Permanganate mixtures are known to be rather unstable. They are highly energetic, and unpredictable. As such, they are not widely employed in pyrotechnic use. They are, unfortunately, used by amateurs who are uninformed as to the dangers of this material. I would avoid using them for rockets. Ignition time on them is difficult to gauge. Much depends on particle size, humidity, temperature and a number of other variables. The best thing to do is stick to potassium nitrate/sugar mixtures, as everyone else here has said. If that's not good enough for you, go check out black powder rockets on the web. There's plenty of info out there.