Introducing BalloonSats
An alternative to using robotics as a vehicle for teaching STEM is the BalloonSat project. One reason BalloonSats may make a superior alternative to robotics is that robotics doesn’t involve as much science and mathematics as a well structured BalloonSat project. And while robots in competition can operate in either autonomously (independent of a human operator) or with operator control (by human control, usually over a radio), BalloonSats can only operate in autonomous mode. Students design and program their BalloonSat to operate sensors and collect data without human intervention.
Description of a BalloonSat
BalloonSats, as Linda Kehr describes them, are model satellites carried under helium filled weather balloons to altitudes in excess of 80,000 feet, a very space-like environment. In fact, BalloonSat flights reach 85,000 feet easily and can reach over 120,000 feet with lighter payloads and larger balloons.
BalloonSats are the first step in the National Space Grant Satellite Program’s strategy, “crawl, walk, fly, run”, whose ultimate goal is to send a student-designed payload to Mars. However, the first step, “crawl” is designed to encourage students to build and fly simple models of satellites, like BalloonSats. It is believed that by getting students involved in a series of more complex projects, more will graduate from STEM programs and enter into aerospace engineering fields.
Description of their construction
BalloonSats are an inexpensive way to access space while still retaining some of design and engineering challenges of satellites (Kohler 2003).
Airframe
BalloonSats are student designed from Styrofoam to carry programmable dataloggers and cameras and typically do not weigh more one pound (Kennon, Roberts & Fuller 2008). Other design challenges may involve volume (not to exceed 1000 cc), minimum datalogging capability (internal and external temperatures over the entire flight) and functional testing preflight. Adhesives used to assemble the BalloonSat airframe from foamcore include silicon rubber glue, hot glue, and JB Weld (an epoxy). Aluminum duct tape is also a popular material to seal the airframe (Koehler 2003).
Figure 1. Example of a BalloonSat. This one is constructed from a sheet of ½” thick Styrofoam, the same material used as insulation of outside house walls. It’s walls are assembled with hot glue and covered in black packaging tape. Photograph from the author’s collection.
Avionics
The datalogger used when BalloonSats were first designed is the Hobo datalogger. Scouts involved with the Glenn Research Center’s BHALF (BalloonSat High Altitude Flight) are beginning to experiment with using BASIC Stamps by Parallax – the same microcontroller used in the Boe-bot robot (BHALF). For Students in CU Boulder’s Gateway to Space course who are ready for a more advanced challenge, the timer is replaced with a programmable BASIC Stamp.
After recovery of their BalloonSat, students connect the datalogger and camera to a PC to retrieve the data and images. Students can perform their own mathematical analysis of the data and images or rely on the software used to program the dataloggers.
Figure 2. An eight-bit Hobo datalogger manufactured by OnSet Computing. This model records internal temperature and an external voltage. Photograph from the author’s collection.
A one time popular camera for BalloonSats was the Canon Elph. These APS film cameras were relatively inexpensive and very easy to modify for operation by intervalometers. The intervalometer is a 555 IC based timer kit soldered together by students. More recently, digital cameras and digital video recorders are included in the BalloonSats.
Preflight testing
Prior to flight, their designers test their BalloonSats. Even though flights cost less than $300, this is still too expensive to launch a BalloonSat that has no guarantee of functioning properly. Typical tests used in Koehler’s program include the following.
Drop Test: BalloonSats land by parachute. At touchdown, the BalloonSat’s speed can easily reach 10 mph. To ensure BalloonSats will remain in one piece during the landing, students drop their BalloonSats from a height that simulates their landing of 10 mph. The height from which a BalloonSat must be dropped to simulate a 10 mph landing can be calculated as shown below.
time of fall = 0.455 seconds
height of drop = 3.34 feet
Cooler Test: Near space gets very cold (the coldest temperature the author’s BalloonSats have measured is -90O F, although -60O F is more typical). To ensure the BalloonSat is build well enough to keep its datalogger contents warm enough to function is to place the BalloonSat inside a Styrofoam ice chest filled with dry ice. The BalloonSat is left inside the cooler long enough to let the interior temperature bottom out (the author uses a time of 20 to 30 minutes).
