Rockets

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 sci.physics,

wrote
on 11 Jul 2003 12:42:21 -0700
:
Marc 182 wrote in message et...
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.

Actually, if the OP could throw the rocks straight up, at least
somebody would get a laugh.
Socks

It's only fun until someone gets a rock in the eye... :-)

--
#191,
It's still legal to go .sigless.

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 |
`------------------------------------------------------------------'

Penguinista wrote:
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.

This is misleading. Let's say you had a very tiny rocket in a
perfect vacuum, and hurled *one atomic nucleus* out of the back end at
extremely high velocity. The rocket would still be propelled in the other
direction, even though the atomic nucleus didn't push against anything,
since the momentum of the system would have to be conserved.

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.

Note that this beanbag system would also propel the kid in a perfect
vacuum, even if the beanbags he threw never collided with (or "pushed
against") each other.
--
-- With Best Regards,
Matthew Funke )

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) writes:

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.

Bull****. Your claim violates the law of conservation of momentum.
(BTW, do you know how an aircraft thrust reverser works ???)

[False comic-book explanation of how rockets work that violates Newton's Laws
sent to /dev/null where it belongs.]


-- Gordon D. Pusch

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

JRS: In article , seen in
news:sci.space.science, Greg D. Moore (Strider)
posted at Sat, 12 Jul 2003 18:09:17 :-

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.

Goddard, IIRC, did so.

It mattereth not from where the mass is ejected; only that it be ejected
backwards.

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

"Matthew F Funke" wrote in message
...
Penguinista wrote:
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.

This is misleading. Let's say you had a very tiny rocket in a
perfect vacuum, and hurled *one atomic nucleus* out of the back end at
extremely high velocity. The rocket would still be propelled in the other
direction, even though the atomic nucleus didn't push against anything,
since the momentum of the system would have to be conserved.

How do you hurl the atomic nucleus out of the back of the rocket w/o somehow
pushing on it?



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.

Note that this beanbag system would also propel the kid in a perfect
vacuum, even if the beanbags he threw never collided with (or "pushed
against") each other.

But again, the kid is pushing on the beanbags. That's the key. For every
reaction there's an equal and opposite reaction. There's no way to make the
beanbag go in one direction w/o pushing on it somehow.


--
-- With Best Regards,
Matthew Funke )

"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.

Greg D. Moore \(Strider\) wrote:
"Matthew F Funke" wrote:
Penguinista wrote:
George Kinley wrote:
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.

This is misleading. Let's say you had a very tiny rocket in a
perfect vacuum, and hurled *one atomic nucleus* out of the back end at
extremely high velocity. The rocket would still be propelled in the other
direction, even though the atomic nucleus didn't push against anything,
since the momentum of the system would have to be conserved.

How do you hurl the atomic nucleus out of the back of the rocket w/o somehow
pushing on it?

I can see that moving a nucleus about in some desired direction can
be considered "pushing on it". However, careful examination of the
response above -- that rockets get thrust "by pushing against the
propellant being thrown out throuygh the engine" -- could be read as
meaning that (a) the rocket is doing the pushing on the propellant *as the
propellant exits* to create forward momentum (which I would agree with,
though I might have worded it differently); or (b) that it is the
propellant being shoved by the rocket against previously-expelled
propellant that moves the rocket forward (which I wouldn't agree with).
Note that I didn't say that the reply above was *incorrect*, just
*misleading* -- it could be misinterpreted as saying something that isn't
quite the case.
Of course, I myself could have been a little less ambiguous in my
last sentence -- that the atomic nucleus didn't push against any
*previously expelled propellant*. The rocket did shove it out the back
end, though, and this shove would make the rocket move forward.

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.

Note that this beanbag system would also propel the kid in a perfect
vacuum, even if the beanbags he threw never collided with (or "pushed
against") each other.

But again, the kid is pushing on the beanbags. That's the key. For every
reaction there's an equal and opposite reaction. There's no way to make the
beanbag go in one direction w/o pushing on it somehow.

No disagreement here. I just wanted to avoid the notion that a
rocket needs to "push against" the ground, air, previously expelled
propellant, or any other thing that is sitting out the back end when the
rocket fires in order to work. (Rockets work better in vacuum than in
air, in fact.)
--
-- With Best Regards,
Matthew Funke )