I thought I could post photos of my altimeter bay using a BT-60 body tube. I am building the same design for a BT-50. I will have to use another design for a BT-20.
This design is simple, yet incredibly secure. The altimeter is securely held inside the smaller tube which locks in place. Assembly in the field can be accomplished in a matter of seconds.
The BT-60 tube is 7 inches long. A standard balsa tube connector is glued into the base with a screw eye in the bottom. Before gluing the balsa into place I do the following.
1) I drill a 3/8 inch wide hole into the center of the balsa about half the length of the balsa.
2) I take a 3/8 inch dowel and cut it to length to fit the drilled hole.
3) I drill a pilot hole into the dowel, put some glue down the pilot hole and then screw the screweye into place.
4) I drill a small hole, about 1/16 of an inch, all the way through the length of the balsa connector. The hole is near the edge of the connector to keep it away from the BT-20 body tube that will hold the altimeter.
5) I thread about 12 inches of kevlar thread through the hole and knot the end on the screweye side of the connector. It helps to place a little white glue on the portion of the thread that has to be put through the hole. The glue stiffens the thread just enough to allow you to push it through.
6) I put a drop of glue on the knot and pull it tight against the balsa. This also seals the hole to keep ejection gasses out of your altimeter bay.
7) I glue the dowel into place.
8) Determine where the altimeter sampling holes need to be drilled.
9) I push the balsa connecter up inside the bay and then drill the holes, drilling into the balsa. The balsa supports the tube and a few small holes don't harm the balsa connector.
10) Pull the connector out and sand the inside and outside of the holes. Clean up as needed.
11) Glue the balsa connector into place.
12) I used Semroc BT-20 to BT-60 centering rings (white fiber rings) to position the altimeter tube inside the bay. The green rings are BT-20 to BT-50 adapter rings which add even more support to the white rings.
13) The white rings each have a hole drilled through them to allow the 12 inch kevlar thread to pass through. A knot was tied into the thread just past the second ring. Make sure you allow enough slack to allow the altimeter tube to pulled all the way out of the BT-60 tube. The altimeter tube is now permanently attached to the BT-60 tube.
14) A hole for the altimeter is cut into the BT-20 tube just over the barometer on the altimeter.
15) I glued a toothpick inside the altimeter tube oposite the sampling hole to keep the altimeter from turning away from the hole.
16) An 18mm thrust ring is glued into the altimeter tube so that when the altimeter is inserted into the bottom of the tube it rests against the thrust ring and the other end is flush with the bottom of the altimeter tube.
17) A small strip of wood will used to hold the altimeter bay into the BT-60 tube. Cut notches into the white rings on opposite sides that will fit over the strip of wood once it is glued into place.
18) Insert the altimeter tube assembly into place. Put glue on the upper 3/4ths of the wood strip and glue it to the inside of the BT-60 tube so that the lower end is resting on the top white ring and away from the notch.
19) After the glue has set a little, rotate the altimeter assembly to make sure it rotates freely and has not been glued in place by the strip.
20) I added a third centering ring to the top of the altimeter tube to strengthen the top of the tube. I stick my finger in here and use it to insert and rtotate the assembly.
21) A thread tied just below this ring ties off the nose come so that if it pops loose it stays with the rocket.
22) A line drawn straight up from the wood strip helps with alignment when installing the altimeter assembly.
For flight, arm the altimeter, and insert through the bottom of the tube. Insert the altimeter assembly into the BT-60 tube lining up the notch in the lower ring with the wood strip . Push down until the upper ring contacts the strip. Rotate 180 degrees and push the second ring past the strip. Rotate 180 degrees again. Insert nose cone and the bay is ready.
For even more security you can tie off the altimeter to the 12 inch long kevlar thread before arming.
This altimeter is not coming out of this rocket until you remove it.
Note: Make sure the BT-20 tube is at least 2 to 2.5 inches shorter than the BT-60 tube. You want to make sure the nose cone can be fully inserted all the way and does not hit the top of the altimeter tube. Keep in mind that if you cut the BT-20 tube too short it can be difficult to install and rotate.