Functional Tests: During its construction and at the competition, the BalloonSat, its datalogger, intervalometer, and camera are tested together to verify they will work without interfering with each other. This means all subsystems must fit inside the airframe without blocking access to the camera power button or the camera’s view outside the airframe.
Description of launch/recovery
BalloonSats are lofted into near space on a helium-filled weather balloon. The entire vehicle consists of a helium-filled weather balloon at the top, a recovery parachute attached below the balloon by a load line of nylon cord, one or more GPS trackers packed inside a Styrofoam enclosure, and one or more BalloonSats (Koehler 2003). The GPS tracker transmits position reports of the balloon over amateur radio frequencies. The system amateur radio operators use to track the location of items (like automobiles) is called the Automatic Packet Reporting System, or APRS. Therefore, a licensed amateur radio operator is required on each near space launch. The expendable parts of the flight are the helium and latex weather balloon and accounts for the $300 price tag for the flight. The radio tracking equipment are repaired, if necessary, so it can track another mission.
The maximum weight on most BalloonSat launches is 12 pounds as long as no single item weighs more than six pounds nor has a surface density greater than one ounce per square inch. Additional FAA rules apply when these limits are exceeded (Federal Aviation Administration, FAR 101). Therefore, to avoid the application of additional FAA procedures, most schools launching BalloonSat limit their flights to 12 pounds total weight.
The typical BalloonSat launch occurs in the morning and requires between two and three hours to complete. The early morning launch permits the balloon to be filled while the winds are generally lower. After release, the typical ascent rate for the weather balloon and payload is 1,000 feet per minute. Latex weather balloons are sold by weight and frequently used balloons are 1200 and 1500 grams. Kaymont is an example of weather balloon dealer located in the United States.
A balloon filling system consisting of a regulator designed for welding gases, oxygen hose, and a length of PVC pipe. The PVC pipe attaches to the end of the oxygen hose and has a diameter less than the diameter of the balloon’s nozzle. The balloon nozzle slides over the PVC pipe and taped securely. Once secured, the balloon is filled with helium. Welding companies are the suppliers of helium required to launch a weather balloon. The helium arrives in welding tanks and they can weigh as much as 120 pounds.
Figure 5. Balloon Filler. The green oxygen hose is 12 feet long and the PVC pipe is 1.25 inches outside diameter. Photograph from the author’s collection.
Figure 6. University of Kansas students filling two latex weather balloons in preparation for BalloonSat launches. Photograph from the authors collection.
A typical flight requires 90-100 minutes to climb to peak altitude and approximately 30 minutes to descend back to the ground on its parachute. Because of the amateur radio equipment onboard, the balloon is tracked and its landing site located. Because of APRS onboard the balloon, students can track the position of the balloon carrying their BalloonSat in real time (Koehler 2003).
Near Space
According to Aerostar, near space begins at an altitude of 50,000 feet. According to the United States Air Force, near space begins at 20 km (65,600) feet, or above class A airspace.
Figure 7. Example of air pressure measured as a function of altitude by a BalloonSat. Environmental sensors from this author’s past near space flights indicate the air pressure drops to 10 mb, or 99% of a vacuum at an altitude of 100,000 feet (Data from the author’s collection).
Figure 8. Example of air temperature measured as a function of altitude by a BalloonSat. Air temperature drops to a low of -60O F in the summer and lower in the winter at the boundary between the troposphere and the stratosphere (Data from the author’s collection).
Figure 9. Example of the relative humidity measured as a function of altitude by a BalloonSat (Data from the author’s collection).
Figure 10. Example of cosmic ray flux measured as a function of altitude by a BalloonSat. The flux of secondary cosmic rays increases as the altitude increases until well into the stratosphere, where primary cosmic rays begin to be detected (From author’s personal data).
Figure 11. Example of an image returned by a BalloonSat showing the blackness of space and the curvature of the earth. A digital camera modified for operation by a programmable flight computer recorded this image of near space at an altitude of 78,000 feet (Image from the author’s collection).
