Ion-propulsion advance

Will ion propulsion replace engine thrusters on future deep-space missions?
By | Published: January 20, 2006
Ion thruster
The DS4G ion thruster undergoes test firing at ESA’s Electric Propulsion Laboratory at ESTEC in the Netherlands.
January 20, 2006
A breakthrough in ion-propulsion technology has resulted in a new ion thruster with more than a tenfold increase in fuel efficiency over the ion engine now in use on the European Space Agency’s (ESA) SMART-1 Moon mission. An Australian National University team designed and built the experimental Dual-Stage 4-Grid (DS4G) thruster in less than 4 months. The DS4G is based on a 2001 concept by British mathematician and ion-propulsion pioneer David Fearn.

A more fuel-efficient ion thruster will make long-duration space flight possible. Roger Walker of ESA’s Advanced Concepts Team says “A future spacecraft using our new engine design wouldn’t just reach the Moon, it would be able to leave the solar system entirely.”

Ion engines use an electric field to accelerate a beam of ions — positively charged particles — away from the spacecraft, thus propelling the craft forward. Ion propulsion engines aren’t new, but designs have steadily improved since NASA began studying the technology in the 1960s. From October 1998 to December 2001, NASA’s Deep Space 1 performed a record 200 starts and operated for 16,246 hours using the NSTAR ion engine. The NSTAR propelled DS1 163,171,352 miles (263,179,600 kilometers) and reached speeds of up to 10,066 mph (4.5 km/s).

What sets ESA’s DS4G apart from traditional ion engines like the DS1 is a two-stage, four-grid ion decoupling and acceleration process that results in longer engine life. Older ion engines use a single-step, two-grid method to extract and accelerate ions out of a reservoir into space. The large voltage difference across the gap between the two grids creates an electric field that extracts and accelerates ions at the same time. At voltage differences near 5,000 volts (5kV), some ions collide with and damage the second grid, thereby reducing engine life.

The DS4G ion engine uses Fearn’s two-stage, four-grid process in which two closely spaced high-voltage grids are separated from two closely spaced low-voltage grids. In stage one, ions are extracted from the first grid pair. Because the grids are both high voltage, the small voltage difference between them results in fewer ion collisions. In stage two, the ions are accelerated toward the second pair of low-voltage grids. The large voltage difference between the two pairs of grids accelerates the ions more forcefully than in other ion engines but with fewer collisions.

During laboratory testing, the DS4G ion engine achieved voltage differences of 30,000 volts (30kV) and fuel efficiency 4 times greater than current state-of-the-art ion engines. The next step is continuous operation of the DS4G over several thousand hours. After rigorous testing, the DS4G ion engine is set to widen the frontiers of space exploration to beyond our solar system.