For this experiment, Tamir made many small 70 gram sticks of propellant (Potassium Nitrate and Sorbitol). In each, the mass ratio of the components of the propellant were varied. The purpose of this group of experiments was to find the burn profile of each propellant ratio before we go through the trouble of putting the propellant in actual engine casings. The resulting capsules of propellant were broken into pieces, taking note of which pieces were broken off of the top of the capsule and which were broken off of the bottom. They were then tested in a running fume hood on a safe stone surface, using a soldering iron as the ignition device.
Something that should be noted: the propellant was made in a double-boiler arrangement, meaning that it is possible that the molten propellant itself was not exposed to the same amount of heat that the hotplate was turned to. (For example: when we say that the propellant was at 400°C, this means that the hotplate was at 400°C. Since the wax acts like a buffer, it may take a long time for it to heat to exactly 400°C, meaning that it may be a while before the propellant becomes exposed to a full 400°C) Some experiments will be done in the near future to determine exactly what the heating profile for wax is.
Propellant ratios are given by (Ratio of Potassium Nitrate by Mass):(Ratio of Sorbitol by Mass)
20:80
20:80
Predictions:
- Basically the same as the 30:70 ratio
Notes on Preparation:
- It was heated to only 400°C in an attempt to avoid what had happened with the 30/70 batch.
Results:
- It is moldable around the center, and hardened at the edges.
- The bottom of the capsule lights but does not have any sustained fizz.
- The top of the capsule will not light at all.
- Black molten stuff and clear molten stuff is made as the propellant melts. The black molten matter is probably melted and charred sugar, while the clear molten matter is probably melted but uncharred sugar.
- The difference in the ratios at the top and bottom of the propellant capsule could not be avoided at lower temperatures, as can be seen by the different burn profiles of different parts of the capsule.
30:70 30:70
Predictions:
- Even worse performance compared to 40:60
Notes on Preparation:
- Almost a completely clear liquid, with some sediments at the bottom.
- The sediments were white and gritty, and were most likely KN.
- It was heated to a very hot temperature (450°C), which may have had something to do with the clear liquid; the SB may have gotten hot enough to begin decomposing.
Results:
- The entire thing is mushy; like a cooled block of caramel. It did not fully harden overnight.
- The highest part of the capsule when the propellant was poured was gooey and could be pulled apart by hand. This part never lit; it only formed some clear liquid and some black stuff (presumably charred SB)
- The bottom of the capsule was gooey and could also be pulled off by hand. It lit and had a sustained burn. It produced a lot of smoke and flame. This is probably because of the sedimentation. It would make sense that the sediments were KN and, if so, that would mean that it would all settle at the bottom of the capsule, skewing the proportions of KN to SB so that more KN was present locally, significantly changing the burn profile.
- This also means that the highest part of the capsule had a disproportionately high amount of SB, and that the true 30/70 ratio may have been modeled incorrectly.
40:60 40:60
Predictions:
- A slower, less explosive burn
Notes on Preparation:
- Melted easier and is more watery
Notes on Result:
- Hard to get started (the soldering iron needed to stay touching the propellant for a while until the burn could sustain itself)
- All other characteristics are the same as the 50:50 mix
50:50 50:50
Predictions:
- Less reactive, longer burn time
Notes on Preparation:
- More watery
Notes on Result:
- It came out well, and rather shiny or polished compared to our usual 65:35 propellant.
- It produced less dense, darker smoke as compared to 65:35
- It didn't produce any yellow blobs
- It produced an initial, short lived flame, but after that the only flame visible was the fuse-like flame.
60:40 60:40
Predictions:
- More or less the same as 65:35, perhaps a bit more reactive
Notes on Preparation:
- Smooth, easy melt at 400°C. It is not as sticky as the propellants made with higher sugar ratios. Also, it is pourable, but not overly viscous.
Results:
- It ignites easily and behaves like the 65:35 ratio (as was expected).
- Some caramelization apparently did occur with this one:
- The top of the capsule will not light at all.
- The bottom is similar to the 65/35 ratio
- The top did not make the flame characteristic of 65/35, but still produced about as much smoke. It seemed to be a little more reactive than the top one.
65:35 (control) 65:35 (control)
Produces a dense white cloud of smoke, purple flame, and spits out yellow blobs that become white as they harden. (these 'blobs' are presumably potassium carbonate, but we aren't sure about that yet)
A note on our usage of 'flame': by 'purple flame', we mean that the air above the propellant was burning a particular color. This differs from the small, red, fuse-like flame that signifies that the propellant is actually burning.
70:30 70:30
Predictions:
- Similar to 65:35, but a bit less reactive
Notes on Preparation:
- It melts at 400°C, but stays clumpy for a long time. Once it does melt, it begins to 'bubble'.
Results:
- Ignites easily.
- The top behaves like the 65:35 ratio, while the bottom makes a whole lot of residue (some of which is extremely hygroscopic). Both the bottom and the top produce approximately the same amount of flame.
80:20 80:20
Predictions:
- Barely reactive (if it ignites at all)
Notes on Preparation:
- It does not want to melt. It clumps at 400°C, but that's it. It was taken out of its pan and then shaped with paper towels into a cylindrical shape. It is very gritty and not sticky at all.
Results:
- It fizzes a bit, releasing small amounts of smoke, but quickly dies off as soon as the heat source is removed.
90:10 90:10
Predictions:
- Not reactive at all
Notes on Preparation:
- The mixture turned into fluffy, crumbly pieces pretty fast. It also had to be shaped with paper towels into a cylindrical shape.
Results:
- It melts. No fizzing.
Conclusions Conclusions
- The molten mixture gets more watery as more sugar is added, and more dry (until it is finally a gritty solid that barely sticks together) as more potassium nitrate is added. This is because the sugar serves as not only the fuel, but the binder.
- The mixture also gets more sticky as more sugar is added.
- The finished propellant is more explosive with more KN (until there isn't enough sugar left for it to react with), and less explosive with more SB (until there is not enough KN to react with it, in which case it doesn't ignite or anything)
