Experiments in Zero Gravity

Gravity often modifies the behavior of solids and liquids in subtle ways. One technique for eliminating its influence is to study materials in free fall. Donald R. Pettit of the Los Alamos National Laboratory and astronaut Joseph P. Allen of the Johnson Space Center in Houston have recently done experiments in free fall in the cargo space of an airplane belonging to the National Aeronautics and Space Administration. They were assisted by Robert K. Williams of the Johnson Space Center.

The airplane flew a series of between 40 and 60 vertical parabolic loops. As the craft neared the top of a loop with a speed of about Mach 5 (half the speed of sound in the surrounding air) the occupants began free fall. The floor of the airplane actually fell away from them, but the sensation was that gravity suddently vanished. This state is referred to as zero g, where g symbolizes the normal strength of gravity.

About 20 seconds later the free fall ended near the bottom of the loop. For the next 50 seconds the airplane pushed upward on the occupants, creating the sensation that gravity was twice its normal strength, a state referred to as 2 g. The speed of the airplane at the bottom of the dive was about Mach 88. The brief time of free fall and the subsequent need to protect oneself and the equipment from the 2-g phase limited the studies to transient phenomena.

In addition to their serious research Pettit and Allen had time for a few recreational experiments. One experiment involved the stability of an egg spinning about its long axis. Try standing an egg on a table and spinning it on either end. A hard-boiled egg spins stably for some tens of seconds, but a fresh egg quickly becomes unstable, falls over and spins for a while about its short axis until friction from the table drains all its energy.

Pettit investigated the spin of eggs at zero g. On each egg he marked a line from end to end to enhance the visibility. Then he carefully spun each egg (in the air) about its long axis with as little initial wobble as possible. The hard-boiled egg continued to spin stably throughout the zero-g phase of the loop. The fresh egg completed about two revolutions and then abruptly began to spin about its short axis. Apparently the fluid in the egg was set in motion by the initial rotation, even at zero g. The fluid motion increased the wobble, making the egg spin about its short axis.

Pettit then did a similar experiment with a closed, transparent container partially filled with water. He released the container at zero g while giving it a spin about its long axis. The container soon began to wobble appreciably, but it never stabilized into rotation about its short axis before the end of zero g.

Pettit and Allen also studied fluid flow in rotating systems. Consider a cylindrical container of water that is placed at the center of a turntable. When the turntable begins to rotate, the wall of the container drags the water in a circle. Eventually the water circulates around the center of the container at the speed of the turntable. During the transition the water is said to be in a spin-up state. Suppose the turntable abruptly stops. The circulation slows and eventually stops. In this phase the water is said to be in a spin-down state.

During spin-up and spin-down an additional flow arises in the water. This secondary flow results from unequal pressures created in the water by the primary flow around the center. In spin-up the secondary flow is downward along the center line, outward along the bottom, upward along the wall and then inward along the top surface. In spin-down the secondary flow is reversed. Evidence for secondary flow is seen in the motion of tea leaves when the tea is in spin-down. The leaves, which initially are strewn over the bottom of the cup, are forced to the center and then abandoned in a pile by the upward flow of water.

What about secondary flow in zero g? Pettit partially filled the transparent, closed container with water, adding half a teaspoon each of waterlogged sawdust and aluminum glitter. (The glitter is available in hobby and art-supply shops.) The sawdust and glitter served as tracers for the secondary flow.

In normal conditions of gravity the secondary flow of spin-down caused the glitter to collect like tea leaves in a small pile at the center of the container. The sawdust circulated in a ring just above the bottom center until near the end of spin-down; then it collapsed onto the pile of glitter. In spin-up the glitter moved to the wall first, followed by the lighter sawdust. At zero g Pettit released the container while spinning it. The water was in spin-up. The sawdust and glitter moved to the wall as before, but this time they did not collect along the bottom edge. The glitter was pressed against the wall and the sawdust moved up along the wall.

To generate spin-down Pettit made the container gyrate and then held it stationary. The sawdust and glitter moved along with the expected secondary flow, but they failed to pile up on the bottom.

Apparently the secondary flow in spin-up and spin-down is the same at zero g as it is at normal gravity. In the effective absence of gravity, however, the sawdust and glitter are no longer confined to the bottom of the container. The secondary flow can carry them upward at the center of the container in spin-down and at the wall in spin-up.

Pettit and Allen did another experiment with the container. When water circulates about a container's long axis at normal gravity, the top surface is concave. Pettit and Allen wondered how the shape would change as the effective gravity varied. They found that at 2 g the concave surface was shallower. At zero g the concavity deepened enough to force all the water into a layer along the wall.

In a final experiment Pettit and Allen tested a yo-yo at zero g and at 2 g. They wondered if it could be made to spin at the end of its string, a trick called sleeping. At normal gravity you must let the yo-yo fall gently to the end of its string to minimize the usual bounce. Gravity holds the yo-yo there while it spins loosely in the loop around the spindle.

At 2 g Pettit easily made the yo-yo sleep. At zero g it refused to sleep even with a gentle toss. It always bounced. The only way Pettit could get it to sleep was to throw it outward and then pull on the string. That made the yo-yo circle around his hand. The resulting effective centrifugal force kept the yo-yo at the end of the string.

Pettit is interested in more experiments that might be done at zero g. If you have any ideas, write to him at the Los Alamos National Laboratory, MS P952, Los Alamos, N.M. 87545.