Charge to the stars

Australian researchers claim to have developed a super-efficient ion drive which could make space exploration cheaper and reduce the cost of running the International Space Station.

Australian researchers claim to have developed a super-efficient ion drive which could make space exploration cheaper and reduce the cost of running the International Space Station as well as maintaining satellites in position.

According to its Canberra-based developers, the Helicon Double Layer Thruster (HDLT) system will make spacecraft travel faster and be five times more efficient than conventional chemical rockets used by space agencies such as NASA.

It could also be configured to burn hydrogen created from the reclamation of fluids such as human waste produced while in space, making fuel cheaper and missions more viable.

The researchers say HDLT systems could be installed on the International Space Station to use waste hydrogen as a propellant when periodically correcting the station’s orbit. Plasma thrusters could also be used to transfer satellites from their drop-off altitudes to operational orbit, and to maintain proper station position.

With the HDLT system, krypton or xenon gas is injected into a ceramic tube called the source, which is attached to a vacuum system and a gas flow control. The source is open at one end and closed at the other. The gas is energised using an RF antenna, turning it into a high-density plasma.

Solenoids are used to shape the plasma to produce a magnetic nozzle in the plasma source. Formed in this way the high-density plasma is prevented from exiting the source by a non-linear plasma effect known as an electric double layer that is located near the exit of the source tube.

It is this electric field that accelerates ions and electrons from the plasma source to very high velocities, creating thrust.

Because the double layer is the result of the relationship between plasma density and the magnetic field, the accelerating grids used in conventional ion drives are not necessary.

Due to the equal flow of ions and electrons from the plasma the system’s exhaust is neutral, so there is no risk of charging up the spacecraft, causing an electrical systems failure, or coating solar panels and impairing their function, unlikealternative designs.

As it is based on magnetic fields rather than the grids or moving parts used by some ion thrusters, there is less likelihood of mechanical failure, said project leader Dr Christine Charles of the Plasma Research Laboratory at the Australian National University’s Research School of Physical Sciences and Engineering.

‘The HDLT is the new generation of magnetic nozzle thrusters. It needs no electrodes to generate the energetic ion beam, as is the case with traditional ion thrusters.’

The developers calculate that a 50 litre xenon tank would last about 180,000 hours, or 20 years of continuous operation. Keeping a satellite at a set orbit would need two five-hour bursts of power a day, giving the tank a 40-year life.

Compared to current propulsion systems, this efficiency would save over 400kg inpropellant payload for each launch – reducing the cost of the launch considerably.

‘This type of thruster is being actively developed by a number of groups worldwide,’ said Charles. ‘Our contribution was to discover that under certain operational parameters, an electric double layer develops in the throat of the magnetic nozzle.

This sudden change in potential accelerates the ions to much higher mach numbers than the simple plasma expansion described by rival researchers. We are working on increasing the thrust and specific impulse.’

Using the spare hydrogen produced by reclaiming fluids such as human waste to power the system would create less thrust than an alternative such as xenon. But it would allow spacecraft to travel further on less fuel, at lower speeds. ‘Hydrogen will increase the specific impulse but, of course, reduce the thrust,’ explained Charles. ‘But it is free, and as there seems to be water on Mars this could be used by humans while producing hydrogen.’