Re: To hover, a reaction-motor pushes on the earth?

[brian whatcott <inet@INTELLISYS.NET>]

At 02:02 8/18/99 -0700, Bill wrote:
> ...
> Everything flys by pushing against the earth?  I strongly disagree.
> ... If a rocket hovers high above
> the ground, and if its exhaust eventually contacts the ground, then the
> rocket is *not* using the surface of the earth in order to remain aloft.


Bill voices the conceptual difficulty of connecting a rocket exhaust
(or a bird's wing flutter) with a ground reaction.
   I find this helpful:
If I eject one molecule downwards at high speed, I can expect it to
share with two molecules at lower speed, and so on in increasing
quantities until at appreciable distances beneath, it will be
instrumentally difficult to detect the small pressure increment over
the large area in question.



> ... Suppose a
> rocket hovers several lunar-diameters above the moon.  We can aim the
> exhaust at the moon, or we can divide the exhaust into two diagonal
> streams which miss the moon. This has little effect on the force-pair
> between the rocket and its exhaust, although if the exhaust-streams miss
> the moon, then the attraction exterted by the rocket upon the moon will
> cause the moon to accelerate towards the rocket (imperceptably, because
> the moon's mass is so enormous.)

> William J. Beaty


I can see that Bill wanted to argue for the case where there
 is no air to dilute the pressure due to the efflux.
We all can accept I'm sure that the rocket is not gaining thrust
by pushing on the air. Rather, we are merely noticing a consequence
 of firing a rocket exhaust towards the Earth (or Moon as the case
 may be.)  If we remove the exhaust several planetoid diameters
but still aim it so that it all impacts the planetary surface,
then its effect though diluted by the large area of application
is the same as if it were close.
    If the exhaust does not impinge upon the planetoid, then it
does not contribute a push.
(Or am I missing something here?)     :-)





brian whatcott <inet@intellisys.net>
Altus OK