|
A countdown is a sequence of backward counting to indicate the time remaining before an event is scheduled to occur. NASA commonly employs the term "T-minus" during the preparation for and anticipation of a rocket launch, and even "E-minus" for events that involve spacecraft that are already in space, where the "T" could stand for "Test" or "Time", and the "E" stands for "Encounter", as with a comet or some other space object. Other events for which countdowns are commonly used include the detonation of an explosive, the start of a race, the start of the New Year, or any anxiously anticipated event. An early use of a countdown once signaled the start of a Cambridge University rowing race.〔Everett, William. ''On the Cam: Lectures on the University of Cambridge in England''. Cambridge, Mass.: Sever and Francis, 1865, (p. 192. )〕 The first known association with rockets was in the 1929 German science fiction movie ''Die Frau im Mond'' (English: ''Woman in the Moon'') written by Thea von Harbou and directed by Fritz Lang in an attempt to increase the drama of the launch sequence of the story's lunar-bound rocket.〔(【引用サイトリンク】url= http://www.wissenschaft-online.de/artikel/636420 )〕〔 A gallery of behind-the-scenes shots of movies featuring space travel or aliens. Page 68, photo caption: "Directed by Fritz Lang (third from right), the silent film "Woman in the Moon" (1929) is considered one of the first serious science fiction films and invented the countdown before the launch of a rocket. Many of the basics of space travel were presented to a mass audience for the first time."〕 == Rocketry == A countdown is a carefully devised set of procedures ending with the ignition of a rocket's engine. Depending on the type of vehicle used, countdowns can start from 72 to 96 hours before launch time. During countdown: * Aerospace personnel bring the rocket vehicle to the launch site and load it with payload and propellants; * Launch-center computers communicate with sensors in the rocket, which monitor important systems on the launch vehicle and payload; * Launch personnel monitor the weather and wait for the launch window; * Security personnel prevent unauthorized persons from entering the "keep-out" area.〔Angelo, Joseph A. ''Space Technology.'' 2003, Greenwood Publishing Group; ISBN 1-57356-335-8, ISBN 978-1-57356-335-2: 143〕 The procedures for each launch are written carefully. For the Space Shuttle, a five-volume set, Shuttle Countdown (KSC S0007), often referred to as "S0007", was used. Rosie Carver, a technical writer for United Launch Alliance, has created at least 15,000 procedures for more than 300 missions since the Solar Maximum Mission, which launched Feb. 14, 1980. These documents are living documents, which reflect new issues and solutions as they develop. Each mission requires approximately 100 procedure books.〔(NASA, "Launching by the Book" ), April 30, 2007. NASA〕 Proceeding with the countdown depends on several factors, such as the proper launch window, weather that permits a safe launch, and the rocket and payload working properly. The launch weather guidelines involving the Space Shuttle and expendable rockets are similar in many areas, but a distinction is made for the individual characteristics of each. The criteria are broadly conservative and assure avoidance of possibly adverse conditions. They are reviewed for each launch. For the Space Shuttle, weather "outlooks" provided by the U. S. Air Force Range Weather Operations Facility at Cape Canaveral began at Launch minus 5 days in coordination with the NOAA National Weather Service Spaceflight Meteorology Group at the Johnson Space Center in Houston. These include weather trends and their possible effects on launch day. A formal prelaunch weather briefing was held on Launch minus 1 day, which is a specific weather briefing for all areas of Space Shuttle launch operations.〔(【引用サイトリンク】date=May 20, 1999 )〕 The launch window is a precise time during which aerospace personnel launch a rocket so the payload can reach the proper orbital destination.〔Angelo 2003: 144〕 A hold is the suspension of the normal countdown process. This can be done to investigate a technical process that has gone wrong, or because of marginal weather at the launch pad. Some holds are planned: they are done so the launch-support computers can run automatic checks on the rocket. Under some circumstances, a countdown may be recycled to an earlier time. When that happens, launch personnel begin following the countdown checklist from the earlier point.〔 During communications for a countdown, the launch team uses acronyms to keep channels open as much as possible. All Firing Room console positions are assigned unique 'call signs' that are used by the team for quick and positive identification of who is talking. For example, dialogue heard during the launch of a Delta II rocket carrying the Kepler Space Telescope on March 8, 2009, included:
In the context of a rocket launch, the "T minus Time" is the time before launch, e.g. "T minus 3 minutes and 40 seconds". The last ten seconds are usually counted down aloud "Ten seconds to liftoff. Nine, eight, seven, six, five, four, three, two, one." After a launch, most countdown clocks begin to show Mission Elapsed Time, which is typically shown as "T plus." The picture to the right shows "+ 00:00:07", approximately seven seconds after liftoff. In Fritz Lang's film ''Frau im Mond'', after each number the phrase "seconds to go" was repeated. When counting down to the launch of an explosive, e.g. when testing a new model of gun, it is customary to omit "5" from the countdown sequence because "five" sounds too similar to 'Fire!'. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Countdown」の詳細全文を読む スポンサード リンク
|