Why does a countdown start from 10 or 5 or 3?

Answer 1

A countdown can start minutes, hours or days before a launch.

The length of the "final countdown" can vary based on the number of pre-launch steps that must be doneimmediatelybefore lift-off.

Space Shuttle Example:

• T-minus 31 sec: If all systems are OK, team issues the "Go" command. At this point launch control is handed off to the shuttle computers
• T-minus 16 sec: Water system starts spraying the launch pad, so it isn'tincinerated
• T-minus 6 sec: Start main engines, because it takes a moment for them to power up
• T-minus 0: The solid rocket boosters are ignited, then lift-off

-BUT- If you are shooting a model rocket in your backyard... feel free to start with any number you want.