In Rocket start countdowns ns noticed that they count through respect to part parameter T as:

T-10 secondsT-9 seconds...T-0 seconds

Why not just count as





The "T" represents the time in ~ which the rocket is booked to be launched.

T minus 4 minutes and 49 2nd indicate 4 minutes and 49 2nd before the launch.

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Eventhough the last ten seconds are counting in many of the situation as 10,9,8,7,....,3,2,1

This an approach is also be provided as "T-PLUS" which shows the time ~ the launch.

And additionally

even "E- minus" for events that show off ubraintv-jp.comcraft the are already in,..... The "E" means "Encounter", as with a comet or some other object.

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answer Oct 19 "13 in ~ 16:44

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T is just a designator because that the key sequence countdown time that serves as the synchronisation device for the majority of systems and also procedures that have to take place before, specifically on and even ~ the countdown sequence reaches T-0. Countdown technically ends through T-0 (T minus zero) and also the count then continues in the positive selection (T plus 1, 2,...), however we usually refer to all of it together the countdown for simplicity sake, even after continuing to count upwards with other sequences taking place post launch.

T-0 does no necessarily median the exact time in this sequence as soon as the liftoff happens, and can additionally stand for the time of main engine ignition, or something similar, though usually close to the really liftoff and rocket"s clearing that the launch tower. There are other time designators used, for example L have the right to stand because that time prior to the beginning day, E for the moment to some event, encounter or ETA (Estimated Time the Arrival), and also so on.

So in short, T it"s just a convention, a solitary letter abbreviation designator because that the system clock (mostly quoted to stand for Time) the a lot of events are synchronised through throughout the term of the launch. Much more about the purpose can be review in this thread: What is the function of having a countdown throughout a rocket launch?

I have actually to point out that T might not be provided at all, for example with the JAXA"s start of the Epsilon rocket, no designator to be used since there was just a solitary system clock come synchronize v (something the didn"t go quite according to plans with the first launch attempt of this new launcher and also the timestamp discrepancy between the onboard timekeeper and also the mission regulate of a couple of milliseconds caused the launch sequence to it is in halted and also postponed to a later date). Second attempt was successful though, yet they still didn"t use the T designator.

Some ILS (International start Services) rocket launches also didn"t use any kind of designator in ~ all, for instance the last ILS Proton launch, and also even despite the English reporter covering the start still used it during live streams (I presume the end of habit), the official announcer in Russian language didn"t use it in ~ all. They additionally didn"t speak "Liftoff" prefer we"d hear some various other announcers say. V the last Soyuz launch, because that example, the main announcer claimed "Fly now!".

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So all this T business is a type of convention that seems to stick best to western launch solution presumably for historical reasons, however it could really be any type of other solitary letter abbreviation (or every little thing is straightforward to pronounce and also won"t be puzzled for other designators) of the main system"s clock, or even none at all when a solitary system clock is supplied to synchronise all the launch subsystems against.