OUTDOORS
Calibrate the G-Meter
Calibrating this combo meter is more complicated
than constructing it, but it’s both cool and educational. To calibrate the G-meter, you first build
a calibration rig. Glue a 12" length of
6" dowel
vertically in a wooden base and mark it the same
way you did the G-meter dowel.
Slide the G-meter spring over the dowel and add
the payload quantity of 1 small washer and 2 large
washers. Check the reading on the scale. That’s 1g,
our baseline. Make a table that translates millimeter
readings on the dowel to corresponding G-forces.
Record the 1g reading, then add 1 more small washer
and 2 more large ones, and record the position on
the scale for 2 g. Continue doing this until you’ve got
readings for 40g (Figure H). Unless you need a lot of
washers, you might opt to conduct this procedure
in the aisle of the hardware store.
Calibrating the Altimeter
Calibrating the altimeter is even cooler, but more
involved. You’ll need access to a vacuum chamber;
if you can’t gain access to a scientific or industrial
vacuum chamber, borrow (or buy) a vacuum food
sealer. You’ll also need an altimeter or barometer;
I experimented with a RadioShack mini digital
weather station, which included a barometer, but
it was a challenge getting it to take instantaneous
readings. I was able to do it by interrupting and
re-energizing the circuit at the moment I wanted the
reading. But I wound up using a good old airplane
altimeter, which was simpler.
If you’re using a vacuum food sealer, put the
G-meter/altimeter in a cage made by cutting two
5" windows in a 12" piece of 2" PVC pipe. The cage
prevents the bag from collapsing around the instrument. Put the reference altimeter or barometer
and our instrument in the vacuum bag or chamber
(Figure I).
Blip the vacuum button to reduce the pressure in
small increments, and record the readings from the
dowel and the commercial barometer or altimeter.
Make an association table between the two, like with
the G-meter. If you’re using a barometer, you can
convert pressure to altitude using the calculator at
csgnetwork.com/pressurealtcalc.html.
Prepare for Liftoff
Make sure both heat-shrink bands are in their starting positions before each flight: the G-meter band
144 Make: Volume
16
at the top of the spring, just under the washers, and
the altimeter band at the top of the dowel, so that it
will be flush with the nose cone disk when you slide
it on. If the nose cone is a little loose, secure it with
masking tape. As with other model rockets, friction-fit the payload atop the booster, connect the elastic
shock cord, and attach the parachute — or else
you’ll be digging your combo meter out of a hole!
Recovery, Reading, Reset,
and Repeat
After “ 3, 2, 1, liftoff!” and whoosh!, recover your
vehicle and peek inside. You may need to loosen
the bottom wood screw in order to read the bands,
then use your tables to translate their positions into
max-g and max-altitude. You’ve got your max-g and
max-altitude recorded on the combo!
A variation: Instead of marking millimeters on the
dowels, inscribe the target units — gs and feet or
meters — directly during calibration. Just take care
not to bump anything while making the marks.
You’ll find that different rockets, engines, and gross
weights all affect the readings. You may need to add
or remove washers and recalibrate to change the
overall range of the G-meter; more powerful engines
need fewer washers to register a reading while
weaker engines need more.
Safe rocketeering!
David Simpson is a private pilot and teaches aviation to
teenage cadets as aerospace education officer for the Civil
Air Patrol in New Jersey. He can be reached at dsimpson@
hydroflightsim.net.
The Key Hole
My first job was as a bicycle mechanic at a
shop where we also cut keys. One fine day, a
fellow came in to have his house key duplicated,
and he asked me to drill a nice big hole in the
key so it would be easy to find in the dark. It
wasn’t long after that when I drilled my own key! My
current key ring has, among others, three identically
shaped Schlage keys, so I cut a little off the wings of
another to make it easy to identify. Try this trick — I’ll
bet you never go back.
—Frank Ford, frets.com
Find more tools-n-tips at
makezine.com/tnt.
