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A parabolic trough is a type of solar thermal energy collector. It is constructed as a long parabolic mirror (usually coated silver or polished aluminum) with a Dewar tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the Dewar tube. The trough is usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day. Alternatively the trough can be aligned on an east-west axis, this reduces the overall efficiency of the collector, due to cosine loss, but only requires the trough to be aligned with the change in seasons, avoiding the need for tracking motors. This tracking method works correctly at the spring and fall equinoxes with errors in the focusing of the light at other times during the year (the magnitude of this error varies throughout the day, taking a minimum value at solar noon). There is also an error introduced due to the daily motion of the sun across the sky, this error also reaches a minimum at solar noon. Due to these sources of error, seasonally adjusted parabolic troughs are generally designed with a lower solar concentration ratio. In order to increase the level of alignment, some measuring devices have also been invented. Parabolic trough concentrators have a simple geometry, but their concentration is about 1/3 of the theoretical maximum for the same acceptance angle, that is, for the same overall tolerances for the system. Approaching the theoretical maximum may be achieved by using more elaborate concentrators based on primary-secondary designs using nonimaging optics. Heat transfer fluid (usually oil) runs through the tube to absorb the concentrated sunlight. This increases the temperature of the fluid to some 400C. The heat transfer fluid is then used to heat steam in a standard turbine generator. The process is economical and, for heating the pipe, thermal efficiency ranges from 60-80%. The overall efficiency from collector to grid, i.e. (Electrical Output Power)/(Total Impinging Solar Power) is about 15%, similar to PV (Photovoltaic Cells) but less than Stirling dish concentrators. Current commercial plants utilizing parabolic troughs are hybrids; fossil fuels are used during night hours, but the amount of fossil fuel used is limited to a maximum 27% of electricity production, allowing the plant to qualify as a renewable energy source. Because they are hybrids and include cooling stations, condensers, accumulators and other things besides the actual solar collectors, the power generated per square meter of area varies enormously. [READ THE REST OF THIS ARTICLE]



An earthquake (also known as a quake, tremor, temblor or seismic activity) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are measured with a seismometer; a device which also records is known as a seismograph. The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale. At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity. In its most generic sense, the word earthquake is used to describe any seismic event whether a natural phenomenon or an event caused by humans that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter. [READ THE REST OF THIS ARTICLE]



The M18A1 Claymore is a directional anti-personnel mine used by the U.S. military. It was named after the large Scottish sword by its inventor, Norman A. MacLeod. It is used primarily in ambushes and as an anti-infiltration device against enemy infantry. It is also of some use against unarmored vehicles. The M18A1 Claymore mine consists of a horizontally convex black plastic case (inert training versions are gray), which is vertically concave. The shape was developed through experimentation to deliver the optimum distribution of fragments at 50m range. The case has the words "Front Toward Enemy" embossed on the front surface of the mine. A simple open sight on the top surface is provided for aiming the mine. Two pairs of scissor legs attached to the bottom support the mine and allow it to be aimed vertically. Either side of the sight are fuse wells set at 45 degrees to the vertical. Internally the mine contains a layer of C-4 explosive on top of which is a matrix of approximately seven hundred 1/8 inch diameter steel balls (about as big as #4 birdshot) set into an epoxy resin. When the M18A1 is detonated, the explosion drives the matrix of 700 spherical fragments out of the mine at a velocity of 1,200 m/s, at the same time breaking the matrix into individual fragments. The spherical steel balls are projected in a 60 fan-shaped pattern that is 2m high and 50m wide at a range of 50m. The force of the explosion deforms the relatively soft steel fragments into a shape similar to a .22 rimfire projectile. The Claymore mine is typically deployed in one of three modes: Controlled, Uncontrolled, or Time-delayed. When in use by the U.S. military, the M18A1 Claymore Anti-Personnel Mine is most often command-detonated. Such use is permitted by the Mine Ban Treaty. However, use of Claymore mines in uncontrolled (tripwire) mode is prohibited by the treaty. Because of this uncontrolled mode, it is frequently listed in efforts to ban anti-personnel mines. [READ THE REST OF THIS ARTICLE]





Project Excelsior was a series of high-altitude parachute jumps made by Colonel (then Captain) Joseph Kittinger of the United States Air Force in 1959 and 1960 to test the Beaupre multi-stage parachute system. In one of these jumps Kittinger set world records for the highest parachute jump, the longest parachute drogue fall and the fastest speed by a human through the atmosphere, all of which still stand. The first test, Excelsior I, was made on November 16, 1959. Kittinger ascended in the gondola and jumped from an altitude of 23,300 m (76,400 ft). In this first test, the stabilizer chute was deployed too soon, catching Kittinger around the neck and causing him to spin at 120 revolutions per minute. This caused Kittinger to lose consciousness, but his life was saved by his main chute which opened automatically at a height of 3,000 m (10,000 ft). Despite this near-disaster on the first test, Kittinger went ahead with another test only three weeks later. The second test, Excelsior II, was made on December 11, 1959. This time, Kittinger jumped from an altitude of 22,800 m (74,700 ft) and descended in free-fall for 17,000 m (55,000 ft) before opening his main chute. Excelsior III gondola at the National Museum of the United States Air ForceThe third and final test, Excelsior III, was made on August 16, 1960. During the ascent, the pressure seal in Kittinger's right glove failed, and he began to experience severe pain in his right hand. He decided not to inform the ground crew about this, in case they should decide to abort the test. Despite temporarily losing the use of his right hand, he continued with the ascent, climbing to an altitude of 31,333 m (102,800 ft). The ascent took one hour and 31 minutes and broke the previous manned balloon altitude record of 30,942 m (101,516 ft), which was set by Major David Simons as part of Project Manhigh in 1957. Kittinger stayed at peak altitude for 12 minutes, waiting for the balloon to drift over the landing target area. He then stepped out of the gondola to begin his descent. The small stabilizer chute deployed successfully and Kittinger fell for 4 minutes and 36 seconds, setting a still-standing world record for the longest parachute free-fall (although some authorities do not count this as a free-fall record because of the use of the stabilizer chute). At an altitude of 5,330 m (17,500 ft), Kittinger opened his main chute and landed safely in the New Mexico desert. The whole descent took 13 minutes and 45 seconds and set the current world record for the highest parachute jump. During the descent, Kittinger experienced temperatures as low as -94 F (-70 C). In the free-fall stage, he reached a top speed of 988 km/h (614 mph). A plaque attached below the open door of the Excelsior III gondola read, "This is the highest step in the world". [READ THE REST OF THIS ARTICLE]



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