The European space agency is ramping up expectations for its Dual-Stage 4-Grid (DS4G) ion thruster. Using a concept developed by UK propulsion theorist David Fearn, the agency’s test model, designed and built by a team from Australian National University, is said to be ten times more fuel efficient than the ion engine used on the SMART-1 lunar mission. In fact, says Roger Walker of ESA’s Advanced Concepts Team, who is technical manager of the project, “Using a similar amount of propellant as SMART-1, with the right power supply, a future spacecraft using our new engine design wouldn’t just reach the Moon, it would be able to leave the Solar System entirely.”
Walker calls the design “an ultra-ion engine,” and ESA talks about using flight models to push into the Kuiper Belt and beyond, or deploying clusters of higher-power versions of the engine on manned Mars missions. All of which is exciting stuff, though it comes with a needed caveat. Currently at the stage of laboratory experiment, the DS4G thruster must now evolve into a flight-ready device, and that means not only intensive design work but thousands of hours of ground testing. Ion engines operate continuously, and must burn in a vacuum at low thrust for long periods to be viable for deep space missions.
The key difference between DS4G and earlier ion designs is that it seems to resolve an issue that limited engine lifetime. Normally, a voltage difference between two perforated grids creates an electric field that extracts and accelerates ions. But the accelerating ions can damage the second grid when voltage differences are high. DS4G uses a two-stage process to extract and accelerate the ions that has produced impressive results without resultant damage to the grid. Here’s what an ESA news release has to say about the early findings:
The test model achieved voltage differences as high as 30kV and produced an ion exhaust plume that travelled at 210,000 m/s, over four times faster than state-of-the-art ion engine designs achieve. This makes it four times more fuel efficient, and also enables an engine design which is many times more compact than present thrusters, allowing the design to be scaled up in size to operate at high power and thrust. Due to the very high acceleration, the ion exhaust plume was very narrow, diverging by only 3 degrees, which is five times narrower than present systems. This reduces the fuel needed to correct the orientation of spacecraft from small uncertainties in the thrust direction.
Centauri Dreams note: Their high specific impulse makes engines like these of great interest for long duration missions (and be sure to compare this work with ESA’s Helicon Double Layer Thruster (HDLT) drive, described here in an earlier post). But electric propulsion systems are not new. In fact, geostationary satellites have used them for orbital maintenance since the early 1980s, and the methods have also been applied to low Earth orbit satellites. Their use as a primary means of propulsion wasn’t demonstrated until NASA’s Deep Space One mission, which used a xenon-based ion engine; ESA’s SMART-1 has been an outstanding proof of concept in its own right.
And then there’s the Japanese Hayabusa, which put four ion engines to work in 20,000 hours of cumulative operation. We’re learning huge amounts about making ion engines better, but even greater changes are clearly in the offing. Electrical power to operate these devices normally comes from solar panels. If we ponder deep space missions, nuclear electric options will have to come into play, and so will a host of new engine designs.