Running out of gas usually means a long walk to a gas station to get more fuel. When a satellite's fuel tank runs dry, though, the mission's over — or it used to be.
Scientists at Purdue University and Lockheed Martin Corp. have devised new techniques that allow engineers to extend the lives of older communications satellites by redistributing the dwindling supply of propellant in the crafts' fuel tanks.
They tested the method on twin communications satellites launched in 1991 and were able to boost the remaining life span for each satellite from six to nine months to about two years.
Steven Collicott, a Purdue professor of aeronautics and astronautics, said that additional time equated to about $60 million in revenue for the broadcast companies that owned the satellites and used them to carry television and radio transmissions.
He worked with Lockheed Martin Corp. researchers to extend the lives of older communication satellites equipped with multiple fuel tanks that hold the propellant their rockets draw on to remain in a stationary orbit far above the planet.
Together, the researchers devised two new methods — one to precisely determine how much fuel remains in each of a satellite's tanks, and another that allows for the propellant levels in all of the tanks to be "rebalanced" or equalized.
Their findings are detailed in a paper published in the July-August issue of the Journal of Spacecraft and Rockets.
Collicott said the developments are important because if one of the tanks on an older satellite runs low of fuel it often allows helium used to pressurize the liquid propellant in all the tanks before engine firings to seep out, making the remaining fuel all but useless.
"You would open the valve and it would be like having no water pressure in your household plumping," he said. "You would open the valve but the liquid just sits there."
Collicott worked with Boris Yendler, a senior thermal system analyst at Lockheed Martin Mission Services in Sunnyvale, Calif., to develop the new fuel-sensing and fuel-redistribution methods. Both ideas originated with Yendler.
The fuel-sensing technique relies on data collected when onboard heaters are periodically turned on to keep the fuel from freezing as a satellite rotates away from the sun. Computer simulations the team developed revealed the three-dimensional distribution of fuel in each tank.
Collicott, Yendler and Timothy A. Martin, an engineer at Lockheed Martin Space Systems Co. in Denver, used that technique on the twin satellites and discovered that one of the four fuel tanks in each satellite was nearly empty.
They then used a method they developed to periodically turn on the heaters inside the tanks still holding abundant fuel and were able to redistribute the fuel into the tank that was running on empty. That tank was kept unheated during the fuel-shifting process.
Yendler said the fuel-distribution method, applied periodically over about a year and a half time period, extended the satellites' operational lives for about two years, until late 2003.
"Without our help they would have been dead," Yendler said. "We kept the fuel balanced to extend their lives."
Once the satellites' fuel was almost spent, he said both satellites were sent into higher orbits, as is the practice with defunct communications satellites.
Jerry Grey, a professor emeritus of aerospace engineering at Princeton University, said the techniques would likely only be useful for older communications satellites with multiple tanks.
Communications satellites occupy a narrow band of space called geosynchronous orbit that rings the equator about 22,500 miles aloft where they keep an exact pace with Earth's rotation so their antennae can remain pointed at the same place for communications purposes.
Grey said those satellites have an average life span of about 15 years, and so keeping them in a successful orbit as long as possible would be of great interest to broadcast companies.
"If they die a year or two early you're definitely losing money because those satellites bring in fairly good revenue," said Grey, a consultant to the Reston, Va.-based American Institute of Aeronautics and Astronautics.