The high regression rates of paraffin, along with the ease of manufacturing and storage, were the main arguments against fuels with lower regression rates such as HTPB or ABS. Having made the choice, 2 issues remaind
To avoid uneven burnback and therefore risk of catastrophic failure, the fuel grain should be smooth and free of air pockets. To achieve this, usually a method called spin-casting is employed. In our case, to avoid building an entire machine, we decided for an alternitive. This would mean pouring the paraffin mixture into a mold and then placing it into a vacuum chamber, which we may be able to borrow, to remove air bubbles. Another alternative is placing it under a hydraulic press, leaving a small hole for air exhaust, pushing the air out.
Regarding the chemical additives, two perspectives are kept in mind. Propellant stability, and regression rate. The regression rate, the speed at which the solid fuel surface recedes, increases if the propellant has a high thermal absorption. Considering the regressions' importance on thrust capability, carbon black coloring was considered. However a decision has not been made.
In regards to stability, an addition of ethylene-vinyl acetate (EVA) was decided. It has previously been used in other hybrid rocket motors, thanks to its adhesiveness and stability, minimizing the risk for cracks during and post production. However, due to its limited and expensive nature, we will harvest it from gluesticks.
The following document attempts at casting experimental paraffin fuel grains. The purpose is to inspect the quality and to concretise the method.
Early attempts used a simple design with a removable bottom, an o-ring to ensure no leakage and a piece fitted to be pressed down the cylinder. This was designed to make a 80mm tall, 45mm in diameter fuel grain with the central hole 15mm in diameter. The purpose of the press was to simulate a hydraulic press since that was the initial plan. This was printed using standard PLA.
Early attempts also used a microwave to melt paraffin, which took a long time and also questionable in terms of safety.
What we discovered from these early attempts:
Later attempts use a larger base (80mm diameter) to facilitate easier removal and the central hole is now 30mm. Also the o-ring was deemed unnecessary and the outer walls are now able to be separated in halfs. For these attempts we used an oven with stovetops. WD-40 was also used in hopes of facilitating easier removal.
What we discovered from these attempts:
At this point it was decided to cast the actual fuel grain in the liner, a phenolic tube. It was also decided that we want empty spaces over and under the fuel grain, the pre-combustion chamber and the post-combustion chamber or mixing chamber.
Since casting within a tube is different, a new design on the mould was required. The new mould consist of 3 parts, the central metallic tube from before, a cylinder (80mm inner diameter, 200mm height, 5mm thick walls), a puck that slides in and a press with long handles. The cylinder rests on the base, a part of the puck, with 4 'hinges' such that when screws are inserted it locks the cylinder in place. The puck extends 45 mm in the cylinder to simulate the empty space. The puck also has a indent 3mm deep to fit the central metal tube. A flat press with a hole in the middle slides along the tube and has two rectangular extruding handles for a total height of 120mm. Utilizing this new design, multiple tests were performed.
Phenolic tube and carbon black has arrived.
We are planning to add only a little paraffin at a time. Let it cool to some degree and then add the next layer in a way that no delamination occurs. This would eliminate the shrinking problem and large voids. It will also be useful when carbon black is added, so that the carbon black does not sink to the bottom and create an uneven distribution.
We also decided to test in larger batches.
Additionaly the inner diameter is increased to 40 mm.
The carbon black used is ordered from https://lerochem.eu/sv/pagrindinis/353-kol-carbon-black-monarch-570-by-cabot-kg.html.
In particular it has small particle sizes, most below 144 nm. It also has decent surface area (B.E.T.) up to 550 m^2/g.
IMPORTANT: Fine powder, handle with care. Easily spreads in air, wear masks or something. It might be fine and smooth, but it is still irritating and gets everywhere. Try to keep it clean, else it will be a pain to clean up.
Pot: 919 g
A glue stick (7,2 x 100mm): 4 g
Glue stick assuming 1150 kg/m^3: 4,68 g
Intended ratio of [paraffin : glue (EVA) : CB] is [90 : 10 : 1]
A ratio of [80 : 20 : 1] might be more desirable, conclusions reserved for actual tests.
Not accounting for CB, we are aiming for 10% EVA. Note that we are assuming a glue stick is 100% EVA, but are actually less than 50% EVA. Might adjust in future depending on results.
Also, note that the following masses are before casting. Loses are incurred as spill and residual in the pot etc. Instead finale weight displays the resulting weight.
Glue: 8 g
Carbon black added: > 5 g
Paraffin: ~250 g
Total weight: 261 g
Final weight: N/A
Notes
Glue weight: 44 g
Carbon black added: 4,5 g
Paraffin: 450 g
Total weight: 494,5 g
Final weight: - g
Notes
Large tests, same as last time, are continued after summer break with the goals of minimizing splash and rapid cooling before tests on phenolic.
