Adept Rocketry - Information on Static Ports
Copyright © 1989-2012, All Rights Reserved
An Altimeter must be installed in a "sealed" chamber with a vent or vents to the outside. A sealed bulkhead below the altimeter chamber is necessary to avoid the strong vacuum caused by the aft end of a rocket during flight. Any leakage around the motor mount or in other areas at the aft end of the rocket will allow the strong vacuum to be partially felt inside the rocket body. In this case an altimeter could incorrectly indicate and record an altitude that is far higher than reality.
A sealed bulkhead above the altimeter chamber is necessary to avoid any pressure fluctuations that may be created at the nose end of the rocket. If the front of the payload section slip fits to another section such as a nosecone, then the fit must be as free as possible from turbulence.
You cannot ... you cannot put an altimeter in a rocket along with the parachute and expect to get accurate readings. Sometimes it may seem to work just fine, and sometimes the values may even seem reasonable, but the readings will always be wrong, and active deployment functions may not perform properly. An altimeter must be mounted in its own specially isolated and properly vented payload chamber or electronics bay. If your rocket does not already have one, then you must create one or add one. See example below.
A breathing hole or vent (also known as a static port) to the outside of the rocket must be in an area where there are no obstacles above it that can cause turbulent air flow over the vent hole. Do not allow screws, ornamental objects, or anything that protrudes out from the rocket body to be in line with and forward of a vent hole. The vent must be neat and burr free and on an outside surface that is smooth and vertical where airflow is smooth without turbulence.
Some rocketeers use multiple static ports (vent holes) instead of just one. Very strong wind blowing directly on a single static port could affect the altimeter. Multiple ports evenly spaced around the rocket tube may help cancel the effects of strong wind, the pressure effects of a non-stable liftoff, or the pressure effects that occur due to flipping and spinning after deployment. If you wish to use multiple ports, then use three or four. Never use two. Ports must be the same size and evenly spaced in line around the tube.
The general guideline for choosing port size is to use one 1/4 inch diameter vent hole (or equivalent area, if multiple holes are used) per 100 cubic inches of volume in the altimeter chamber. For instance, An eight-inch long four-inch diameter tube has a volume of about 100 cubic inches. Use one 1/4 inch port, or three or four 1/8 inch ports evenly spaced around the tube. An altimeter chamber two inches in diameter and eight inches long (25 cubic inches) needs one 1/8 inch vent hole or three or four 1/16 inch vent holes. Try to keep hole sizes within -50% or +100% of the general guideline. Do not make the holes too small, and especially do not make them too large. In general, the vent holes need never be smaller than 1/32 inch. Also, the vent hole diameter need never be less than the thickness of the body tube.
Adept Rocketry completed the research on static port sizes in 1990. The information provided here has remained the industry standard ever since those early years.
When possible, vent holes should be a minimum of 4 body diameters below the junction of the nosecone with the rocket body. This is necessary with high performance (high speed) rockets. The tremendous pressure on the nosecone leeches down the rocket body as much as three or four body diameters before it dissipates. However, with lower speed rockets, the "minimum of 4 diameters" rule may be reduced to one or two.
An Altimeter must be installed in a "sealed" chamber with a vent or vents to the outside. You cannot put an altimeter in a rocket along with the parachute and expect to get accurate readings. An altimeter must be mounted in its own specially isolated and properly vented payload chamber or electronics bay. If your rocket does not already have one, then you must create one or add one.
If your rocket does not already have an altimeter chamber or payload bay, then one way you may add one is by creating what is known as an "extended nosecone." This approach works exceptionally well with small hobby rockets, but it also works quite well with larger rockets.
The upper end of a typical rocket carries the parachute, and usually with a lot of room to spare. The spare room above the parachute can be used to create a new isolated payload chamber.
Cut apart the rocket tube several inches down from the top end. A coupler tube is then inserted part way into the upper portion and glued into place. Fashion a bulkhead from wood, and glue it into place inside the coupler tube. Add a metal loop for the shock cord.
This new assembly is treated as an "extended nosecone."
Put your altimeter inside this new chamber along with some protective padding. The vent hole/s should be away from the nosecone as far as possible, just above the top end of the coupler tube.
Assure that the nosecone is a snug fit. You can use small screws on larger rockets. Secure a small line to the inside of the chamber, and tie it to the altimeter and nosecone just in case of separation.