Thomas J. Curtiss

Tensile Tests of Paraffin Wax for Hybrid Rocket Fuel Grains

The tensile strength, elastic modulus and percent elongation of paraffin wax doped with small concentrations of low density polyethylene (LDPE) where measured. The centrifugal casting process was used to create paraffin wax / LDPE wax hybrid rocket motors. Tensile tests were then performed on samples taken from these test motors. Concentrations from 0 % to 4 % LDPE added to the paraffin wax were tested. The tensile strength, and elastic modulus increased with increasing concentration of LDPE. The results show that paraffin wax motors can be created that have a greater elastic modulus and similar tensile strengths compared to current solid rocket motors based on HTPB rubber motors. However the paraffin wax motors were found to be less elastic than HTPB with a much lower percent elongation. Void formation, tiny bubbles in the paraffin wax created during the casting process, were found to affect the tensile properties measured.

Characterization of the Performance of Paraffin / LiAlH4 Solid Fuels in a Hybrid Rocket System

This investigation examined the burning characteristics of paraffin-based solid-fuel grains doped with various additive percentages (up to 28%) of lithium aluminum hydride (LiAlH4). In addition, the test sequence included examination of a paraffin-wax based fuel formulation containing 10% triethylaluminum and another formulation containing 10% diisobutylaluminum hydride. The fuel grains were cast into paper phenolic tubes and then tested in a cartridge-loaded hybrid rocket system. It was found that under similar test conditions, increased LiAlH4 additive increased the overall chamber pressure throughout the duration of the test, caused by an increase in the ratio of flame temperature to the molecular weight of the products. Due to deposits of unburned and unreacted fuel in downstream sections of the hybrid rocket motor, an accurate correlation between increased additive percentage and regression rate was not able to be found. It was determined that a new set of fuel grain formulations with changes to the overall fuel matrix (e.g., higher melting point wax) and/or changes to the energetic additive particles (e.g., reduced particle size) will allow for more accurate regression rate calculations and more favorable combustion characteristics. Despite the necessary modifications to the fuel formulations, the results from this series of tests showed that nearly all these solid-fuel formulations burned similarly. Qualitative comparisons of each type of fuel formulation proved to be a beneficial method for improving the solid-fuel formulations for future tests for hybrid rocket motor applications.

Testing Hypergolic Ignition of Paraffin Wax / LiAlH 4 Mixtures

,The hypergolic ignition of paraffin wax doped with LiAlH4 was investigated. The paraffin wax/LiAlH4 mixture was found to be hypergolic with 363 K - 373 K nitric acid at concentrations above 30 % LiAlH4. The mixture was also found to be hypergolic under certain conditions with 296 K nitric acid. The exact ignition mechanism was explored with chemical kinetic modeling and experimentation. Further investigation finds the doped wax to be hypergolic with several other strong acids; sulfuric acid and hydrochloric acid, but not the weak acid, acetic acid. Hypergolic ignition of paraffin wax with N2O4 was observed but a reliable ignition system was not found.

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 CenterPerforated 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.

Generation of intense, hexapole-selected, supersonic beams of fluorocarbon radicals: CF, CF2, and CF3

Intense molecular beams of fluorocarbon radicals were generated with a corona discharge passing through a supersonic jet expansion of 15% C2F6 in Ar. An electrostatic hexapole separated the polar neutrals entrained in the beam by selectively focusing them through a small aperture. Molecules were separated according to the ratio of their mass to permanent electric dipole moment (m/μ) and according to their rotational state. Our most dramatic success was in isolating CF radicals in a single rotational state: |JΩMJ〉=|1/21/21/2〉. The pure, state-selected CF radical beam was characterized by a narrow velocity distribution (Δv/v∼14%) and high flux density (J=8×1011 radicals cm−2 s−1). Adjustments in the hexapole voltage and discharge conditions produced variations in the beam composition of 100% CF, 0% CF2, 0% CF3 to 20% CF, 0% CF2, 80% CF3, and 60% CF, 15 CF2, 25% CF3 with no other detectable components (e.g., F and C atoms, C2F6).