Drop tube

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.

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.

The 105-meter drop tube at Marshall Space Flight Center produces 4.6 seconds of weightlessness when it is fully evacuated.

[1][2] Negating the physical space needed for the initial acceleration, this technique doubles the effective period of weightlessness.

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.

A view down the tube of NASA Glenn 's 5 second Zero Gravity Facility.
A view down the shaft of NASA Glenn's 2.2 second drop tower. A large (deflated) air bag is at the bottom.