(Washington, DC) -- The Naval Research Laboratory's (NRL's) Naval Center
for Space Technology has designed, built and deployed a small satellite to
research the gravity-gradient dynamics and survivability of a tethered system in
space, known as the Tether
Physics and Survivability (TiPS) experiment.
The TiPS satellite was deployed today into a circular orbit, at an altitude
of 1022 km (552 nmi) and inclination of 63.4 degrees. A deployment sequence
jettisoned the TiPS satellite from a host vehicle and then separated its two end
masses from each other. When fully deployed, the 53.5 kg (118lb.) tethered
system consisting of two small end-masses is connected by a 4 km (2.5 miles)
nonconductive braided tether.
NRL, NASA, and an international network of Satellite Laser Ranging (SLR)
stations are tracking the position of each end-mass to study the dynamics and
survivability of tethered systems. Telemetry generated during the separation of
the two end-masses was received by the United States Air Force (USAF).
The National Reconnaissance Office (NRO) is a sponsor of the TiPs program.
Research and funding for this low-cost, passive experiment began in 1995. The
NRO helped fund this project to increase understanding of gravity gradient
tether dynamics for space operations. In addition, NRO engineers hope the
research will assist the scientific community in evaluating survivability of
future tethered vehicles in low-earth orbit.
Mr. Robert Towsley, TiPS Systems Engineer at NRL, explains "Tethered
systems are a new and relatively untested space technology. The TiPS spacecraft
is designed to characterize tether dynamics and survivability in the space
environment. Specific dynamics issues of tethered systems include libration
amplitudes and stability, while tether issues include internal damping and
flexibility." Dr. Shannon Coffey, Mission Operations Manager of NRL's
TiPS, adds "From a survivability aspect, TiPS' tether is susceptible to
space debris damage. The tether, roughly 2 mm in diameter, can be severed by a
particle as small as 1mm travelling at a relative velocity of 14km/s (31,318
mph)."
Mr. William Purdy, NRL's TiPS Program Manager says, "The TiPS endmasses
are similar in size and weight, which dictates that both will undergo
significant dynamic motion with respect to the center of mass of the system.
The heaviest end mass, affectionately dubbed "Ralph," weighs 37.7 kg
(83.2 lb.). Ralph contains all of the electronic components, which include the
telemetry system, turncount recorder, and temperature sensors. The telemetry
system, supplied by NASA, is powered by a nonrechargeable battery, which
operated for the first eight hours of the satellite's life. The lighter
end-mass dubbed "Norton" weighs 10.3 kg (22.8 lb.). The tether weighs
5.5 kg (12.0 lb.), and was coiled on a spool inside Ralph, much like a spinning
reel."
Ralph and Norton separated at a relative velocity of approximately 5.1 m/s
(16.7 ft/s). The passive deployment scheme utilized a small Marman clamp and
ten spring-loaded cartridges. The initial separation energy was designed to
deploy about 2 km of the tether, at which time gravity-gradient forces assisted
to unwind the remainder.
The TiPS orientation is controlled by gravity gradient forces. The TiPS
satellite orbits the Earth with a nominal vertical orientation, Ralph being
closest to the earth. About this nominal orientation, the tethered system
undergoes libration, a pendulum-like motion, about the center of mass.
A globally distributed SLR network, including NASA, DoD, and cooperating
international stations, are tracking the TiPS satellite. These SLR stations
provide very accurate, submeter-resolution, range data for Ralph and Norton.
The SLR information is being used to determine the orbit and attitude of the
tethered system. Ms. Amey Peltzer, NRL's TiPS SLR Mission Specialist reports, "This
is the first time SLR data has been converted into attitude and orbit knowledge,
to validate analytical models and simulations of a tethered system."
Retroreflectors are mounted on the exterior surfaces of both Ralph and
Norton, for long-term passive monitoring of the tethered system. End-mass
discrimination is accomplished by coating the retroreflectors of Ralph to
reflect only one of the two transmitted laser wavelengths. The uncoated
retroreflectors on Norton reflect both transmitted wavelengths.
And Mr. Towsley adds, "TiPS is a low-budget, quick-reaction experiment.
To date, mathematical models have been developed to predict the motion and
survivability of tethers in space, but little long-duration on-orbit data have
been obtained. The data from TiPS will be used to verify and improve
understanding of the physics of tethered systems in space as well as the
mathematical models."
The Naval Research Laboratory is
the Department of the Navy's corporate laboratory. NRL conducts a broad program
of scientific research, technology and advanced development. The Laboratory,
with a total complement of nearly 4,000 personnel, is located in southwest
Washington, DC, with other major sites at the Stennis Space Center, MS; and
Monterey, CA.
