physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

and materials science, a drop tower or drop tube is a structure used to produce a controlled period of

weightlessness Weightlessness is the complete or near-complete absence of the sensation of weight. It is also termed zero gravity, zero G-force, or zero-G. Weight is a measurement of the force on an object at rest in a relatively strong gravitational fie ...

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 NASA John H. Glenn Research Center at Lewis Field is a NASA center within the cities of Brook Park and Cleveland between Cleveland Hopkins International Airport and the Rocky River Reservation of Cleveland Metroparks, with a subsidiary facilit ...

'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 An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...

or evacuated to create a low-pressure environment. Following any desired preprocessing (e.g.

induction heating Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an induction coil that creates an electromagnetic field within th ...

to melt a

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...

), 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

camera A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...

s and

pyrometer A pyrometer is a type of remote-sensing thermometer used to measure the temperature of distant objects. Various forms of pyrometers have historically existed. In the modern usage, it is a device that from a distance determines the temperature of ...

s. Drop towers are also commonly used in

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...

research. For this work,

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

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 a

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 An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engine ...

or with space-based laboratories aboard the

Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program ...

or the

International Space Station The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ( ...

. The duration of

free-fall In Newtonian physics, free fall is any motion of a body where gravity is the only force acting upon it. In the context of general relativity, where gravitation is reduced to a space-time curvature, a body in free fall has no force acting on i ...

produced in a drop tube depends on the length of the tube and its degree of internal evacuation. The 105-mete
drop tube
at Marshall Space Flight Center produces 4.6 seconds of weightlessness when it is fully evacuated. In the drop facility

Fallturm Bremen Fallturm Bremen is a drop tower at the Center of Applied Space Technology and Microgravity at the University of Bremen in Bremen. It was built between 1988 and 1990, and includes a 122-metre-high drop tube (actual drop distance is 110 m), in whic ...

University of Bremen The University of Bremen (German: ''Universität Bremen'') is a public university in Bremen, Germany, with approximately 23,500 people from 115 countries. It is one of 11 institutions which were successful in the category "Institutional Strategi ...

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 (The 2.2 Second Drop Tower). Much of the operating cost of a drop tower is due to the need for evacuation of the drop tube, to eliminate the effect of aerodynamic drag. Alternatively the experiment is placed inside an outer box (the drag shield) for which, due to its weight, during its fall the reduction of acceleration due to air drag is less.

Historical uses

Though the story may be apocryphal, Galileo is popularly thought to have used the

Leaning Tower of Pisa The Leaning Tower of Pisa ( it, torre pendente di Pisa), or simply, the Tower of Pisa (''torre di Pisa'' ), is the ''campanile'', or freestanding bell tower, of Pisa Cathedral. It is known for its nearly four-degree lean, the result of an unst ...

as a drop tower to demonstrate that falling bodies accelerate at the same constant rate regardless of their mass. Drop towers called

shot tower A shot tower is a tower designed for the production of small-diameter shot balls by free fall of molten lead, which is then caught in a water basin. The shot is primarily used for projectiles in shotguns, and for ballast, radiation shielding, ...

s were once useful for making

lead shot Shot is a collective term for small spheres or pellets, often made of lead. These were the original projectiles for shotguns and are still fired primarily from shotguns and less commonly from riot guns and grenade launchers, although shot shell ...

. A short period of weightlessness allows

molten Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which inc ...

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

to solidify into a quasi-perfect sphere by the time it reaches the floor of the tower.

List of drop towers

The vacuum-dynamic stand in Academician V.P.Makeyev State Rocket Centre NASA Glenn Research Center 2.2 Second Drop TowerNASA Glenn Research Center Zero-G Research Facility
*Micro-Gravity Laboratory of Japan (MGLAB) (Closed June 2010) * HASTIC 50m Drop Tower, Sapporo *

Applied Dynamics Laboratories drop tower for spinning spacecraft
*Experimental drop tube of the metallurgy department of Grenoble

* ttps://www.pdx.edu/dryden-drop-tower/ Dryden Drop Tower, Portland State University, Maseeh College of Engineering and Computer Science*National Microgravity Laboratory (China) * The pagoda at the

Royal Botanic Gardens, Kew Royal Botanic Gardens, Kew is a non-departmental public body in the United Kingdom sponsored by the Department for Environment, Food and Rural Affairs. An internationally important botanical research and education institution, it employs 1,10 ...

was used during the Second World War as a drop tower for testing bomb designs. *

Queensland University of Technology Queensland University of Technology (QUT) is a public research university located in the urban coastal city of Brisbane, Queensland, Australia. QUT is located on two campuses in the Brisbane area viz. Gardens Point and Kelvin Grove. The univ ...

drop tower, Brisbane (Closed 2014)

References

{{Reflist Laboratory equipment Towers Weightlessness

Typical operation

Historical uses

List of drop towers

See also

References