Things that go "bump" in the flight

[Sooner Boomer]

It was great to see so many figh power flights at Airfest! There was one thing that happened frequently. I asked, and got several answers, some contradictory. During the motor burn, sometimes almost from the start, I could hear a sound - a bump (or thump or pop) - several times during the flights. Sometimes only two or three times, sometimes almost continuously. What causes this?

I've been told:
the motor is spitting the grain liner
it's voids in the grain (or other irregularities)
it's slag being spit out of the nozzle

I never see any change in the flame or in the smoke trail when this happens. I have never seen this happen when a motor burns in a test stand

Ideas? (and can you back up your opinion with data/observations, or other facts/)

 

[dhbarr]

Seems reasonable that anything which briefly clogs the nozzle would make a pop. I'd lean toward casting tube bits, as sugar motors are very slaggy with smaller nozzles but I don't recall them popping a bunch more.

An easy test would be to find a reliably poppy motor and glue the grains in. Pop disappears, there you have it.

 

[ECayemberg]

Spitting the casting tube(s). Tapered core grains are more likely to do so; bonding grains to liners prevents this.