July 09, 2009

NanoFET: Nano-particle field extraction thruster funded by the Air Force

Funded by the Air Force, Brian Gilchrist and his colleagues are developing a new type of thruster that uses nanoparticles as propellant. Much of the engine is etched directly onto a wafer-thin piece of silicon via micro-electromechanical systems technologies, known as MEMS, that are more commonly used in the semiconductor industry. Measuring no thicker than a half-inch (1 centimeter, including the fuel) and with tens of thousands of accelerators able to fit on an area smaller than a postage stamp, these “stick-on” thrusters could power tiny spacecraft over vast distances.

Previous claims for nanoFETs [2007 paper]
- nanofets could deliver up to 10 times as much thrust as an ion engine
- nanofet systems can span an Isp range of 100 to 10000 s at greater than 90% thrust efficiency with three types of carbon nanotube particles
- advantages offered by nanoFET’s potential for high efficiencies, lower thruster specific mass, and longer operational lifetimes are both mission enhancing and enabling.
-Having EP systems with long operational lifetimes is important for future missions that require continuous propulsion capability for tens to hundreds of kilo-hours. The nanoFET concept’s operational lifetime is not driven by the primary life-limiting factors of state-of-the-art EP systems. Since the nanoparticles are charged electrostatically rather than ionized as in ion or Hall thrusters, greater reliability and efficiency can be achieved. Without the need to ionize propellant, nanoFET does not experience charge exchange (CEX) collisions between high energy charged and slow moving neutral particles.

The technology is called a “nano-particle field extraction thruster,” or nanoFET. The tiny thrusters that work much like miniaturized versions of massive particle accelerators. The device uses a series of stacked, micron-thick “gates” that alternate between conductive and insulating layers to create electric fields. These small but powerful electric fields charge and accelerate a reservoir of conductive nanoparticles, shooting them out into space and creating thrust.

“In that a particle accelerator uses an electrical field to propel charged particles to high speeds — that’s exactly what we’re doing,” Gilchrist said.

A 19 page pdf from 2008 AIAA paper "Nanoparticle Field Extraction Thruster (nanoFET): Introduction to, Analysis of, and Experimental Results from the “No-liquid” Configuration"

This paper introduces a nanoparticle field extraction thruster (nanoFET)
concept that does not depend on the liquid delivery of micro and nano-particles for
extraction and acceleration. The no-liquid approach potentially provides important
advantages such as allowing the use of smaller particles for propellant, which may
offer a greater specific impulse. The most likely developmental obstacles are the
adhesion of the particles to the source electrode and the cohesion between the
particles. Adhesion and cohesion models are presented along with proposed
methods of overcoming each.

A method of using the applied charging electric field to overcome the adhesion
force is investigated, which predicts that it may be possible to remove particles with diameters down to hundreds or even tens of nanometers from a planar electrode
with only the application of a high strength electric field. To investigate this particle removal model, eight test cases, involving 4 particle sizes and 2 electrode materials, are presented.

A method of transporting the dry particle propellant through an ultra-fine sieve
prior to the charging and accelerating stages is investigated as a method of
overcoming the cohesion between the particles. A simple proof-of-concept
experiment is presented which indicates that this method is capable of breaking the
cohesion force under appropriate conditions, which helps to guide future research.

Brian Gilcrist is also working with the Electrodynamic Applications company on nanofet

Developmental Progress of the Nanoparticle Field Extraction Thruster, July 2008 [16 page pdf]

Recent experiments in microgravity and on the ground have yielded promising results for NanoFET’s development. For the liquid-NanoFET configuration, the NanoBLUE microgravity flight results suggest that the electric field threshold for liquid surface instability is increased for smaller channels. Higher particle charging electric fields may thus be possible for channels at the MEMS scale, resulting in a larger range of specific charges and propulsion performance. While slot orifice geometries may be easier to microfabricate than large numbers of circular orifices, the trade-off must be evaluated between manufacturing ease and the reduction in the maximum allowable charging electric field relative to an array of circular orifices.

For the dry-NanoFET configuration, preliminary ground test results have demonstrated the ability to reduce particle liftoff electric fields with the use of inertial accelerations provided to the charging electrode. This phenomenon provides the dry-NanoFET design with added flexibility to tune its performance. Further studies are needed to better understand the particle adhesion and cohesion forces in the NanoFET system and their impacts on NanoFET’s design and operations.

An assumed PPU efficiency of 0.95 for the nanoFET system results in internal efficiencies over 85% for an Isp range of 100 to 10,000 seconds using three different types of carbon nanotubes. 800-V to 10-kV accelerating potentials are used for carbon nanotubes of (1) 5-nm diameter and 100-nm length, (2) 1-nm diameter and 100-nm length, and (3) 1-nm diameter and 3.5-mm length. Emitter inefficiencies are principally due to viscous drag and charge loss to the liquid, and efficiency losses associated with particle impingement on the gate structures and beam divergence are expected to be no worse than those of existing EP systems. From Equation (4), such high internal efficiencies associated with nanoFET translate to thrust-to-power ratios, particularly at low Isp, that are greater than state-of-the-art EP thrusters.

For applications that do not demand the entire 100 to 10,000 seconds Isp range, a wide Isp range can still be achieved with a single nanoparticle type, which simplifies the overall system integration. For example, a dielectric liquid configuration can potentially use carbon nanotubes with 1-nm diameter and 400-nm length and acceleration potentials ranging from 400 V to 10 kV to span an Isp range of 800 to 4,000 seconds at over 85% internal efficiency.

No other state-of-the-art ion or hall thrusters in Figure 2 are designed to span such a large Isp range at high efficiencies and high thrust-to-power ratios. For low-Isp, high thrust-to-power maneuvers, nanoFET would outperform arcjets by achieving greater thrust for the same power. At high Isp, nanoFET’s projected performance is comparable to field emission electric propulsion (FEEP) thrusters operating in ion mode. However, in the low-Isp regime, FEEP thrusters must operate in colloid mode, resulting in a dispersion in the specific charge distribution and less thrust controllability compared to nanoparticles with precise charge states.

The significance of such a wide Isp range at high efficiencies for nanoFET is that it provides mission designers with tremendous flexibility. Consider a robotic probe or a freighter vehicle to a planetary body. During interplanetary cruise, nanoFET would operate in a high-Isp mode to minimize the propellant cost. Once within the planetary body’s gravity well, nanoFET could switch to a low-Isp, high thrust-to-power mode to provide greater thrust capability. This flexibility also provides a wider margin for both robotic and crewed missions to accommodate offnominal and abort scenarios, adjust the flight time, and perform dynamic retasking to take advantage of in-flight opportunities. To achieve comparable capabilities with other EP systems across the entire Isp range, multiple engine types would have to be used, which tends to increase the mass of the propulsion system while complicating spacecraft integration and design

A patent from 2005 by Brian Gilcrist and others for Scalable flat-panel nano-particle MEMS/NEMS thruster.

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