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Re: Sometimes physics is fun



This is all correct, but has some logistical problems.

The main reason that simple pressure rockets tend to outperform chemical
rockets over short distance is the ignition delay. Over such a short
distance, this whole thing is going to be over in a fairly small fraction
of a second - most chemical engines will still be sitting on the start
line waiting for a fuel ignition.

Once ignited, model chemical engines *tend* to produce relatively low
pressures. Increased pressure (created, for example, by plugging up the
nozzle a little) can be a runaway effect, it causes a faster burn rate -
so higher pressures... Hence, the relatively low accelerations seen in
the safe Estes engines.

It is certainly possible to produce engines that burn muuuch faster, but
not particularly safe. As an extreme, you could always strap a stick of
dynamite to the back, that would at least take care of your oppositions
entries :)
(assuming they are still on the start line when you get ignition)

.... 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