Tethered Satellite Investigation Report is Released
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From: Peter Langston <psl>
Date: Tue, 4 Jun 96 18:03:36 -0700
Subject: Tethered Satellite Investigation Report is Released
Forwarded-by: firstname.lastname@example.org (David D. Lang)
Forwarded-by: "Pemberton,Nate" <XD000JYJ@macpo.macnet.is.lmsc.lockheed.com>
Forwarded-by: McCandless,Wayne on Tue, Jun 4, 1996 12:12
From: NASA HQ Public Affairs Office on Tue, Jun 4, 1996 11:40 AM
TETHERED SATELLITE INVESTIGATION REPORT IS RELEASED
NASA and the Italian Space Agency (ASI) today released the
report of the investigative board appointed to determine
factors which resulted in the Feb. 25 tether break and loss of
the Tethered Satellite during the STS-75 Space Shuttle mission.
Findings of the board, included in a 358-page document,
identified primary causes which accounted for the tether break
during deployment of the Tethered Satellite.
"The tether failed as a result of arcing and burning of
the tether, leading to a tensile failure after a significant
portion of the tether had burned away," the report concludes.
The arcing occurred because either external foreign object
penetration (but not orbital debris or micrometeoroids) or a
defect in the tether caused a breach in the layer of
insulation surrounding the tether conductor. The insulation
breach provided a path for the current to jump, or arc, from
the copper wire in the tether to a nearby electrical ground.
The board found that the arcing burned away most of the
tether material at that location, leading to separation of the
tether from tensile or pulling force. The break occurred when
approximately 12.2 miles (19.7 km) of tether was unreeled, in
a period when the tether was experiencing normal stresses of
approximately 15 pounds (65 newtons).
In addition to the two primary causes for the tether
break, the board cited, as one contributing factor, that "the
degree of vulnerability of the tether insulation to damage was
not fully appreciated." The board noted that the actual
environment that the tether was exposed to in flight made it
more vulnerable to damage than was expected. And, it noted
that the high voltages under which the system was operating
could, over a period of time, have reduced the ability of the
tether insulation to withstand electrical breakdown due to
contamination found in the tether.
"The tether itself was a remarkable engineering
achievement," said Ken Szalai, who chaired the investigative
board, "and produced some startling scientific discoveries."
Scientific papers recently presented at an American
Geophysical Union conference reported that currents generated
by the tether were three times higher than theoretical models
had predicted prior to the flight.
"Constructing a tether that was strong, lightweight and
electrically conducting took the project into technical and
engineering areas where they had never been before," said
Szalai. "Now, with 20/20 hindsight, they know where the system
is vulnerable and can improve the design."
The Tethered Satellite System is a joint NASA-ASI system
that was flown aboard Space Shuttle Columbia in an experiment
to better understand the electrically charged environment of
Earth's ionosphere, and how tether systems behave in it. ASI
had the responsibility of providing the satellite, while NASA
had the responsibility of the Deployer, which includes the
tether, and the overall responsibility for payload integration
and operations. The provision of science investigations was
shared by ASI and NASA.
The system was generating 3,500 volts DC and up to 0.5
amps of current during satellite deployment. That high level
of electrical energy resulted from the length of conducting
tether extending from the Shuttle, coupled with the 17,500-
mile-per-hour speed at which the Shuttle and tether were
cutting through Earth's magnetic field lines.
The board found sufficient evidence to identify two
possible causes of the breach in the insulation -- foreign
object damage, or a defect in the tether itself. Debris and
contamination found in the deployer mechanisms and in the
tether itself could have been pushed into the insulation layer
while the tether was still wound on its reel. The
investigation found evidence of damage to copper wire in the
tether, and also established that normal forces on the tether
while on the reel could push a single copper strand or foreign
debris through the insulation.
The arcing, which began in an intricate part of the
Tethered Satellite System known as the lower tether control
mechanism, sputtered intermittently for nine seconds as the
moving tether passed through deployer mechanisms and then into
the boom area of the tether system. At the time, tether was
continuing to play out at one meter per second, or slightly
more than three feet per second.
"This arcing produced significant burning of most of the
tether material in the area of the arc," the board found. The
tether was designed to carry up to 15,000 volts DC and handle
tensile forces of up to 400 pounds (1780 newtons). It used
super-strong strands of Kevlar as a strength-providing member,
wound around the copper and insulation. However, postflight
inspection of the tether end which remained aboard Columbia
showed it to be charred. The board concluded that after
arcing had burned through most of the Kevlar, the few
remaining strands were not enough to withstand forces being
exerted by satellite deployment.
Extensive, rigorous tests performed in support of the
investigation established that undamaged tether would not arc,
even when subjected to electrical potentials much higher than
the 3500 volts experienced during the mission.
The board was able to exonerate a number of factors which
clearly did not cause the break. These factors include the
satellite, the science equipment hardware and operations,
which were being conducted prior to the break, in addition to
micrometeoroids or orbital debris impact, and electrical storm
The investigation panel made several detailed
recommendations which it said should be followed for any
future space missions involving electrodynamic tether systems
such as that flown aboard Columbia. These include more
precautions to ensure any such tether systems in the future do
not suffer from possible debris or contamination damage and
specific attention during design to minimize the possibility
of high-voltage arcing.
The board offered, in the form of observations, its
assessment that the STS-75 tether problem "is not indicative
of any fundamental problem in using electrodynamic tethers."
It also noted that in spite of the break, a "significant
amount" of scientific data was obtained from the Tethered
Satellite operations during STS-75.
The nine-member independent review panel was formed in
consultation with ASI and appointed by NASA's Associate
Administrator for the Office of Space Flight, Wilbur Trafton,
shortly after the tether break. The board was chaired by Ken
Szalai, director of the Dryden Flight Research Center,
Edwards, CA, and included representation from NASA and the ASI.
© 1996 Peter Langston