To be fair, speed is relative. Imagine a plane flies at 500 km/h and is pursued by another plane at the same speed. If the first plane fires a rocket backwards that accelerates for a total of 200 km/h, then for an observer on the ground the rocket will still do 300 km/h, in the same direction as the planes. However, the guys in the second plane will see a rocket approaching them at 200 km/h.
Wind resistance, aerodynamics, etc. will have an impact, but it can work.
The missile would have to cancel out the speed of the plane before achieving any meaningful acceleration.
To be fair, speed is relative. Imagine a plane flies at 500 km/h and is pursued by another plane at the same speed. If the first plane fires a rocket backwards that accelerates for a total of 200 km/h, then for an observer on the ground the rocket will still do 300 km/h, in the same direction as the planes. However, the guys in the second plane will see a rocket approaching them at 200 km/h.
Wind resistance, aerodynamics, etc. will have an impact, but it can work.