Glue weight: 67,58 g
Carbon black added: 4,50 g
Paraffin: 382,40 g
Total weight: 455,48 g
Final weight ~ 405 g
Intended ratio: [85 : 15 : 1]
The more precise values were achieved by a new scale, MAXIM-700.
More glue sticks (with other dimensions) were sourced from another workshop.
Notes process
Notes results
Glue weight: 122,1 g
Carbon black added: 8,16 g
Paraffin: 690,5 g
Total weight: 812,76 g
Final weight ~615 g
Intended ratio: [85 : 15 : 1]
This tests the new amount of fuel to be used in the rocket. As such the cylinder (phenolic sub) was elongated. Other changes to the mould is deeper hole in the puck and the grippers around the puck was removed.
This time the central rod will be continued to be heated between pours using a heat gun. Pours will be strictly kept above 95°C and time between purs will be kept more stricty using a alarm instead of a visual film on the paraffin. Also aluminium around the mould was abolished as it is thought thta the effects are minimal.
Notes process
Notes result
A arbitrary whole cylinder fuel grain we are planning to drill a hole through instead. It is the same composition as No.3 and No.4. Poured in one go and tried to use clamps to add pressure, this way tghe fuel is free to rotate. By rotating it as it cools, the carbon black can be evenly distributed. However it was poured too hot (105°C) and deformed the plastic, causing our seals to no longer be water-tight. So the press was changed and the paraffin had cooled down a lot (guesstimate: 60°C). It should instead be 85°C or less. This will not cause any issues either since the central metal rod was removed.
The drilled hole was not as even and centered as we had hoped. Drilled with drill press.
Three smaller fuel grains to be drilled into as practice and tests towards a perfected hole. At the same time the method above to abolish layering will be further tested.
4.6 - Will be poured at 90°C, pressed and regulary rotated. (red)
4.7 - Will be poured at 80°C, pressed and regulary rotated. (blue)
4.8 - Control. Will be poured at 65°C, not pressed or rotated.
We know 105°C is too high, so we are testing 90°C. If that is too high we will see how 80°C does. Altough we will aim for higher temperatures for the actual fuel grain in th ephenolic tubve as theose genrally show better results. The control is poured at a very low temperature to see if this lowers shrinking and prevents carbon black from sinking via quick solidification.
The new presses were misprinted, so impromptu presses were old pucks with holes. These were not long enough and so No.4.8 was skipped and even then No.4.6 was not properly pressed. No.4.6 and No.4.7 were pressed immediately after being poured and then rotated. Both show signs of deformation, so it is concluded that the pressure we exert on the moulds are too much at these temperatures.
The press with a large empty space amde it hard for all the air to get out and resulted in voids. Also air got in through the bottoms, resulting in shrinkage there. Will use longer pucks.
Glue weight: 58,6 g
Carbon black added: 3,7 g
Paraffin: 336 g
Total weight: 498,3 g
Final weight ~ g
Intended ratio: [85 : 15 : 1]
We will use the method above, using proper press this time. We will also lower the pressure and temperatures, altough suboptimal on the microscopic scale, it might show better results on the general shape of the grain especially in these plastic moulds.
Notes
Notes results
We have decided against drilling the central hole as it would be problematic and not saving a lot of time anyway.
Once again the inner diameter is now 40mm, we will use the solid rod for now. Make sure to heat it up before pouring in fuel to avoid rapid cooling.
Remelted from previous batches.
Final weight ~ g
Intended ratio: [85 : 15 : 1]
We will attempt a higher temperature and pressure using thicker moulds. We will also attempt topping-off if the top and/or bottom is not flat and without voids.
Notes
May 20th 2025
Casting in the phenolic tube hinges on the fuel sticking to the phenolic, which is yet to be tested. Altough fuel grains are hard to remove, it is unknown whether or not it holds during a launch.
If it is possible to borrow a vacuum chamber, we may use that to remove air bubbles, but the shrinkage of paraffin might still be an issue. Also it is hard to predict how the fuel grain will behave in the mould.
The layering method eliminates shrinking and large voids. Small bubbles might still persist and this method might be hard to combine with the vacuum chamber or hydraulic press. It is unknown wether or not the small bubbles will present a problem.
We are planning to apply constant pressure after the last layer using the hydraulic press to create a nice edge. Hopefully this also removes some air bubbles in the process.
Future experiments should test casting on an actual phenolic tube and also test the behaviour in vacuum as well as the effectiveness of the hydraulic press.