Rockets

[George Kinley]

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

[Joe Strout]

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

[Timo Nieminen]

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

[Gordon D. Pusch]

"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;'

[Marc 182]

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

[Gregory L. Hansen]

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

[Don Stauffer]

Think about it this way. Suppose you have a tube closed at one end,
other end open, and you have a source of gas that has enough flow and
pressure to maintain a fair pressure in the tube.

Now, the force on the tube walls on the cylindrical part (the part
without the end pieces) all cancel out because the force on every piece
is cancelled out by the same force on the piece diametrically opposite.

However, the pressure on the closed end creates a force that is NOT
cancelled out by the open end. So the gas goes roaring out of the tube,
but the force on the closed end 'pushes' the rocket forward.

Now, real rockets have a 'nozzle' that makes better use of the gas
flowing through it, pushing on walls of nozzle. So thrust of rocket
comes from forces on nozzle, and on end of chamber opposite opening in
nozzle.

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

--
Don Stauffer in Minnesota

webpage- http://www.usfamily.net/web/stauffer

[John Schoenfeld]

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.

[HOST Comp JimS]

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

Others have already answered the question well, but I just
wanted to say that this is a question that many people
wondered about or misunderstood in the early days of
rocket development. I think I've read quotes from some
prominent people in those days who, to their embarassment
later, declared publically that rockets wouldn't work in space.

Of course many things about space travel can be a little hard
to understand at first, since vacuums, weightlessness, etc.
are things that we normally don't have to think about in our
daily lives.

Jim

[Penguinista]

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.

[Ian Stirling]

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.

[Greg D. Moore \(Strider\)]

"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.

[Gareth]

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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[The Ghost In The Machine]

In sci.physics,
>
wrote
on 11 Jul 2003 12:42:21 -0700
>:
> Marc 182 > wrote in message >...
>> 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.

[Joe Strout]

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

[Joann Evans]

Don Stauffer wrote:
>
> Think about it this way. Suppose you have a tube closed at one end,
> other end open, and you have a source of gas that has enough flow and
> pressure to maintain a fair pressure in the tube.
>
> Now, the force on the tube walls on the cylindrical part (the part
> without the end pieces) all cancel out because the force on every piece
> is cancelled out by the same force on the piece diametrically opposite.
>
> However, the pressure on the closed end creates a force that is NOT
> cancelled out by the open end. So the gas goes roaring out of the tube,
> but the force on the closed end 'pushes' the rocket forward.
>
> Now, real rockets have a 'nozzle' that makes better use of the gas
> flowing through it, pushing on walls of nozzle. So thrust of rocket
> comes from forces on nozzle, and on end of chamber opposite opening in
> nozzle.

And indeed, as an atmosphere *impedes* the exhaust gases, rockets
work *better* in vacuum, and are the only way known in current physics
to make something move in a vacuum. (I suppose if you tried to light a
chemical rocket deep in the atmosphere of Jupiter, where atmospheric
pressure equalled combustion chamber pressure...you go nowhere.)

Then we get into more complex issues of optomizing the expansion
ratio of bell nozzles for the altitude of operation (which is always
changing, if the idea is to leave the atmosphere, and not cruise at a
single altitude), two position bell nozzles, aerospikes, and so on....

[Greg D. Moore \(Strider\)]

"Joann Evans" > wrote in message
...
> Don Stauffer wrote:

> And indeed, as an atmosphere *impedes* the exhaust gases, rockets
> work *better* in vacuum, and are the only way known in current physics
> to make something move in a vacuum.

Not quite: Solar sails are known to work and VASIMAR probably works.

[Matthew F Funke]

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 )

[Penguinista]

Ian Woollard wrote:
> Joann Evans > wrote in message >...
>
>>rockets ... are the only way known in current physics to make
>>something move in a vacuum.
>
>
> That's not actually true; electromagnetic tethers and solar sails also
> work among several other options.

To be general, momentum transfer, rockets being currently the best
developed means.

[John Schoenfeld]

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.

[Gordon D. Pusch]

(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;'

[Dr John Stockton]

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.

[Joann Evans]

"Greg D. Moore (Strider)" wrote:
>
> "Joann Evans" > wrote in message
> ...
> > Don Stauffer wrote:
>
> > And indeed, as an atmosphere *impedes* the exhaust gases, rockets
> > work *better* in vacuum, and are the only way known in current physics
> > to make something move in a vacuum.
>
> Not quite: Solar sails are known to work and VASIMAR probably works.

Momentum is still transferred, even in solar sails, and VASIMR is
still a rocket.

Just not a chemical one.

[Greg D. Moore \(Strider\)]

"Gordon D. Pusch" > wrote in message
...

> VASIMR (note corrected spelling) stands for "Variable Specific Impulse
> Magnetoplasma _ROCKET_" VASIMR is definitely still a rocket, and still
> derives its thrust by heating and ejecting mass backward in its exhaust
plume.
>
> Perhaps you are confusing VASIMR with M2P2 or a magsail...

Ack, you're right. I was thinking M2P2. Thanks for the correction.


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

[Greg D. Moore \(Strider\)]

"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 )

[Greg D. Moore \(Strider\)]

"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 \(Strider\)]

"Joann Evans" > wrote in message
...
> "Greg D. Moore (Strider)" wrote:
> >
> > "Joann Evans" > wrote in message
> > ...
> > > Don Stauffer wrote:
> >
> > > And indeed, as an atmosphere *impedes* the exhaust gases, rockets
> > > work *better* in vacuum, and are the only way known in current physics
> > > to make something move in a vacuum.
> >
> > Not quite: Solar sails are known to work and VASIMAR probably works.
>
> Momentum is still transferred, even in solar sails, and VASIMR is
> still a rocket.

