I'm building a short stubby rocket and plan to mount an altimeter in the
fin can area between two of the fins. The body tube is 5.5" diam and 21"
long. The distance between the centering rings is about 11" and there
are 4 fins total.
My plan is to cut a long narrow hatch (apx 1.5 x 7.5") between the
centering rings and mount the altimeter and battery to the inside of the
hatch. I can put the baro vent hole very near the top of the fins so
there shouldn't be much disturbance.
Is this a bad idea? I've seen this done before but it was usually a more
square shaped hatch. I don't think this will weaken the structure too
much because where it's located, but before I go cutting out a major
section of the body tube, I want to be sure.
I did the same thing on my LOC Fantom. I was worried about the tube
buckling at the cut out so I glued in longitudinal (vertical ribs that ran
from the motor mount tube out to the airframe tube. These ribs were 1/8 inch
plywood. I put one of these vertical ribs every 90 degrees. At the cutout
location I made sure that the ribs were set far enough in to allow the
building of the closure lip explained hereafter. I glued wide popsicle
sticks (tongue depressor size) around the inside edge of the cut out using
Devcon 5 minute epoxy. I let half of the stick width hang out over the
cutout edge to support the closure. For the curved part of the tube I just
uses multiple short lengths of popsicle sticks to make the curve. After the
epoxy dried I put some plastic saran wrap around the loose piece of airframe
I had cut out. I then put 5 minute epoxy gel all over the lip I had created
with the popsicle sticks and pressed the saran wrapped closure into the
epoxy gel so that the edges were flush with the airframe tube. Note: I
rounded the corners of the closure prior to wrapping it up. The gel then
filled in the corners of the airframe cut out to comply with the rounded
corners of the closure. The gel also filled in the dips and valleys created
when I used short pieces of stick to form the curved part of the closure
lip. When it was set I removed the closure. The saran wrap allowed this
removal. I had a perfect fit for the closure and a lip that backed it up.
I then assembled the closure to the airframe and drilled holes around the
edge of the closure for my screws. I glued brass nut on the back side of
the cut out lip to accept the screws. I used brass because I had a Magnetic
Apogee Detector that I did not want steel screw interfering with the earths
magnetic field. It worked great and is very strong.
Good luck with what ever method you use.
After posting the previous message It dawned on me that at a fin can the
fins would act as the vertical ribs I previously described. So no need to
add additional vertical ribs. My cut out happened to be above the fins so I
had to add the longitudinal ribs.