Editing Countdown (section)

== Rocketry ==
{{rquote|right||[[Willy Ley]], 1968<ref name="ley196810">{{Cite magazine
 |last=Ley
 |first=Willy
 |date=October 1968
 |title=The Orbit of Explorer-1
 |department=For Your Information
 |url=https://archive.org/stream/Galaxy_v27n03_1968-10#page/n93/mode/2up
 |magazine=Galaxy Science Fiction
 |pages=93–102
}}</ref>|quote=People involved in countdowns always say that the last twenty minutes are the worst. By that time everything that needs doing has been done, and therefore everybody has twenty minutes in which to think of what may not have been done, or else what could possibly go wrong.}}
{{See also|Built-in hold}}
A countdown is a carefully devised set of procedures ending with launch of a rocket. Depending on the type of vehicle used, countdowns can start from 72 to 96 hours before launch time.

There are two countdowns proceeding simultaneously:
* The T-minus clock, a schedule of planned activities prior to launch (T−0), and
* The L-minus clock, a mechanical countdown to the time of launch (L−0).

Except for the last few minutes, which are highly automated and rigid, scheduled activities rarely take exactly the scheduled time, and the T-minus clock only corresponds approximately to the time until launch.

A ''hold'' is the suspension of the normal countdown process, during which the T-minus clock is stopped and no planned activities take place. This can be done to investigate a technical process that has gone wrong, or to intentionally delay the launch, e.g. because of bad weather at the launch pad.  Most countdown schedules also include some pre-planned ''[[built-in hold]]s''.  These provide an opportunity to perform non-launch activities, handle unexpected issues, or to catch up on the schedule if it is running long.

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.<ref name="Angelo 2003: 144"/>

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.<ref>{{Cite book |last=Angelo |first=Joseph A. |title=Space Technology |publisher=[[Greenwood Publishing Group]] |year=2003 |isbn=978-1-57356-335-2 |pages=143}}</ref>

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.<ref>{{Cite web |date=April 30, 2007 |title=Launching by the Book |url=http://www.nasa.gov/centers/kennedy/launchingrockets/carver.html |url-status=dead |archive-url=https://web.archive.org/web/20070502071641/http://www.nasa.gov/centers/kennedy/launchingrockets/carver.html |archive-date=May 2, 2007 |website=[[NASA]]}}</ref>

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.

[[File:Countdown clock at NASAs Kennedy Space Center on 28 April 2011 (5665401600).jpg|thumb|Countdown clock at NASA's [[Kennedy Space Center]] at L-11 hours (28 April 2011) of STS-134, {{OV|105}}.]]
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 included weather trends and their possible effects on launch day. A formal prelaunch weather briefing was held on Launch minus 1 day, which was a specific weather briefing for all areas of Space Shuttle launch operations.<ref>{{cite web |date=May 20, 1999 |title=Kennedy Space Flight Center Release 39-99: Space Shuttle weather launch commit criteria and KSD end of mission weather landing criteria |url=http://www-pao.ksc.nasa.gov/kscpao/release/1999/39-99.htm |url-status=dead |archive-url=https://web.archive.org/web/20090626180630/http://www-pao.ksc.nasa.gov/kscpao/release/1999/39-99.htm |archive-date=June 26, 2009 |website=Nasa.gov |publisher=NASA}}</ref>

The launch window is a precise time during which aerospace personnel launch a rocket so the payload can reach the proper orbital destination.<ref name="Angelo 2003: 144">Angelo 2003: 144</ref>

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.<ref>{{cite web |url=http://science.ksc.nasa.gov/shuttle/countdown/launch-team.html |title=NASA, "The Space Shuttle launch team" |publisher=Science.ksc.nasa.gov |date=1967-11-09 |access-date=2014-06-28 |archive-date=2015-06-19 |archive-url=https://web.archive.org/web/20150619025300/http://science.ksc.nasa.gov/shuttle/countdown/launch-team.html |url-status=dead }}</ref> For example, dialogue heard during the launch of a [[Delta II]] rocket carrying the [[Kepler Space Telescope]] on March 8, 2009, included:
<blockquote>{{dialogue
|time=Time
|lc=Launch Control (LC)
|osm=Operations Safety Manager (OSM)
|time|T minus 3 minutes
|lc|OSM, third stage {{abbr|S&A|safe-and-arm device}} arm permit to close.
|osm|Closed.}}<br/>{{dialogue
|lc=LC
|ssc=System Sequence Controller (SSC)
|prop1=Prop 1
|lc|SSC, third stage S&A armed.
|ssc|Armed.
|lc|[[Propellant|Prop]] 1, vehicle fuel tank press open
|prop1|Open.
|lc|Fuel umbilical purge to open.
|prop1|Open.
|lc|SSC, vent 1 heater control exit.}}<br/>{{dialogue
|time=Time
|lc=LC
|osm=OSM
|ssc=SSC
|prop1=Prop 1
|md=Mission Director
|ssc|Exit.
|lc|SSC, vent 2 heater control exit.
|ssc|Exit.
|lc|NSC reports spacecraft is go.
|md|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.
|prop1|Pressurized.
|lc|Prop 2, top first stage LOX to 100 percent levels.}}<br/>{{dialogue
|time=Time
|prop2=Prop 2
|lc=LC
|ssc=SSC
|rco=Range Control Officer (RCO)
|md=Mission Director
|prop2|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.
|rco|Range go for launch.
|md|LC (Viera), you're go for launch.
|lc|Roger.<ref>NASA Channel, 8 March 2009, 10:40 Eastern Standard Time</ref>
}}</blockquote>

[[File:STS-121 Launch.jpg|thumb|Seven seconds after launch of [[STS-121]], the [[Press Site-Clock and Flag Pole|countdown clock]] at [[Launch Complex 39|LC-39]] at the [[Kennedy Space Center]] seen counting up, rather than down.]]
In the context of a rocket launch, the "L minus Time" is the physical time before launch, e.g. "L minus 3 minutes and 40 seconds". "T minus Time" is a system to mark points at which actions necessary for the launch are planned - this time stops and starts as various hold points are entered, and so doesn't show the actual time to launch. 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 adjacent picture shows "+00:00:07", approximately seven seconds after liftoff.

The time T−0 is specifically the moment of ''launch commit'', when it is no longer possible to prevent liftoff.  This is necessarily slightly before the moment the rocket actually lifts off the launch pad. Liquid-fueled rocket engines (which can be turned off after ignition) are normally lit a few seconds before T−0 and brought to full throttle around T−0.  Solid rocket motors, which cannot be extinguished, are lit at T−0 and achieve full power a few seconds later.  If a rocket has hold-down clamps which can withstand full engine thrust, the moment the clamps are released defines T−0.

In [[Fritz Lang]]'s [[film]] ''[[Frau im Mond]]'', after each number the phrase "seconds to go" was repeated.