DARPA Readies Demonstration of Radically New In-Space Propulsion

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

Calledthe High Delta-V Experiment, or HiDVEfor short, the program aims within the next year or so to complete a grounddemonstration of an unconventional propulsion system that uses the heat of thesun to produce enough thrust to push a 10-15 kilogram satellite into a neworbit. If the ground demo goes well, DARPA would look to press on with anin-space demonstration on a dedicated microsatellite.

Whilespacecraft designers are constantly finding new ways to pack more capabilitiesinto small satellites, matching these little wonders with an affordable launchremains a challenge. Typically, very small satellites have to make due withsecondary launch opportunities and frequently are dropped off in non-optimalorbits. Since very small satellites more often than not are built without anymeaningful propulsive capability, they must remain there.

U.S. AirForce Lt. Col. Fred Kennedy, DARPA's HiDVE program manager, believes that solar thermalpropulsion, which uses the warmth of the sun to heat an onboard liquid such aswater or ammonia to very high temperatures, and vent it through a nozzle can change that. "We're trying to say you canthrow up little systems ... drop them off in an orbit you wouldn't want them tobe in, and let them perform orbit transfers to get them where they need tobe," Kennedy said.

The keyenabling technologies for such a system, according to Kennedy, include veryhigh temperature materials and innovative solar receiver and concentratordesigns.

In lateSeptember, DARPA selected two firms, Pratt & Whitney Rocketdyne and SpaceDev, tospend the next six months working on competing solar thermal propulsiondesigns. Including some subsystem validation work, under contracts valued at$4.9 million and $3.7 million respectively. Kennedy said if the next six monthsshows that the type of solar thermal propulsion capability DARPA wants isdoable, HiDVE would press ahead with a full-up grounddemonstration six to nine months later. After that, DARPA could press aheadwith an in-space demonstration, assuming funding availability and positiveresults from the ground demo, Kennedy said.

Mark Sirangelo, SpaceDev's chairmanand chief executive, said the Poway, Calif.-based space technology companyexpects to "complete a full-on system design" under the initialsix-month contract, which also calls for the initial design of a 15-kilogramsatellite platform capable of hosting the solar thermal propulsion system. Sirangelo said SpaceDev and teammatesGeneral Atomics and BAE Systems would be ready before the end of 2010 to flightdemonstrate their system and satellite.

Pratt& Whitney Rocketdyne spokesman Brian Kidder saidOct. 23 that the Canoga Park, Calif.-based propulsion company was still severaldays away from being ready to discuss its HiDVEaward, noting that the company's press release on the win still was beingdrafted.

While SpaceDev and Pratt & Whitney were reluctant to tiptheir hands in what still remains a competition, Kennedy was willing to speakin generalities about the technology and itspotential.

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

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

"Youcan get close to cryogenic performance on a very small vehicle," Kennedysaid of solar thermal's potential.

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

"Atmost, you are probably talking about cold gas thrusters with very low specificimpulse. At most, you might look at monopropellant hydrazine, for example. Youmight get a couple hundred seconds of specific impulse out of it. That doesn'tgive you a lot of options. And even if you were to go to monopropellanthydrazine, you are talking about fairly expensive, hard to test systems that small satellite vendors simply don't want to go off andbuy because they tend to be a significant portion of their total budget."

Solarelectric propulsion, despite being highly efficient, is not a good solution fortiny satellites, Kennedy said, because they require more electrical power thansmall satellites can be expected to generate.

Kennedyis speaking as somebody who has spent a lot of time thinking about the options.His earned his doctorate in 2004 from the United Kingdom's University of Surrey with a thesisthat examined solar thermal propulsion as one way to improve themaneuverability of small satellites.

Solarthermal propulsion is not new.

Theconcept dates back to the 1960s, Kennedy said, but much of the focus to datehas been on very large systems capable of powering vehicles the size of thespace shuttle or larger. Pratt & Whitney tested a solar thermal heatexchanger with the Air Force Research Laboratory in the 1980s, he said, andNASA's Kennedy Space Centerdid some work with the technology in the 1990s. But the technological andfinancial barriers were always too steep to move ahead with an on-orbit flighttest of any sort.

When itcomes to proving the value of solar thermal propulsion, Kennedy said, startingsmall appears to be the way to go.

But thepayoff could be big.

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

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Small satellites could get a novel propulsionsystem developed by DARPA.

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