Analysis, design and experimental test of an altitude control system and a station keeping model for a near space platform

Over the last few years, there has been an increasing focus on stratospheric balloons as a cost-effective alternative to LEO satellites, as a result of the growing demand for simple solutions able to commercially compete with the space segment. At the present time, three main kinds of balloon are commercially available: Zero-Pressure balloons, Super Pressure balloons and rubber balloons. Rubber balloons represent the cheapest solution by far, for this reason they are widely used for university research programs and for activities that may vary from meteorology, Earth-observation, telecommunications, scientific experiments and space hardware testing. Unfortunately, a common mission using those balloons lasts only for a few hours, during which the balloon ascends with a constant speed until bursting occurs, usually at altitudes above 30 km. For missions of a longer duration zero-pressure or super-pressure balloons would be preferable, those can autonomously stabilize in the atmosphere at a certain altitude, increasing the duration of mission up to several days. The problem of using those balloons is mainly monetary, since they are more expensive than a sounding balloon and the budget of most university groups is limited. In this context, the purpose of the present work is to develop and test the preliminary version of a stabilization system for a high altitude sounding balloon, to create a low-cost platform capable of performing long-term missions. The stabilization methodology behind the system involves the use of two actuators, releasing either liquid ballast or helium, to modify altitude and velocity of the balloon by changing its buoyance. Furthermore a preliminary version of a station keeping model will be developed, allowing the balloon to float over a pre-defined geographical area for a limited amount of time by exploiting the different wind layers directions into the stratosphere. The first part of this work presents a mathematical formulation to better comprehend the motion of balloons into the atmosphere, the formulation includes a dynamic, thermal and actuation model. Then the process for the selection and development of a ballast release system is presented, the latter, after being carefully tested, is placed by side with the helium release system designed by Matteo da Valle to complete the balloon altitude control system (BACS). The tests on the assembled system are fully described and the results are discussed. The following section of the work regards the development of the control law. This has been divided in two models, the first is the altitude control system, which will allow the balloon to maintain a predetermined altitude using the gas and ballast actuators, the second instead is the Station keeping model, which exploits the layers of wind so that the balloon is able to float above a predetermined area by changing its floating altitude. A statistical analysis is also presented, it was conducted by using the university simulator to identify the best launch periods and how the variation of certain parameters affect the effectiveness and duration of the control system. Finally, in the concluding section, considerations on the system are made, concentrating on the simulation results and future works