Eggtimer Rocketry Eggtimer User Manual page 42

"-" outputs of the deployment channel. If you leave out the resistor, the servo will not work,
because the resistor pulls the control line down to ground while the channel is turned off; it will
also fail the continuity check too. The Black lead of the servo will go the the "-" lead of the
deployment battery, and the Red lead of the servo will go to the "+" lead of the deployment
battery. Note that most servos will run fine on a 3.7v Li-Po cell, a 3.6v NiMh pack, and any
battery up to 6V. You can also use a small PX28 6v battery, the Eggtimer limits the travel time
to 3 seconds, so it will probably last awhile. Nevertheless, like any deployment battery, you
need to check it with a DVM before each flight.
Air Pressure Ports
For the altimeter to work properly, there must be an undisturbed exposure to the outside air
pressure. This is a little trickier than it sounds, because the air is whizzing by the rocket at
several hundred feet per second during flight, so as you may remember from Bernoulli's
principle in Physics this is going to cause the pressure at that location to drop if the pressure
cannot be equalized. The trick is to make the air pressure ports large enough so that sufficient air
can enter the payload bay, but not so small that it drops with air velocity. If you make the ports
too large, then crossflow air currents can disturb the airflow and your readings will be very
"noisy" during the boost and coast phases when the rocket is moving at high speed.
There has been a lot written about sizing air pressure ports for model rocket altimeters, but most
of the literature agrees that the total area need to be about 1/500 of the volume of the payload
bay. For example, if you have a 3" rocket with a 9" payload bay, that's about 63.5 square inches,
so the total area of the ports should be about .12 square inches (about 1/8 square inch). We
recommend that you use three ports, equally spaced around the diameter of the tube, so each port
would be about 1/24 square inch, or about ¼" in diameter. You need these ports to be at least
four body tube diameters from the tip of the nose cone to prevent possible pressure build-ups
during high speed flight, particularly during mach transition, but this is usually not an issue with
most multiple deployment rocket configurations because the payload bay is typically located
behind the main parachute bay.
A quick formula for the diameter of the holes, assuming that you are using three is:
1) Length of Payload Bay in inches: __________________
2) Diameter of body tube in inches: __________________
3) Square of #2
4) Multiply #1 and #3
5) Divide by 1500
6) Take the square root:
This is the diameter of the holes you need
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