The Drop Tube Facility is located in the MSFC Dynamic Test Stand which was used for vibration testing of the Saturn rocket and Shuttle. The Drop Tube Facility consist of sections of 26 centimeter diameter stainless steel pipe vertically assembled into a tube of 105 meters in length. Small samples with diameters up to 8 millimeters can be melted at the top and dropped. For an evacuated Tube, minimal free-fall times of 4.6 seconds produce a quiescent, micro-gravity environment. Vacuum levels of less than a billionth of an atmosphere are achievable. Inert gases can be used in the Tube to provide convective cooling of the sample; however, the gas can produce a drag de-acceleration on the droplet on the order of tens of milli-gravities. Whichever environment the sample is dropped, a totally containerless surrounding is provided.
Containerless melting and solidification of materials provide a capability of reducing the surface contamination of materials from crucible walls and of greatly increasing the amount of undercooling. Undercooling is the process of reducing the temperature of a liquid below its "normal" (i.e., containerized) solidification temperature without it freezing. The low-gravity environment provides internal fluid dynamic benefits of reduced bouyancy and density-driven convection. Both benefits allow researchers to study basic solidification kinetics, liquid-liquid separation, and formation of new materials.
The Drop Tube presently has two furnaces for melting materials at the top level. These are an electromagnetic levitation (EM) and electron beam (EB) device. The EM furnace consist of a Lepel RF generator connected to a coil which lifts and melts semi-spherical metallic specimens. When the appropriate temperature is reached, the power is shut off and the drop falls out of the bottom of the coil. The EB furnace uses samples in the form of wire which when lowered into the beam melts into a liquid drop on the wire's end. The drop falls off when its mass surpasses the force of its surface tension on the wire. Processing temperatures in the Drop Tube are typically greater than 1000C because of the limitations in the cooling rates in the attainable free-fall time. However, high-temperature material processing is not the only use of the Drop Tube Facility.
Other avenues of research using a drop tube are available. The MSFC Drop Tube has various sized ports on all 15 levels for accommodating instrumentation. With video techniques, droplet dynamics such as oscillation damping, deformation, and bifurcation points could be studied for most liquids in any gaseous environment. Research can be performed on samples that splat at the bottom to study rapid quenching effects or to perform high-speed photography of the splat shape. With an appropriately built calorimeter, thermophysical properties of high-temperature materials can be obtained in an undercooled state. Engineering test can also be performed in the Drop Tube such as the recently completed vibration test of the rope, or tether, to be used on the Tethered Satellite Missions for NASA.
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Principal Investigator: Dr. Michael Robinson/MSFC [email protected]
