I realized, after making that "thing" out of a construction tube and some balsa/plywood (see this post) that I had to rethink my design entirely. I'd also read that even though anybody can fly G class motors without a license, flying four of them at a time wasn't exactly a shortcut to more power with less paperwork.
So I went back to the drawing board and took a look at what a system with only one rocket motor would look like. Details after the break.
To the left is a sketch, roughly to scale, of what the new system will look like. One motor at the bottom centered in a larger tube using a centering ring. Electronics are mounted in the main body, and right now I'm evaluating placement of the parachute system on the outside, to avoid the necessity for a deployable nose cone. A couple of the components are drawn to scale on the right side.
DESIGN NOTES:Using only one rocket motor should help me meet the FAA regulation that balloon payloads must be less than 4 pounds (interesting sidenote, apparently that regulation comes from the average weight of geese. Airplane jet engines are designed to withstand a strike from a 4 lb bird so it was assumed they'd be able to take the strike of a 4 lb payload). Of course, I'm not sure I'll be totally FAA clear since I am igniting a rocket at altitude. That will all be cleared up in due time.
For the single tube, I'm not sure if I'll need a 2.5" or 3" tube. The 2.5" might not be large enough for the flip cam (which I'll now be mounting inside the rocket facing down), which would necessitate a 3" tube. For now I'll order both and see which one fits. The rocket motor, which is only 1" in diameter, will be placed at the bottom (where else?) in a tube specifically designed for 29mm engines and centered in the larger tube using a centering ring. Given the amount of space there, I'm wondering if I could get away with putting two or maybe even more motors down there, but then you get into issues of centering them and the whole issue of whether I need a license or not.... I'll deal with that later. By the way, I'm getting all these components from www.apogeerockets.com.
The rest of the components, however, should fit fine one way or another. Securing them will be its own problem but we'll deal with that later. The GPS unit will be mounted to the outside so it gets good reception, and also to counter the weight of the parachute system on the other side.
I should also note that I ordered a component from Seeedstudio that allows Arduino to write to an SD card but it's shipping from Hong Kong to it'll take a while to get here. Once it arrives though, I'll be able to hook up the SD card, Arduino, and GPS and take them for a spin around the neighborhood to see how good this system actually works for tracking location and altitude.