Satellite rescue

Researchers have used a new technique to extend the service life of two communications satellites.

The two satellites would have been shut down prematurely and wasted remaining fuel if not for the new technique developed by researchers from Purdue University and Lockheed Martin, said Steven Collicott, a Purdue professor of aeronautics and astronautics.

Communications satellites, which are maintained in orbit about 22,500 miles above the Earth by the firing of small rocket thrusters, must be replaced shortly before they run out of fuel. Enough fuel must remain to get the satellites out of orbit to make room for their replacements.

While most new satellites have just one fuel tank, some older ones have four. So if one of the tanks empties before the others, the satellite loses control and has to be decommissioned, said Dr Boris Yendler, senior thermal system analyst at Lockheed Martin Mission Services in Sunnyvale, California.

The Purdue and Lockheed Martin engineers not only determined precisely how much fuel remained in each tank, but they also used a technique to "rebalance," or equalise, propellant levels in all of the tanks.

Using the technique, the engineers kept two communications satellites operating an additional six months, which equated to a savings of about $60m in revenue for the broadcast companies that owned the satellites.

'The work, led by Dr Yendler, represented the first time such a fuel gauging and rebalancing has been carried out in commercial communications satellites,' Collicott said.

The new technique works by using a computational simulation to determine the precise 3D distribution of fuel in each tank. Then, engineers analyse data from the satellite showing how the tanks respond when they are heated by onboard heaters. The heaters are routinely used to keep fuel from freezing as a satellite rotates away from the sun. But data related to how the tanks respond to the heating can be also analysed to reveal precisely how much fuel remains in each tank.

Since the technique uses temperature data that are already routinely collected, it requires no additional satellite hardware and can be applied to existing spacecraft.

After applying their technique to the two satellites, the researchers discovered that one of the four fuel tanks in each satellite was nearly empty.

'Because of the design of the propulsion system, if one tank had emptied out that would have ended the profitable life of the satellite, wasting all the fuel in the other three tanks,' Collicott said.

The reason for this is that the system relies on compressed helium to push fuel through a valve in each tank. When the valves are opened, fuel flows into rocket thrusters, which are fired periodically to reposition the satellites.

'The tanks are pressurised with the helium. If one tank runs out of fuel, the next time the valve in that tank is opened to ignite the rocket thrusters, the helium from that tank mixes with fuel going to the thrusters from the other tanks, preventing the thrusters from firing and shutting down the propulsion system,' Collicott said.

Once the imbalance in fuel is determined, however, the propellant can be equalised in all four tanks by a process called "thermal pumping."

'In thermal pumping, if you heat up the three tanks containing more fuel and you keep the emptier tank cooler, the helium gas in the three warmer tanks expands, pushing the liquid fuel into the emptier tank,' Collicott said. 'The difficult part is figuring out how much heating is needed and how long it will take to move the fuel into the emptier tank. My collaborators detected the fuel imbalance and designed a way to reprogram the heaters so that we could even out the propellant load over all four tanks and therefore maximise the mission life.'

Communications satellites cost about $100m and sometimes as much money to launch them into a geosynchronous orbit. They generally have a 15-year lifetime, bringing in $5m to $10m a month in revenue.

'Understandably, broadcast companies are very concerned about when a satellite needs to be replaced,' said Collicott, who has been developing the simulation since the early 1990s. 'A lapse in service would have tremendous impact on their business.'

The satellites eventually were shut down in mid-2003.