Fuel Additives

Several fuel additives will be investigated for favorable augmentation of the paraffin's thermal and mechanical properties. Paraffin alone is a brittle, translucent solid. This means that thermal radiation from the combustion zone is allowed to penetrate the fuel grain. radiant heating within the fuel grain can reduce the performance of the hybrid system in terms of total impulse since the paraffin will be depleted too quickly and incompletely combusted with the oxidizer. In the worst case, this radiation heats the liner and melts the fuel from the outside in. These effects make pure paraffin fuels poor for long duration firings. Also, paraffin has a propensity to crack under the mechanical loads and thermal gradients found withing a large scale hybrid rocket motor. High regression rate fuels in general exhibit problems in that the high O/F ratios required for efficient combustion often cannot be met because of limitations associated with oxidizer injection and a surplus of unburned fuel.

In theory, the addition of a light metal to the fuel could solve several of these problems, while significantly improving the density specific impulse performance of the fuel. The metal opacifies the fuel, reflecting thermal radiation, preventing it from penetrating the fuel grain. Also, the metal lowers the optimal O/F ratio in terms of characteristic velocity, meaning that the fuel can be more completely combusted with achievable oxidizer flows, allowing for increased burn durations.

 

In reality, however, the metal may not have enough "residence time" (Fintel) in the combustion chamber of a smaller motor to effectively contribute to performance. Aluminum, in particular, may not contribute substantially to performance because it forms an inhibiting oxide coating. Magnesium doesn't form an inhibiting coating and it offers a theoretical performance comparable to that of Aluminum. Magnesium will be tested as a possible additive. At the very least, the metallized propellant will reflect the majority of the thermal radiation that could compromise the fuel grain. Another drawback of metallizing the fuel is that rates of conductive heat flux into the grain are increased, possibly having a similar effect as the radiation.

Graphite is another additive that opacifies the fuel. Unlike a metal, graphite absorbs thermal radiation and transfers it to the surrounding fuel. This effect results in a higher, pressure-dependent, regression rate. The drawback is that graphite also conducts a substantial amount of heat back into the fuel grain. Activated charcoal and carbon black display similar thermal absorption properties to graphite except activated charcoal exhibits a massive surface area due to high porosity on a nano scale. Hence, activated charcoal is an effective insulator. Moreover, activated carbon's micropores provide good conditions for adsorption to occur, making it a potential catalyst in some reactions. The use of activated carbon in conjunction with nitrous oxide and a metallized fuel may speed up the combustion process, decreasing the necessary residence time for the magnesium within the motor.

Finally, the fuel grain itself can be strengthened against the mechanical and thermal loads with the addition of polymeric fibers like cellulose. Anything from dietary fiber to a cloth fiber matrix may supply this added structural integrity. These combustible additives will be investigated in the hybrid rocket's fuel grain. Adding these polymers will present unique challenges to fuel grain fabrication techniques.