Session
Technical Session 11: Propulsion
Location
Utah State University, Logan, UT
Abstract
The Propulsion Research Laboratory at Utah State University (USU) has recently developed a promising High-Performance "Green" Hybrid Propulsion (HPGHP) technology that derives from the novel electrical breakdown property of certain 3-D printed like acrylonitrile butadiene styrene (ABS). This electrical breakdown property has been engineered into a proprietary, power-efficient system that can be cold-started and restarted with a high degree of reliability. One of the issues associated with ABS as a propellant is its low burn rate. It is well documented in technical literature that hybrid rocket systems generally have fuel regression rates that are typically 25-30% lower than solid fuel motors in the same thrust and impulse class. Lowered fuel regression rates tend to produce unacceptably low equivalence ratios that lead to poor mass-impulse performance, erosive fuel burning, nozzle erosion, reduced motor duty cycles, and the potential for combustion instability. To achieve equivalence ratios that produce acceptable combustion characteristics, hybrid fuel ports are often fabricated with large length-to-diameter ratios. The resulting poor volumetric efficiency is incompatible with Small Satellite (SmallSat) applications.
This paper presents preliminary results from a collaborative development program between the University of Miami (UM) and USU. In this reported work, modern extrusion and 3-D printing techniques are used to fabricate sample ABS fuel grains with varying levels of copper-metallization. Hybrid-ABS fuel grains were printed from Cu-infused feed stock with 2%, 4%, and 6% Cu-mass concentrations. As baseline control, 100% pure ABS fuel grains (0% Cu) were also printed. Heat conduction via the additive copper (Cu) provides an efficient heat transfer mechanism that augments surface convection from the flame zone. Forced convection, the primary mechanism for pyrolysis for hybrid fuels, is generally inefficient due to "wall-blowing" associated with the radially emanating mass flow from fuel pyrolysis. Wall-blowing pushes the flame zone away from the fuel surface and significantly reduces the rate of enthalpy exchange. Homogeneously mixing a high conductivity metal such as Cu into the ABS fuel provides an efficient heat transfer mechanism, and allows radiant heat from the flame zone to be transferred deep into the fuel material. This process significantly increases the pyrolytic efficiency of the fuels.
The Cu-infused fuels were tested at USU using a legacy 12-N hybrid thruster system. Fabrication and manufacturing methods are described, and results of hot fire tests are presented. The top-level conclusion is that Cu-infusion of the printed fuels measurably increases the fuel regression rate, allowing for a higher thrust level with no increase in the required volume. The Cu-infusion has negligible impact on the propellant characteristic velocity and the overall system specific impulse. The increased burn rate and overall increase in solid-fuel density resulting from Cu-infusion allows a measurable increase in the propellant impulse-density. This increase in volumetric efficiency is potentially significant for small spacecraft applications where available space has a premium value. Follow-on methods that infuse lower-molecular weight and higher thermal conductivity materials like graphene and carbon-nanotubes are proposed.
Cu-Enhanced 3-D Printed Fuels for Green SmallSat Propulsion
Utah State University, Logan, UT
The Propulsion Research Laboratory at Utah State University (USU) has recently developed a promising High-Performance "Green" Hybrid Propulsion (HPGHP) technology that derives from the novel electrical breakdown property of certain 3-D printed like acrylonitrile butadiene styrene (ABS). This electrical breakdown property has been engineered into a proprietary, power-efficient system that can be cold-started and restarted with a high degree of reliability. One of the issues associated with ABS as a propellant is its low burn rate. It is well documented in technical literature that hybrid rocket systems generally have fuel regression rates that are typically 25-30% lower than solid fuel motors in the same thrust and impulse class. Lowered fuel regression rates tend to produce unacceptably low equivalence ratios that lead to poor mass-impulse performance, erosive fuel burning, nozzle erosion, reduced motor duty cycles, and the potential for combustion instability. To achieve equivalence ratios that produce acceptable combustion characteristics, hybrid fuel ports are often fabricated with large length-to-diameter ratios. The resulting poor volumetric efficiency is incompatible with Small Satellite (SmallSat) applications.
This paper presents preliminary results from a collaborative development program between the University of Miami (UM) and USU. In this reported work, modern extrusion and 3-D printing techniques are used to fabricate sample ABS fuel grains with varying levels of copper-metallization. Hybrid-ABS fuel grains were printed from Cu-infused feed stock with 2%, 4%, and 6% Cu-mass concentrations. As baseline control, 100% pure ABS fuel grains (0% Cu) were also printed. Heat conduction via the additive copper (Cu) provides an efficient heat transfer mechanism that augments surface convection from the flame zone. Forced convection, the primary mechanism for pyrolysis for hybrid fuels, is generally inefficient due to "wall-blowing" associated with the radially emanating mass flow from fuel pyrolysis. Wall-blowing pushes the flame zone away from the fuel surface and significantly reduces the rate of enthalpy exchange. Homogeneously mixing a high conductivity metal such as Cu into the ABS fuel provides an efficient heat transfer mechanism, and allows radiant heat from the flame zone to be transferred deep into the fuel material. This process significantly increases the pyrolytic efficiency of the fuels.
The Cu-infused fuels were tested at USU using a legacy 12-N hybrid thruster system. Fabrication and manufacturing methods are described, and results of hot fire tests are presented. The top-level conclusion is that Cu-infusion of the printed fuels measurably increases the fuel regression rate, allowing for a higher thrust level with no increase in the required volume. The Cu-infusion has negligible impact on the propellant characteristic velocity and the overall system specific impulse. The increased burn rate and overall increase in solid-fuel density resulting from Cu-infusion allows a measurable increase in the propellant impulse-density. This increase in volumetric efficiency is potentially significant for small spacecraft applications where available space has a premium value. Follow-on methods that infuse lower-molecular weight and higher thermal conductivity materials like graphene and carbon-nanotubes are proposed.
