Testing of hybrid rocket fuel grains at elevated temperatures with swirl patterns fabricated using rapid prototyping technology

Hybrid rocket fuel grains fabricated with rapid prototyping technology enable the use of complex internal structures and port geometries. Using rapid prototyping to print features that introduce flow disturbances and increased surface area can result in improved regression rate and combustion efficiency without the need for difficult machining and casting procedures. In some small-scale hybrid rocket applications, such as small satellites or CubeSats, a lack of robust environmental control might require the motor be used at elevated temperature. Additional increase in regression rate can result from firing the rocket motor with an elevated initial fuel grain temperature, however, due to slumping in liquefying hybrid rocket fuels this is also typically accompanied by a decrease in combustion efficiency. In order to characterize the performance of various fuel grains at elevated temperatures, printed fuel grains with a heterogeneous paraffin and acrylic matrix supplied by The Aerospace Corporation were compared with cast paraffin grains using the Long-Grain Center-Perforated hybrid rocket motor (LGCP) at the Pennsylvania State University’s High Pressure Combustion Laboratory (HPCL). Results from the LGCP testing showed the effects of initial temperature on regression rate and combustion efficiency. The calculated regression rate and combustion efficiency for each fuel grain was compared to previous testing at Penn State and a correlation previously developed for room temperature paraffin fuels. Regression rate increases of over 20% were found for the heated fuel grains, both printed and cast. As expected, the cast paraffin fuel grains experienced a decrease in combustion efficiency as unburned paraffin wax was expelled from the rocket. The printed fuel grains, however, maintained the combustion efficiency of a room temperature cast paraffin fuel grain. The addition of swept honeycomb cell structures utilizing rapid prototyping technology reduced paraffin slumping and allowed more complete combustion at elevated fuel grain temperatures.