Re: Rocket action

[brian whatcott <inet@INTELLISYS.NET>]

At 08:52 12/2/99 -0500, Rick Tarara wrote:
> It is easy to 'see' that the rocket can continue to accelerate under a
> sustained thrust, but less obvious how the center of mass (rocket starting
> in space, originally at rest) remains fixed and at rest, especially
> once the speed of the rocket exceeds the speed of the exhaust.  ;-)
> ///
> Richard W. Tarara

On Fri, 3 Dec 1999, brian whatcott wrote:
> I believe Rick would want to reconsider this mind experiment.

 At 15:53 12/9/99 -0500, Robert Cohen wrote:
> Why reconsider it?  Is something wrong with it?

On Thu, 9 Dec 1999, brian whatcott wrote:
> I have written up my objection to the list two or three
> times by now.    Is the objection not intelligible?
>
> The idea of a time progression until a rocket outraces its
> exhaust is a physics textbook fairy tale in essence, if you
> consider that its exhaust can be throttled to low velocity
> at some indefinitely early stage.
At 13:22 12/10/99 -0500, Robert Cohen wrote:
> I'm sorry but you've lost me.  What is it, specifically, about Rick's mind
> experiment that you object to?  Is it his statement that the center of
> mass remains fixed?  It seems reasonable to me and I'd like a clear
> explanation for why it is wrong.  By the way, I've only been a member of
> this list for a short while and so I may have missed your earlier posts
> (although I checked the archive and couldn't come up with anything).
>
> | Robert Cohen


I should first apologize for my response which was far too terse.

Next I should mention the facet of Rick's post that Robert repeats
- the non intuitive but correct idea of a stationary center of mass
for a speeding rocket and exhaust - is what initially caught my eye,
as a matter of honesty.

But Robert's patient questioning reminds me of what reserves of verbal
resilience, a teacher is called on to exhibit - in contrast with
my often expressed impatience.

Trying to rephrase more clearly:
If a rocket's reaction drive accelerates in a vacuum, there is a
pretty math model development that can show a relation for equality
of rocket speed and exhaust speed which is relevant to a rocket with
constant thrust. This model is beloved of physics textbook writers.

   In practise, there are liquid fueled rockets (some of which can
be throttled) to which this pretty model is less relevant.

Specifically: an attempt to relate conditions for which a rocket's
speed forward is faster than the speed of its efflux backwards
is oversimplified, if one can throttle down the thrust with reduced
efflux speed at any point.

In summary: if a rocket can be throttled, it is not necessary to wait
for the rocket's forward speed to increase enough for it to exceed the
exhaust speed. One can instead, simply reduce the exhaust speed.

Hope this is clearer.

On an unrelated note: <rant on> I reformatted this note in classical BBS
'bulletin board' style. You have no difficulty in realising who wrote
what in this style.

I can't imagine why - but the Usenet Newsgroup posts, and email software
in general use the much more primitive method of front loading the author
identifications, and adding an extra rank of qoutation markers for each
contribution  - guaranteed to confuse new users. </rant>


Sincerely

brian whatcott <inet@intellisys.net>
Altus OK