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Pentagon Working on Solar-Powered Satellite Engines

Small satellites could soon get a boost from a novel in-space propulsion system under development at the U.S. Defense Advanced Research Projects Agency (DARPA).

Called the High Delta-V Experiment, or HiDVE for short, the program aims within the next year or so to complete a ground demonstration of an unconventional propulsion system that uses the heat of the sun to produce enough thrust to push a 10-15 kilogram satellite into a new orbit.

If the ground demo goes well, DARPA would look to press on with an in-space demonstration on a dedicated microsatellite.

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While spacecraft designers are constantly finding new ways to pack more capabilities into small satellites, matching these little wonders with an affordable launch remains a challenge.

Typically, very small satellites have to make due with secondary launch opportunities and frequently are dropped off in non-optimal orbits.

Since very small satellites more often than not are built without any meaningful propulsive capability, they must remain there.

U.S. Air Force Lt. Col. Fred Kennedy, DARPA's HiDVE program manager, believes that solar thermal propulsion, which uses the warmth of the sun to heat an onboard liquid such as water or ammonia to very high temperatures, and vent it through a nozzle can change that.

"We're trying to say you can throw up little systems ... drop them off in an orbit you wouldn't want them to be in, and let them perform orbit transfers to get them where they need to be," Kennedy said.

The key enabling technologies for such a system, according to Kennedy, include very high-temperature materials and innovative solar receiver and concentrator designs.

In late September, DARPA selected two firms, Pratt & Whitney Rocketdyne and SpaceDev, to spend the next six months working on competing solar thermal propulsion designs.

Including some subsystem validation work, under contracts valued at $4.9 million and $3.7 million respectively.

Kennedy said if the next six months shows that the type of solar thermal propulsion capability DARPA wants is doable, HiDVE would press ahead with a full-up ground demonstration six to nine months later.

After that, DARPA could press ahead with an in-space demonstration, assuming funding availability and positive results from the ground demo, Kennedy said.

Mark Sirangelo, SpaceDev's chairman and chief executive, said the Poway, Calif.-based space technology company expects to "complete a full-on system design" under the initial six-month contract, which also calls for the initial design of a 15-kilogram satellite platform capable of hosting the solar thermal propulsion system.

Sirangelo said SpaceDev and teammates General Atomics and BAE Systems would be ready before the end of 2010 to flight demonstrate their system and satellite.

Pratt & Whitney Rocketdyne spokesman Brian Kidder said Oct. 23 that the Canoga Park, Calif.-based propulsion company was still several days away from being ready to discuss its HiDVE award, noting that the company's press release on the win still was being drafted.

While SpaceDev and Pratt & Whitney were reluctant to tip their hands in what still remains a competition, Kennedy was willing to speak in generalities about the technology and its potential.

"It's essentially a steam kettle," he said. "What you are trying to do is construct a very, very small, perhaps thimble-size, heat exchanger and flow a low molecular weight fluid through it — be it water, ammonia or methane — heat it up to very high temperatures, typically 2,000 to 3,000 degrees Celsius, exhaust it through a nozzle, and that's how you get your thrust."

How much thrust?

"You might be able to achieve specific impulses of 400 meters-per-second or better this way," Kennedy said, noting that the space shuttle's liquid hydrogen-fueled main engines deliver 455 meters-per-second of specific impulse.

"You can get close to cryogenic performance on a very small vehicle," Kennedy said of solar thermal's potential.

Conventional satellite propulsion methods are not a good fit for very small satellites, according to Kennedy, because they tend to be too low performance to deliver a sufficient change in velocity, or delta-v.

"At most, you are probably talking about cold gas thrusters with very low specific impulse. At most, you might look at monopropellant hydrazine, for example. You might get a couple hundred seconds of specific impulse out of it.

"That doesn't give you a lot of options. And even if you were to go to monopropellant hydrazine, you are talking about fairly expensive, hard to test systems that small satellite vendors simply don't want to go off and buy because they tend to be a significant portion of their total budget."

Solar-electric propulsion, despite being highly efficient, is not a good solution for tiny satellites, Kennedy said, because they require more electrical power than small satellites can be expected to generate.

Kennedy is speaking as somebody who has spent a lot of time thinking about the options. His earned his doctorate in 2004 from England's University of Surrey with a thesis that examined solar-thermal propulsion as one way to improve the maneuverability of small satellites.

Solar thermal propulsion is not new.

The concept dates back to the 1960s, Kennedy said, but much of the focus to date has been on very large systems capable of powering vehicles the size of the space shuttle or larger.

Pratt & Whitney tested a solar thermal heat exchanger with the Air Force Research Laboratory in the 1980s, he said, and NASA's Kennedy Space Center did some work with the technology in the 1990s.

But the technological and financial barriers were always too steep to move ahead with an on-orbit flight test of any sort.

When it comes to proving the value of solar thermal propulsion, Kennedy said, starting small appears to be the way to go.

But the payoff could be big.

"It's all part and parcel of DARPA's general strategy of making space more flexible," he said. "We're trying to create a more flexible space architecture, and one of the ways you do that is by taking some of these small systems, which for a long time people thought couldn't really do much of anything, and make them really useful in the larger architecture. This is one way that we think we can do that."

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