Well.... I could probably try making something for you but it would only be a basic terminal based program. Can't do any fancy GUI stuff mainly because I don't know how to use the libraries (I'm thinking QT library will be needed).
Anyways.... Barring that it should be simple enough to rig a program up that counts down. Now as for the launching when it reaches 0 thing. That might be a bit more involved. Maybe sending some sort of output from the terminal to set the rocket to launch. I'm thinking you'd probably have to have some sort of hardware solution for that to send the signal to launch the rocket.
Something more low level might be to get a protoboard, some 74SL series logic chips, an lcd output and physically making a counter that counts and when it reaches 0 then you send a signal (1) to the model rocket which fires. You'll probably need some sort of microcontroller though and as for programming you have to do it at a very low level so someone who knows ASM or C should be able to program it.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE