The poor man’s space program
How amateurs are designing and building functioning models of satellites and launching them into a space-like environment
by L. Paul Verhage
|While the costs of materials and launch are kept low, the results returned are priceless. Where else can one return images of the inky blackness of space and the curvature of the Earth’s horizon at a thousandth the cost of launching a comparable satellite into Earth orbit?|
Located inside the airframe is the near spacecraft’s avionics. The avionics may be as simple as a GPS tracker or be as complex as a complete flight computer. Simple trackers and complex flight computers rely on amateur radio and Automatic Packet Reporting System (APRS) to locate the near spacecraft. The APRS tracker consists of a radio modem, called a terminal node controller (or TNC), an amateur radio, usually a handheld radio (also known as a HT), and a GPS receiver. The GPS receiver constantly outputs position reports, usually at either one- or two-second intervals. The TNC collects position reports from the GPS receiver, formats it for transmission over amateur radio, and then operates the HT. The result is a scratchy sounding burst of data only understandable to another TNC. Ground stations using the same setup connected to a laptop or PC receive the data, decode it, and then display it on a moving map. The result is a depiction of the current position and movement of the near spacecraft in reference to local roads and terrain. The position reports are accurate enough to recover the near spacecraft after its balloon bursts and it parachutes into the landing zone.
Flight computers are built around microcontrollers and are programmed in advance of the mission. The flight computer operates experiments on each flight, either in a programmed sequence or based on the occurrence of events during the mission. Results from experiments are either transmitted to ground stations or stored onboard the near spacecraft for later retrieval. Popular microcontrollers used in flight computers include the Rabbit, PIC, and Basic Stamp.
A recovery parachute is a must. The recovery parachute is either purchased assembled from an amateur rocketry store or sewn at home. Parachutes sewn at home are made from ripstop nylon and use twill tape to reinforce its seams. Parachute shroud lines are made from cord-like Spectra kite line. The minimum parachute diameter depends on the payload weight and fabric porosity, but typically parachutes are around six feet in diameter.
Weather balloons come in a variety of sizes. The most popular size is made from 1200 grams of latex. Filled with helium to a volume of over 300 cubic feet (8.5 cubic meters), the balloon stands some 2.1 meters tall and is capable of lifting 7.2 kilograms. The large size of the balloon and its low weight means the balloon must be launched early in the morning before the winds have a chance to pick up. Since the filled balloon may cost $125, there is a great incentive to launch the balloon without bursting it.
From top of the balloon to the bottom of the last module, the stack approaches a height of 15 meters. It’s a big beast these people launch. In many cases the stack is raised to its full height on lanyards before its launch. Once everyone is clear and all near spacecraft systems are confirmed to be working, the lanyards are released and the stack begins climbing into near space. In some cases though the entire stack is released without first raising the balloon. In this style of launch, the climbing balloon pulls the rest of the stack out of the hands of the launch crew. After liftoff, the stack typically ascends at a rate of 300 to 360 meters per minute. Within thirty minutes of launch, the stack is passing above commercial jets. The spacing between aircraft, the small size of the balloon, and the large volume of air space, means the chance of a collision or even a near approach is insignificantly small. However, just to be sure, near-space groups file a NOTAMS before launch. With the larger balloons, a near spacecraft spends two-thirds of its mission above commercial aircraft.
|Within thirty minutes of launch, the stack is passing above commercial jets. With the larger balloons, a near spacecraft spends two-thirds of its mission above commercial aircraft.|
As the balloon ascends and the atmospheric pressure drops, the balloon is forced to expand in volume. Weather balloons can reach diameters greater than six meters before bursting. From the ground, a large weather balloon at an altitude of 30 kilometers appears as a star (if you know where to look). At the end of its ascent, the balloon bursts from the lack of air pressure in near space. The parachute opens immediately upon balloon burst, but because of the extremely low air pressure, the near spacecraft descends at speeds in excess of 160 kmph! What remains of the balloon after burst is dragged earthward by the descending near spacecraft. At lower altitudes the balloon remains, now looking more like a tangle of cooked spaghetti, falls over the side of the parachute canopy. Landing is at a safe nine kmph or lower.
Once the near spacecraft’s final position report is received, recovery crews begin their final drive to the near spacecraft. The flight of a near spacecraft can take recovery crews through some seldom-traveled regions of the state. Rarely does the near spacecraft land near the road; usually recovery crews must hike out to retrieve it, through a cut wheat field or through canyons and over mountains.
Whether you like to construct models, sew fabric, design and build electronics, program microcontrollers, drive road rallies, hike, or analyze data, you’ll find something in your local amateur near-space program. Amateur near-space truly is the poor man’s space program. Onwards and upwards!
Sidebar: Amateur near-space groups
Here is a list of most of the active amateur near-space groups in the United States:
Ralph Wallio maintains a list of theory and current mission records. Check his website out for a flavor of what these groups are accomplishing.