hi, i would be very glad if some one Could tell me the algorithm ,to fire a missile at another missile falling down in the y axis with certain velocity. i want to know the speed at which my missile should go so as to shoot down the enemy missile. Thank you very much.

It depends at what TIME you want it to be intersected. Half way on its fall? 1/4 on its fall?

Basically what you do is find out how long it takes to get to that position, then find the velocity required by the other missile to to that point in that time. There you go.