It's the propellant. Even with magnelite ignitors I can't get them to go at the same time. I can be reasonably sure I'll get both. But the greens come to pressure slightly longer in some. You can minimize this by getting motors out of the same lot, but it just seems to be a characteristic of greens. Whites do it too, my I200's didn't come up at the same time. Most motors have a slight lag from ignition to hitting pressure. You want motors where that lag on both will overlap on the rail. too fast like W9 and the rocket is gone before #2 has a chance... too slow and you get my H250G cluster, where #2 comes up to pressure at 30 feet and it hooks, or just never lights.
I wouldn't say it's unsafe....you just need to be careful. Good ignitors, and long long leads. I use the red caps on the nozzles and it'll take the ignitor for a ride until it goes. I hit one G76G cluster with magnelight and 2 foot igniters wrapped in parallel on a loose whip. I feel good about that setup, but it did do a little wiggle going up.
Blues seem to be the sweet spot for clustering. F42, G80 and G138 I got both sides to light on the rail. My next flight will likely be H238T's
I haven't tried it, but red's seems to lag on ignition too much to cluster well.
I haven't tried white Gs, others seem to have good luck with them. Fresher will be better. Also sand the cores on the upper grain and leave the dust in.
and I will be checking that site.
