Aug 2, 2021 08:30 PM
(This post was last modified: Aug 3, 2021 07:49 PM by C C.)
Microwave-powered rocket propulsion gets a boost
https://www.nanowerk.com/news2/space/newsid=58559.php
RELEASE: Sending a rocket into space typically requires about 90% of the rocket’s initial weight to be fuel. This limitation could be overcome by wirelessly transmitting the needed power to the rocket through a beam of microwave radiation. A research team from Japan has investigated the viability of using such microwave-powered propulsion for real-world applications.
In a study published this month in the Journal of Spacecraft and Rockets, researchers led by the University of Tsukuba have demonstrated wireless power transmission via microwaves for a free-flying drone and determined the efficiency of this process.
Previous analyses of this kind were carried out decades ago and mostly considered microwaves of a low frequency (a few gigahertz; GHz). Given that the power transmission efficiency increases as the operating frequency is raised, the team behind this latest research used microwaves with a relatively high frequency (28 GHz). The team’s drone weighed roughly 0.4 kilograms and hovered for 30 seconds at a height of 0.8 meters above the source of the microwave beam.
“We used a sophisticated beam-tracking system to ensure that the drone received as much of the microwave power as possible,” says Kohei Shimamura, lead author of the study. “Moreover, to further increase the transmission efficiency, we carefully tuned the phase of the microwaves using an analog phase shifter that was synchronized with GPS units.”
The researchers measured the efficiencies of the power transfer through the beam (4%), the capture of microwaves by the drone (30%), the conversion of microwaves to electricity for propulsion (40%), and other relevant processes. Based on this information and an analytical formula, they calculated the overall power transmission efficiency in their experiment to be 0.43%. For comparison, in a previous study, the team measured the total transmission efficiency for a fixed-position (rather than free-flying) drone to be 60.1%.
“These results show that more work is needed to improve the transmission efficiency and thoroughly evaluate the feasibility of this propulsion approach for aircraft, spacecraft, and rockets,” explains Shimamura. “Future studies should also aim to refine the beam-tracking system and increase the transmission distance beyond that demonstrated in our experiment.”
Although microwave-powered rocket propulsion is still in its early stages, it could someday become a superior way to launch rockets into orbit given the high onboard-fuel demands of conventional propulsion techniques.
The tangled physics of knots, one of our simplest & oldest technologies
https://massivesci.com/articles/knot-str...shoelaces/
EXCERPT: . . . knots are quite ancient, predating both the use of the axe and of the wheel and potentially even the divergence of humans from other apes. After all, ropes and cords are practically useless without being tied to something else, making one of the most ancient technologies still remarkably relevant today. But these tie-offs can be a problem, since knots actually decrease the strength of a rope.
[...] So how can we tell, just with our eyes, whether one knot is stronger than another? Perhaps most intuitively, knots with more strand crossings tend to be stronger. Additional points of contact likely create more friction — more resistance to coming undone. A smaller knot with fewer crossings will also usually have tighter bends than a larger knot with more crossings. This increases the tension imbalance in the fibers of the rope with the smaller knot and making it more likely to break, a catastrophe for a climber dangling off the end of a safety line or a fisherman whose dinner escapes when the knot breaks.
The second characteristic of a strong knot is known as “twist fluctuations,” which accounts for whether a single rope strand undergoes twisting as it snakes through the knot. Twist fluctuations are essentially a proxy for the knot’s stability, since a strand that is forced to twist will “lock” on itself (like a square knot) while a strand that is free to roll won’t keep its shape (like a granny knot). The concept of twist fluctuations also explains why there is a scientifically correct way to tie your shoes: the knot is less likely to slip if you alternate which strand goes on top when forming the two parts of the knot, which is the concept behind a square knot. You can tell right away if your shoes are tied correctly just by looking. If the knot tends to lie vertically along the shoe, it's a weaker granny knot, whereas if the knot is horizontal, it's the stronger square knot.
The last easily countable knot attribute is “circulation,” or whether adjacent rope strands move in the same direction as the knot is pulled tighter. This can happen in multiple parts of a single knot, and the more rope strands moving in opposite directions, the stronger the knot will be. For example, in a standard granny knot, rope strands slide along each other in opposite directions as the knot is tightened, generating friction that increases the knot's strength. If instead you take a granny knot and pull on diagonal ends, the exact opposite happens. (This knot actually has a different name, the grief knot.) Adjacent strands now move in the same direction, and the knot tends to slip.
