Calculator paper tape case, many layers taped, no glue - 2 mm thick. 6 mm ID, 50 mm overall length. 3 mm diameter nozzle hole, penetrating only a mm or so into grain. Cat little nozzle 8 mm thick, bulkhead of paper wadding, no hot-melt glue.
Propellant of meal blackpowder, my best stuff, the kind that has in the past blown the motor to pieces in core-burning configuration. The purpose of this test was to assess if end-burning configuration would produce usable thrust (and also to test the hypothesis that over-pressure has been my main problem).
Meal-NC tissue paper fuse, no priming. Kebab stick.
It flew OK.
At last, a black powder motor that didn't explode! Well, OK there has been a few that haven't, but not for a while. The performance wasn't fantastic, it got caught by a gust of wind and flattened out in flight, flying down-range a good 200 metres.
Burn time was about 2 seconds, the rocket landed in the ocean and was not recovered. The main lesson learnt is that core-burning pressures are just too great for my primitive cases and bulkheads. (Although note that this case was deliberatly poor as a worst-case experiment.)
The thrust produced was marginal, a short core cavity should improve this without blowing the motor up. The motor is too small to add a pressure gauge or other instrumentation, so it is back to the cut-n-try method to find just how deep I can make the core before it blows up.
I am too chicken (and underfunded) to do a closed bomb pressure/rate test on my propellant, I could use published figures for blackpowder to /design/ the motor but I am still lacking data on my casings. I might have a go at testing my case material to get a rough figure for its tensile strength. Although I haven't had a hoop stress failure for a while, and better cases are on the way. My main problem seems to be bulkhead blowout which suggests some kind of grain/case bonding failure.
Ironically my larger motors seem to be more reliable, perhaps because of the consistancy of their ramming. The small test motors are fiddly to make and probably don't get rammed as consistantly as larger devices. Larger devices in end-burning mode may work better, but they are heavier, cost more for experimental shoots, and make a bigger bang when they fail. :-)
| title | type | size |
|---|---|---|
| Rocket Launch | video/x-msvideo | 1.854 Mbytes |