In early May of this year we launched a flight that achieved float with a latex balloon for over eight hours at an altitude of 28km. It remained airborne for over fourteen hours. This flight utilized a tandem balloon configuration and a Doongara Balloon Cut-Down device.
We went over to the Sisters Airport to take part in a STEM project initiated by ISTAR in 2015. It included a number of volunteers and two chemistry classes from the Sisters High School. The students had designed two different payloads with a variety of experiments that were conducted during the flight. After helping to launch the student-oriented balloons, we explained how tandem ballooning works and then launched this tandem balloon flight.
The flight was calculated to have an initial ascent rate of 4m/s (787fpm) from the combined free-lift of both balloons. Once an altitude of 17.5km was achieved, the Doongara Balloon Cut-Down device released the tow balloon. This reduced the free-lift from 966g to 24g, resulting in a new, slower ascent rate of only 1m/s (197fpm).
In the case of this flight, the balloon achieved float around 28km and maintained it for the duration of the day. It only started to descend when the sun set and the loss of solar radiation cooled the helium sufficiently to upset this balance. The flight then descended at 2m/s (394fpm) until it landed, a little over fourteen hours after launch.
Theory of Achieving Float With A Latex Balloon
Float with a latex balloon can be achieved because of how the internal pressure varies with the balloon size. The pressure inside of the balloon increases until it is fully inflated and no longer has flaccid material present. After this point, the pressure inside of the balloon starts to decrease as the latex membrane uniformly stretches and becomes thinner. As the latex membrane starts to reach its maximum limit of stretch, the pressure inside starts to increase. If this trend is continued, the balloon will finally burst when the internal pressure exceeds what the latex is able to withstand.
With a slow ascent rate of 1m/s, the subtle changes in the density of the helium due to the varying internal pressure of the balloon permit a stable float. As the balloon ascends, so will the internal pressure which increases the density of the helium lift gas and causes the balloon to become less buoyant. As the balloon descends, the internal pressure decreases which decreases the density of the helium lift gas and causes the balloon to become more buoyant. Over this small but usable range the latex balloon acts like a super pressure balloon.
Equipment For Achieving Float With A Latex Balloon
A 200g Hwoyee balloon was used for the tow balloon which was released at an altitude of 17.5km. A 600g Hwoyee was used for the main balloon that carried the balloon on to float at an altitude of 28km.
A Doongara Balloon Cut-Down device was used to separate the tow balloon from the payload at 17.5km. Using a Doongara permits a more accurate separation altitude than allowing the tow balloon to simply burst. It also ensures all of the tow balloon is released and that balloon remnants post-burst do not remain attached to the payload and change its expected mass.
A flight computer from a Boomerang Balloon Flight Control System was used as the payload to provide a long-distance command and telemetry link. Using its 70-centimeter radio, ranges in excess of 200km were possible with an omnidirectional antenna, and ranges in excess of 300km were possible with a 13dBi yagi antenna.