How To Mount FireFly

The firefly is a nice little altimeter that most of the TARC teams use for the TARC competition .
I have seen them mounted in many different ways in many different parts of the rocket - the nose cone, most typically a standard payload section and even seen them mounted in the booster next to the engine (which I would definitely not recommend)! 

To mount them in the payload section, which it sounds like you have (4" dia x 7" long), I have seen them simply taped to the side of the payload bay (not recommended), but a design I like is to get a small section of BT20 body tube which is aprox 0.7" inner diameter (or something similar) maybe an inch and a half or so long and that gets glued to the inside of the payload bay so it does not interfere with the shoulder of the nose cone or coupler below. The top of the little inner tube should be easily accessible by removing the nose cone. Cut some balsa to fit the base of the inner tube and glue it in the bottom. Then cut another section of balsa (or a plastic nose cone or coupler plug will also work great) that can plug up the top. It needs to be tight enough so the altimeter can't fall or get bumped out. Then you can drill a pressure equalization hole thru the side of the payload bay and into the smaller Firefly holder tube. A single 3/32" dia hole should be fine. Just make sure it does not line up with any lugs or anything that would interfere with the airflow over the side of the rocket. Finally you can use some packing foam (or similar) on the top and bottom of the altimeter to sandwich it and keep it from moving around in the tube. Just don't smother it which might interfere with the barometric sensor. 

Let me know if you need more clarification or a picture or something. 

- H

Yes - exactly.  You could also just drill a hole directly thru the body tube and into the inner tube, where you have your glue arrow. That would give you quicker air compensation. But I think your design should be fine. Anyone else care to weigh in?