The capsule needs to perform the following functions:
- Provide a platform for cameras, sensors, avionics and whatever experiments you are conducting
- Keep electronic devices within their operating temperature range. Temperatures can get extremely cold (-70F) which can cause electronics to malfunction, cameras to freeze, etc..
- Provide impact protection upon landing. Even with a parachute successfully deployed, the capsule can hit the ground at 20 mph
In addition it's critical that the capsule is lightweight as possible such that there is remaining lift to carry your payload. Weather balloons commonly used for high altitude flight have very limited lift capabilities, so you'll need to design your capsule carefully. You can see the lift capacity of these balloons here: http://bit.ly/11bsn29
The guide, Near Space, by L. Paul Verhage was an invaluable resource in the design and construction of our capsule.
We started with a 3/4 inch 12.5 x 12.5 x 7 Styrofoam Cooler purchased from Mr Box Online. Styrofoam is easy to work with and provides amazing insulating capabilities. At times during the flight there was a 70 degree difference between the outside and inside of the capsule.
We stitched a covering for the cooler using Nylon Rip Stop fabric and attached key ring on the four corners with 1&1/2" Dacron Tape. The top of the cooler (aka "The Hatch") was secured to the body with Velcro Brand Sew-On Tape.
We built two small platforms using plywood and Styrofoam to hold the experiments and attached these to each side of the capsule.
On the bottom of the capsule we attached a high-gain 900mhz antennae.
We used Mil-Spec 550 Paracord Type III line to attach the key rings to the radar deflector. We then attached a parachute to the radar reflector then used 30' of line to connect the parachute to the balloon. Between each attachment we used heavy-duty fishing swivels.
We attached a 100dB Piezo 2-Tone Buzzer from Radio Shack to the outside of the capsule to aid in the recovery process.
On the front of the capsule we created an instrument panel with switches for the avionics and buzzer. We also included LEDs to indicate power and radio transmission. The control panel was a bit frivolous and we wouldn't recommend including this into the capsule as the power switches created an unnecessary potential failure point. On the other hand it looked pretty cool and the kids loved playing with it.
Overall we were very happy with how the capsule worked out!