Magnetic Apogee Detection Sensor

Posted on Feb 5, 2014

Figure shows a simplified representation of the Earth`s dipole field, it resembles the field due to a bar magnet. The field lines originate at the south magnetic pole and terminate at the north magnetic pole. The angle that this field makes when it intersects the Earth`s surface is known as the inclination angle. Figure 2 shows a contour plot of the inclination angle over the continental US.

Magnetic Apogee Detection Sensor
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This angle varies from about 60 degrees in the south to about 75 degrees in the north. Since the magnetic field has a large vertical component, it is possible to use it as a reference to sense a rocket`s orientation and to set off an ejection charge when the rocket tips over at apogee. A common misconception is that a mercury switch could be used to detect this orientation more easily. A mercury switch uses the Earth`s gravity as a reference and senses down  with the blob of mercury. Acceleration due to gravity with the rocket sitting on the launch pad and accelerations while in flight are indistinguishable with the mercury switch. Once the rocket launches, it no longer has the Earth`s gravity as a reference and it will respond to any acceleration or deceleration. After the motor burns out, the rocket will begin to decelerate due to air resistance but the mercury in the switch will keep on moving. This is the same behavior that turning the switch over while standing on the ground would have. In effect, this switch will believe it is upside down and will trigger an ejection charge immediately after motor burnout, not a desirable situation. The Earth`s magnetic field is independent of the rocket`s speed or acceleration. In order to use the magnetic field as a reference, a sensor is desired that will output a voltage proportional to the angle of the sensor relative to the field, V=Vmax cos q. When this voltage falls below a...

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