Solar Balloon Flight Dynamics During the 2024 Total Solar Eclipse

In recent years, solar balloons have become a popular method for performing high altitude observations. Heliotropes, a type of solar balloon, are relatively inexpensive and easy to manufacture. These light-weight balloons rely on solar radiation to float within the lower stratosphere (~ 20 km) and can float for multiple hours. During flight, solar radiation heats the ambient air within the balloon which generates buoyant lift. Shortly before sunset the balloon begins to descend as there is no longer sufficient radiation to maintain the float altitude. As part of the NASA Nationwide Eclipse Ballooning Project (NEBP), Oklahoma State University (OSU) used the 2024 total solar eclipse to study its effects on the solar balloon flight dynamics. Two balloons were launched from Broken Bow, OK, which was within the path of totality, and each balloon carried two different payloads. The first payload was designed by OSU specifically for the eclipse and was placed 15 ft below the mouth of the balloon. It was made of a styrofoam box that included three exterior cameras, two BME280 sensors, and a pyranometer. The BME280 sensors recorded temperature, pressure, and relative humidity. Of the two, one was located inside the payload, primarily to monitor the internal temperature, and the second was used to record the surrounding atmospheric conditions. The external sensor was placed within a custom, 3D printed cover. This cover had exposed slats on all four sides, but only the bottom was left exposed, and the rest were sealed. A pyranometer was placed on top of the payload to record solar radiation. Due to supply issues, one balloon had a LI-COR LI200R pyranometer and the other had a SP-510-SS made by Apogee Instruments. With this design, the balloon flight profile could be characterized relative to the rapid change in solar radiation due to the eclipse. The second payload was located approximately 10 ft below the first payload, 25 ft from the mouth, and carried an infrasound sensor. Infrasound is sound pressure waves with frequencies that are below the threshold of human hearing (< 20 Hz). This sensor was reconfigured to have four inlets by using tee connectors and plastic tubing, which has been shown to reduce wind noise. A separate research team from Sandia National Laboratories launched four of their own solar balloons for the eclipse. These balloons were launched from Junction, TX, also within the path of totality.

Data from these balloons were analyzed to determine the impact of reduced insolation on balloon buoyancy and resulting flight dynamics utilizing EarthSHAB. EarthSHAB is an open-source software that was developed to predict the trajectories of solar balloons. By inputting the balloon diameter and flight line weight, a trajectory can be generated prior to the launch. This greatly aids in recovery planning and setting conditions for the High-Altitude Balloon (HAB) Bounder. This device is a balloon cut-down system that uses preset conditions, such as a time limit or coordinate-based geofence, to disconnect the flight line from the balloon. The key aspect of this software is its heat transfer model. It calculates the overall radiant and convective heat loads on the balloon, resulting in a flight profile. Combining this with a National Oceanic and Atmospheric Administration (NOAA) weather forecast, the predicted trajectory of a balloon is generated. The total eclipse offered a chance to see how the predicted trajectory from EarthSHAB compared to the actual flight path of the eclipse balloons. This presentation will compare the predicted and actual trajectories of the OSU balloons, in addition to investigating their behavior during the eclipse. Furthermore, the flight dynamics of both the OSU and Sandia balloons will be analyzed.

Acknowledgements
“This work was funded, in part, by NSF and NASA as part of the NEBP, as well as the Gordon and Betty Moore Foundation under grant GBMF11559 (doi.org/10.37807/GBMF11559). We would like to thank the Choctaw Nation for their hospitality and use of their facilities for the OSU launches. Thank you to the Laboratory Directed Research and Development program for supporting the launch efforts of Sandia National Laboratories.”