Dude, see what is throwing, it is rate of change of momentum, or force, that needs acceleration. The aircraft is not in much acceleration at the point when it launches a missile. Now let's assume it is at 1.5 M and hence the initial velocity of Brahmos missile is 1.5 M. Now the engine will kick in after a time but even then let's assume it kicks in on the same moment. The missile will gain velocity due to the ramjet engine on board, but the ramjet has a maximum topspeed of say 3M. Eventually brahmos will stabalize on it. If MKI was to accelerate very quickly and then launch a say hypothetical scramjet missile, it can hypothetically it can take speeds up to 5-7M because the scramjet engine has the capacity to reach those speeds, I think something like that is what zircon/ Brahmos II ALCM will do. Even in the case of Brahmos II I think the missile will be launched from a cruising speed probably with a rocket booster.