DIY

OUTDOORS

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

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