A propellant-free superconducting solenoid thruster driven by geomagnetic field.

INTRODUCTION: Space travel nowadays relies on physical ejection of propellants, which is challenged by reachable distance of a vehicle in desirable time. In contrast, electromagnetic propulsion was proposed to be a potential solution without need of carrying bulky mass of propellants, by using force interaction of local magnetic dipoles with the external natural magnetic field. Further development of this technique, however, has been daunted by extremely small magnetic induction that can be obtained. OBJECTIVES: To generate a significant thrust by a system with a reasonable scale, we propose an alternative concept of design, based on the variation of local magnetic dipole moments that has not been considered. METHODS: A magnetic dipole is created by wrapping a solenoid around an iron core. It is varied spatially by changing the cross-sectional area of the solenoid, hence giving a gradient of magnetic dipole moment. The interaction force is measured by an in-house force sensor based on a cantilever, which has a high sensitivity of one micro-Newton. In addition, numerical simulation is used to calculate the magnetic field and created force via the Maxwell stress tensor. RESULTS: As shown by experimental measurements and numerical simulations, a substantially larger magnitude of force is obtained on the solenoid with varying cross-sectional area, indicating a much stronger interaction with the geomagnetic field. Furthermore, to enhance electric current with negligible dissipation, a superconducting solenoid can be adopted at low temperature in space. With readily attainable conditions of operation, we demonstrate generation of a thrust comparable to that of present electric propulsion thrusters which are deemed as the most promising techniques for long-term space travel. CONCLUSIONS: By incorporating supplementary means, we provide a breakthrough solution for constructing an efficient thruster with minimal energy consumption and nearly null propellant load for near-Earth transportation and deep-space exploration.

Pinching Dynamics and Satellite Droplet Formation in Symmetrical Droplet Collisions.

In head-on collisions between two droplets, reflexive separation is frequently formed, showing tentative coalescence followed by disintegration into two primary drops. With higher impact inertia relative to surface tension, characterized by a Weber number (We), more satellite droplets are created between the primary drops. In the symmetric configuration, the existing phenomenological models indicate the absence of satellite droplets at the onset We when the coalesced drops start to break. Supported by experimental and simulation evidence, here we demonstrate the exclusive formation of at least one droplet after pinch of the thread connecting the colliding drops. In accordance with the universal features of a thinning liquid filament approaching singularity as predicted by scaling theories of pinch-off, the mechanism of satellite droplet formation in the symmetrical impact of droplets is clarified. Via slight breaking of the symmetry, no satellite droplet can be observed, thus providing a possible interpretation for the discrepancy in the literature and implications for controlling undesirable drop formation.

Droplet impact upon a wet surface with varied fluid and surface properties.

The transition boundaries of various regimes characterizing the impact outcomes of a droplet upon a liquid layer of small thickness were investigated experimentally. With careful control of film thickness, the onset of these regimes in terms of a Weber number (We), which expresses the ratio between the droplet inertia and surface force, specifically when the layer depth is smaller than the droplet diameter, has been further clarified, as compared to prior studies. Several turning, non-monotonic trends between We and the film thickness normalized by the droplet diameter, H, were thus identified as H≲1. Furthermore, by adding various percentages of glycerine, the effects of liquid viscosity were revealed, which inhibited disintegration into secondary droplets. We also added surfactant to change the surface tension, leading to further complication of the collision outcome that would be related to the interaction between the crater and the bottom surface. The material effect of the solid surface was hence studied for further demonstration of such interplays. The results showed that increasing viscosity would essentially delay the occurrence of these transitions whereas reducing surface tension might encourage the onset. Therefore a possibility of using additives to manipulate the collision outcomes, while not changing much the constituent fluid properties, is presented.

Binary droplet collision at high Weber number.

By using the techniques developed for generating high-speed droplets, we have systematically investigated binary droplet collision when the Weber number (We) was increased from the range usually tested in previous studies on the order of 10 to a much larger value of about 5100 for water (a droplet at 23 m/s with a diameter of 0.7 mm). Various liquids were also used to explore the effects of viscosity and surface tension. Specifically, beyond the well-known regimes at moderate We's, which exhibited coalescence, separation, and separation followed by satellite droplets, we found different behaviors showing a fingering lamella, separation after fingering, breakup of outer fingers, and prompt splattering into multiple secondary droplets as We was increased. The critical Weber numbers that mark the boundaries between these impact regimes are identified. The specific impact behaviors, such as fingering and prompt splattering or splashing, share essential similarity with those also observed in droplet-surface impacts, whereas substantial variations in the transition boundaries may result from the disparity of the boundary conditions at impacts. To compare the outcomes of both types of collisions, a simple model based on energy conservation was carried out to predict the maximum diameter of an expanding liquid disk for a binary droplet collision. The results oppose the dominance of viscous drag, as proposed by previous studies, as the main deceleration force to effect a Rayleigh-Taylor instability and ensuing periphery fingers, which may further lead to the formations of satellite droplets.