class!

Best,
- Joe

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| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'

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.

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html

In article Pine.LNX.4.50.0307110958020.29738-100000
@kolmogorov.physics.uq.edu.au, says...
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).

Or increase the speed. Stand on the skateboard and fire a shotgun,
you'll move.

Marc

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.

John Schoenfeld wrote:

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.

How? Won't that break conservation of momentum?

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In article ,
(John Schoenfeld) wrote:

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.

No you couldn't; that would violate conservation of momentum. In the
system you've described, the center of mass of the entire system would
(through some trickery of pipes and combustion chambers) accelerate
itself off in one direction. That's impossible -- you might as well
propose a perpetual motion machine.

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).

Only if by "extremely difficult" you mean "impossible."

Also note that you don't even need mass to escape from a rocket.

Yes, you do (if only by definition of a rocket). Of course there are
other ways to accelerate something, e.g. by pushing it with an external
force (like a sail) or by pushing against an external medium (like an
airplane). But then it's not a rocket.

Inertial propulsion is not prohibited in physics if you think about
it.

Um, yes, it is.

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.

There is conservation of momentum, and you just proposed to violate it.
This is a stiction (static friction) drive, and works only when in
contact with some other body. The movement comes from the difference
between static and sliding friction. And, BTW, if you want a drive that
only works when in contact wiith a larger body, there are much better
ones (the wheel comes to mind).

Such a drive is utterly useless in space, however. Jerk your rocket
around all you want, it'll never make any net progress at all.

Cheers,
- Joe

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'

Joe Strout wrote in message ...
In article ,
(John Schoenfeld) wrote:

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.

No you couldn't; that would violate conservation of momentum. In the
system you've described, the center of mass of the entire system would
(through some trickery of pipes and combustion chambers) accelerate
itself off in one direction. That's impossible -- you might as well
propose a perpetual motion machine.

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).

Only if by "extremely difficult" you mean "impossible."

Certainly not. You could add holes along the side of the rocket so
that the air-pressure can let the explosion exit without affecting the
overall net upwards momentum. Although this would be inefficient, it
demonstrates that the CAUSE of upwards acceleration is IRRESPECTIVE of
the exit direction of expelled mass.

Also note that you don't even need mass to escape from a rocket.

Yes, you do (if only by definition of a rocket). Of course there are
other ways to accelerate something, e.g. by pushing it with an external
force (like a sail) or by pushing against an external medium (like an
airplane). But then it's not a rocket.

Inertial propulsion is not prohibited in physics if you think about
it.

Um, yes, it is.

No it is not. Imagine a stationary black-box floating in space. One
wall of the box is hard iron and the opposite side is ellastic. If a
ball is thrown from the middle at the hard iron wall there will be a
high-impulse transfer of momentum from the ball to the box. Relative
from the center of the box (which at this point is moving), the ball
now approaches the opposite ellastic wall in which it inevitably
collides with and transfers the same momentum but in the opposite
direction bringing the box to rest again. However, the elastic wall
collision was low-impulse and took longer for the momentum to be
conservered. Irrespective of momentum conservation, there is an
overall displacement.

At this point we have the box at rest yet it is displaced from its
original position, however in future time this same effect will occur
but in the opposite direction and thus the overal motion of this
contraption would be to OSCILLATE about the original position. So
technically speaking, its not inertial propulsion yet as the center of
mass is constant.

So the third and final requirement would to have a constant stream of
balls colliding just as the first one thus always staying one step
ahead of the "backwards oscillation phase".





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.

There is conservation of momentum, and you just proposed to violate it.
This is a stiction (static friction) drive, and works only when in
contact with some other body. The movement comes from the difference
between static and sliding friction. And, BTW, if you want a drive that
only works when in contact wiith a larger body, there are much better
ones (the wheel comes to mind).

Such a drive is utterly useless in space, however. Jerk your rocket
around all you want, it'll never make any net progress at all.

Sure you can, read above.

"John Schoenfeld" wrote in message
om...

Certainly not. You could add holes along the side of the rocket so
that the air-pressure can let the explosion exit without affecting the
overall net upwards momentum. Although this would be inefficient, it
demonstrates that the CAUSE of upwards acceleration is IRRESPECTIVE of
the exit direction of expelled mass.


No you can't since the "explosion" exiting would now push against the rocket
from all sides and the rocket would not move.

No it is not. Imagine a stationary black-box floating in space. One
wall of the box is hard iron and the opposite side is ellastic. If a
ball is thrown from the middle at the hard iron wall there will be a
high-impulse transfer of momentum from the ball to the box.

First, you've already ignored, "what throws the ball." Hint, if it's
attached to the box (saw a cannon) the box will move backwards until the
ball hits the front wall. At that point the box will move forward and end
up where it started. No net movement.

If the cannon is not attached, it will be propelled backwards, hitting the
back wall, forcing the box backwards. The ball hitting the front will then
push the box forwards the same amount. No net movement.

Relative
from the center of the box (which at this point is moving), the ball
now approaches the opposite ellastic wall in which it inevitably
collides with and transfers the same momentum but in the opposite
direction bringing the box to rest again. However, the elastic wall
collision was low-impulse and took longer for the momentum to be
conservered. Irrespective of momentum conservation, there is an
overall displacement.


No, there isn't. The box returns to its original position.

At this point we have the box at rest yet it is displaced from its
original position, however in future time this same effect will occur
but in the opposite direction and thus the overal motion of this
contraption would be to OSCILLATE about the original position. So
technically speaking, its not inertial propulsion yet as the center of
mass is constant.


THat part is right. All it will do is oscillate.

So the third and final requirement would to have a constant stream of
balls colliding just as the first one thus always staying one step
ahead of the "backwards oscillation phase".


Again, this simple doesn't work.






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.

There is conservation of momentum, and you just proposed to violate it.
This is a stiction (static friction) drive, and works only when in
contact with some other body. The movement comes from the difference
between static and sliding friction. And, BTW, if you want a drive that
only works when in contact wiith a larger body, there are much better
ones (the wheel comes to mind).

Such a drive is utterly useless in space, however. Jerk your rocket
around all you want, it'll never make any net progress at all.

Sure you can, read above.

In article ,
John Schoenfeld wrote:
...
... ellastic. ...
...
... ellastic ...
...

I have often wondered why idiosyncratic misspelling
so often coincides with crackpotism. Anyone know?
(It is certainly a helpful shibboleth.)