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In physics and materials science, a drop tube or drop tower is a structure used to produce a controlled period of weightlessness for an object under study. Air bags, polystyrene pellets, and magnetic or mechanical brakes are sometimes used to arrest the fall of the experimental Cargo. In other cases, high-speed impact with a substrate at the bottom of the tower is an intentional part of the experimental protocol.
Not all such facilities are towers: NASA Glenn's Zero Gravity Research Facility is based on a vertical shaft, extending to below ground level.
Drop towers are also commonly used in combustion research. For this work, oxygen must be present and the payload may be enclosed in a drag shield to isolate it from high-speed "wind" as the apparatus accelerates toward the bottom of the tower.
Fluid physics experiments and development and testing of space-based hardware can also be conducted using drop towers. Sometimes, the ground-based research performed with a drop tower serves as a prelude to more ambitious, in-flight investigations; much longer periods of weightlessness can be achieved with Parabola-flight-path Vomit Comet or with space-based laboratories aboard the Space Shuttle or the International Space Station.
The duration of free-fall produced in a drop tube depends on the length of the tube and its degree of internal evacuation. The
target="_blank" rel="nofollow"> drop tube at Marshall Space Flight Center produces 4.6seconds of weightlessness when fully evacuated. In the drop facility Fallturm Bremen at University of Bremen, a
Accessed:KÖNEMANN, T., VON KAMPEN, P., and RATH, H.J. (2010). The drop tower bremen – experiment operation, in COSPSAR, Proceedings of the 38th COSPAR Scientific Assembly, Bremen, Germany, 15–18 July 2010. volume 38 of COSPAR, Plenary Meeting. p. 3588. Available from: http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010cosp...38.3588K&link_type=ARTICLE&db_key=AST&high= Accessed: Negating the physical space needed for the initial acceleration, this technique doubles the effective period of weightlessness. The NASA Glenn Research Center has a 5second drop tower (The Zero Gravity Facility) and a 2.2second drop tower (The 2.2Second Drop Tower).
Much of the operating cost of a drop tower is due to the need for vacuum of the drop tube to eliminate the effects of aerodynamic drag. Alternatively, the sample may be placed inside an outer box (called a drag shield) designed to prevent drag.
Drop towers called were once useful for making lead shot. A short period of weightlessness allows molten lead to solidify into a quasi-perfect sphere by the time it reaches the floor of the tower.
target="_blank" rel="nofollow"> NASA Marshall Space Flight Center Drop Tube Facility is currently mothballed
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