User`s manual

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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, a 3” rocket with a 9” payload bay has about 63.5 square inches of volume, 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 where the pressure buildup is more gradual.
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
For example, a small 5” payload bay 1.5” inches in diameter (inside of a BT-60 coupler…)
1) Length of Payload Bay in inches: 5
2) Diameter of body tube: 1.5
3) Square of #2 2.25