The Hydra have 2 attacks: one with rockets and one with the rocket who stay on the ground. That rocket is for one thing: If a rocket follow you (e.g.: rockets from Area 59) throw one of this rocket and the rocket that follow you, will go after the one sitting on the ground.
The attack 1, is using Left-CTRL and the attack 2 is Right-CTRL.
If you use attack 1, you can press and hold SPACE for aim the target (e.g.: a vehicle, a plane etc),wait until the cursor is make red and then press Left-CTRL. The rocket will go after the vehicle.
P.S.: My english is not very good, but I think you will understand. Bye!