Looks like I misread what Terry meant in saying "BP is not particularly
sensitive to pressure." I thought he was referring to pressure during burn
as opposed to the pressure of packing/tamping it. My bad.
On Thu, Apr 20, 2017 at 8:22 AM, Dave McMillan <skyefire@xxxxxxxxxxxx>
wrote:
On 4/18/2017 3:09 PM, Terry McCreary wrote:
Hello Dave,
I am extremely wary of PVC casings; many years ago I saw one fragment into
quite sharp pieces from about 50 ft away, and one is more than enough for
me... I know that any number of people have used and even recommended PVC
for such applications. I'm not one of them.
That being said, black powder motors have been made for literally
centuries by tamping the powder into (PAPER) casings. BP is not
particularly sensitive to pressure. And sugar propellant (without sulfur)
is probably significantly less sensitive than BP. For me, it's the casing
that makes me cringe.
I'll admit, my biggest "cringe moments" came from the lack of any
safety warnings (like, "DO NOT be clever and *mix* the powders in the
blender!", or "don't try using a metal tube and metal tamping rod", or
"minimum safe distance"). And the way he was tamping the mix with those
open wineglasses full of propellant mix in the background, *bouncing* and
almost falling over....
Still, I've certainly seen *worse*. And given this person's track
record, I suspect he's being more careful than he appears. I just cringe a
bit thinking about ignorant kids trying to copy his example and inevitably
trying shortcuts.
On a semi-related note, another garage experimenter cut an Estes motor
in half and filmed the burn with a high-speed camera
<http://sploid.gizmodo.com/someone-cut-a-model-rocket-engine-in-half-so-you-can-se-1794422133>.
This one is less cringe-worthy, except when he was actually cutting the
engine (he did wear a full faceshield while doing it, though, and kept his
hands away from the potential ignition event, so he wasn't being careless.
Risky, maybe, but not *careless.*)
One interesting side effect of the setup he used is that it
effectively turned the motor from an end-burner into a "side burner," due
to lack of seal between the acrylic and the motor. The delay and ejection
charges also appear to get short-circuited and ignite during the primary
burn.
