Glenn
Research Center
Microgravity Science Division
The demonstrator provides approximately 0.1g for 0.6sec. It consists of four essential parts: (1) an experiment platform, (2) a drop structure to hoist the platform to the top and then release it into a free-fall drop, (3) a deceleration system at the bottom to catch the experiment, and (4) a video cassette recorder and a monitor to record the experiment during the drop and replay it afterwards.
(1) The experiment platform consists of a video camera and one of several interchangeable physics or chemistry experiments (described later). An eye bolt is fastened to the platform such that it can be hoisted to the top of the drop structure via a rope and pulley.
(2) The drop structure provides a rigid means by which one can easily raise and release the experiment platform. The structure is segmented for easy assembly and provides a fall distance of approximately 2m. Based on this height, one can calculate the time it will take for the package to hit the bottom (neglecting air resistance), from the following equation:
where h is the height that the platform falls, g is the Earth's local gravitational acceleration (=9.8m/s2) and t is the time of free-fall. Hence, for h=2m, the platform will fall for approximately 0.6 seconds.
(3) The deceleration system consists of a shipping crate lined with foam to cushion the impact of the video camera and experiment payload when they hit the bottom. The drop structure is mounted on internal side wall of the crate. The crate is also designed to hold the drop structure and experiment payloads during shipping.
(4) The video system consists of a video recorder and a monitor which are connected to the output of a video camera mounted on the experiment platform. It is used to record the experiment during free-fall. Since the test time is so short, it is very helpful to use a video recorder with a frame-by-frame jog shuttle (or slow motion) to slow down the video to clearly show what happens during the drop test.
The sample experiments shown here illustrate the behavior of fluids, flames, and mechanical systems in microgravity. After each description, a picture of the experiment, followed by its appearance in normal gravity (1g) and then micro-gravity (ug) are given.
The weight (W) of a body of mass (m) in a gravitational field of strength (g) is given by W=mg. In microgravity, g is virtually eliminated and therefore the weight of the body is also eliminated. To demonstrate this principle, a mass on a scale is dropped. The two counterbalancing forces in this experiment are (1) the gravitational force acting on the mass and (2) the force of the spring in the scale. During the drop, g tends towards zero and the restoring spring force pushes the indicator from the original weight of the body toward zero. Shown in figure 3.
The fluid interface experiment highlights the role of surface tension in the absence of gravity. In 1g, the effect of surface tension is evident only near the container walls and most of the surface looks flat. In reduced gravity, surface tension leads the liquid to assume a very different shape. Specifically, the liquid creeps up the walls of the container by capillary forces; this is most evident in the corners. Given enough ug time, the liquid would wet the walls of the vessel, leaving a bubble of air in the center. Shown in figure 4.
The candle flame experiment demonstrates the effect of buoyant convection and its absence on combustion phenomena. In normal gravity (1g), the combustion gases are much hotter, and thus lighter than the surrounding air. Buoyancy causes the hotter, less dense combustion gases to rise, giving the candle flame its vertically-elongated, conical shape. However, during the drop experiment, the hot gas expands in all directions. As a result, the flame becomes shorter and wider. In longer periods of reduced gravity, the flame becomes spherical and entirely blue. This was observed in a candle flame experiment performed on the Space Shuttle (USML-1/STS-50, June-July, 1992). Shown in figure 5.
Two magnets are oriented with like polarities opposing one another (i.e., N-N or S-S). The lower magnet is fixed to the experiment platform while the upper magnet is freely supported on a lever arm. In 1g, the upper magnet is levitated by the magnetic repulsive force which is balanced by the gravitational force pulling the upper magnet downward. During the drop, the magnetic repulsion becomes dominant and the upper magnet moves rapidly away from the lower magnets. Shown in figure 6.
An object can be subjected to microgravity by either placing it (1) at a location where gravity is naturally small, (2) between large masses where the gravitational attraction from each body is balanced, or (3) in free-fall. Gravity does not "disappear" in outer space, but reaches throughout space to hold planets in their orbits and the stars within galaxies. However, a location far removed from large masses, for example between stars (or even galaxies), would also have a weak gravitational field.
Regardless of the distance from large masses (such as the Earth), an object may be put into microgravity by allowing it to freely fall without the influence of other forces. In this situation, the object will be weightless because weight is a measure of the object's resistance to gravity. While the object is freely falling, there is no resistance to gravity, and the object becomes weightless.
If the object is at rest (or a constant velocity), there must be a force equal but opposite to that of gravity, based on Newton's laws. For a person climbing a rope, that opposing force results from the rope pulling the person upward, just as gravity pulls downward. If the rope breaks, the resistance disappears and the person will fall in a low-gravity condition; in other words, the person would be in microgravity. Gravity is such that objects fall at the same acceleration regardless of their mass. So the person and the broken end of rope would fall together, and relative to the falling person, the broken end of the rope would be floating.
An accident where a painter fell of a roof inspired Einstein to develop the theory of general relativity, which explains (among other things) that an object in free-fall will experience the same microgravity conditions as an object in a weak gravitational field. He considered this to be his greatest scientific achievement [Wheeler, 1990].
(2) Drop Structure
A drop structure is useful, but not required for reduced-gravity demonstrations.
The structure merely provides a convenient way of hoisting and releasing the experiment
platform. Instead, someone could simply hold the experiment platform in their
hands and let go to initiate a drop. If an increased fall distance is required,
the experiment platform could be dropped by someone standing on a chair or stepladder.
Alternately, in some rooms it may be possible to hang a pulley from the ceiling
(e.g., from a basketball hoop).
(3) Deceleration System
The experiment platform can be caught in a large cardboard box filled with some
cushioning. The Reduced-Gravity Demonstrator uses layers of foam, but down pillows
(fluffed up of course) or styrofoam "peanuts" would probably be even
better. Instead of slowing down the platform, the foam tends to simply make it
bounce. Alternately, it may be possible to simply pad the experiment platform
itself and drop it directly onto the floor.
(4) Video System
For the demonstrator to work, you must be willing to drop a video camera on the
experiment platform. Video cameras are relatively rugged and are constantly dropped
in the NASA microgravity facilities. A board camera (literally a circuit board
with a lens) is a good option, since it is both small and cheap (i.e., as little
as $200). Note that dropping a camcorder is discouraged, since the recording mechanism
is relatively fragile.
Since the experiment only takes a fraction of second, it is very useful to be able to play back the tape in slow motion. It is most convenient if the video recorder has a jog shuttle allowing you to control the play back rate.
NASA scientists are using microgravity experiments to study fluid physics, material science, combustion science, biotechnology, and life science. The microgravity experiments are conducted in several different facilities, all of which depend on the experiment achieving a state of free-fall. On the Earth, drop towers provide a few seconds of free-fall, while aircraft and sounding rockets, following parabolic paths, provide additional microgravity time. Experiments requiring extended time in microgravity are conducted on spacecraft such as the Space Shuttle. The astronauts appear to "float" in the Shuttle because the Shuttle and the astronauts are falling towards the Earth as they orbit it.
