Researchers at the Georgia Institute of Technology have won
a $6.5 million grant to develop improved components that will boost the
efficiency of electric propulsion systems that are used to control the
positions of satellites and planetary probes.
Focusing on improved cathodes for devices known as Hall
effect thrusters, the research would reduce propellant consumption in
commercial, government and military satellites, allowing them to remain in
orbit longer, be launched on smaller or cheaper rockets, or carry larger
payloads.
Researchers Jud Ready and Mitchell Walker prepare a carbon nanotube field emitter sample for measurements in the High-Power Electric Propulsion Laboratory of Georgia Tech’s School of Aerospace Engineering.
Sponsored by the U.S. Defense Advanced Research Projects
Agency Defense Sciences Office (DARPA-DSO), the 18-month project seeks to
demonstrate the use of propellant-less cathodes with Hall effect thrusters.
"About 10 percent of the propellant carried into space
on satellites that use an electric propulsion system is essentially wasted in
the hollow cathode that is part of the system," said Mitchell Walker, an
assistant professor in Georgia Tech's School of Aerospace Engineering and the
project's principal investigator. "Using field emission rather than a
hollow cathode, we are able to pull electrons from cathode arrays made from
carbon nanotubes without wasting propellant.
That will extend the life of the vehicle by more efficiently using the
limited on-board propellant for its intended purpose of propulsion."
To maintain their positions in space or to reorient
themselves, satellites must use small thrusters that are either chemically or
electrically powered. Electrically-powered thrusters use electrons to ionize an
inert gas such as xenon. The resulting ions are then ejected from the device to
generate thrust.
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Assistant Professor Mitchell Walker and Graduate Student Logan Williams examine a 10-kilowatt Hall effect thruster.
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In existing Hall effect thrusters, a single high-temperature
cathode generates the electrons. A portion of the propellant—typically about 10
percent of the limited supply carried by the satellite—is used as a working
fluid in the traditional hollow cathode.
The DARPA-funded research would replace the hollow cathode with an array
of field-effect cathodes fabricated from bundles of multi-walled carbon
nanotubes.
Powered by on-board batteries and photovoltaic systems on
the satellite, the arrays would operate at low power to produce electrons
without consuming propellant.
Walker and collaborators at the Georgia Tech Research
Institute (GTRI) have already demonstrated field-effect cathodes based on
carbon nanotubes.
This work was presented at the 2009 AIAA Joint Propulsion
Conference held in Denver, Colo.
The additional funding will support improvements in the devices, known
as carbon nanotube cold cathodes, and lead to space testing as early as 2015.
"This work depends on our ability to grow aligned
carbon nanotubes precisely where we want them to be and to exacting
dimensions," said Jud Ready, a GTRI senior research engineer and Walker's collaborator on
the project. "This project leverages our ability to grow well-aligned
arrays of nanotubes and to coat them to enhance their field emission
performance."
In addition to reducing propellant consumption, use of
carbon nanotube cathode arrays could improve reliability by replacing the
single cathode now used in the thrusters.
"Existing cathodes are sensitive to contamination,
damaged by the ionized exhaust of the thruster, and have limited life due to
their high-temperature operation," Ready noted. "The carbon nanotube
cathode arrays would provide a distributed cathode around the Hall effect
thruster so that if one of them is damaged, we will have redundancy."
Before the carbon nanotube cathodes developed by Georgia
Tech can be used on satellites, however, their lifetime will have to be
increased to match that of a satellite thruster, which is typically 2,000 hours
or more. The devices will also have to withstand the mechanical stresses of
space launches, turn on and off rapidly, operate consistently and survive the
aggressive space environment.
Part of the effort will focus on special coating materials
used to protect the carbon nanotubes from the space environment. For that part of the project, Walker and Ready are collaborating with Lisa Pfefferle in
the Department of Chemical Engineering at Yale University.
The researchers are testing their cathodes with the same
Busek Hall effect thruster that flew on the U.S. Air Force's TacSat-2
satellite. In addition, the cathodes
will be operated with Hall effect thrusters developed by Pratt & Whitney
and donated to Georgia Tech. The
researchers are also collaborating with L-3 ETI on the electrical power system
and with American Pacific In-Space Propulsion on flight qualification of the
hardware.
The ability to control individual cathodes on the array
could provide a new capability to vector the thrust, potentially replacing the
mechanical gimbals now used.
The use of carbon nanotubes to generate electrons through
the field-effect process was reported in 1995 by a research team headed by Walt
de Heer, a professor in Georgia Tech's School of Physics. Field emission is the extraction of electrons
from a conductive material through quantum tunneling that occurs when an
external electric field is applied.
The improved carbon nanotube cathodes should advance the
goals of reducing the cost of launching and maintaining satellites.
"Thrust with less propellant has been one of the major
goals driving research into satellite propulsion," said Walker, who is
director of Georgia Tech's High-Power Electric Propulsion Laboratory. "Electric propulsion is becoming more
popular and will benefit from our innovation.
Ultimately, we will help improve the performance of in-space
propulsion devices."
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