Where did the term "T-minus" come from?

Where did the term "T-minus" come from and or originate from? Why can't explosive people skip it and just count down?

2 Answers

  • tl;dr
    Lv 6
    10 years ago
    Favorite Answer

    People tend to think the "T" stands for "time", or perhaps "takeoff" which is understandable, but these are merely guesses.

    In fact, the "T" stands for "test" (as in: any scheduled test that is significant). I wouldn't make this claim without proof, which can be found here [1]. This is a transcript on NASA's website, "B" is a launch director at NASA, Mike Leinbach. If anyone would know, he would.

    "A:What does the "t" stand for?

    B:I joined the space program about 20 years ago and I asked this question of my, of my elders in the program when I did join, and it turns out that T stands for test. Because it's not always related to time. It could be the start of a particular test in our Orbiter Processing Facility that is independent of the time of day."

    The "T" (or test) referred to in a rocket launch is a specific event in the launch: *liftoff*. I have to stress that "liftoff" is the operative term, as there are several stages involved in a launch.(proof here:[2], quoted below)

    So why "minus?" because the time being recorded is in fact "mission elapsed time", which is counted relative to liftoff, which is time-zero. Time *before* liftoff is "negative" mission elapsed time (t-minus), and time *after* liftoff is "positive" mission elapsed time (t-plus).

    NASA is reported to have used the term X-minus in a 1958 launch, which is explained in this way on a forum entry discussing the subject [2]. (I have numbered the various procedures ending in liftoff):

    "The "X-minus" count figured from the V-2 days, when time had to be allowed for the launch controller to (1)manually observe [visually observe] the engine flame during the low-thrust segment of the engine start sequence. (2)The "firing command" was given at X-minus zero. (3)The propellant pre-valves opened at X-plus zero, starting the engine start sequence. (4)Full thrust ignition did not occur until X-plus 14 seconds. (5)Liftoff, at what would later be called T-zero time, occurred at X-plus 15.75 seconds for Explorer I."

    I don't have any specific information as to why they feel the need to keep track of every second of MET (mission elapsed time), including "negative" MET, even to the point of calling out the last few seconds, but I *guess* that it's because everyone needs to be certain that a complex series of specific actions take place at specific times, or people could die.

    So now you know, and you have reliable proof, too! Tell your friends, as this question has been asked repeatedly on 'Answers and around the internet, but apparently no one has ever researched it. Now come on, slide me them 10 points.

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  • 10 years ago

    The "T" is something like the time the shuttle has been in the air. Since the countdown to launch occurs before the aircraft is up, the time is "in the negative" or "minus." After the launch, they switch to "T-plus" and record how long the mission takes.

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