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Re: Sometimes physics is fun



On 10/28/2003 12:35 PM, J. Green wrote:
>
> This is a pretty loosely organized event focused on a bunch of people
> having fun, and most questions about the rules seem to be made up on
> the spot with little thought.

Hmmm. High concentrations of energy with little
thought as to the consequences.

Anybody who thinks this is "fun" needs adult
supervision, stat.

This is more akin to a pipe-bomb building contest
than to a regular pinewood derby.

On 10/28/2003 02:14 PM, Gary Turner wrote:
> On such a short track, it will likely come down to whoever can
> maximize acceleration.

Yup. Provided the thing stays in the general
vicinity of the track.

> .... look into the water rocket idea. Accelerations are huge and
> will easily outperform a chemical rocket over that sort of distance.

I find that rather implausible.

The ultimate performance of a rocket, hydraulic or
otherwise, is limited by the bursting pressure of
the chamber. The hydraulic rocket suffers a
performance degradation factor f, on the order
of f=.75, because the pressure at the end of the
thrust phase is about half of the initial pressure.

A chemical rocket need not suffer any such degradation.
To see this, consider the intermediate case, namely
a water-rocket pressurized by a chemical reaction.
If the reaction goes to completion at t=0, the
situation is just like an ordinary water rocket.
Things can only get better if the reaction continues
longer, maintaining peak chamber pressure until the
water is exhausted.

It is true that a water rocket optimized for drag
racing will outdo a modest chemical rocket designed
to develop "only" 10 or 15 Gees. But that's just
a truism and an optimization issue ... you can
design rocket engines that burn muuuuch faster
than anything Estes sells ... water rockets have
no fundamental performance advantage.

Amusing water-rocket page, including diagrams of
the Ian Clark hold-down mechanism:
http://users.hubwest.com/gordo/waterrocket.html