Imagine sitting on the floor of a DC-8 airplane, 26,000 feet above the Gulf of Mexico.
The roaring of the plane's engines begins to diminish, and suddenly, everything around
you, yourself included, starts floating toward the ceiling. Short of actually going into space, this is the closest that civilians will ever get to experiencing the
weightlessness that astronauts encounter on Shuttle missions. Because of our experience
with the 1997 NASA Academy and with some help from NASA's Reduced-Gravity Student
Flight Opportunity Program, we had the chance to experience firsthand the weightless
environment associated with space travel.
Our project started in the summer of 1997 when Jennifer Probst was a NASA Academy
Research Associate at the NASA/Goddard Space Flight Center (GSFC), with Brian Roberts
acting as her Principal Investigator. The project is part of Brian's Masters of
Science thesis at the Space Systems Laboratory at the University of Maryland. The goal of
his thesis research is to develop a new wrench for astronauts to use while conducting
spacewalks or extravehicular
activity (EVA).
Depending on the pitch or spacing of the teeth on conventional ratcheting wrenches,
sometimes more than one-eighth of a backward turn can be required to engage the next
ratchet tooth, which causes the wrench to work inefficiently in confined spaces.
Furthermore, the bulkiness of an astronaut's spacesuit gloves causes additional range-of-motion
problems when performing EVA tasks. The torque associated with turning a traditional
ratchet wrench back against the "clicks" of the gears, or backdrive torque, before tightening a bolt again sometimes causes fasteners to actually be loosened while
trying to tighten them in space.
NASA/GSFC has patented a special three-dimensional (3-D) rolling sprag technology
that, when used in place of a traditional ratchet mechanism, permits construction
of compact locking mechanisms that can withstand large loads; require very small,
almost infinitesimal throw angles; and have very low backdrive torques. Locking occurs as a
result of the wedging action between the tapered periphery of the 3-D roller locking
sprag and a grooved race. Just like its two-dimensional counterpart, which has been
in use for over 100 years, and unlike traditional ratchet mechanisms, the 3-D sprag requires
no lubricants. This could allow a wrench using sprags to remain on-orbit indefinitely
aboard the International Space Station, rather than being ferried to and from orbit every year for testing, as is done with current ratchet wrenches. The resulting
economic and logistic savings from such a wrench could be substantial.
During the summer of 1997, Jennifer designed and built a test set-up that will be
used to test a commercial version of the sprag wrench to failure. Near the end of
the summer, to verify that the test stand could be used to actually "break" commercial
wrenches, she tested three brands of commercial ratchet wrenches. Basically this meant
pulling on the handle of the wrench until something broke and measuring the amount
of torque required to break it. The test set-up worked beautifully and will be used
this fall to failure test commercial-style sprag wrenches.
However, the research did not end there. We decided to test a space version of the
sprag wrench aboard the KC-135. We built a Shuttle-like wrench used by astronauts
today and replaced the ratchet mechanism with sprags. The goal was to evaluate the
EVA sprag wrench while performing typical EVA tasks and then compare it with the current EVA
ratchet wrench used by today's spacewalking astronauts.
We both flew the first day as test subjects. Brian had the opportunity to fly again
a second day as a journalist and observe two other test subjects perform tasks in
the weightless environment. All four test subjects thought the EVA sprag wrench
outperformed the ratchet wrench. The biggest difference was the amount of perceived fatigue
associated with using both wrenches---the sprag wrench seemed to require very little
effort to operate.
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| Jenn takes a moment to smile for the camera. | Brian floats in micro-g. |
Because of the success of those two days of tests, NASA has accepted the sprag mechanism
for a flight aboard the Space Shuttle this October on mission STS-95. In addition,
the Hubble Space Telescope (HST) project at GSFC is interested in flying an EVA sprag wrench on the third HST servicing mission, currently scheduled for the year 2000.
The sprag wrench's ability to accomplish the HST servicing tasks will be tested at
JSC's Neutral Buoyancy Laboratory in Houston this fall. To think that this all started
out as a NASA Academy project!