I find it interesting that this should come up at this
time because I'm almost finished a similar project and
should be testing it in a week or two.

Mine is based on the basic stamp II (because I have a
friend who wanted to participate in it and is a
microcontroller beginner), and ended up being 1" by
2.5" (plus 1 23A battery, about the size of an N cell).
It uses the ADXL190 +/- 80G accelerometer from Analog
Devices. Weighs only about 20 grams (with battery).
Using the built-in serial interface, I can download
each flight's data to a terminal emulator on my Palm
M100.

I considered both types (pressure and accelerometer)
and the accel type really seems more practical to me.
No worry about air pressure changes due to velocity,
etc. I should be able to get a full profile of flight
acceleration, velocity, and altitude.

I'd be curious, Dave, to know what the problems with
accelerometers are that you mention. I have done quite
a bit of thinking about sources of error, and the
largest source seems to be angle error, which works out
to only about 6 meters off (for a 10 sec time to
apogee) even for 10 degrees off. Unless you have a
strong wind, I don't think you would get that far
off.In addition, a pressure sensor (is seems to me)
would be very difficult to calibrate very accurately,
whereas an accelerometer can be very accurately
calibrated by just turning it upside-down and then
right-side up again several times and averaging them
together to find out the +/- 1G response.

Sean




At 07:06 PM 12/30/00 -0500, you wrote:
It certainly does, at burnout, or max V.
Accelerometer approach has it's problems too.

There's also a pressure spike around mach that can
cause problems.
If you're just logging, it's not such a problem, but
many designs detected
max altitude by indicated baro pressure, or zero
acceleration (as opposed
to negative during coast phase) and got fooled.

Deploying a parachute at trans-sonic speeds is
generally not a good idea.

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