Nuts & Volts - February 2004

Near Space

and shut down telemetry from your near spacecraft (ever see the movie, Lost In Near Space?).

There are two things you can do to prevent this. The first is to use lithium cells. The chemistry of lithium cells holds up to cold much better than most other battery chemistries. The second is to cover the exterior of the airframe in dark fabrics. A jacket of dark fabric absorbs solar radiation and will passively heat the interior of the module. Besides warming the module, a fabric jacket will also protect the MLI and airframe exterior from abrasion during landing. It's also a great place to attach link lines between modules. I use ripstop nylon for my fabric jackets, which I call abrasion jackets. You've most likely seen ripstop nylon used in fabric kite sails.

Avionics for Near Space

The electronics used to operate a near spacecraft are called avionics (aviation electronics). For your first near space mission, I recommend using a simple amateur radio tracker. A simple and inexpensive amateur radio tracker consists of a terminal node controller connected to a GPS receiver and a handheld, two-meter, amateur radio (see Resources).

The terminal node controller (TNC) is a modem built for radio use. When used in avionics, it accepts sentences from a GPS receiver and formats them for transmission over the radio.

Afterwards, it keys the radio and sends the proper tones. A similar set-up on the ground decodes the tones and displays the data on a laptop or PC. This method is referred to as the Automatic Packet Reporting System (APRS) and is very popular with the amateur radio community.

Because of APRS, the position of the near spacecraft in three dimensions, its speed, and its heading are known to chase and recovery crews.

The simple tracker costs around $250.00, but don't let the cost scare you. Assembling your tracker is a one-time expense, because it is used on every mission. Besides, the cost of a tracker is less than the cost of a good set of golf clubs. Not only does a tracker cost less, but the aggravation associated with it is less than the aggravation associated with the same set of golf clubs.

FEBRUARY 2004

The next step up from a radio tracker is a flight computer. Most of the flight computers used by amateurs today are based on programmable microcontrollers like the PIC, Rabbit, or BASIC Stamp.

Upgrading from a radio tracker to a flight computer doesn't add much to the cost of avionics, but it does permit complex experiments and mission profiles that aren't available with a simple tracker. More information on flight computers is available from groups like ANSR, Project Traveler, or myself.

Your experiments will change on each mission, but your modules shouldn't. Give some thought to designing and building a generic style of airframe and flight computer. This way experiments are designed to meet the standards for the modules, rather than having modules designed to match the experiment. Be sure to document these standards. This is a much faster approach to flying missions, making failures less likely. Now your near spacecraft is more like a Space Shuttle than a Mercury space capsule. NV