January 23, 2013
Credit: Credit: DARPA
The U.S. Defense Advanced Research Projects Agency (Darpa) plans to show that a robotic vehicle can remove the antenna from a retired spacecraft in graveyard orbit, and attach systems to it to rebuild a functioning geostationary communications satellite, in an orbital demonstration planned for 2016.
Begun in July last year, the four-year, $180 million Phoenix program aims to “increase the return on investment for Defense Department space missions by really lowering the cost,” says David Barnhart, Darpa program manager. The program centers on developing “a different way of building spacecraft, with some level on on-orbit assembly to add hardware and use what is already up there.”
Although antennas average only 2-3% of a communications satellite’s mass, the cost of the spacecraft increases proportionally with aperture size, in turn driving the size of the booster and cost of launch into GEO. “The dollars per specific mass is not that high for an antenna, but if you have to send up a large satellite on a large booster the cost is very high,” he says.
An example is the $350 million sticker price on NASA’s TDRS-K data-relay satellite, with its two 4.9-meter-dia. unfolding parabolic antennas, and the likely $200 million price tag for its launch this month via Atlas V. “The cost depends on the size of the antenna. With a large commercial aperture, say 18 meters, this architecture could provide a potential 10 times reduction,” he says.
While it can take up to 15 years for fuel reserves to be depleted and solar arrays degraded to the point where a satellite has to be retired, Darpa calculates the antenna structure could last more than a century. The agency believes the cost of providing satellite communications could be reduced dramatically by removing and repurposing an existing antenna by attaching “satlet” modules manufactured at high volume and low cost and launched cheaply and quickly as piggyback payloads on commercial satellites.
“If we cannot replace the appropriate functions and mass to control the aperture, the concept does not make sense,” Barnhart says. The plan is to achieve this through aggregation, sending up satlets that each perform a function, such as attitude control, momentum management or power generation, provided by the original satellite.
“We will create an aggregate set of hardware. The question is how many of these very small things do we need and can we aggregate them at low cost?” Barnhart says.
In the Phoenix concept, a robotic servicer/tender would be launched into GEO. The satlets, along with tools for the servicer’s robot arms, would then be packed into payload orbital delivery systems (PODS) and delivered to GEO as hosted payloads on commercial satellite launches.
“The tempo at which the satlets are going up becomes critical,” Barnhart says. With 10-15 GEO commercial launches a year on average, “that’s a one-a-month tempo on which we can take advantage of any excess mass as hosted hardware.”