Balcony Shoot #12 - Clark's Giant Steel Gerb Composition Test

Date: 2003-10-10


Small half core-burning, half end-burning gerb as an experiment to test composition properties. 5 mm ID, 50 mm long, choked with cat litter nozzle, 2.4 mm ID nozzle and core, 25 mm deep.

Fused with meal prime filling the core and fuse taped across mouth of nozzle. It took 3 tries to get it lit, one with straight blackmatch, one with meal-NC twisted fuse, and last with meal-NC fuse and meal prime.

The composition is:

10 Potassium Nitrate

2 Iron (20-40 mesh)

2 Red Gum

It is slightly sticky, even bone-dry, and rams very well, not unlike non-melted candy propellant. I am sure moistening with ethanol would be a good idea for larger devices, but for anything under an inch ID it seems redundant.


It sucked!

Well, as an effect, it obviously needs to be huge to work well. The very small internal surface area produced a pretty tiny plume. The end burning part just sat and smoked.

It clearly doesn't scale down well at all. This isn't suprising, the full size device is 50 mm ID with a huge cavity that runs the full length of the tube and it still burns for a long, long time.

Even this short burn absoutely roasted the 5 mm thick fully PVA pasted walls of the tube, the chared and shrunk from the dehydration, especially in the extended end-burning phase. The case didn't breach though, and similar dimensions (OD = 3*ID) should be suitable for a scaled up version.

I think high-carbon steel (ie sparkler steel filings) would work much better, giving bright dividing sparks. My Iron is fairly pure, not a grey cast-iron dust, and gives long-lasting orange sparks that don't divide. When my few pounds of sparkler steel arrive from Syklighter I'll give it another go with a 25 mm case.


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