As the knots get bigger and more complicated, the same rules apply... (MORE - details)
https://www.nanowerk.com/news2/space/newsid=58559.php
RELEASE: Sending a rocket into space typically requires about 90% of the rocket’s initial weight to be fuel. This limitation could be overcome by wirelessly transmitting the needed power to the rocket through a beam of microwave radiation. A research team from Japan has investigated the viability of using such microwave-powered propulsion for real-world applications.
In a study published this month in the Journal of Spacecraft and Rockets, researchers led by the University of Tsukuba have demonstrated wireless power transmission via microwaves for a free-flying drone and determined the efficiency of this process.
Previous analyses of this kind were carried out decades ago and mostly considered microwaves of a low frequency (a few gigahertz; GHz). Given that the power transmission efficiency increases as the operating frequency is raised, the team behind this latest research used microwaves with a relatively high frequency (28 GHz). The team’s drone weighed roughly 0.4 kilograms and hovered for 30 seconds at a height of 0.8 meters above the source of the microwave beam.
“We used a sophisticated beam-tracking system to ensure that the drone received as much of the microwave power as possible,” says Kohei Shimamura, lead author of the study. “Moreover, to further increase the transmission efficiency, we carefully tuned the phase of the microwaves using an analog phase shifter that was synchronized with GPS units.”
The researchers measured the efficiencies of the power transfer through the beam (4%), the capture of microwaves by the drone (30%), the conversion of microwaves to electricity for propulsion (40%), and other relevant processes. Based on this information and an analytical formula, they calculated the overall power transmission efficiency in their experiment to be 0.43%. For comparison, in a previous study, the team measured the total transmission efficiency for a fixed-position (rather than free-flying) drone to be 60.1%.
“These results show that more work is needed to improve the transmission efficiency and thoroughly evaluate the feasibility of this propulsion approach for aircraft, spacecraft, and rockets,” explains Shimamura. “Future studies should also aim to refine the beam-tracking system and increase the transmission distance beyond that demonstrated in our experiment.”
Although microwave-powered rocket propulsion is still in its early stages, it could someday become a superior way to launch rockets into orbit given the high onboard-fuel demands of conventional propulsion techniques.
The tangled physics of knots, one of our simplest & oldest technologies
https://massivesci.com/articles/knot-str...shoelaces/
EXCERPT: . . . knots are quite ancient, predating both the use of the axe and of the wheel and potentially even the divergence of humans from other apes. After all, ropes and cords are practically useless without being tied to something else, making one of the most ancient technologies still remarkably relevant today. But these tie-offs can be a problem, since knots actually decrease the strength of a rope.
[...] So how can we tell, just with our eyes, whether one knot is stronger than another? Perhaps most intuitively, knots with more strand crossings tend to be stronger. Additional points of contact likely create more friction — more resistance to coming undone. A smaller knot with fewer crossings will also usually have tighter bends than a larger knot with more crossings. This increases the tension imbalance in the fibers of the rope with the smaller knot and making it more likely to break, a catastrophe for a climber dangling off the end of a safety line or a fisherman whose dinner escapes when the knot breaks.
The second characteristic of a strong knot is known as “twist fluctuations,” which accounts for whether a single rope strand undergoes twisting as it snakes through the knot. Twist fluctuations are essentially a proxy for the knot’s stability, since a strand that is forced to twist will “lock” on itself (like a square knot) while a strand that is free to roll won’t keep its shape (like a granny knot). The concept of twist fluctuations also explains why there is a scientifically correct way to tie your shoes: the knot is less likely to slip if you alternate which strand goes on top when forming the two parts of the knot, which is the concept behind a square knot. You can tell right away if your shoes are tied correctly just by looking. If the knot tends to lie vertically along the shoe, it's a weaker granny knot, whereas if the knot is horizontal, it's the stronger square knot.
The last easily countable knot attribute is “circulation,” or whether adjacent rope strands move in the same direction as the knot is pulled tighter. This can happen in multiple parts of a single knot, and the more rope strands moving in opposite directions, the stronger the knot will be. For example, in a standard granny knot, rope strands slide along each other in opposite directions as the knot is tightened, generating friction that increases the knot's strength. If instead you take a granny knot and pull on diagonal ends, the exact opposite happens. (This knot actually has a different name, the grief knot.) Adjacent strands now move in the same direction, and the knot tends to slip.
As the knots get bigger and more complicated, the same rules apply... (MORE - details)