Communications is one of three areas in smallsat technology that will be flight-tested with new funding from NASA. The agency's chief technologist received a heavy response to its request for proposals in communications, proximity operations and propulsion, and expects to make selections for the 2-3-year effort before the end of August.
Sensors, software and thrusters for proximity operations could enable tiny inspectors to fly safely around larger spacecraft—including the ISS—to routinely document their physical condition and pinpoint debris damage or mechanical problems soon after they occur. Some of the software work is already underway inside the station with the Synchronized Position Hold, Engage & Reorient Experimental Satellites (Spheres) control-software testbeds: three volleyball-sized balls designed to give programmers a quick check of their algorithms in microgravity (AW&ST June 25, p. 44).
Because of the size and safety limitations that launch-service providers impose on secondary payloads, propulsion has been a particularly difficult problem for small-spacecraft designers, with the difficulty increasing as the size decreases. The Swedish Space Corp.'s NanoSpace unit has used micro-electromechanical systems (MEMS) fabrication technology to develop miniature thrusters that have been tested in orbit on the Prisma satellite testbeds (AW&ST May 7, p. 21).
At least one proposal in the OCT competition involves an update on the colloid thrusters tested in the 1960s and '70s and dropped in favor of ion propulsion because they just did not work as well. But now, says Paulo Lozano, an associate professor of aeronautics and astronautics at the Massachusetts Institute of Technology, advances in propellant chemistry and MEMS production is enabling development of “electrospray thrusters” that emit ions when subjected to an electric charge, instead of the heavier droplets emitted in the older technology. Using coulomb liquids—electrically conductive liquid salts composed of molecular ions—wicked by capillary action through a plate of tiny emitters produced with proprietary MEMS techniques—the thrusters produce a spray of ions when an electric charge is passed across them. The approach eliminates the need for pumps, valves and other moving parts, and generates specific impulses of 1,500-5,000 sec., depending on the propellant.
“It basically works like a candle,” Lozano says, noting that the thrusters operated with 80% efficiency. “The 'wax' of the candle is the propellant, and the 'wick' is just the transport medium, and the 'flame' is the thrusting mechanism. So it's very similar, except that we evaporate ions, and in the process of evaporating the ions we also accelerate them to very high speeds.”
In the longer term, engineers are studying ways to combine spacecraft so a small satellite can disperse cubesats after launch and then serve as a “mother-ship” communications hub. NASA's Nanosail-D flew to orbit in a P-POD inside the agency's Fast, Affordable, Science and Technology Satellite (Fastsat) developed by Dynetics as a way to get payloads to orbit on a freeflier in fewer than two years after authority to proceed. With the solar-sail demonstration, it also showed that a larger spacecraft can safely jettison a smaller one.
Now the Huntsville, Ala.-based company is looking for new uses for the Fastsat capability.
“The Communications Relay for the Arctic Domain is the next generation of that concept, where we would actually be a mother ship and deploy multiple cubesats to fly in formation with Fastsat and provide a larger coverage ring,” says Mike Graves, manager of the Space Vehicles Department at Dynetics. “So you have the host mother ship in the middle and then you've got in formation flight maybe four 3U cubesats, each one of those having its own localized communications capability and sensors, and then you send it back to the mother ship for the large data bandwidth to the ground.”