When the space shuttle Columbia blasts off tomorrow, March 3, it will carry an MIT experiment that will help scientists understand more about how large structures vibrate in space and how astronauts physically affect their environment as they go about their daily activities.
The results will aid engineers in designing future space structures like the scheduled international space station, and help quantify the forces exerted by astronauts on the shuttle itself as they perform daily tasks. Some researchers have concerns that such forces, while trivial in a laboratory on Earth, could disrupt sensitive experiments in space. Until now, however, these forces have never been measured.
This will be the second flight for the experiment-called MODE, for Middeck O-gravity Dynamics Experiment. The first, in September 1991, studied the mechanical and fluid behavior of components similar to those that could be used in future space systems. (The MODE team received a group achievement award from NASA for their work on the first flight.)
This time around the scientists, led by Professor Edward F. Crawley of aeronautics and astronautics, will continue studies of how large structures vibrate in space. They will also use the MODE equipment to study the human forces described above, and to support a NASA experiment that will try to identify the vibration characteristics of the space station when it is in orbit.
"So we're meeting an original goal of MODE: the hardware we developed can be used to support other experiments," said Marthinus C. van Schoor, a visiting scientist in aeronautics and astronautics and co-principal investigator for the study on structural vibrations.
The flexibility of the MODE hardware slashes costs for new studies. For example, the study on human forces will cost about $50,000 as opposed to the estimated $500,000 it would have taken to "develop it from scratch," Dr. van Schoor said. Dava J. Newman, an assistant professor in the department and co-principal investigator for the study on human forces, noted that as far as she knows the human force study will be one of the most inexpensive shuttle experiments ever flown.
For the continuing study on how large structures vibrate in space, the astronauts will unfold in the shuttle middeck cabin a truss structure similar to the structure that will support the space station's solar array. When fully deployed this structure, which resembles a construction crane boom, is 72 inches long with an eight-inch square cross-section. Sensors distributed along the structure will collect data on how it responds to vibrations produced by an actuator. That data, in turn, will be sent to MODE's experiment support module, which controls the experiment and stores the resulting data.
The scientists are excited about the MODE reflight because, for one thing, they will have much more time to collect data. "On the first flight we only had an eight-hour test period," Dr. van Schoor said. "For this flight the structural part of the experiment is scheduled for 40 hours, so we will have enough time to investigate fully the structure's vibratory characteristics."
Most of that time will be spent gathering more data. Such information is critical to developing computer models that can predict how a given structure will behave in space. Tests on the ground and simulations of what will happen in zero gravity are useful to that end, but the resulting model must be verified-or modified-by data from space.
A case in point: during the last flight the scientists found that the model they'd based their structural experiment on "didn't accurately simulate some zero-gravity effects," Dr. van Schoor said. As a result, they were unable to collect certain data. Since then, however, "we have used the first flight's data to refine our models, so we can with confidence use this reflight to collect the data that we missed."
The scientists will also expand the study to test more configurations of the truss structure. For example, they'll test a new joint added to the structure-a scaled space-station alpha joint. "The alpha joint gives you the ability to change the angle of a [space station] solar panel so it's aligned with incoming rays," Professor Newman said.
Dr. van Schoor also noted that the study on structural vibrations will be the focus of a real-time videoconference for children. "The astronauts will give a demonstration of the experiment in space for the kids to watch. They will show them how the structure deploys, and explain why the vibrations of structures are important to space flight."
The MIT measurements of the typical forces exerted by astronauts will be the first of their kind. While similar force measurements were made in the 1970s for astronauts aboard Spacelab, "they represented the extreme," said Professor Newman. "For example, the astronauts were pushing off the floor and soaring to the ceiling as fast as they could. We want to measure the forces associated with everyday, common activities."
To that end the scientists have developed three instrumented sensors: a handhold, a foot-loop, and a push-off pad that will be mounted in the galley area of the shuttle. Each sensor will also be connected to MODE's experiment support module. Then, for a 67-hour period, the module will collect data on the forces applied as the astronauts use the sensors to get around or anchor themselves.
The sensors have the same dimensions as the "real" handholds and footloops they're patterned after, so "ideally the crew members will become used to them, and in a sense `forget' about them," Professor Newman said. As a result, "the data gathered should accurately reflect the true level of disturbances arising from normal crew motion."
The MIT principal investigators involved in MODE emphasized the involvement of many MIT students. Since the project began at least 15 undergraduates have worked on it and eight graduate students (six masters students and two doctoral students).
The MODE experiment is primarily funded through a NASA program designed to develop small, technologically innovative experiments via collaborations between industry, academia and government. The In-Space Technology Experiment Program (IN-STEP) awarded the MODE experiment to MIT's Space Engineering Research Center, of which Professor Crawley is the director-in 1987.
From there, MIT selected Payload Systems, Inc., of Cambridge as the prime subcontractor responsible for hardware fabrication, certification and mission support. The company, founded by Byron K. Lichtenberg, an MIT alumnus who was a payload specialist aboard a 1983 flight of NASA's Spacelab, is headed by Javier de Luis, also an MIT alumnus. NASA's Sherwin Beck of the Langley Research Center in Hampton, VA, manages the contract. The crew force measurements are funded by NASA's Office of Space Systems Development.
A version of this article appeared in the March 2, 1994 issue of MIT Tech Talk (Volume 38, Number 24).