# What is meaning of T in rocket launch countdown?

In Rocket Launch countdowns I noticed That they count with respect to some parameter T as:

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

Why not simply count as

10 9 8 . . 0

?

http://www.nasa.gov/mp3/590318main_ringtone_135_launch.mp3

• They don't say all those T minuses. You need to listen again. They say "T minus ten, nine, eight, ...". – Mark Adler Feb 16 '14 at 16:23

The "T" stands for the time at which the rocket is scheduled to be launched.

T minus 4 minutes and 49 second indicate 4 minutes and 49 second before the launch.

Eventhough the last ten seconds are counted in most of the case as 10,9,8,7,....,3,2,1

This method is also be used as "T-PLUS" which shows the time after the launch.

And also

even "E- minus" for events that involve spacecraft that are already in space,..... the "E" stands for "Encounter", as with a comet or some other space object.

External source

http://www.nasa.gov/mission_pages/epoxi/epoxi20101004.html

• Example with STS Terminal countdown: T-0 was SRB ignition. Detailed countdown from T-6h. – mins Jun 6 '15 at 9:26
• I heard they don't say "five" because it sounds like "fire" but why don't they say 6 or 4 either? – cantsay Jun 14 '15 at 17:53

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

T-0 does not necessarily mean the exact time in this sequence when the liftoff happens, and can also stand for the time of main engine ignition, or anything similar, though usually close to the actual liftoff and rocket's clearing of the launch tower. There are other time designators used, for example L can stand for time before the launch day, E for the time to some event, encounter or ETA (Estimated Time of Arrival), and so on.

So in short, T it's just a convention, a single letter abbreviated designator for the system clock (mostly quoted to stand for Time) that a lot of events are synchronised with throughout the duration of the launch. More about its purpose can be read in this thread: What is the purpose of having a countdown during a rocket launch?

I have to mention that T might not be used at all, for example with the JAXA's launch of the Epsilon rocket, no designator was used since there was only a single system clock to synchronize with (something that didn't go quite according to plans with the first launch attempt of this new launcher and the timestamp discrepancy between the onboard timekeeper and the mission control of a few milliseconds caused the launch sequence to be halted and postponed to a later date). Second attempt was successful though, but they still didn't use the T designator.

Some ILS (International Launch Services) rocket launches also didn't use any designator at all, for example the last ILS Proton launch, and even though the English reporter covering the launch still used it during live streams (I presume out of habit), the official announcer in Russian language didn't use it at all. They also didn't say "Liftoff" like we'd hear some other announcers say. With the last Soyuz launch, for example, the official announcer said "Fly now!".

So all this T business is a sort of convention that seems to stick best to western launch systems presumably for historical reasons, but it could really be any other single letter abbreviation (or whatever is easy to pronounce and won't be confused for other designators) of the main system's clock, or even none at all when a single system clock is used to synchronise all the launch subsystems against.

• +1. I bet it's related to the U.S. military convention of coordinating times relative to "D day", "H hour", etc. – ruakh Oct 19 '13 at 22:02
• Yes, a bad habit. It is an annoying bad habit, since the Russian launches have their own tradition going continuously from the pre-Sputnik times. See the questions discussing the "key to start" on this site. – horsh Oct 19 '13 at 22:26

The 'T' is the point in which the spacecraft receives the 'Go for Launch' command. At the point where 'T-0' touched, the rocket takes off from the pad. Also, as 'Hash' has mentioned above, you can use the 'T Plus' measurement after the mission has begun.

This convention has been used throughout NASA's history, and even when the Saturn V was still around, launching astronauts to the moon in the Cold War. Most of the progression NASA has made has been driven by the Space Race, which was like a competition against the USA and the Soviet Union (which is today Russia).

I hope this helps you!

• Welcome to space and thank you for taking the time to answer. But I am unsure how this answer adds anything more than the existing answers to the question. – James Jenkins Feb 16 '14 at 12:01
• For the Shuttle, the last 31 seconds of the countdown were automated and executed by the computer onboard the orbiter which took over the Launch Processing System (LPS) after the Ground Launch Sequencer (GLS) has allowed autosequence. Nobody had to send a Go after this milestone. The LPS (ground system) could recycle at T-20 mn or abort the automated sequence. The ignition of the main engines occured at T-6.6, and for the boosters at T-0. More. – mins Jun 6 '15 at 9:15