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  #21  
Old July 15th 03, 02:45 PM
Gregory L. Hansen
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Default Rockets

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."
  #22  
Old July 16th 03, 02:38 AM
Greg D. Moore \(Strider\)
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"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 )


  #23  
Old July 16th 03, 09:24 AM
Bruce Janson
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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.)
  #24  
Old July 17th 03, 12:28 AM
johnhare
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"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

  #26  
Old July 17th 03, 01:18 AM
Joann Evans
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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.....

  #27  
Old July 17th 03, 01:18 AM
Joann Evans
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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....

  #28  
Old July 31st 03, 02:07 AM
V-Man
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They don't - rockets need air to push against, which is why they do not work
in a vaccum.
Ergo, we have never successfully launched anything into a very high orbit
yet - we just can't get the rockets fast enough before they run out of air
to push against.


Uhm, I'm sorry, I *must* ask - are you a "Flat Earth"-er?

Or something similar? You see, we didn't have Kozmo the Mysterious levitate
the Lunar Lnaders off the moon in 69, 70, 71, & 72. They used - GASP!- rocket
motors.

And there is no air on the moon, in it's orbit, etc. Rockets were used,
again, to break the moon's gravity and return the astronauts from it.

Newton's Third Law (I think it's the third) - every action will have an equal
and opposite reaction. That is how rockets work. Air needs not apply.


Play more with Claymore! V-Man
Living Vicariously through my Characters...
  #29  
Old July 31st 03, 12:10 PM
Brett Mount
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V-Man can see Paradise by sci.space.science's light:

}They don't - rockets need air to push against, which is why they do not work
}in a vaccum.
}Ergo, we have never successfully launched anything into a very high orbit
}yet - we just can't get the rockets fast enough before they run out of air
}to push against.
}
} Uhm, I'm sorry, I *must* ask - are you a "Flat Earth"-er?

Of course he is- have you ever been anywhere that wasn't flat?

Seriouslu, do you really need a smiley to indicate a humourous post these
days?

--
Regards

Brett
"In the Doraverse, there are very few female opinions that matter"
  #30  
Old August 1st 03, 06:06 AM
V-Man
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Seriouslu, do you really need a smiley to indicate a humourous post these
days?


Yes, when it's delivered is such a deadpan manner. You & Gordon both know
there are people that seriously believe that we never landed on the moon.


Play more with Claymore! V-Man
Living Vicariously through my Characters...
 




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