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I have seen Ariane-5 lift off at T+07 seconds and not at (or close to) T=00 seconds. The following statement is from the Ariane-5 User's Manual (Page 35, Section 2.3 - Typical Mission Profile):

The engine of the cryogenic main core stage, Vulcain 2, is ignited at H0+1s. Until H0+7.05 seconds, the on-board computer checks the good behavior of the engine and authorizes the lift-off by the ignition of the two solid rocket boosters.

Most of the rockets to the best of my knowledge lift-off exactly (or around) T=00 seconds. Engine ignition takes place at somewhere like T-0x seconds. Then they attain full thrust close to T=0 seconds. Either SRBs ignite and hold down clamps release or the launch clamps release and lift-off take place. Why does Ariane-5 liftoff at T+7.05 seconds instead of T=0 seconds? Why so much delay? Instead, they could have started the engine ignition and the following checks a bit earlier (like engine ignition at T-6.05 seconds, and SRB ignition and lift-off at T=00 seconds). What is the reason for this? Are there any other launch vehicles which lift-offs well after T=00 seconds?

I thought all Ariane Space launch vehicles (Soyuz, Vega, Vega-C, etc.) do the same thing, but the following ascent profile of Soyuz shows that it lifts-off at T=00 seconds.

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(Image from Soyuz User's Manual - Page 35)

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    $\begingroup$ related: What is the purpose of having a countdown during a rocket launch? The time in the countdown corresponding to T=0 may correspond to anything you want, this is just a convention. $\endgroup$ – Manu H Oct 16 at 11:47
  • $\begingroup$ How do you define lift off? release from launchpad (last thing holding the launcher gone) ? nominal take off thrust reached ? Launcher not touching the launchpad (any part of the launcher? last umbilical released?) $\endgroup$ – Manu H Oct 16 at 11:53
  • $\begingroup$ @ManuH, According to me (and I think it will be for most of the people), lift-off is when there is no physical contact between the rocket and the ground facilities. I think two of your questions (in your second comment) mean the same, since rockets are released by the clamps only when full thrust is achieved after engine ignition. $\endgroup$ – Intellex Oct 16 at 12:24
  • $\begingroup$ I'm not quite sure clamps are released as soon as full thrust is achieved, especially for liquid propellant first stage such as delta VI heavy. $\endgroup$ – Manu H Oct 16 at 15:19
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Liftoff at T-0 is common, but not universal. For Ariane 5, T-0 is the moment of ignition of the Vulcain engine. In a nominal countdown, checkout takes 6 seconds, and the solids are ignited when checkout finishes, leading to liftoff at T+7. I suspect there is some flexibility built into this: if the checkout takes longer, this is easily accommodated and liftoff will be at T+8, for instance. T-0 is then the last fixed point in the countdown.

Ariane 5 does not reuse any infrastructure from other space agencies, so ESA/CNES/Arianespace were free to design a countdown that works for them.

The sources I've found (not many) don't give a reason for this choice.

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