Traditional hybrid rocket fuels suffer from poor regression rates, and hence poor thrust. Additionally, they have relatively low efficiency. Paraffin has been explored by many as a potential solution to the hybrid fuel problem, as it regresses quickly and has a fairly high specific impulse, burning similarly to other hydrocarbons like kerosene.  It is also incredibly easy and safe to cast into grains, and leftovers can be remelted and recast to enable efficient production.

 

However, paraffin fuel grains have the unfortunate tendency to slough off large fragments during the burn as the paraffin melts, leading to large combustion instabilities and losses of propellant. With the regression rate as high as it is, it is also common to see a nontrivial amount of the paraffin droplets released into the combustion chamber escape through the nozzle without combusting, leading to a characteristic "after burning" plume. Paraffin is also more difficult to reliably ignite in a rocket motor than alternatives like HTPB or PBAN. Finally, non-actively-pressurized hybrid rocket engines suffer from a severe shift in oxidizer to fuel ratio and combustion chamber pressure over their burn which lead to performance losses and instability due to flow separation in the nozzle. 

 

SquidWorks has taken these issues and iteratively introduced a series of solutions to address them through testing on our Mobile Vertical Test Stand. Collecting combustion chamber pressure, oxidizer tank pressure, oxidizer temperature, high fidelity thrust readings, and multiple slow motion video, FLIR video, and high speed photography data sets during each test with the Mobile Vertical Test Stand at either of our two test sites helps give us the data we need to make intelligent decisions about adjustments to be made in the fuel grains for the next test.

 

SquidWorks Ignition Medium (SWIM)

The first problem to consider when dealing with paraffin fuel grains in a hybrid motor system is ignition. For our first test fire, we cast an thin layer of paraffin with suspended solid rocket fuel around the top few inches of the fuel grain. SWIM v1 worked very well, providing rapid ignition of the paraffin fuel grain, however, it complicated the production process, and brought in new safety concerns for dealing with the relatively volatile solid propellent infused paraffin. From past testing our members had conducted, we knew that ABS tended to ignite readily in hybrid motors with a relatively simple pyrotechnic igniter. With this in mind, we have integrated in a very thin, burnaway ABS liner that covers the top 5 inches of each 18 inch grain's port- SWIM v2. This ignition medium layer is printed as part of the 3D printed ABS liner that makes up the outermost layer of each of our fuel grains, requiring no added steps in production and no new volatile materials.

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Anti-Sloughing Baffles

The next major problem to consider with paraffin grains is the sloughing off of fuel grain fragments. Previous research at MSU lead to mixed results over the use of azimuthally partitioned fuel grains. While sloughing was seen to be controlled better with a larger number of partition cells, performance decreased. After several tests to determine an appropriate geometry, SquidWorks has come to use a series of baffles that extend partially into the fuel grain and provide structural support where the fuel is most likely to separate from the grain. Thrust stability has increased dramatically since the introduction of the baffles, and performance has increased as well. 

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Additives

Pure paraffin makes a good fuel, but it can be improved upon greatly with the use of additives. We are currently in the process of testing several different additives including atomized aluminum and potassium nitrate in several different mixture ratios to explore the best avenue for performance enhancement in our hybrid rocket engines. Our best results so far have been with aluminized paraffin, achieving 110% of the rated impulse of the best performing commercial fuel grain available for the motor we are using.  With over 6000 N-s of impulse, our fuel grains provide the best performance for the price (~$20 for the fuel grain itself in materials).

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We are currently exploring fuel mixtures containing solid oxidizer additives. During our first test, we were able to achieve nearly constant combustion chamber pressure and thrust over the course of the burn, as the liquid oxidizer flow dropped, and the solid oxidizer embedded in the grain began to react. Relatively small amounts of solid oxidizer are used (less than 20% by mass), meaning that the grains are not easy to ignite outside of an engine and do not require any special storage or handling procedures like solid motors. Testing is ongoing to combine the stabilizing properties of solid oxidizer enhanced grains with the impulse boosting afforded by atomized aluminum.

 

Having large amounts of particulate additives requires solidifying the paraffin suspension quickly to prevent settling. Cooling from the outside, however, results in delamination of the fuel from the grain casing, resulting in excessive sloughing of propellant during the burn. With this in mind, our vacuum-driven, grain-filling apparatus cools the paraffin to the point of solidification within 10 minutes from the inside using a liquid-cooled port mold.

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