Inasmuch as a photon torpedo is "not a missile." You can't have impulse without thrust. You can't have thrust without a reaction. A drive system that expels a reaction mass to produce thrust is, essentially, a rocket. Impulse engines are not that simple, and they do a lot more than simply "expel thrust." That, probably, is the DIFFERENCE between an impulse engine and a conventional rocket (much like the difference between, say, a rocket and a turbojet).