Nov 4, 1996

Chuck Reveals Invisible Vibrations




November 5, 1996

Interferograms - images built by having light waves interfere with each other - compare how warm fluid flows on the ground (bottom) and doesn't in space (top). Click to view a larger image and explanation.

How do you prove that you can hold a space experiment better than rock-steady? You do it with mirrors.

A major research area for NASA is using the weightlessness of space to grow advanced materials -- such as crystals of proteins -- that are difficult or impossible to grow on Earth. Yet, even in orbit a spacecraft has small vibrations that disturb crystals (for research on new electronics devices or for medical research) that grow as slow as an inch a week (3.8 mm/day). To smooth the ride, Marshall developed the Suppression of TrAnsient Events by LEvitation (STABLE) system which uses electromagnets to suspend equipment inside a rack and act as shock absorbers.

STABLE was developed jointly by NASA's Marshall Space Flight Center and by McDonnell Douglas Corp. in Huntington Beach, Calif.

But how to prove that STABLE works as advertised? You can see part of the answer by dropping ice into a glass of warm water. Different fluids usually have different indices of refraction, meaning they bend light differently. That index of refraction will change when a fluid changes temperature, so light bends where warm and cold layers meet. It's like injecting a tracer without disturbing the experiment.

To turn that view of hot and cold liquid into an accurate picture they could measure, Marshall scientists built a device, nicknamed Chuck, using mirrors to combine images of two identical fluid cells into one image. The cells, 4 cm square and 1 cm thick (1.6 x 0.4 in.), were filled with a halocarbon fluid (related to Teflon) that would react to heat more slowly than water.


Chuck flight hardware
Chuck's hardware was small enough to fit in a briefcase - its footprint is less than 24 by 21 cm (9.3 x 8.3 in.) and height is about 6 cm (2.4 in.). As an extra challenge, the experiment was developed in less than six months so it would be ready in time for the STABLE mission.

One fluid cell had a small heater and four temperature monitors, and each cell was placed in front of a mirror. A beam-splitter (a half-silvered mirror diagonally across a cube of glass) split a laser beam and sent half to each cell. The mirrors behind the cells reflected the light back to the beam-splitter which recombined the two beams into one beam focused on a TV camera.

Light behaves as waves that combine to build a higher peak if they are in step, or to level each other out if they are out of step. Any change in the two light beams will cause the waves to interfere with each other and form light and dark bands called an interference pattern. Thus, adding an image of the test cell (with heater) to an image of the control cell (no heater) produces interference patterns that depict any change in the test cell.

This kind of interferometer was invented by astronomer A. A. Michelson in 1881 to prove that light is carried through space by a fluid called ether (by 1887, Michelson and E. W. Morley wound up proving that ether does not exist). Since then, Michelson interferometers have been used in countless experiments requiring measurements so sensitive that light itself must be the yardstick.

And that's how Chuck worked inside STABLE aboard the STS-73 Space Shuttle mission in October 1995. On Earth, when the heater was turned on the warm fluid would float to the top, as shown in the interferogram at the top of this page. In space, warm fluid would shift (float is not exactly correct) in response to any residual accelerations that got through while STABLE was on or off. Chuck produced an image that can be translated into measurements of the vibration.

Even the pictures themselves clearly show a difference between tests on Earth -- a stream of warm liquid rises to the top -- and in space -- the warm fluid just forms a bubble within the colder fluid.
Preflight simulation of fluid flow
Computer simulations depict the differences in fluid flow on the ground and in space. Click for an explanation and a larger image.

Results from the mission are under study, but scientists are pleased with Chuck's performance. It was the first experiment focused on how heat moves through a fluid under extremely low vibrations, and it provided a benchmark for assessing math models used in materials experiments.

More details on Chuck are available in a recent paper:

Ramachandran, N., Baugher, C. R., Rogers, J., Peters, P., Roark, W., and Pearcy, G. "Thermal diffusion experiment Chuck - Payload of STABLE." Proceedings of SPIE Conference on Space Processing of Materials, Denver, Aug. 4-9, 1996. , Ed. N.Ramachandran. SPIE 2809: 367-378. (Note: Chuck is paper 2809-49).