Direct simulation of meteoroids and space debris flux on LDEF spacecraft surfaces

The meteoroid flux on all faces of the long duration exposure facility (LDEF) is predicted by a direct simulation Monte Carlo (DSMC) model, which for the first time provides a self-consistent method to model the collision behaviour between both meteoroids and debris with oriented spacecraft surfaces. This new model includes the modified Divine's meteoroid population, and Taylor's velocity distribution, to include the effects of planetary shielding and gravitational enhancement by the Earth. Results obtained when only meteroid impact is considered show good agreement with observed data and provide some correlation with previous models. When the space debris population is also included, the total particle flux on different faces of LDEF fits well with the observed measurements. Information concerning Earth shielding, gravity capturing and atmospheric effects can be obtained by comparing the ratio of the number of meteoroids moving towards the Earth to the total number of the meteoroids, obtained from the DSMC model with measured data. Approximately 25% of the meteoroids flux is predicted as not returning into the interplanetary space due to these effects

Performance modulation of colloid thrusters by the variation of flow rate with applied voltage.

The effect of voltage on flow rate within cone jet mode electrospraying has been investigated, with particular emphasis on the effect of emitter geometry. Using a high fidelity flow meter a set of experiments investigated the effect of the emitter geometry on the flow rate relationship to voltage, in cone jet mode electrospray. It is demonstrated that there are various parameters that influence the flow rate sensitivity to voltage, including the inner and outer diameter of the emitter, and the emitter to extractor distance. By a simple derivation,the latter two parameters relationship is explained by the variation of theoretical electric pressure with voltage, as the geometry is varied. The theoretical and experimental agreement has important implications for variable throttling of thrust in colloid thrusters, and could lead to the optimization or selection of a particular thrust variation

Design and fabrication of the thruster heads for the MicroThrust MEMS electrospray propulsion system

Microfabricated electrospray thrusters are widely acknowledged as one of the most promising technologies for the propulsion of small spacecraft. Their relative simplicity, high efficiency (>70%), low footprint (M <500g, V <10cm3) and large potential specific impulse (>3000s) enable the creation of a miniature system capable of providing up to 5km/s∆ V to 3U CubeSats. We report here on our latest efforts in the development of such a thruster system, completed within the MicroThrust (www.microthrust.eu) project. While a companion paper will present early test results of the thrusters, this paper will focus on their design and fabrication. We use MEMS microfabrication to manufacture internally fed capillary emitters from silicon. This permits the high fluidic impedance required to get the necessary low flow rates associated with pure ionic mode operation, in addition to allowing the fabrication of large arrays of perfectly aligned, nearly identical emitters. We present for the first time the wafer-level integration of an acceleration stage, with individual electrodes operating on up to 127 emitters on a single chip. By adding the accelerator, we increase both the specific impulse and thrust generated by the emitters, while also increasing the thrust efficiency by electrostatic focusing the spray. We have fabricated chips with varying emitter density (213 and 125 emitters per cm2) and have successfully tested passively fed emitter arrays, obtaining up to 35μA of current at +875V for a 91 emitter arra

MicroThrust MEMS electrospray emitters - integrated microfabrication and test results

With the growth of interest in small satellites (<10kg), there is a particular need to provide a propulsion element for this class of spacecraft. Microfabricated electrospray thrusters offer a solution to this problem. By using ionic liquids as the propellant solely ions can be emitted, resulting in a large specific impulse[1] . The thrust from an individual emitter is though a fraction of a N. However by using well-established MEMS technologies thousands of capillary emitters can be manufactured within an area of a few cm2, increasing the thrust to the mN level. We report on results from the Microthrust FP7 Project1,where the aims are to manufacture and test a complete breadboard thruster system based upon microfabricated thruster chips, alongside the design of a flight system that could enable a CubeSat to leave earth orbit. Prior to this project we had developed a number of manufacturing processes for specific thruster elements[2,3] . We report here on a new generation of microfabricated emitters, and their relative performance. The emitters consist of 70μm high etched-Silicon capillaries with outer diameters tapering to less than 10μm. Previous designs included 5μm silica microspheres within the 18 to 24μm internal diameter of the emitter to increase the hydraulic impedance[4]. However the filling factor of these microspheres in individual emitters differed; therefore a new generation of emitters having more similar impedance and with 5 -10μm internal diameters and hole depths of 100μm have been manufactured. Previously the etched-Silicon extractor chip was aligned to the emitter chip using 200μm ruby spheres [2] . Due to assembly difficulties this has been replaced with a polymer- based wafer bonding interface, allowing for simplified assembly and a wafer -scale fabrication process. These emitters have been tested in both uni-polar and bi-polar mode, using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4). The tests herein have been achieved without an acceleration stage. The Time-of-Flight data shows a mixed ion-droplet regime, approaching a Purely Ionic Regime (PIR) at low flow rate