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Re: Sometimes physics is fun



Gary Turner's comparison of water rockets and chemical rockets coincides
with my experience. When the water rocket is released, the water is
ejected and the rocket is 30 feet into the air before the chemical
rocket (eg Estes) has cleared the launch rod.

How do I know this? Years of experience.

The Estes rockets are a lot of fun, go higher, have more crowd appeal,
and have nice parachute recovery. But a 2-liter bottle rocket using
water and compressed air will beat the Estes rocket off the launch pad.
It isn't even a close contest. You can observe the Estes rocket
accelerate off the launch pad. The bottle rocket is already coasting
before you realize it has accelerated.

As Gary said, you can put a bomb on the bottom of a rocket if quick
chemical detonation is what you want. Or, as John Denker implied, you
can put a shotgun shell under the rocket. In those cases the
"projectile" is also coasting before you realize it has accelerated.
John is certainly correct that chemical detonations can be incredibly
fast.

Therefore I think the distinguishing factor between John's statements
about chemical-reaction propulsion versus Gary's statements about
compressed-air-water propulsion boils down to making a distinction
between a chemical rocket (controlled burn) versus a chemical gun
(maximum detonation rate).

I realize the distinction can be muddy.

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu