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.