G-METER AND ALTIMETER
Double-duty aerospace instrument on
a shoestring budget. By David Simpson
Here’s an aerospace instrument you can build for
$5 that will measure the crushing forces that a
model rocket withstands and the rarified strata it
attains. It isn’t exactly six-sigma technology in
terms of accuracy, but it’s darn fun.
Photograph by David Simpson
The device, which you install in the rocket’s
payload compartment, uses 2 small bands of
heat-shrink tubing that slide over a dowel to record
the maximum G-force and altitude attained. As
the rocket accelerates, the G-force band is pushed
down by washers on a spring, and as the rocket
rises, the altitude band is pushed down the rod by
the expansion of a pressure chamber made from a
pill bottle and a rubber-balloon membrane.
The force of landing doesn’t disturb the positions
of the bands, which are heat-shrunk snugly over the
rod and stay in place thanks to their relatively high
coefficient of friction and low mass.
The “secret sauce” for both readings is the calibration step, where you mark positions on the dowel
with their corresponding G-force and altitude levels.
To calibrate the G-force meter, we stack increasing
weights onto the spring and gauge its compression.
The altimeter we calibrate using a kitchen vacuum
food sealer and a commercial altimeter or barometer.
Build the Altimeter
Make the flexible membrane by cutting a 2"-diameter
circle from a rubber balloon. Stretch it flat over the
open end of the pill bottle, and secure it by winding
button thread around several times, near the top. Tie
off the thread and coat it with a thin layer of wood
glue or epoxy, then trim away the excess rubber.
Cut a disk the same diameter as the pill bottle out
of ¼" balsa or aircraft plywood. Drill a
5" pilot hole
in the center of the disk and glue it to the bottom of