Spacecraft propulsion

Summary

Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric entry. Several methods of pragmatic spacecraft propulsion have been developed, each having its own drawbacks and advantages. Most satellites have simple reliable chemical thrusters (often monopropellant rockets) or resistojet rockets for orbital station-keeping and some use momentum wheels for attitude control. Russian and antecedant Soviet bloc satellites have used electric propulsion for decades, and newer Western geo-orbiting spacecraft are starting to use them for north–south station-keeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used ion thrusters and Hall-effect thrusters (two different types of electric propulsion) to great success. Hypothetical in-space propulsion technolo

Official source

https://en.wikipedia.org/wiki/Spacecraft_propulsion

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Swimming by reciprocal motion at low Reynolds number

Peer Fischer, Tian Qiu

Biological microorganisms swim with flagella and cilia that execute nonreciprocal motions for low Reynolds number (Re) propulsion in viscous fluids. This symmetry requirement is a consequence of Purcell's scallop theorem, which complicates the actuation scheme needed by microswimmers. However, most biomedically important fluids are non-Newtonian where the scallop theorem no longer holds. It should therefore be possible to realize a microswimmer that moves with reciprocal periodic body-shape changes in non-Newtonian fluids. Here we report a symmetric 'micro-scallop', a single-hinge microswimmer that can propel in shear thickening and shear thinning (non-Newtonian) fluids by reciprocal motion at low Re. Excellent agreement between our measurements and both numerical and analytical theoretical predictions indicates that the net propulsion is caused by modulation of the fluid viscosity upon varying the shear rate. This reciprocal swimming mechanism opens new possibilities in designing biomedical microdevices that can propel by a simple actuation scheme in non-Newtonian biological fluids.

Nature Publishing Group2014

The coordination of body and limbs motions contributes to effective propulsion in limed locomotion. However, there is lack of understanding that characterizes the mechanism underlying the body-limb coordination. The aim of this study involves clarifying the mechanism through mathematical modeling and dynamic simulation. To this end, a body-limb coordination model was designed based on a simple rule by effectively using a pivot turn. The model successfully generated locomotion similar to primitive tetrapod walking, e.g. salamander and Polypterus walking based on their physical properties.

IEEE2017

Dynamic Assessment of Source–Load Interactions in Marine MVDC Distribution

Drazen Dujic, Francisco Daniel Freijedo Fernández, Uzair Javaid, Wim Van der Merwe

Medium-voltage direct-current (MVDC) distribution is a possible replacement due to the advancements in power electronic technologies, for existing medium-voltage ac distribution on ships. The new systems based on MVDC are expected to increase fuel efficiency, remove bulky low frequency transformers used for voltage coordination, and integrate storage technologies. These expected benefits of MVDC come with challenges such as stability and reliability of the new distribution system. In this paper, the effect of three different source-side converters, based on commercially available technology, on the MVDC distribution grid and their interactions with the constant power loads (propulsion drives) are investigated. Additionally, the effect of variations in the filtering effort and the distribution lengths on the system stability is analyzed using the impedance-based stability assessment.

Institute of Electrical and Electronics Engineers2017