THE IDEA:
The idea is that you have an HE grenade or some other grenade, and you need to throw it FAR. With a rocket grenade, you throw it, and once it's in the air it shoots flame out the back and goes flying off. It would have a slight arc but not much. It wouldn't blow up untill it hit something. This idea also works if you want to blow stuff up from far away. Maybe like an ion grenade, bu that's a different story.
THE SCRIPT:
The script should be fairly simple. one way to go would be to make it so that when you throw the grenade it goes REALLY far with a slight arc and then add a flame emmitter or something like it to the back of the grenade. I don't know how you wold make it detonate on impact but it's probably possible.
THE MODEL:
You could use the HE grenade model for simplicity or you could go all-out and make your own hi-tech model.