Aerodynamic and engineering design of a 1.5 s high quality microgravity drop tower facility. (December 2016) - Belser, Valentin
Abstract: Microgravity experiments are essential for research in space science, biology, fluid mechanics, combustion, and material sciences. One way to conduct microgravity experiments on Earth is by using drop tower facilities. These facilities combine a high quality of microgravity, adequate payload masses and have the advantage of virtually unlimited repeatability under same experimental conditions, at a low cost. In a collaboration between the Institute of Space Systems (IRS) at the University of Stuttgart and Baylor University (BU) in Waco, Texas, a new drop tower is currently under development at the Center for Astrophysics, Space Physics and Engineering Research (CASPER). The design parameters of the drop tower ask for at least 1.5 s in free fall duration while providing a quality of at least 10 −5 g. Previously, this quality has only been achieved in vacuum drop tower facilities where the capsule experiences virtually zero aerodynamic drag during its free fall. Since this design comes at high costs, a different drop tower design concept, which does not require an evacuated drop shaft, was chosen. It features a dual-capsule system in which the experiment capsule is shielded from aerodynamic forces by surrounding it with a drag shield during the drop. As no other dual-capsule drop tower has been able to achieve a quality as good as or better than 10 −5 g previous work optimized the design with an aerodynamic perspective by using computational fluid dynamics (CFD) simulations to determine the ideal shape and size of the outer capsule and to specify the aerodynamically crucial dimensions for the overall system. Experiments later demonstrated that the required quality of microgravity can be met with the proposed design. The main focus of this paper is the mechanical realization of the capsule as well as the development and layout of the surrounding components, such as the release mechanism, the deceleration device and the drop shaft. Because the drop tower facility is a complex system with many interdependencies between all of the components, several engineering challenges had to be addressed. For example, initial disturbances that are caused by the release mechanism are a common issue that arises at drop tower facilities. These vibrations may decrease the quality of microgravity during the initial segment of free fall. Because this would reduce the free fall time experiencing high quality microgravity, a mechanism has been developed to provide a soft release. Challenges and proposed solutions for all components are highlighted in this paper. Highlights: Drop tower layout is shown featuring a non-vacuum dual capsule design at a low cost. Drop capsules were aerodynamically optimized to achieve quality of better than 10 − 5 g. Molten rod release mechanism allows disturbances to be damped within less than 0.1 s. Performance of deceleration device with polystyrene beads was tested in small scale. Deceleration method shows promising performance although smaller beads are required.