Colloid Thruster Fundamentals
A colloid thruster uses electrostatic acceleration of charged species for propulsion. The species can be either charged droplets, solvated ions or a combination thereof. The colloid thruster uses a technique known as electrospray atomisation, whereby an electrolytic fluid (with conductivities ~1 S/m) housed in a fluid reservoir is injected, from an emitter, into an electric field (~108V/m) generated by an electrical potential difference between the emitter and an extraction grid. The fluid, under the influence of the electric field, forms into a structure known as a Taylor cone which then breaks up to form a charged spray (with charge to mass ratios ~1-100kC/kg). The charged spray is then accelerated in a static electric field generated by an electric potential difference between the extraction grid and an acceleration grid. This charged spray is then emitted from the spacecraft to produce the required thrust. A neutraliser is also required to neutralise the spacecraft due to the emitted charged spray. A typical colloid thruster configuration is shown below.
Basic architecture of the electrostatic colloid thruster system
Applications and Capabilities
A colloid thruster system can be effectively used for the following tasks:
Satellite Attitude Control Applications:
- Bang-bang system
- Wheel unloading
- Low perturbation compensation
Satellite Orbit Control and Transfer Applications:
- Drag make-up
- Formation Flying
- Orbital Debris
- Low-to-Medium Earth Orbit Transfer
- Geostationary Transfer Orbit to Moon Transfer
MicroThrust Baseline Technology
The concept for MicroThrust is a Colloid Thruster using voltage-driven fluid handling, with arrays of individually addressed MEMS capillary emitters with integrated extraction electrodes. This approach allows for a simple architecture, since no pumps are required: all fluid handling is done by capillary and electrostatic forces. Hence, the complexity of the complete propulsion system is radically reduced, leading to lighter, more compact, and reliable spacecraft development. The baseline propellant is the ionic liquid EMI-BF4, though other propellants are under investigation within the research programme.
The major technology issues for a miniaturized thruster technology
The MicroThrust team will transform the propulsion concept into a reality by addressing each of the outstanding technology issues: electrospray control; beam neutralization; materials compatibility; microfabrication; MEMS integration, fluidics and packaging; highly compact and efficient DC/DC conversion. The team has already shown that that a colloid propulsion system has the potential for a high level of integration, however this will only be achieved fully if we change significantly the subsystem approach. Thus the integration between propellant storage, propellant distribution, propellant flow rate control and propellant utilisation are inherently embedded in our design methodology. This approach is of course radically different not only for “conventional” propulsion technologies, including advanced electric propulsion units, but our approach is also at variance from the other groups engaged in the development of colloid thruster systems. Whilst developing the necessary understanding of the propellant flow process, closely associated with fundamental physical processes including wetting, electrowetting and underlying electrospray characteristics, we also need to confirm advanced approaches in micro-manufacturing, micromachined component assembly and packaging technologies in the context of the demands of high voltage isolation, and high voltage switching and control to deliver the aims and objectives of this consortium.
Progress Beyond the State of the Art
MicroThrust breaks new ground with each component of its thruster system, and with the overall system architecture. Because of the dramatic mass and volume reduction planned, all components of the thruster, as well as the overall concept, are major steps forward from what is flown today.
MicroThrust will deliver advances beyond the state of the art in the fields of:
- Micromachined ion sources
- Micromachined fluid handling
- Integrated high voltage power supplies
- Electron sources (neutralizers)
- Colloid thruster physics
- System engineering of colloid thruster
- Mission analysis for low-thrust trajectories
MX5050 Wafer Bonding
Test assembly with 19-emitter array
Test vacuum chamber