Radiation Heating Effects on Oxidizer-to-Fuel Ratio of Additively Manufactured Hybrid Rocket Fuels

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Radiation Heating Effects on Oxidizer-to-Fuel Ratio of Additively Manufactured Hybrid Rocket Fuels

Whitmore, S. A. / Merkley, S.

Recently, multiple authors have investigated the use of additive manufacturing for precision fabrication of hybrid rocket fuels. Extensive regression rate data for 3-D printed motors with thrust levels from 5 to 900 N, using both thermoset and thermoplastic feed-stock materials, have been collected. A class of printed thermoplastics based on acrylonitrile-butadiene-styrene was discovered to present unique burn characteristics not been previously observed with many legacy hybrid rocket-fuel materials. These printed thermoplastics are shown to produce fuel regression rates that strongly correlate to motor diameter, with smaller motors tending to burn strongly fuel rich with time. This burn behavior contrasts with the known burn behaviors of most thermoset hybrid fuels whose regression rates are only weakly dependent on motor size. Nearly all thermoset hybrid fuels tend toward a lean oxidizer-to-fuel ratio as burn lifetime progresses. To describe the anomalous behavior of 3-D printed thermoplastics, an extension to the classical Marxman regression rate model was developed to account for radiant heat transfer to the material surface. The model extension is empirically verified in this report and demonstrates that observed anomalous behaviors of certain 3-D printed thermoplastics result from dominant radiant heat-transfer mechanisms that are less significant for traditional hybrid fuel materials.