Using Electronic Altimeters for Model Rockets
There are a few ways to determine the altitude of your rocket. In a previous article we discussed using inclinometers and trigonometry. In this article we will discuss using electronic altimeters for model rockets to measure the altitude of flights.
Figure 1 - Basic Pressure Altimeter
Using Electronic Altimeters for Model Rockets
The most common electronics device carried aboard rockets are electronic altimeters. In recent years, electronic altimeters have become more and more plentiful and as a result less expensive than when they were first introduced. As well the size of the altimeter has decreased significantly adding as little as 6 grams of weight to the rocket.
Altimeters employed in rocketry may be used to control the ejection charge. But most of all they are used to measure the apogee, or highest altitude, achieved in a flight. Simple altimeters indicate measurement through a series of beeps. Some altimeters may be connected to a computer for data extractions. Still other use a small display screen on the altimeter itself to indicate measurements taken. Figure 1 shows a simple altimeter that relays the apogee of the flight through a series of beeps.
To measure altitude either of two basic methods are employed: measuring acceleration or measuring air pressure.
Pressure Altimeters
Pressure altimeters use atmospheric pressure to determine altitude. The lower pressure high above the ground is compared to ground level in order to determine the altitude.
Sealed aneroid (defined as using no liquid) wafers are exposed to outside air and the pressure difference causes the wafers to expand or contract. This change is then measured and used to determine altitude.
Figure 2 - Installing an altimeter into an electronics bay
Pressure altimeters must measure the outside air. Thus vents (holes) must be made in the rocket or the payload section holding the altimeter. Pressure altimeters may also be affected by pressure fluctuations on rockets that reach speeds in excess of Mach 1 (the speed of sound) and low pressure caused by weather. However, modern pressure altimeters take these factors into account and adjust for it. For example, some altimeters will not measure altitudes of less than 15 meters or 50 feet to account for low pressure due to weather.
Accelerometer Altimeters
Accelerometer altimeters measure the change in velocity (the acceleration) of the rocket. By knowing this and the time, accelerometer altimeters are then able to determine the apogee of the flight.
These type of altimeters do not need to be vented to the outside air. However, they are less accurate when the rocket does not fly perfectly straight.
Dual Deployment
For bigger or high-power rockets, altimeters may be used to control the ejection charge used to deploy the parachute. Often this involves having two ejection charges fired, one for pushing out a drogue parachute and the other for deploying the main parachute. By pushing out a drogue the rocket is positioned such that it is falling horizontally. This keeps the rocket from picking up too much speed in descent and allows the main parachute to be fired out sideways and not down.
Figure 3 - Altimeter Data
Dual deployment allows for more accurate landings as the main parachute is fired at a relatively close altitude to the ground. This cuts down on drift significantly. Multiple altimeters are often used in dual deployment systems. Usually the altimeters come from different manufacturers. This cuts down on the risk of an altimeter not firing due to sensitivities based on manufacturer. Dual deployment altimeters are often installed in a separate payload bay. Figure 2 shows such an installation.
Measuring More Than Altitude
As mentioned above, some altimeters may be used to measure more than just the apogee of the flight. Some altimeters, such as the Entacore AIM USB altimeter, have USB connections that allow their data to be extracted to a computer and displayed graphically (see figure 3).
Figure 3 is a extraction of data from a rocketry flight using this altimeter. As the AIM USB altimeter is used for dual deployment in high-power rockets it may be configured for deployment times using the computer program provided.
Through the use of its small LCD screen the Altimeter Two from Jolly Logic is able to give flight measurements such as top speed, engine burn time, ejection altitude, coast time to apogee, descent speed, peak and average acceleration, apogee to ejection time and the total flight duration.
Additional uses for altimeters
Altimeters that offer more than just the altitude may often be used in non-flight applications. For example, how about taking one on a roller coaster ride! That first drop may provide some interesting data. Measuring velocity, acceleration, and height are valuable parameters to have for interesting classroom discussions.
Purchasing an Altimeter
Careful research should be taken to determine the altimeter that is best for your application. Factors such as cost, functionality and size may be considered.
In the case of dual deployment altimeters, care must be taken to choose one with a good reputation as a failure of a dual deployment altimeter could prove tragic. Quite often redundancy in the form of multiple altimeters and timers is employed in a high-power rocket to reduce the risk. We currently do not carry dual deployment type altimeters in the Sigma Rockets Online Store.
Below I have listed two altimeters, Altimeter One and Altimeter Two from Jolly Logic, that we currently do offer. Altimeter One provides a simple way to measure and display apogee of a flight and Altimeter Two offers more flight analyses as described above. These have a protective casing to cover the circuit board instead of exposing it as most altimeters do. Please note however, that despite looking like USB memory sticks, the altimeters use the USB connection merely for charging their internal batteries and do not transfer flight data to the computer. Flight data is displayed through the use of their LCD screens.
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©2012 Sigma Rockets and Aerospace Inc.
