Active Guidance for High Altitude Balloon Payloads

Introduction High-altitude balloon (HAB) missions provide a cost-effective platform for scientific experiments and atmospheric research in the upper atmosphere and near-space environments. Traditional HAB payload recovery methods rely primarily on passive parachute descent, often resulting in unpredictable landing locations due to wind and other atmospheric effects. This uncertainty increases the cost and complexity of payload retrieval operations and can limit the reusability of sensitive scientific instruments. The Floating Longhorn HAB Project aims to address these recovery challenges by developing an autonomously guided payload recovery system designed to enhance landing precision and facilitate rapid retrieval. The Floating Longhorn node integrates an autonomously actuated suite of grid fins to control position during free fall, a parachute-based recovery subsystem, and avionics to autonomously navigate towards a predetermined landing zone and facilitate communications with the ground. Unlike traditional HAB payloads that drift with limited control upon release, the Floating Longhorn project leverages active control and real-time guidance algorithms to significantly reduce landing dispersion and improve ease of recovery. System Overview The Floating Longhorn system consists of three primary subsystems: deployment, guided descent, and recovery. Each subsystem is designed to operate within the total 6lb weight limit imposed by the FAA on unmanned free balloons. Deployment Subsystem: The deployment subsystem secures the node to the balloon and controls its release at the designated altitude of 6km. This system uses a hot-wire cutdown mechanism to release the node from the balloon. When our altimeter detects the target altitude, an onboard battery passes a high current through a nichrome wire that is wrapped around a synthetic rope. The high current causes the nichrome wire to quickly heat up, melting through the synthetic rope and dropping the node from the balloon. Guided Descent Subsystem: Upon release, the node enters a free fall phase before stabilizing using a set of four grid fins mounted radially around the node. These grid fins are mounted to servo motors, giving control over the grid fins’ angles of attack. By selectively actuating the grid fins, we can steer the node toward a designated landing zone. The guidance, navigation, and control (GNC) algorithms, implemented via an onboard Arduino, continuously compute the optimal grid fin deflection angles based on real-time data from our avionics. The avionics suite includes an inertial measurement unit (IMU), GPS module, and barometric altimeter which all provide necessary data for the GNC algorithms. Recovery Subsystem: At a preprogrammed altitude of 0.25km, the main parachute deploys, significantly reducing descent velocity for a controlled landing. Parachute deployment relies on dual-redundant pyrotechnic charges triggered when an altitude of 0.25km above ground level (AGL) is detected. At this altitude, the flight computer sends a signal to the e-matches in the main and backup charge wells, igniting the black powder in them and pressurizing the aft section of the node. This pressure will sever the shear pins that connect the aft bulkhead to the node’s airframe and eject the parachute. Future Tests To ensure mission success, the Floating Longhorn HAB project will incorporate a testing campaign including laboratory experiments and full-scale flight trials. Preliminary FEA, CFD, and other simulations have proven valuable, but a physical testing campaign will aid in ensuring the mission’s success. This will include the following: Wind Tunnel Testing: Grid-fin prototypes will be tested in a low-speed wind tunnel to validate the aerodynamic properties predicted by CFD under varying angles of attack. These results will inform future grid-fin sizing considerations and refine the lift and drag curves produced with CFD. Wind tunnel testing on the entire node will also help provide a more accurate drag coefficient for the entire vehicle to help refine our other simulations. Ejection Testing: Ejection testing on the ground of a fully packed and prepared node can be tested by manually sending current through the e-match or by manually triggering our avionics’ recovery event protocol. This will validate the size of our pyrotechnic charges, parachute packing method, and separability of our aft bulkhead. Drop Testing: “Lower” altitude drop testing from heights such as 6km can serve to validate the guided node platform and ensure the system’s viability before applying it to high-altitude balloon systems. Conclusion The Floating Longhorn High-Altitude Balloon Project demonstrates an innovative approach to payload recovery by integrating grid-fin-based aerodynamic control with autonomous guidance and parachute landing. Through rigorous analysis, prototyping, and flight testing, the project seeks to establish a viable method for achieving high-precision landings for HAB payloads. Future small-scale flight tests will provide critical insights into system performance, paving the way for future developments in guided descent technologies for high-altitude and space-based applications.