Re: Rocket action

[brian whatcott <inet@INTELLISYS.NET>]

At 21:05 12/3/99 -0800, Roger Freedman wrote:
> I'm assuming that your question is about a rocket that starts from rest, so
> the question can be restated as "If a rocket starts from rest, can its
> speed v ever exceed the speed v_ex of the exhaust relative to the rocket?"
> The answer is to this question is "Yes, and by a lot!" (In other words, the
> original statement in your message just isn't so.)
>
> The final speed v will exceed the exhaust speed v_ex if the initial
> combined mass m_0 of the rocket and the propellant is greater than the
> final mass m of the rocket by a factor of at least e = 2.71828 . . ., that
> is, if m_0/m > e. As an example, the main engines of the Space Shuttle use
> a mixture of liquid hydrogen and liquid oxygen ////
> Cheers,
> Roger Freedman

This was a splendid exposition which integrated the description of
the model terms and their labels neatly in the text that developed the
concepts.

   But this does not stop me nit-picking, because that is my nature.
Let me proceed....

Solid fuel rockets usually can't be throttled, and liquid fuel rockets
 often can. You probably recall commentaries from the high pillar of
fire which talk of throttling the Shuttle up to 102% and so on.

 The presence of a throttle admits the possibility that one could
dribble a few percent of thrust at low velocity. In this way, almost
any rocket speed could be made to exceed its exhaust velocity.
The condition is not at all restrictive.
 That was the nit that came to mind.

 The title of your book is evidently catchy.
 I bid for some items at a college auction today.
I came by a copy of University Physics (Models & Applications) by
Crummett & Western who provided a comparable development of the
rocket equation.

I had forgotten just how sumptuous and interesting and profusely
illustrated these texts now are. (I  expect this describes your text
too, if it has reached many editions?)

Sincerely



brian whatcott <inet@intellisys.net>
Altus OK