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