No one is arguing that momentum isn't transferred. Only that your statement
that rockets the only known way was incorrect.

And of course I was incorrect about VASIMR.


>
> Just not a chemical one.

[Matthew F Funke]

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 )

[Gregory L. Hansen]

In article >,
Greg D. Moore \(Strider\) > wrote:
>
>"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.

F=dp/dt, so p=\int F*dt. Push hard for a short time, push softly for a
long time. Either way the problem can be turned to one of conservation of
momentum and you don't have to worry about the details of the push.
Universality is one of the great things about conservation laws.

--
"When fighting with sharpened Bronze, or harder Metals from the Heavens,
it is Wise to kick thy Opponent, be he a Chaldean or a man of Uruk, in his
Man Sack, that thou mayst defeat him more handily than by Arms. So sayeth
INNAMURUTUSHIMMILODEK, who hath slain threescore Ammelekites."

[Will R]

>"Joann Evans" > wrote in message
...
>> Don Stauffer wrote:
>
>> And indeed, as an atmosphere *impedes* the exhaust gases, rockets
>> work *better* in vacuum, and are the only way known in current physics
>> to make something move in a vacuum.
>
>Not quite: Solar sails are known to work and VASIMAR probably works.
>
>
>

Technically, one might argue that since a solar sail needs light pressure to
function, it needs more than just a vacuum. It needs a well lit vacuum. :) I
think Rockets count as our only way to get around in a "perfect" vacuum, where
you have no extra photons, or magnetic fields to make use of in your
propulsion.
------------------
Woooogy
I have to go back in time to pretend to be myself when I tell myself to tell
myself, because I don't remember having been told by myself to tell myself. I
love temporal mechanics.

[Greg D. Moore \(Strider\)]

"Matthew F Funke" > wrote in message
...
>
> 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.)


Right. I think ultimately we're in agreement here.


> --
> -- With Best Regards,
> Matthew Funke )

[Joann Evans]

Ian Woollard wrote:
>
> Joann Evans > wrote in message >...
> > Ian Woollard wrote:
> > >
> > > Joann Evans > wrote in message >...
> > > > rockets ... are the only way known in current physics to make
> > > > something move in a vacuum.
> > >
> > > That's not actually true; electromagnetic tethers and solar sails also
> > > work among several other options.
> >
> > Okay, but Newton's Third Law is still involved, momentum is still
> > transferred.
>
> Big deal, Newton's third law and momentum transfer happen when I play
> basketball. That doesn't make me a rocket either.

Are you playing it in free fall? Throw that basketball, and see what
happens to you. Basketballs are acceptable reaction mass.

> Rockets have one or more rocket engines. Neither an electromagnetic
> tether nor a solar sail have rocket engines.

Repeating: okay.

I agree with that. Strictly speaking, this is not a rocket.

[Christopher M. Jones]

"Joann Evans" > wrote:
> And indeed, as an atmosphere *impedes* the exhaust gases, rockets
> work *better* in vacuum, and are the only way known in current physics
> to make something move in a vacuum.
[snip]

This depends much more on the definition of vacuum than on
anything else. If your definition is the ordinary definition
then this is not true, as solar sails, mag sails, and
electrodynamic tethers can change momentum in the "vacuum"
of space. If your definition is a gasless, "perfect vacuum",
then there are still solar sails and electrodynamic tethers.
Only when you consider absolute, perfect vacuums without
light, or any force gradients would your statement be true.

[Bruce Janson]

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.)

[johnhare]

"Bruce Janson" > wrote in message
...
> 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.)
>
Speaking for myself, the poor spelling and crackpotism
are both caused by a self inflicted education. I've been thinking
about suing myself for the defficiencies in learning.

John Hare

[Gordon D. Pusch]

(Bruce Janson) writes:

> 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.)

For one thing, crackpots typically are ill-educated to un-educated ---
which they often view as giving them an "advantage," since to their minds
it means they are not "hidebound" or "constrained" by "conventional thinking."

Regardless of their level of formal education, crackpots generally grossly
over-estimate their own mental competence, typically considering themselves
to be "brilliant" or even "geniuses," when in reality they are of sub-normal
intelligence, and are often incapable of forming even a coherent sentence,
let alone a coherent thought.

It is an interesting phenomenon that the incompetent generally
over-estimate their own abilities, while the highly skilled generally
under-estimate them. See "Unskilled and Unaware of It: How Difficulties
in Recognizing One's Own Incompetence Lead to Inflated Self-Assessments,"
<http://www.apa.org/journals/psp/psp7761121.html>.


-- Gordon D. Pusch

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

[Joann Evans]

Joe Strout wrote:

[snip]

> > 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


One more reason for cheap access at least to LEO: Everyone who thinks
they've got a reactionless drive can take a prototype up there, put it
outside the ship, and then they can put up, or shut up.

(I know, there are cheaper ways of doing this [suspend it, and see if
you get a unidirectional deflection, instead of gyrating or oscillating
around the perpindicular], but my approach is instinctively unambiguous.
This, after all, is where the thing is supposed to work.)

It would be even more interesting, if the losers have to find their
own way back to the ground.....

[Joann Evans]

John Schoenfeld wrote:

[snip]

> > 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".

I think some past claims of reactionless drives that allegedly
reduced their weight (though never to zero, it seems) on scales, had
more to do with a similar phenomenon in the springs of the scale, then
actually providing a net upward force. Time your oscillations right, and
you can fool the scale, but not Mother Nature....