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'[OT] Re: Water Rockets - measuring altitude and ac'
1999\06\28@200049 by Russell McMahon

picon face
If you can be bothered with the expense this should work well -

Pressure transducer (Motorola and many others) frequency modulating
an audio oscillator modulating a small transmitter (just about a 1
transistor bug would do for the range you'll get) - send this to a
ground based received - either listen to tone or record on tape or if
you are keen demodulate on the spot to altitude. Weight of the above
should be minimal. For maximum altitude you actually get more
altitude with a small amount of ballast *      in a water rocket so
this could serve that task. Ballast seems to peak altitude at about
5% of gross weight. For a 1.5l bottle (half full at launch) this is
about 40 grams. It should be easy enough to fit the above into this
mass budget.

This method has the advantage of allowing you to also have some
attempt at measuring acceleration. Won't discuss this here - gets too
long for such an OT matter (although a PIC could well be used.).

You could simply measure pressure change with PIC A2D and log and
recover and dump after flight. Saves a tx and rx and ... (but its
less fun).

I find PET bottles will got to about 130psi but are getting very sad
by then :-)

Audio osc can be set to give wide frequency range with small altitude
variation - stability need only be short term.
At 300 feet (which is probably the best you'll get without very
special care and multiple stages) the pressure change is only
about -1.4% (-0.496% for 100 feet). Best altitude I have read of is a
bit over 1000 feet with a 2 stage rocket. Stage 2 was a long thin
polycarbonate tube previously used to hold fluorescent tubes.
A spear on the way down !

Pressure is about P0 x 1/(2^(h/14000))
P0 is ground level Pressure
ie pressure halves every 14,000 feet with logarithmic decrease.

Russell McMahon

{Original Message removed}

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