Shock tests of recrystallized candy rocket propellant
Static-compression tests of recrystallized candy rocket propellant reveal it to be very strong in this regard. In my most recent of such tests, I placed a sample cylinder 5/8 inch in diameter by 3/4 inch long under a static load of 500 pounds, and kept it there for a full minute. The sample did not crack, crumble, break, ignite, or fail in any catastrophic way. It deformed only very slightly, measuring just under 3/4 inch after compression.
(for details, see: http://members.nbci.com/jyawn/compression/index.htm)
I have no doubt that this fuel could be induced to fail under a static compression load, but it has exhibited such strength as to suggest that failure is more likely to be due to different factors, such as shock.
When I have a large piece of candy and want smaller pieces, one reliable method is to put it in a plastic bag, lay it on a hard surface, and hit it with a hammer. It takes a pretty good whack, but it works quite well. So I know that this candy can fail from a sudden compression load.
I had envisioned rigging up a known weight to be dropped from a known
height on a known sample of candy, but had not quite figured it all out.
Recently, Al Bradley suggested a variant - to drop the fuel sample from
a known height. With this insight I realized that I could make a simplified
test of this sort very quickly.
My plan is to roll 100 grams of recrystallized KN/sucrose into a ball, let it cool, and drop it from a measured height onto a solid surface. The test will be repeated, increasing the drop-height until some kind of failure occurs.
This ball is actually 106 grams. I let it cool for five hours before performing the drop tests. It is wrapped it in one layer of Handi-Wrap to keep it dry and clean, and tied with a string. I will drop it wrapped, so the parts don't get scattered and dirty.
The receiving end is one of those concrete foundation blocks used in the static compression tests. I level it to make sure the ball strikes it squarely, and suspend a surveyor's tape next to it so I don't have to guess the height.
In the first test (above) I dropped the ball from 3 feet. It bounced several times, but was not visibly damaged.
In the second test, I dropped the ball from five feet. The ball bounced, but also had a "spall" fracture
Click here to view a 15-second video of this test (344k)
Third test is from 7 feet. I re-wrapped the ball so that it would hit on the side opposite the fracture from the last test.
Click here to view a video of this test (344k)
This time it cracked, three big pieces and several little ones. The video is revealing because of the sound. It made a sharp "klink" the first time it hit the concrete but the second time sounded like a raw egg. This makes me wonder if the sample broke on the second bounce, which would suggest that the fractures were cumulative.
And yes, I took the rope-swing down from the platform.
Click here to see me play monkey (344k)
Click here to see me play monkey some more (344k)
The rope broke on the last swing. Too bad I did not have the camera set to catch it. But I am OK, thanks for asking.
Now for my favorite part - the candy will be healed!
Well, maybe its my second-favorite part. Firing rockets also ranks pretty high.
After heating the broken shards to 200 degrees the sample is rolled into a ball again, ready for re-testing. And after all the tests are done, I can make an engine with it. Recrystallized KN/sucrose can be re-heated many times with no apparent degradation. Not only is candy environmentally friendly, it can be recycled.
So what is the significance of this? Obviously this candy can fail if struck hard enough. Is this significant in a rocket engine? Seems like this depends upon the design. Several Bates grains in an engine might bang into one another upon ignition. I suspect that once each grain is fully ignited the flame-front would tend to "pad" the grains - the burning fuel exerting enough pressure to prevent them from making violent contact with each other. Perhaps some combustible padding could be placed between the grains, like fuse paper or black match.
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