Brief:
For altitude measurements on large model and HPR rockets having no payload compartment, I have devised a simple 18mm x 9" "parasite pod" containing a Perfectflite micro altimeter (model Alt15K/WD) that can be temporarily attached to the main body tube of the rocket with masking tape. I have used it successfully to measure drag coefficient (Cd) on a 4" rocket with 5 different motors from G-J at speeds from 144 mph to 511 mph.

Construction:
The pod is made from a 7" length of Estes 18mm tubing, which just fits the Perfectflite Alt15K/WD. The forward end is capped with a small conical Estes nose cone glued permanently in place and the aft end is plugged with an ogive Estes cone that is held in place with masking tape. The whole affair is aligned along the midline of the rocket about 2/3 of the way down from the tip and securely fastened in place with strips of masking tape. Two 1/16" diameter vent holes are poked through the tubing so the altimeter can do its thing.

After making the required series of altitude measurements to allow for flight testing, the masking tape is carefully peeled away, taking care not to damage the finish, which usually doesn't happen, because that's what masking tape is for! For a 4" rocket, the 18mm "Parasite Pod" increases cross-sectional area by only about 3%. For a 3" rocket this would be about 6%, while, for 6" rockets and above the increase is negligible (less than 1%). Even for a large rocket (4" or greater) with a payload compartment, the 18mm Perfectflite altimeter is so dinky, it would be a good idea to use the "Parasite Pod" taped internally, so it doesn't rattle around in there! I originally designed the pod for just this purpose.

Flight:
The Parasite Pod was used for a complete flight testing program on a new design HPR tube fin rocket ("J-Boat") designed for G through J power and easy L1-L2 certification. The parasite pod verified H-power flights for "J-Boat" to 900 and 990 feet on a CTI H143SS-6 and Ellis Mountain H275-6, respectively. Flight on a J-400SS-9 gave a height measurement of 1858'. G80-4 and CTI I287SS-8 gave climbs of 466' and 1506', respectively. The resulting Cd values and speeds obtained from these altitude measurements are displayed on the graph, showing a nice, linear velocity dependence to Cd for "J-Boat", with a range of 1.35 to 1.65 between 200 mph and 500 mph. Not surprisingly, since the standard equation for Cd contains a velocity term in it! The "pod" and its tape attachment method held up just fine to 25G boosts on the H275 and J400SS and low transonic speeds (511 mph).

Summary:
PROs: This is an effective way to use an altimeter on a rocket with no payload bay and is easy to put together, can be removed easily, and transferred to another rocket in the field. The altimeter is quickly and easily loaded into the "pod" while attached to the rocket, which is important since you have 30 seconds to pack it in its holder after inserting the battery before the Perfectflite Alt15K/WD arms itself. You have the option to attach permanently with epoxy if desired too.

CONs: This setup is useful only for altitude measurements as there are no connections for dual deployment wiring. It is best for rockets of 3" diameter of greater, otherwise the additional drag will degrade rocket performance. You can't leave it attached for long periods or tape adhesive will mar the finish.