Ooeer... it isn't rocket science!
Titanic's rockets were fired from dedicated sockets attached to the vessel at an angle. A lanyard was required to fire them. There was probably some detonation 'bang' as they shot upwards to a considerable height, then a further 'bang' as they exploded into white stars. Apart from my guess as to the detonation 'bang', the above is as per the Cotton Powder Co. spec.
One would not want to insert a further unfired rocket into firing socket until same had cooled down after a previous firing and heat generated.
However, after 12.25am etc on Titanic on the 15th April, in the freezing cold conditions, this would have been of no worry, and a 5 minute delay or longer between firings, could have been considerably increased, and alternative socket fixings for firing utilised port and starboard, so that the distress rockets could have been fired alternatively from each side's socket in very quick succession, and there were ample stocks of boxes to fire off.
Boxhall knew the ship was going to founder and sink. He should have kept firing all those rockets in quick succession. The Titanic had no further need to retain any stock of distress rockets - she was doomed and sinking. If he was not personally able to give attention to this, he should have delegated far more frequent firing of all the stocks of rockets to Rowe and others.
It does not require any skill to insert a rocket into a socket, stand back, and pull the lanyard to fire it.
In the middle of the night, with a dangerous ice field stopping The Californian and knowing Titanic was fast approaching (from other Marconi reports from other ships) an extensive ice field some 20 miles southwards if both ships were on the correct tracks, Stone sees a 'flash' then 7 white rockets that 'burst', and Gibson sees 3 of these that exploded into white stars when he rejoined Stone on the flying bridge.
My personal view is that Stone ought to have realised the importance of the signals he saw, and had Boxhall done his job properly, with more rockets fired at closer intervals than there would be no doubt whatsoever.
Cheers,
Julian