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Darwinian spacecraft engine to last twice as long

Mon, 05/24/2010 - 8:24pm
New Scientist

SPACE agencies may one day have Charles Darwin to thank for the longevity of their spacecraft. The life expectancy of a popular type of ion engineMovie Camera has been almost doubled using software that mimics the way natural selection evolves ever fitter designs.

Electrostatic ion engines are becoming popular in space missions. Instead of relying on burning large amounts of heavy liquid propellant for thrust, they use solar power to ionise a small supply of xenon gas. A high voltage applied across a pair of gridded electrodes sends the positively charged ions rushing at high speed towards the negative electrode. Most ions pass through the grid, generating thrust.

However, some ions collide with the grid itself, causing it to gradually wear out, says Cody Farnell, a space flight engineer at the University of Colorado in Fort Collins. Simulations suggest grids in a typical NASA engine will last 2.8 years - but Farnell wondered whether changing the grid's design could extend its lifespan.

He used evolution-mimicking software called a genetic algorithm (GA), and started by instructing the algorithm to randomly generate values corresponding to the geometry of the grid and the voltages applied to it. These values can be thought of as analogous to genes.

Each combination of values was then fed into a simulator to give an idea of the grid's performance and its expected lifetime. If the performance was promising, the "genetic material" was subjected to further random changes, or mutation, and this process was repeated until no more improvements were forthcoming.

After 100 generations, the GA spawned a geometry/voltage set that boosted the ion engine grid's lifetime to 5.1 years - at least in the simulator (Journal of Propulsion and Power, DOI: 10.2514/1.44358). Factors optimised included grid hole diameter, hole spacing and the thickness of the grids. The engine could be improved further, says Farnell, by evolving the other parts too.

After 100 generations, the algorithm spawned a design that almost doubled engine lifetime

Propulsion engineer Steven Gabriel at the University of Southampton in the UK is developing a triple-gridded ion engine. He says Farnell's work could have major implications if real-world tests of his design match the simulations.

Issue 2761 of New Scientist magazine

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