how about using a circuit like this.
first, apologise that i don't have time to draw the schematic now, i will draw it as soon as i have time, or maybe someone has time to do that.
use a 555 timer to generate about 1Hz with it. connect the output to a 4017 decad counter. use a transistor and relay connected to Q9 wich will trigger the rocket. then for the other outputs you can connect some leds. this will give you indication on how much time it is left. you could use different colour leds, use the first 3 red, next 3 yellow, and last 3 green. this way the leds will turn on in order.
you will have like this: RED, RED, RED, YELLOW, YELLOW, YELLOW, GREEN, GREEN, GREEN, LUANCH!!!!
or you can use less leds to know how much time there is left.
i'll be back soon with some schematic.
also, what will be used to launch the rochet ? is it a resisance that will heat up and turn on the engine? how much current will it draw?
also, another mechanism must be used to turn off the launching circuit after the launch takes place.
i would also use a beeper, or buzzer connected to the output of the 555 to have an acustic indication on the process.