Solar-electric propulsion (SEP) is high on NASA's list of things to do in its growing effort to develop technologies that will support long-term human space exploration. And within that arena, figuring out how to deploy large, lightweight solar arrays in space is a key enabler. Even after building the International Space Station with its 115-ft.-long array wings, the agency sees the technology readiness level (TRL) of deploying big arrays for exploration beyond low Earth orbit at 3 or 4—a long way from the demonstrated operational capability represented by TRL 9.

Results are starting to come in under the relatively open-ended technology development effort launched at the beginning of the Obama administration. While Congress has not approved the billion-dollar funding levels for the work the White House wanted, it has sprung enough money to make a start. Now five companies have come back with concept reports on what it would take to build and fly a solar-powered space tug testbed by the end of the decade, at a cost of $200 million. It remains to be seen if a testbed actually will be built, given the ongoing funding uncertainty in these parlous fiscal times. But the concept studies should help NASA's Office of the Chief Technologist (OCT) better understand what needs to be done, and just how much it will cost.

“We need that data to help make sure that we're doing the technology development in the right areas and that we're reducing the right risks,” says Mike Gazarik, the OCT space technology program director. “Those studies are going to help us make sure that in our investments in the near future, we're investing in the right areas.”

The basic SEP concept is nothing new, and is already at work in space. Solar arrays turn sunlight into electricity, which powers a low-thrust, high specific-impulse ion engine or Hall thruster to propel a spacecraft. Many communications satellites use it for north-south stationkeeping in geostationary orbit, and NASA's Dawn probe is set to move out Aug. 26 under SEP from orbit around the Main Belt asteroid Vesta toward an encounter with the dwarf planet Ceres (AW&ST May 21, p. 16).

NASA is spending about $3 million on the initial SEP studies, which were originally set up by the technology element in the Human Exploration and Operations Mission Directorate. The five companies selected to prepare concepts—Analytical Mechanics Associates Inc., of Hampton, Va.; Ball Aerospace & Technologies Corp., Boulder, Colo.; Boeing Integrated Defense Systems, Huntington Beach, Calif.; Lockheed Martin Space Systems Co., Littleton, Colo., and Northrop Grumman Aerospace Systems Corp., Redondo Beach, Calif., each received as much as $600,000 for the work, which could lead to a role in the flying testbed work by the end of the decade.

Near-term, NASA probably will spend its money on ground demonstrations of the deployment mechanisms for large solar arrays like those shown in this testbed concept that Ball Aerospace submitted with its proposal. In the image, a Ball space tug uses circular MegaFlex arrays from ATK. The company also had good luck with Deployable Space Systems' MegaROSA circular arrays, according to Bill Deininger, a staff consultant at Ball.

“Basically what NASA has told us is the technology readiness of the solar array is down around 3 or something like that,” he says. There needs to be some ground-demonstration [engineering demonstration units] built to verify that the deployment mechanisms are as robust as we think they are.”

Gazarik says one data point likely to emerge from the concept studies is a tradeoff between circular and linear arrays. To save weight, both types probably will use folding solar array blankets, but with much lighter support and deployment structures than the linear arrays on the ISS. Other areas to be tackled are the efficiency of the solar cells in the blankets—how much sunlight they can convert to power—and the power electronics needed to switch solar power on and off and to condition it for the electric propulsion systems. “That turns out to be pretty challenging,” Gazarik says.

After the array-deployment tests on the ground, budget permitting, there could be a deployment test “near or around” the space station, possibly making use of the vehicles that deliver supplies and/or crew to it, says Gazarik. The final phase would be the $200 million free-flying space tug test-bed, notionally set for launch in 2018 but highly dependent on funding.

Ultimately NASA foresees using SEP to preposition cargo for human crews arriving later at asteroids and Mars. While the instruments and other systems on the Juno probe now en route to Jupiter are powered with three large linear arrays (AW&ST March 21, 2011, p. 50), Gazarik says studies have showed SEP will be particularly useful at those nearer targets.