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Electric propulsion could launch new commercial trend
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: March 19, 2012


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WASHINGTON -- Boeing's announcement last week of a contract to build up to four communications spacecraft with all-electric propulsion for Asian and Mexican customers could shift the commercial satellite industry's trend for larger, heftier platforms requiring powerful rockets to launch them into orbit, officials said.


Artist's concept of the Boeing-built Satmex 7 satellite in orbit. Credit: Boeing
 
Each of the Boeing-built satellites will weigh less than 4,000 pounds -- less than 2 metric tons -- and still offer the power and communications throughput of heavier spacecraft.

The deal includes firm orders for two satellites, Satmex 7 and ABS-3A, for launch in late 2014 or early 2015 on a Falcon 9 rocket from Cape Canaveral, Fla. Two other unidentified satellites for Satmex and Asia Broadcast Satellite would launch together in 2015 on another Falcon booster.

The satellites will not carry liquid propellant or conventional rocket jets, but they will feature tanks of xenon gas and ion thrusters.

Boeing satellites have used electric propulsion since 1997, but until now the ion thrusters were only used to hold a spacecraft's position in geostationary orbit, an arc 22,300 miles over the equator where satellites appear to hover above a fixed position on the planet.

Communications satellites are typically placed in geostationary orbit to maintain continuous coverage for their customers.

Despite the innovative, ultra-efficient ion thrusters, Boeing satellites continued using conventional liquid propellants for their orbit-raising engines. After launching from Earth on a large rocket, communications satellites are usually released in temporary oval-shaped, elliptical parking orbits because most launchers are not powerful enough to deploy a sizable craft in its operating position 22,300 miles up.

Controllers on the ground must use a satellite's own propulsion system to circularize its orbit and move it over the equator. The orbit-raising process takes a few weeks for most satellites with liquid propulsion, which uses combustion.

After 15 years of electric propulsion in its communications satellite program, Boeing says it is now ready to offer an all-electric version of its Boeing 702 spacecraft platform.

"About 15 years ago, we launched the first electric propulsion system," said Stephen O'Neill, president of Boeing Satellite Systems International Inc. "We've got systems up there that are commercial, government and classified, and we haven't lost one minute of operation in orbit yet. That led us two years ago to make a significant investment in electric propulsion."

The xenon ion propulsion system is 10 times more efficient than chemical thrusters. The system works by accelerating electrically-ionized xenon gas through a thrust chamber at more than 60,000 mph. The thrust of an ion engine is much lower than chemical propulsion, but they can fire for thousands of hours and consume less propellant.

Orbit-raising with only electric propulsion could take six months, delaying the start of service for new commercial communications satellites. It's a trade-off for private operators, which must give up quicker revenue from a new satellite for the reduced launch costs or extra communications payload of a lighter spacecraft.

Electric orbit-raising has been demonstrated before. A communications satellite for the U.S. Air Force used small electric thrusters in 2010 and 2011 to reach a high-altitude geostatioanry orbit after its conventional liquid-fueled engine failed.

The Air Force's first Advanced Extremely High Frequency, or AEHF, spacecraft was built by Lockheed Martin Corp. Lockheed Martin-built communications satellites also feature electric thrusters for small in-orbit adjustments, but they use liquid propulsion for major maneuvers to change their orbits.

Joseph Rickers, president of Lockheed Martin Commercial Space Systems, said his company has not decided to produce an all-electric satellite yet, despite its success with the AEHF 1 satellite's orbit-raising.

"We certainly have electrical propulsion, but not at that price point," Rickers said. "The jury is still out as to whether we're going to go that way or not."

Space Systems/Loral, another leading commercial satellite builder based in Palo Alto, Calif., is developing an ion thruster system to shorten the orbit-raising period to as low as three months, according to John Celli, the company's president.

"We will [offer all-electric propulsion] in a few months when our development of an efficient propulsion engine is going to be available," Celli said. "Right now, it's in qualification."

"I don't know how sensitive the customers are to the time it takes to do electric orbit-raising," Celli said. "Right now, it looks like five to six months, so we're trying to reduce that substantially. If you reduce six months to three months, that's three months more revenue, so that has to be of some value."

According to Boeing, the new Boeing 702SP, which stands for small platform, satellite model will offer between 3 kilowatts and 8 kilowatts of power. Many other satellites with that power range are up to 2,000 pounds heavier at launch than the Boeing 702SP.

"Instead of having all the mass of a liquid propulsion system, and all the structure to support that mass, all of a sudden we can sell a customer a 3 kilowatt to 8 kilowatt solution that weighs less than 2 metric tons," said Stephen O'Neill, president of Boeing Satellite Systems International Inc. "Stack them together and you can put them on multiple launch vehicles. It just so happens our customer made a decision to put them on Falcon 9. You can also dual-launch them on almost anything else."


File photo of a Falcon 9 rocket. Credit: SpaceX
 
An upgraded version of the Falcon 9 rocket, which would be employed for communications satellite launches, can haul up to 10,000 pounds, or 4,500 kilograms, to geostationary transfer orbit. It lifts less mass than any of its major competitors, but it is also less expensive.

Arianespace's Ariane 5 rocket is tailored for tandem launches of satellites weighing about 6 metric tons and 3 metric tons. The Proton rocket and Breeze M upper stage, marketed by International Launch Services, can conduct single or dual launches with a total payload up more than 6 metric tons.

Sea Launch's Zenit rocket is available for missions with single payloads of more than 6 metric tons, and China's Long March 3C launcher has a lift capacity of 5,500 kilograms to geostationary transfer orbit.

The Sea Launch Zenit rocket blasts off from a mobile platform on the equator in the Pacific Ocean, and the Ariane 5 launch base in French Guiana is just north of the equator. Equatorial launches are beneficial for geostationary orbiting satellites because they require less powerful rockets and less liquid propellant to reach a spacecraft's final station.

But the advantage of low-inclination launch sites may be diminished by the use of ion propulsion because it requires less xenon gas propellant than the load of denser liquid fuel aboard other satellites.

There is a mass penalty for satellites launching from higher inclinations. The spacecraft must fly aboard a larger launcher or carry more fuel in its tanks to make up the difference and reach its ultimate destination in space.

"It gives a different cost equation message to our operators because now they're looking not just at the cost of the satellite, but on-orbit, what is the total cost of that system," O'Neill said.

"Do they want to have something that has liquid propulsion, or do they want to have something that is electrical propulsion that is between one-third and one-half the mass? And one-third to one-half the mass launched to orbit is a significant financial difference, and it changes the equation to its business model," O'Neill said.

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