Shake, Rattle And Hum - Measuring Vibrations On Board The Shuttle

November 21, 1997

If you happened to watch the crew of Space Shuttle Columbia exercising this morning, you probably would not have noticed that they were also road testing a new set of shock absorbers on their bicycle ergometer.

They aren't trying to smooth the ride for themselves, but for experiments aboard the Shuttle - and aboard International Space Station in years to come.

Measuring which method works best is the job of Richard DeLombard, discipline scientist for the Space Acceleration Measurement System (SAMS) aboard Columbia. DeLombard, who works at NASA's Lewis Research Center in Cleveland, Ohio, is part of a large team measuring things that go bump in the night.

"One of the things that SAMS is doing on this mission is cooperating with the crew to measure and characterize the exercise so we can make it as quiet as possible," DeLombard said.

This is important to scientists using the U.S. Microgravity Payload (USMP-4) to conduct experiments pushing the limits of what we know in physics and materials sciences.

The ergometer that the crew is using on this mission can be fitted with two different sets of springs between the exerciser and the deck. The SAMS team checks to see which works better, including differences with astronauts who work out more or less vigorously than others. The astronauts can watch, too, through a computer display that lets them watch the results of their won activities. Mission scientist Pete Curreri said the display, first used on USMP-3, quickly led to a quieter mission when the crew could see their own movements displayed as SAMS data. From that, they figured out how to adjust their activities to minimize their impact, literally, on the mission.

The problem is more subtle than developing the best low-impact exerciser for the crew: all mechanical devices generate their own vibrations.

"Zero gravity" does not exist. Since Earth's gravity holds the Shuttle in orbit, "free fall" is a more appropriate description of what is happening (Einstein proved that free fall in a vacuum and zero-gravity are equivalent). Scientists call it microgravity because anything aboard a spacecraft experiences a tiny bit of acceleration.

That would be okay, except that the spacecraft itself vibrates. Motors pump fluids and run fans, valves open and close, antennas point and repoint, and so on.

In everyday life, we ignore the refrigerator or air conditioner switching on and off. But for microgravity scientists, even properly functioning equipment can be like an unbalanced washing machine rattling the house.

That's where SAMS comes in. With sensors deployed around the Shuttle, SAMS measures the intensity of vibrations at different frequencies.

SAMS sensors comprise three miniature pendulums, each about the size of a pin, using Newton's first law of motion: an object at rest (or in motion) will stay that way until something pushes it.

"When an acceleration vibrates the case, this pendulum has inertia and is displaced," he explained. The movement is sensed and an electromagnet adjusts to push the pendulum back into place. How much force is needed is a measure of the acceleration that moved the pendulum in the first place.

"This is somewhat like driving in a car and you go around the corner," DeLombard explained. "You go around a corner and feel yourself pulled to the side and use your muscles to pull yourself upright."As always in science, the fine details count for everything. The SAMS sensors measure more than just the movement, but how fast it happens, continuously. Because a spacecraft is not connected to the Earth, vibrations range through the Shuttle.

Some vibrations are damped by the structure. Others are fed by motors that work constantly, like the Shuttle's data relay antenna. To keep the antenna shaft from sticking, the pointing motor constantly vibrates at 17 cycles per second (17 hertz or Hz) which is felt throughout the Shuttle.

DeLombard showed a plot of vibrations measured with a SAMS unit that has just logged a half-billion miles aboard Mir. When the Shuttle is docked to the more massive Mir, the antenna's 17 Hz vibration is clearly detected aboard Mir.

On the other hand, while watching payload bay TV cameras panning across the Earth, DeLombard worried that the way they wobbled might affect some of the experiments. Working with flight controllers at Johnson Space Center, he was able to determine that they had no noticeable effect on the microgravity experiments.

"It just confirmed that it had no effect on our experiments," he said, "which is just as important as being able to say that something had an effect."

Everything is recorded so that when scientists analyze samples and data after the mission, they can explain their results with certainty.

The records from some 18 missions also help in planning each new one. DeLombard's team has compiled a special catalog of activities and the vibrations they produce (even a lopsided orbit can change microgravity levels). With that in hand, scientists have been able to request that experiments be run while the crew is sleeping, or that certain Shuttle systems not be used while experiments are running.

And DeLombard can use it in tracking down a vibration on concern at 56 Hz.

"It's a bit of a detective mystery, trying to figure out the sources of these disturbances," he said.With the catalog, and working with the science teams, the SAMS team will track it down and add it to the list.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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