Countdown

Countdown

A countdown is a sequence of counting backward to indicate the seconds, days, or other time units remaining before an event occurs or a deadline expires. Typical events for which a countdown is used include the launch of a rocket or spacecraft, the detonation of a bomb, the start of a race, and the start of the New Year. In an early attestation of a countdown, it started a rowing race at Cambridge University.[1] The first known association with rockets was in the 1929 German science fiction movie Die Frau im Mond by Fritz Lang to increase the drama of the launch sequence of the story's lunar rocket.[2]

Contents

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.[3]

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", is 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.[4]

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 begin 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 is held on Launch minus 1 day, which is a specific weather briefing for all areas of Space Shuttle launch operations.[5]

The launch window is a precise time during which aerospace personnel launch a rocket so the payload can reach the proper orbital destination.[6]

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.[7]

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.[8] For example, dialogue heard during the launch of a Delta II rocket carrying the Kepler Space Telescope on March 8, 2009, included:

Time: T minus 3 minutes.
Launch Control (LC): OSM, third stage S&A arm permit to close.
OSM: Closed.
LC: SSC,third stage S&A armed.
SSC: Armed.
LC: Prop 1, vehicle fuel tank press open.
Prop 1: Open.
LC: Fuel umbilical purge to open.
Prop 1: Open.
LC: SSC, vent 1 heater control exit.
SSC: Exit.
LC: SSC, vent 2 heater control exit.
SSC: Exit.
LC: NSC reports spacecraft is go.
Mission Director: Kepler spacecraft is go.
LC: SSC - FTS bat one and two heater controls heaters off.
SSC: Off.
LC: Prop 1, pressurized first stage LOX tanks to relief.
Prop 1: Pressurized.
LC: Prop 2, top first stage LOX to 100 percent levels.
Prop 2: Up and down, 100 percent.
Time: Ninety seconds.
LC: SSC, hydraulic external power to on.
SSC: External.
Time: Eighty seconds.
LC: RCO, report range go for launch.
Range Control Officer (RCO): Range go for launch.
Mission Director: LC (Viera), you're go for launch.
LC: Roger.
[9]

Seven seconds after launch of STS-121, the countdown clock at LC-39 at the Kennedy Space Center seen counting up, rather than down.

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 "10, 9, 8, 7, 6, 5, 4, 3, 2, 1, Lift off!" 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 "+ 00:00:00:07", 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!'.[citation needed]

Film

At the start of films, a countdown sequence is printed on the head leader, to aid in synchronizing the film. In film (but not television) the countdown is in units of feet rather than time units.

The countdown ends just as "two" is displayed so that the countdown isn't shown if the film is put on screen a little earlier than intended. Although this sequence was usually not intended for broadcast, being a cue device for a projectionist or TV producer, it is sometimes used in modern-day spoofs to signify old film footage.

New Year's Eve

In many New Year's Eve celebrations, there is a countdown during the last seconds of the old year until the beginning of the new year. In Times Square on New Year's Eve, besides the 60-Second Toshiba Countdown, there are six hourly countdowns in which people can practice their countdown skills from 6 P.M. to 11 P.M. Additionally, countdowns are often used to count down the days before special events, such as major holidays like Chinese New Year, Christmas, and important town meetings. Sometimes personal countdowns may be used, via phone or computer applications, to count the days to one's birthday or anniversary.

See also

References

  1. ^ Everett, William. On the Cam: Lectures on the University of Cambridge in England. Cambridge, Mass.: Sever and Francis, 1865, p. 192.
  2. ^ "Spektrum der Wissenschaft" - DenkMal-Frage: "Was verdankt die Raumfahrt dem Stummfilm "Die Frau im Mond" (1929) von Fritz Lang?" Wissenschaft-online.de
  3. ^ Angelo, Joseph A. Space Technology. 2003, Greenwood Publishing Group; ISBN 1573563358, 9781573563352: 143
  4. ^ NASA, "Launching by the Book", April 30, 2007. NASA
  5. ^ Kennedy Space Flight Center Release 39-99, "Space Shuttle weather launch commit criteria and KSD end of mission weather landing criteria", May 20, 1999 NASA.org
  6. ^ Angelo 2003: 144
  7. ^ Angelo 2003: 144
  8. ^ NASA, "The Space Shuttle launch team"
  9. ^ NASA Channel, 8 March 2009, 10:40 Eastern Standard Time

External links


Wikimedia Foundation. 2010.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • Countdown — am Platz des Himmlischen Friedens zu den Olympischen Sommerspielen 2008 in Peking Der Countdown (seltener: Count down) (engl. „herunterzählen“, „das Zurückzählen“) ist die getaktete (kurz vor dem Ende im Sekundentakt) Bekanntgabe der bis zum… …   Deutsch Wikipedia

  • Countdown to 3 — was a TV special that broadcast on ABC1 to celebrate the opening of the ABC s 3rd channel, dedicated to children s programs, at 5:00 pm on Friday, December 4, 2009. Description After a brief countdown from 10 to 4 and the logo over a stickered… …   Wikipedia

  • countdown — 1953, Amer.Eng., in early use especially of launches of rockets or missiles, from COUNT (Cf. count) (v.) + DOWN (Cf. down) …   Etymology dictionary

  • countdown — ► NOUN 1) an act of counting down to zero, especially before the launch of a rocket. 2) the final moments before a significant event …   English terms dictionary

  • countdown — ☆ countdown [kount′doun΄ ] n. the schedule of operations just before the firing of a rocket, the detonation of an explosive device, etc.; also, the counting off, in reverse order, of units of time in such a schedule …   English World dictionary

  • Countdown — Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom. Countdown signifie compte à rebours en anglais. Countdown est la version britannique du jeu télévisé Des chiffres et des lettres (depuis 1982). Countdown… …   Wikipédia en Français

  • countdown — n. 1) a final countdown 2) a countdown from; to (the countdown to zero has begun) * * * [ kaʊntdaʊn] to (the countdown to zero has begun) a final countdown a countdown from …   Combinatory dictionary

  • Countdown — Count|down [ kau̮nt dau̮n], der und das; [s], s: a) (beim Abschuss einer Rakete) bis zum Zeitpunkt null zurückgehende Ansage der Zeit als Einleitung eines Kommandos zum Start: der Countdown läuft. b) Gesamtheit der vor dem Start auszuführenden… …   Universal-Lexikon

  • Countdown — Личный номер Lichniy nomer Título Countdown Ficha técnica Dirección Evgeny Lavrentev Producción Serguei Gribkov Yuri Sagaidak Vladimir Trushchenkov …   Wikipedia Español

  • countdown — [[t]ka͟ʊntdaʊn[/t]] N SING: also no det A countdown is the counting aloud of numbers in reverse order before something happens, especially before a spacecraft is launched. The countdown has begun for the launch later today of the American space… …   English dictionary

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”