NASA funds multijunction solar propulsion project
Fuel-efficient electric thrusters that use compound semiconductor technology to propel spacecraft between orbits has received a funding boost from NASA.
NASA s Glenn Research Center has awarded solar array developer Entech a contract worth $0.6 million under its small business technology transfer program. The award represents the second phase of a project to develop and ground-test the Texan firm s stretched lens array technology, which focuses sunlight onto multi-junction cells with high efficiency.
The solar arrays are required as part of an electric propulsion system to power so-called "space tugs" for future missions that NASA is planning, such as carrying cargo between low-Earth and lunar orbits.
While chemical rockets that blast gases out of the back of a spacecraft are usually used to move cargoes about in space, electric propulsion offers a much more fuel-efficient, albeit slower alternative - suitable for missions that do not carry human passengers.
The approach could also offer massive savings in costs by reducing the weight of the launch payload, much of which is taken up by chemical propellants.
Electric propulsion works by applying a high voltage to xenon gas. First, the xenon atoms are ionized, before the electric field accelerates these charged particles away from the spacecraft - pushing the vessel in the opposite direction.
A solar array based on multi-junction cells is the most efficient way to generate this electric field. In fact, the approach was first used in 1998, says Entech president Mark O Neill, as a thruster for the Deep Space I mission to investigate Comet Borrelly.
Crucially, the multi-junction cells also produce a high voltage that propels the xenon ions away at very high speed.
Now, NASA is hoping to develop the technology so that it can be used to move larger cargoes about in space. "They're now looking at taking something from a low-Earth orbit into a geostationary orbit," O Neill said. "The big pay-off would be re-usable tugs that could be used to propel cargo from the Earth to the Moon, or to Mars."
Entech s key advantage is the lightweight nature of its stretched lens arrays, which focus sunlight onto the multi-junction solar cells in a highly efficient manner. These arrays have also been used in terrestrial application of concentrator cells, currently the subject of much interest from chip manufacturers and the venture capital community.
