Countdown
Overview
 
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
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

 or spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

, the detonation of a bomb, the start of a race, and the start of the New Year
New Year
The New Year is the day that marks the time of the beginning of a new calendar year, and is the day on which the year count of the specific calendar used is incremented. For many cultures, the event is celebrated in some manner....

. In an early attestation of a countdown, it started a rowing race at Cambridge University. The first known association with rockets was in the 1929 German science fiction movie Die Frau im Mond by Fritz Lang
Fritz Lang
Friedrich Christian Anton "Fritz" Lang was an Austrian-American filmmaker, screenwriter, and occasional film producer and actor. One of the best known émigrés from Germany's school of Expressionism, he was dubbed the "Master of Darkness" by the British Film Institute...

 to increase the drama of the launch sequence of the story's lunar rocket.
A countdown is a carefully devised set of procedures ending with the ignition of a rocket's engine.
Encyclopedia
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
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

 or spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

, the detonation of a bomb, the start of a race, and the start of the New Year
New Year
The New Year is the day that marks the time of the beginning of a new calendar year, and is the day on which the year count of the specific calendar used is incremented. For many cultures, the event is celebrated in some manner....

. In an early attestation of a countdown, it started a rowing race at Cambridge University. The first known association with rockets was in the 1929 German science fiction movie Die Frau im Mond by Fritz Lang
Fritz Lang
Friedrich Christian Anton "Fritz" Lang was an Austrian-American filmmaker, screenwriter, and occasional film producer and actor. One of the best known émigrés from Germany's school of Expressionism, he was dubbed the "Master of Darkness" by the British Film Institute...

 to increase the drama of the launch sequence of the story's lunar rocket.

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.


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
Technical writer
A technical writer is a professional writer who designs, creates, and maintains technical documentation...

 for United Launch Alliance
United Launch Alliance
United Launch Alliance is a joint venture of Lockheed Martin and Boeing. ULA was formed in December 2006 by combining the teams at these companies which provide spacecraft launch services to the government of the United States. U.S...

, has created at least 15,000 procedures for more than 300 missions since the Solar Maximum Mission
Solar Maximum Mission
The Solar Maximum Mission satellite was designed to investigate solar phenomenon, particularly solar flares. It was launched on February 14, 1980....

, 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.

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.

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

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
Delta II
Delta II was an American space launch system, originally designed and built by McDonnell Douglas. Delta II is part of the Delta rocket family and was in service from 1989 until November 1, 2011...

 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
John C. Stennis Space Center
The John C. Stennis Space Center , located in Hancock County, Mississippi, at the Mississippi-Louisiana border, is NASA's largest rocket engine test facility.- History :...

,third stage S&A armed.

SSC: Armed.

LC: Prop
Propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...

 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.



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
Mission Elapsed Time
Mission Elapsed Time is used by NASA during their space missions, most notably during their Space Shuttle missions. Because so much of the mission depends on the time of launch, all events after launch are scheduled on the Mission Elapsed Time. This avoids constant rescheduling of events in case...

, 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
Fritz Lang
Friedrich Christian Anton "Fritz" Lang was an Austrian-American filmmaker, screenwriter, and occasional film producer and actor. One of the best known émigrés from Germany's school of Expressionism, he was dubbed the "Master of Darkness" by the British Film Institute...

's film
Film
A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...

 Frau im Mond
Frau im Mond
Woman in the Moon is a science fiction silent film that premiered October 15, 1929. It is often considered to be one of the first "serious" science fiction films...

, 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!'.

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
Parody
A parody , in current usage, is an imitative work created to mock, comment on, or trivialise an original work, its subject, author, style, or some other target, by means of humorous, satiric or ironic imitation...

 to signify old film footage.

New Year's Eve

In many New Year's Eve
New Year's Eve
New Year's Eve is observed annually on December 31, the final day of any given year in the Gregorian calendar. In modern societies, New Year's Eve is often celebrated at social gatherings, during which participants dance, eat, consume alcoholic beverages, and watch or light fireworks to mark the...

 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
Chinese New Year
Chinese New Year – often called Chinese Lunar New Year although it actually is lunisolar – is the most important of the traditional Chinese holidays. It is an all East and South-East-Asia celebration...

, Christmas
Christmas
Christmas or Christmas Day is an annual holiday generally celebrated on December 25 by billions of people around the world. It is a Christian feast that commemorates the birth of Jesus Christ, liturgically closing the Advent season and initiating the season of Christmastide, which lasts twelve days...

, and important town meetings. Sometimes personal countdowns may be used, via phone or computer applications, to count the days to one's birthday
Birthday
A birthday is a day or anniversary where a person celebrates his or her date of birth. Birthdays are celebrated in numerous cultures, often with a gift, party or rite of passage. Although the major religions celebrate the birth of their founders , Christmas – which is celebrated widely by...

 or anniversary
Anniversary
An anniversary is a day that commemorates or celebrates a past event that occurred on the same day of the year as the initial event. For example, the first event is the initial occurrence or, if planned, the inaugural of the event. One year later would be the first anniversary of that event...

.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK