In physics and materials science, a drop tower or drop tube 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 payload. 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. Typical operation For a typical materials science experiment, a sample of the material under study is loaded into the top of the drop tube, which is filled with inert gas or evacuated to create a low-pressure environment. Following any desired preprocessing (e.g. induction heating to melt a metal alloy), the sample is released to fall to the bottom of the tube. During its flight or upon impact the sample can be characterized with instruments such as cameras and pyrometers. 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. See a video of a microgravity combustion experiment in the NASA Glenn Five Second Drop Facility at . Fluid physics experiments and development and testing of space-based hardware can also be conducted using a drop tower. Sometimes, the ground-based research performed with a drop tower serves as a prelude to more ambitious, in-flight investigation; much longer periods of weightlessness can be achieved with parabolic-flight-path aircraft 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 105-meter drop tube at Marshall Space Flight Center produces 4.6 seconds of weightlessness when it is fully evacuated. In the drop facility Fallturm Bremen at University of Bremen a catapult can be used to throw the experiment upwards to prolong the weightlessness from 4.74 to nearly 9.3 seconds. Negating the physical space needed for the initial acceleration, this technique doubles the effective period of weightlessness. The NASA Glenn Research Center has a 5 second drop tower (The Zero Gravity Facility) and a 2.2 second drop tower