From one, many
How clusters of small satellites could replace large spacecraft
by Jeff Foust
|“How can small satellites carry enough delta-v to maneuver on demand without growing into large satellites?” Payton asked.|
Payton identified three keys areas where smallsats compete with larger spacecraft: sensor technology, maneuverability, and communications. Sensor technology, he said, is making great strides forward, putting more capable systems into smaller packages, and thus allowing smallsats to carry out missions that once required larger satellites. However, these sensors generate massive data sets, requiring high-bandwidth communications systems to transmit the data back to Earth. Such systems, though, are large and require a lot of power, making it difficult to host them on smallsats. “These large communications pipes and small satellites are generally mutually exclusive, especially from geosynchronous” orbit, he said.
Maneuverability raises another challenge for small satellites. The ability to move around in orbit, either to observe a target of interest on the ground or to evade threats, is a major part of the overall ORS paradigm. Such maneuvers can consume significant amounts of propellant, which, in turn, reduces the useful lifetime of the satellite unless there’s some way to service satellites, as the DARPA Orbital Express mission tested earlier this year. “How can small satellites carry enough delta-v to maneuver on demand without growing into large satellites?” Payton asked. “How can we retain the lifetime of a small satellite and still retain its maneuver capability?”
The solution Payton proposed involves offloading some of the key functions of individual satellites into an on-orbit infrastructure that allows smallsats to remain small while performing more ambitious missions. “How about if… we created an on-orbit infrastructure that would provide the high-speed mission communications as a service, as a utility: a wireless, high-speed broadband local area network,” Payton explained. Such a system would be analogous to mobile phone systems: each phone need only be powerful enough to communicate with a nearby cell tower, which then routes the call to its destination, allowing the handset to remain small.
In this approach, a number of small “mission” satellites, equipped with sensors and short-range communications systems, would operate in the vicinity of a larger “communications concentrator” spacecraft. The concentrator spacecraft would serve as the hub of the local area network, accepting data from the mission spacecraft and transmitting it back to Earth using its own high-bandwidth communications system.
Payton said the communications concentrator was one of two types of “utility” spacecraft that would support smallsats. The other would be a mission support spacecraft that would refuel the mission smallsats and also transfer them to new orbits quickly. He described a scenario where a spacecraft needs to move from one location in geosynchronous orbit (GEO) to another quickly: such fast transits, at the rate of 18 degrees of longitude per day, require a delta-v of 110 meters per second, an amount that would significantly shorten the life of any spacecraft. In Payton’s scenario, a mission support spacecraft would dock with a smallsat, fill up the smallsat’s propellant tanks, then use its own engines to inject the smallsat on a new trajectory. In the case where a smallsat was moving from one GEO slot to another, all the smallsat would have to do is fire its own thrusters to stop at the desired location where, if available, another mission support spacecraft could dock with the smallsat to top off its tanks again.
This approach, called a “Space-Based Group”, was initially studied by the Pentagon when the Space Based Infrared System (SBIRS), the next generation of missile early warning spacecraft, underwent a Congressionally-mandated Nunn-McCurdy review because of cost overruns a couple years ago. “We wanted an out-of-the-box alternative that would meet or exceed the technical requirements of the existing SBIRS-High program,” Payton explained.
|“Space-Based Group is a very different paradigm,” Payton concluded. “It enables small satellites to be the peers of big spacecraft.”|
While SBIRS is back on track, the concept remains under study for use in future systems as a means of allowing groups of small satellites to replace a larger satellite, providing additional mission flexibility. “Instead of building a single NPOESS spacecraft,” he said, referring to the next-generation National Polar-orbiting Operational Environmental Satellite System weather satellite program, which has itself run into its share of technical problems and cost overruns, “with 11 separate, many times incompatible, sensors on a school-bus-sized spacecraft, we could build three or four smaller spacecraft, simpler spacecraft, each hosting their own mission sensors” and flying in formation with the utility spacecraft. “This would greatly simplify systems engineering on the spacecraft.”
Payton added that Space-Based Group would not require any revolutionary technological advances. “We can do all this with today’s technology,” he said, adding that he thought this architecture could be implemented when it came time to replace the SBIRS system.
“Space-Based Group is a very different paradigm,” Payton concluded. “It enables small satellites to be the peers of big spacecraft.”
The general concept of taking the components of a large spacecraft and spreading them among several smaller spacecraft is called fractionation. The type of fractionation that Payton proposed with the Space-Based Groups concept is relatively mild, though, compared to what one DARPA program now in its earliest stages has in mind.
Last month DARPA issued a broad agency announcement (BAA) for a program it calls System F6. (The “F6” is derived from a number of terms used to describe the program: future, fast, flexible, fractionated, and free flying.) The goal of the effort is to develop the technologies needed to replace a single “monolithic” satellite with a free-flying cluster of smallsats that each provide a specific subsystem: communications, power, sensors, et cetera. The individual smallsats would create a “self-forming network of spacecraft nodes” that together act like a single satellite, Owen Brown, F6 program manager, explained at an industry day meeting for the program last month.
Radical fractionation like this has a number of advantages. If a particular subsystem fails in orbit, that smallsat can be replaced with a new one, restoring the overall cluster’s functionality; by comparison, if a subsystem on a monolithic satellite fails, the entire spacecraft much be replaced at greater expense. Fractionated systems can also mitigate launch risks by spreading component launches over a number of smaller launch vehicles instead of one large one. If the smallsats share a common bus or other components, economies of scale can drive down production costs compared to building a few larger spacecraft. (One part of the F6 program will be to develop quantitative models of the net value of such fractionated systems.)
In its BAA solicitation, DARPA has identified a number of key technologies needed for an F6 system to be successful. These include networking and wireless communications capabilities among the spacecraft nodes, distributed computing, wireless power transfer, cluster flight operations, and the development of a spacecraft “black box” for each node to diagnose and recover from failures.
|The goal of the F6 effort is to develop the technologies needed to replace a single “monolithic” satellite with a free-flying cluster of smallsats that each provide a specific subsystem: communications, power, sensors, etc.|
DARPA plans to award contracts for studying the F6 concept this fall; the deadline for responses to the BAA is early September. The agency foresees the first launch of an F6 demonstration system within four years, involving a minimum of two spacecraft each weighing no more than 300 kilograms. DARPA director Tony Tether, in brief comments at the beginning of the industry day, called F6 a “high priority project” with “plenty of money set aside”, although he didn’t specify an exact figure; DoD budget documents requested nearly $17.8 million for F6 in fiscal year 2008 and $21.2 million in 2009.
While F6 might seem like a direct competitor to Space-Based Group, Payton said the two are very different. “This is not DARPA tough,” he said of the Space-Based Group concept. “We intentionally designed this to be much easier than that.” DARPA, he said, “has been doing a superb job in their space efforts; it’s simply that we intentionally architected this [Space-Based Group] so that it’s not that demanding.”