|
NanoFET micropropulsion
Introduction
Most of today's rocket engines rely on chemical propulsion.
All current spacecraft use some form of chemical rocket for launch and most
use them for attitude control as well (the control of the angular position
and rotation of the spacecraft, either relative to the object that it is
orbiting, or relative to the celestial sphere). Real rocket scientists
though are actively researching new forms of space propulsion systems. One
heavily researched area is electric propulsion (EP) that includes field
emission electric propulsion (FEEP), colloid thrusters and other versions
of field emission thrusters (FETs). EP systems significantly reduce the
required propellant mass compared to conventional chemical rockets,
allowing to increase the payload capacity or decrease the launch mass. EP
has been successfully demonstrated as primary propulsion systems for NASA’s
Deep Space 1, Japan’s HAYABUSA, and ESA’s SMART-1 missions. A new EP
concept proposes to utilize electrostatically charged and accelerated
nanoparticles as propellant. Millions of micron-sized nanoparticle
thrusters would fit on one square centimeter, allowing the fabrication of
highly scaleable thruster arrays.
Field emission thrusters are not suitable for launching a
spacecraft into orbit. Their intended purpose is to provide attitude
control and acceleration. Orbiting spacecraft are subjected to a variety of
forces while circling an object in space: drag, gravity, solar pressure and
magnetic streams all must be compensated for in order to maintain the
desired orbit. The very low and highly controllable thrust levels provided
by various electric propulsion technologies enable a new category of
missions which otherwise would not be possible.
A new electrostatic thruster technology is under
development at the University of Michigan's Plasmadynamics
and Electric Propulsion Laboratory, using nanoparticles as propellant
with micro- and nano-electromechanical systems (MEMS/NEMS). Termed the
nanoparticle field extraction thruster – nanoFET – this highly integrated
propulsion concept is a high efficiency, variable specific impulse engine
type that can be readily scalable for a large range of future space science
and exploration missions ("Nanoparticle Electric Propulsion for Space Exploration";
pdf download, 460 KB).
The nanoFET utilizes highly scalable MEMS/NEMS
structures to feed, extract and accelerate nanoparticles through
micron-sized thrusters. The nanoparticles to be used as propellant can be
of various geometries and materials.
nanoFET characteristic size scales (Image: University of
Michigan Department of Aerospace Engineering)
Here is how it works: Conductive nanoparticles would be
transported to a small liquid-filled reservoir by a micro-fluidic flow
transport system. Particles that come into contact with the bottom
conducting plate would become charged and pulled to the liquid surface by
the imposed electric field. If the electrostatic force near the surface can
cause charged nanoparticles to break through the surface tension, field
focusing would quickly accelerate the particles through the surface. Once
extracted, the charged nanoparticles would be accelerated by the vacuum
electric field and ejected, thus generating thrust.
One intriguing aspect of nanoFET is that it uses MEMS/NEMS
technology to enable a "flat-panel" thruster design that
incorporates power processing as well as nanoparticle manufacture, storage,
feed, extraction, and acceleration. This results in a modular and
geometrically scaleable propulsion system, from watts to megawatts,
allowing the decoupling of thruster design from spacecraft design.
Another advantage of this system is that it affords a much
broader set of missions with a single engine type – nanoFETs have an
unprecedented thrust-to-power ratio for electric propulsion systems; they
can adjust specific impulse over a large range from 100s to 10,000s; they
show a high efficiency range of over 90% over the entire specific impulse
range; they do not have the life-limiting factors common in ion thrusters.
The system is also very flexible with regard to the size
and type of particles that can be used. Almost any conductive nanoparticle,
such as carbon nanotubes, fullerenes, as well as metal nanospheres and
nanowires could be used. Currently, the researchers are experimenting with
silver, nickel and copper nanoparticles ranging in size from 5 nm to 70 nm.
So far, the experimental results have validated the
theoretical models and represent a significant step towards proving the
fundamental feasibility of nanoFET
By Michael Berger, Copyright 2007 Nanowerk LLC
|
