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  #1  
Old July 10th 03, 03:20 PM
George Kinley
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Default Rockets

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


  #2  
Old July 10th 03, 11:13 PM
Joe Strout
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Default Rockets

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

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'
  #3  
Old July 11th 03, 01:01 AM
Timo Nieminen
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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
  #4  
Old July 11th 03, 04:39 AM
Gordon D. Pusch
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Default Rockets

"George Kinley" writes:

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


From the Laws of Physics. Specifically, from Newton's Law of Action and

Reaction --- See http://curious.astro.cornell.edu/question.php?number=100.

A rocket throws mass out it's back --- that's the action. In order to
conserve momentum, the rocket has to have an equal and opposite recoil ---
that's the reaction.

Rocket do not need an external atmosphere to "push against" ---
the mass of their own exhaust is quite sufficient.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

  #6  
Old July 11th 03, 02:12 PM
Gregory L. Hansen
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Default Rockets

In article ,
George Kinley wrote:
Hi,
if there is no Atmosphere, where do rockets that go in Space get thrust from




From the engine.


Combustion creates pressure inside the engine, which pushes against all
the walls, and against the front... except there's a big hole in the back,
so nothing presses against the back.

--
"Is that plutonium on your gums?"
"Shut up and kiss me!"
-- Marge and Homer Simpson

  #7  
Old July 11th 03, 05:48 PM
John Schoenfeld
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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.
  #8  
Old July 11th 03, 07:48 PM
Penguinista
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Default Rockets

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


Goddard was pestered with the same question. The answer is simple, by
pushing against the propellent being thrown out through the engine.

Consider a kid on a very low drag sled and a pile on beanbags. By
throwing the beanbags in one direction, he can build up speed in the
other direction.

  #9  
Old July 12th 03, 05:41 PM
Ian Stirling
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Default Rockets

In sci.space.tech 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


Err, no it's not.
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


In some ways, this is a not utterly inaccurate description of the
rocket combustion chamber, up to the throat.
However, at the throat the rules change.
The gas enters the widening nozzle, and rapidly expands.
The expanding gas causes a pressure against the nozzle walls, and
the component of the force which acts opposite to the direction of
the exhaust is transferred through the nozzle walls.
If you just have a hole in the combustion chamber, the exhaust
pretty much spreads over a hemisphere, and you get relatively
little thrust.
A deLaval (sp?) nozzle, produces a much higher thrust, as the
exhaust is not a hemispherical expanding gas, but a relatively
cold jet going in the opposite direction.

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).


This is totally incorrect.
A few seconds with a balloon, a bit of tape to tape the neck down to the
balloon should convince you of this.


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.



This is also totally incorrect.
Conservation of momentum means that you have to expel something.

--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
"The theory of everything falls out trivially." -- Etherman, sci.physics kook.
  #10  
Old July 12th 03, 07:09 PM
Greg D. Moore \(Strider\)
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Default Rockets


"John Schoenfeld" wrote in message
om...
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.


Umm. No. It's directly related to the direction of the mass expulsion.

I could build a rocket that expels its
mass upwards and it would still accelerate upwards.

Good luck.

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).


Nope, because as soon as you did that, your piping would have be acting just
like the top of the combustion chamber.


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.


Yes, you do need mass to escape. You can't design a rocket that pushes UP
more than down unless it can eject mass out the back.


 




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