Busek Co. Inc.

Colloid thrusters emit charged droplets that are electrostatically accelerated to produce thrust. A colloid thruster is an Electrospray device that consists of a hollow needle-like emitter and one or more coaxial electrodes that are commonly termed extractor and accelerator. By applying sufficient voltage between the extractor and the emitter, the mildly conductive propellant deforms into a cone. A submicron-sized jet of propellant forms at the cone apex which subsequently breaks up into droplets/colloids. Under certain conditions a colloid can get so small that it consists of a single ion. The charged particles can be then further accelerated by applying voltage between the extractor and accelerator electrodes. The beam current per emitter (IB) is typically sub micro Ampere and is primarily dictated by the propellant flow rate. The thrust per emitter (T) is very small (of the order of a micro-Newton) and can be precisely regulated by controlling the current (flow rate) and/or the applied voltages (VB).

To a large extent the VB and IB are independent of each other and can be independently controlled. Hence the delivered thrust can be precisely controlled with extreme accuracy (< 0.1 μN) and temporal stability. This makes colloid thrusters ideal for interferometric missions where spacecraft position must be controlled to within a few nanometers.

Busek is also developing other, less precise colloid thrusters that can deliver thrust up to 1 mN with reduced cost and complexity.

The components of the colloid thruster system that produces 5 to 30 μN with 0.1 μN adjustability and low thrust noise are shown on the right. The system is intended for the JPL Space Technology 7/Disturbance Reduction System mission which is a precursor for a large scale interferometric mission called LISA.

Among the technology breakthroughs in this colloid system is the propellant flow controlling, piezo actuated microvalve. Its extreme precision in adjusting the flow rate to achieve set point colloid beam current, and hence thrust, is shown in the figure below.

LISA Mission concept, courtesy of NASA

ST7-DRS Colloid cluster with four thrusters

Integrated System

Piezo Actuated Micro Valve Performance