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We use time-of-flight and energy analysis techniques to measure in a vacuum the charge, specific charge and stopping potential of primary and satellite droplets generated by electrosprays of tributyl phosphate solutions. This information, of interest in itself, is subsequently analysed to obtain the following relevant parameters of the jet emanating from the Taylor cone: the velocity of the fluid at the breakup point, the voltage difference between the liquid cone and jet breakup location, and the most probable wavelength for varicose breakup. A large fraction of the electrospray needle voltage is used to accelerate the jet. Indeed, for the solutions of lowest electrical conductivities studied here, the voltage difference between electrospray needle and jet breakup location becomes approximately 90% of the needle voltage. In addition, the pressure of the jet fluid at the breakup point is negligible compared to its specific kinetic energy. The specific charge distribution function of the main droplets produced in the varicose breakup is remarkably narrow. Hence, the limiting and commonly accepted case of varicose breakup at constant electric potential is not consistent with this experimental observation. On the other hand, a scenario in which the electric charge is bound to the jet surface seems to be a good approximation to simulate the effect of charge on capillary breakup. It is also found that the effect of viscosity on the formation of droplets is paramount in electrosprays of moderate and high electrical conductivity. We expect that these measurements will guide the analytical modelling of cone-jets.

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The objective o f the NASA JPL Space Technology 7 -Disturbance Reduction System (ST7-DRS) mission is to demonstrate: (I) test mass trajectory control that deviates from purely gravitational trajectory by less than 3~1 0 . '~ ~n / s e c~/ H z~~~ [l+(V3 mHz)*] over a frequency range from 1 mHz to 30 mHz and (2) spacecraft position control within l O n m / H~~.~ over the same frequency range. To achieve these objectives, two technologies must be advanced and tightly integrated. These technologies are the Gravitational Reference Sensors (GRS) being developed by Stanford University and the Colloid Micro-Thruster (CMT) propulsion system in development at Busek Co. Inc. The DRS system will he integrated by JPL and fly on an ESA spacecraft called the LISA Pathfmder. This paper reports on the progress in the propulsion technology area.The 1O:l dynamic thrust range, the continuous thrust adjustability and noise requirements represent unique challenges that require significant advances in several technology areas, including extremely precise propellant flow control (

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