lecture23_25 - 16.522, Space Propulsion Prof. Manuel...

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16.522, Space Propulsion Prof. Manuel Martinez-Sanchez Lecture 2 3 -25: COLLOIDAL ENGINES APPENDIX A1. INTRODUCTION . Colloidal thrusters are electrostatic accelerators of charged liquid droplets. They were first proposed and then intensively studied from around 1960 to 1975 as an alternative to normal ion engines. Their appeal at that time rested with the large “molecular mass” of the droplets, which was known to increase the thrust density of an ion engine. This is because the accelerating voltage is V = mc 2 2 q , where m is the mass of the ion or droplet, and q its charge, and c is the final speed. If c is pre-defined (by the mission), then V can be increased as m/q increases; this, in turn, increases the space charge limited current density (as V 3/2 ), and leads to a thrust density, F A = ε o 2 4 3 V d 2 , (d=grid spacing), which is larger in proportion to V 2 , and therefore to m / q ( ) 2 . In addition to the higher thrust density, the higher voltage also increases efficiency, since any cost-of-ion voltage V LOSS becomes then less significant η = V V + V LOSS . In a sense, this succeeded too well. Values of droplet m/q that could be generated with the technology of the 60’s were so large that they led to voltages from 10 to 100 KV (for typical Isp 1000 s.). This created very difficult insulation and packaging problems, making the device unattractive, despite its demonstrated good performance. In addition, the droplet generators were usually composed of arrays of a large number of individual liquid-dispensing capillaries, each providing a thrust of the order of 1 µ N. For the missions then anticipated, this required fairly massive arrays, further discouraging implementation. After lying dormant for over 20 years, there is now a resurgence of interest in colloid engine technology. This is motivated by: (a) The new emphasis on miniaturization of spacecraft. The very small thrust per emitter now becomes a positive feature, allowing designs with both, fine controllability and high performance. (b) The advances made by electrospray science in the intervening years. These have been motivated by other applications of charged colloids, especially in recent years, for the extraction of charged biological macromolecules from liquid samples, for very detailed mass spectroscopy. These advances now offer the potential for overcoming previous limitations on the specific charge q/m of droplets, and therefore may allow operation at more comfortable voltages (1-5KV). With regard to point (a), one essential advantage of colloid engines for very small thrust levels is the fact that no gas phase ionization is involved. Attempts to miniaturize other 16.522, Space Propulsion Lecture 23-25 Prof. Manuel Martinez-Sanchez Page 1 of 36
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thrusters (ion engines, Hall thrusters, arcjets) lead to the need to reduce the ionization mean free path 1 σ ion n e by increasing n e , and therefore the heat flux and energetic ion flux to walls. This leads inevitably to life reductions. In the colloidal case, as we will
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This note was uploaded on 11/07/2011 for the course AERO 16.512 taught by Professor Manuelmartinez-sanchez during the Fall '05 term at MIT.

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lecture23_25 - 16.522, Space Propulsion Prof. Manuel...

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