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The Defence Advanced Research Projects Agency (DARPA), the Pentagon death-boffin hothouse where ideas don't have to be mad to get funding but it doesn't hurt either, has issued a new request for proposals.

This time, the groundbreaking military gambler-profs are after hugely more flexible ion engines for use in spacecraft.

In an announcement posted last week, the agency notes that: "There is increasingly a desire to deploy satellites and spacecraft whose missions and requirements may be changed routinely throughout their operational life to support changing national needs."

This probably refers to spy satellites, much in demand by the American security and defence establishments. Spy satellites typically need to alter their orbits in order to get a good view of specific places or targets, and this uses up fuel. Once a surveillance bird has run out of propellant, it will only get into the right place at the right time again by sheer luck; its useful life is over. Spy spacecraft being expensive things, it makes sense to strive for fuel efficiency.

Electrically-powered ion thrusters can offer excellent propellant efficiency in terms of how much a satellite can change its vector per unit of fuel it spits out, because such systems throw exhaust at very high velocities. In technical terms, this is known as operating with a high specific impulse. These engines are normally configured to use a lot of energy (in spacecraft terms) and hurl tiny flows of fuel out with great violence, achieving only a minuscule thrust but using hardly any fuel. In the last decade, ion thrusters have been used on a number of satellites as an alternative to traditional bipropellant chemical jobs.

That's all very well if you're happy to spend weeks adjusting a spy-bird's track, or you just want to maintain an existing orbit; and such might well be the case. This sort of performance is very appropriate for getting and staying in position to monitor, oh, let's say construction work at uranium refinement facilities over a long period.

But on the other hand you might want to shift a satellite in a matter of days or hours in many cases - as when tracking shipments of weapons or when monitoring temporary camps or meetings. This will be a problem for existing electric thrusters.

"Currently, electric propulsions systems are limited in their ability to vary [specific impulse] while onflight. Tasking the satellite to do a manoeuvre other than it was designed for is either beyond the capability of the spacecraft (e.g. insufficient power and thrust), or extraordinarily wasteful of precious propellant (insufficient specific impulse)," DARPA says.

The Pentagon kill-science people would like to see ideas that could let an electric drive operate at high total thrust and low-but-decent specific impulse for urgent jobs, but also shift/maintain orbits slowly using very little propellant in normal ion-drive style. They expect this to involve "Field Effect Electric Propulsion (FEEP) or Colloid propulsion systems", micro-scale electrical thrusters which have been proposed for use in very small "microspacecraft" or "microsatellites".

DARPA and the US military are very interested in microsatellites, but there are indications that they would also like to scale up these technologies for use aboard more normal-sized craft. Under phase III of the request for proposals, they want to see their future thrusters using 100W of power.

Biggish comms satellites often have an order of magnitude more juice than this, but it isn't a "micro" power load by any means. Certain classes of surveillance sat - such as radar ocean-surveillance jobs - must by their nature have large amounts of power available.

As to how much funding could be available and what sort of timescale they're after, the DARPA programme chiefs are keeping their options open.

"The amount of resources made available... will depend on the quality of the proposals received and the availability of funds," they say.

As for timing, the Pentagon spy-sat designers would evidently like their 100W impulse-adjustable drive as soon as possible. Timetables calling for longer than three years overall will need to show "specific measurable milestones".

Full details here (pdf). As ever, it's important to remember that very few DARPA plans ever come to fruition - but those that do (the internet, night vision, etc) can be very effective. ®

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