[from occupied ga]

One word, “Hydrogen

Better still mix it with about 1/3 volume of O2, get a long string, glue black powder or pyrodex around the last 8 inches or so of the string, tie the pyrodex end of the string to the baloon (or tape it), start the string smoldering at the other end, and let it go. You can hear a large one from over a mile away. (The things one does as a college student)

[Myrddin]

My amusement consisted of soaking cotton string in a potassium nitrate solution, then letting it dry to make a slow, smoldering fuse. Muriatic acid (used for pool service) and aluminum foil liberates plenty of hydrogen to fill a few balloons. I would tie the slow smoldering cotton string fuse to the actual fuse of an M80, then turn the package loose with 3 big hydrogen balloons. It achieved pretty good altitude before the slow fuse lit the M80 fuse.