Altimeters

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I use Missile Works RRC² altimeters for parachute deployment. They come in two "flavors". One is the RRC². It is roughly 6 inches long and allows the battery to be mounted directly onto the altimeter itself. The other is called the RRC²X. It is shorter in length and allows the battery to be mounted remotely. It also has an option that allows the altimeter to be used at higher altitudes.

RRC² Altimeter		View user manual for the RRC²
RRC²X Altimeter		View user manual for the RRC²X

Missile Works RRC2X altimeter, battery and battery holder.

This photo shows the Missile Works RRC²X altimeter, a battery holder, a 9V battery and a US Quarter (for scale). What is not shown here is a switch that is also needed for turning the altimeter on and off. Missile Works supplies a nice little rotary switch that is reliable in the high vibration and high-G environment of rocket flight. You can view it right here.

Wildfire altimeter unit with igniter wires connected.

Close up of the Wildfire altimeter unit.

These three pictures show a completed altimeter unit. This particular unit is for my Wildfire rocket. This unit uses two altimeters, two 9V batteries, and two on-off switches. The two systems are completely independent and serve as mutually redundant back-ups for each other. The parts are all mounted to a small wood block. The block has a hole drilled down the middle of it so that it can be mounted directly onto the threaded rod inside the altimeter bay.

The two on-off switches are the black cylindrical parts mounted in aluminum brackets. The brackets present the face of the switch to the outer edge of the altimeter bay. This makes it very convenient to activate the switch by passing a small screw driver through one of the altimeter static port holes.

The switch is really just a 110-220V selector switch, but it is very useful here because it has high detent forces that ensure the internal contacts will stay mated against each other during rocket flight.

Comparison to Altitude Reported by GPS

Wildfire and Vulcan have both made two flights with a GPS receiver on-board. Each of these rockets also carries two Missile Works RRC² altimeters. Just for fun I have compared the peak altitude reported by the GPS to the peak altitude reported by the altimeters. As seen in the table below, the difference was less than 2% on all four flights.

Since these rockets each carry two altimeters, it was also possible to compare the two altimeter readings themselves. That result is also in the table above. In one case It was almost as large as 3%. I have made many other flights that also carried two altimeters. Those flights are listed in the table below.

Comparison of Altimeter Pairs

This table compares the altitude reported by two altimeters that were both flown on the same rocket. There are four different rockets in this table, each with two Missile Works RRC² altimeters for a total 8 different altimeters. This shows how well the altimeters match under "real world" flight conditions. Usually the altitudes reported by the two altimeters were very close to each other. Often well within 1%. The standard deviation of the differences is 1.8%. That would be a good estimate of the typical expected difference.

This plot shows the percent difference between altimeter readings as listed in the table above.

Related Altimeter Topics

Click here to see how to determine the correct size for altimeter pressure ports.