Hardware Reference
In-Depth Information
The Raspberry Pi camera is the main camera for the payload in this hack, although it
also holds a Nikon Coolpix attached through the Pi's remaining USB port and con-
trolled by gphoto.
Gather Gas, Balloon, and Parachute
Once the payload is built, you can determine its weight, which lets you calculate the
amount of lifting gas it requires. You need one cubic foot of helium per ounce of pay-
load. Thus, for a four-pound payload, you will need about 64 cubic feet of helium.
However, that amount of helium is just enough for neutral buoyancy, so you will need
an additional 1-1.5 pounds of lift to actually lift the payload. This amount of lift will give
you an ascent rate of about 1,000 feet per minute. For a four-pound payload, that's
5.5 pounds of lift needed, for a total of 88 cubic feet of helium.
Manufacturers specify balloon size based on the amount of lifting gas. For this exam-
ple of a four-pound payload and 88 cubic ft of helium, an 600-gram balloon will achieve
altitudes of 70,000 feet or above. Smaller balloons will pop at a lower altitude, and
larger balloons tend to pop at a higher altitude. However, if you choose a balloon that's
too large, it might never pop, and you end up with a “drifter” that can hang around the
sky for days, so it's better to choose a smaller balloon until you get experience with
more launches.
Now that you know the weight of the payload, you can also calculate the size of the
parachute, which you need for a slow and safe descent. Parachute companies de-
scribe parachute size based on a reasonable descent rate, typically 10-15 feet per
second. We recommend
The Rocketman
,
Ky Michaelson, a former Hollywood stunt-
man and stunt equipment designer who now runs a business selling balloon and rock-
et parachutes and related materials. You can read more about recovery parachutes
on
his website
.
For this size payload, a three- to four-foot parachute is appropriate. Balloon and par-
achute should be attached with shroud lines that are 50 pounds tensile strength or
less (per FAA guidelines).
Install Software
You need to load a few things onto your Pi before you're ready to launch:
• I2C libraries (for talking to the GY-80 sensor board)
• GPSd (daemon for decoding GPS data streams)
• GPIO libraries (for controlling status LEDs and sensing switch inputs)
• gphoto (for controlling the Nikon camera—see
“Geting Started with gPhoto” on
page
258
for further help)
