I have a question about rocket launches. I have noticed that sometimes the count down reaches zero just as the rocket lifts off, though the engines had ignited before that. And others I see that it reaches zero when the engine is ignited, and liftoff occurred some seconds after. Now the shuttle has both. The Main engines are up and running before the SRBs ignite and lift off occurs at SRB ignition. I noticed that the Arian doesn't start ignition of the main engine till the count reaches zero. Is there any particular reason for this or is it just a mater of choice. I would think it's a mater of the characteristics of the engine and or the mission as to when zero occurs.