Space technology equates to rocket science in the public imagination, and up until fairly recently, the propulsion of spacecraft has relied exclusively on chemical rockets. But new propulsion technologies are now emerging, albeit restricted to in-orbit manoeuvres of Earth satellites, and the slow but steady acceleration of unmanned interplanetary probes.
For example, the European Space Agency’s Smart-1 moon mission was powered by a propulsion system that ejected a stream of xenon ions from the spacecraft. Other technologies in development include solar sails that make use of radiation pressure on huge mirrors which unfurl once the spacecraft has been placed in orbit.
Astronautical engineers in the University of Michigan’s Plasmadynamics and Electric Propulsion Laboratory are now looking to use nanoparticles as a propellant with micro- and nano-electromechanical systems. Termed nanoparticle field extraction thrusters, or nanoFETs, the devices take conductive nanoparticles, and transport them to a liquid-filled reservoir by means of a micro-fluidic flow transport system similar to the labs-on-a-chip designed for medical diagnostics and environmental monitoring.
Nanoparticles come into contact with the bottom conducting plate of the reservoir, become charged, and are pulled to the surface of the liquid by an electric field. At a critical field strength, the electrostatic force near the liquid surface causes the particles to break through the surface tension. They are then focussed and accelerated by the vacuum electric field, and ejected out of a tube around a micrometre in diameter.
Michigan University’s Brian Gilchrist claims that a system based on nanoFETs could deliver up to 10 times as much thrust as an ion engine of similar size. Another advantage of nanoFETS over ion engines is that the latter use a grid of electrodes that are steadily eroded by the battering they receive from the streaming ions. With nanoFETs, the electrodes are built into the walls of the thruster channels, and are therefore not exposed to such continual bombardment.
The main engineering challenge for designers of nanoFETs is figuring out how to get the particles to each of the many tiny thrusters, For this reason, the concept has faced criticism on grounds of complexity, and the potential for problems such as the clogging of channels and valves in the plumbing system.
Gilchrist is undeterred by such objections, and claims that complexity could be reduced at a spacecraft system level. “The nanoFET concept fully takes advantage of semiconductor processing technology and current initiatives in the nanosciences and microfluidics,” says Gilchrist. “Functionally, we will have low-pressure storage, microfluidic transport, a combined extraction and acceleration zone, and high-voltage power supply/control electronics. From a complexity standpoint, we should compare favourably with other approaches.”
Gilchrist refers also to a highly scaleable design that can cover a broad range of powers and specific impulses, simplifies spacecraft design and widens possible mission scenarios.
If the technology works as intended, efficiencies of around 90% could be achieved; almost twice that of existing electric propulsion systems. Thruster performance will be determined by a number of factors including nanoparticle type and size, and the researchers are currently looking at which particles are best suited to particular mission stages and scenarios. It might even be possible to synthesise nanoparticles on board the spacecraft.
Mitat Birkan, who is responsible for the development of space electric propulsion within the US Air Force, comments: “By changing the size of nanoparticles, a range of thrust/specific impulse can be obtained, and the technology may therefore be useful for several space applications. The concept needs to be further investigated in order to understand and predict the thruster operation, but it is a high pay-off concept because the propellant is nanoparticles, which can be easily stored.”
The research is funded by the NASA Institute for Advanced Concepts.
Further reading: Nanoparticle Electric Propulsion for Space Exploration, Liu et al., Space Technology and Applications International Forum 880, 787 (2007) (subscription required)
Article first published in Nanomaterials News.