Rapid prototyping techniques, materials, and apparatus have evolved to the point that they can be considered for the fabrication of hybrid rocket motor fuel grains. The Aerospace Corporation has initiated a test program to investigate the potential benefits to hybrid motor performance from the complex motor grain shapes enabled by these manufacturing techniques. Stereolithography has been used to produce several small proof-of-concept motors, each having complex 3-dimensional oxidizer flow passages that could not be produced using conventional hybrid manufacturing techniques. A new small hybrid motor test stand has been developed to explore the potential benefits that these techniques may offer. Preliminary results indicate that several of the problems that limit the use of hybrid rocket motors might be alleviated by the design freedom afforded by these new fabrication capabilities. I. IntroductionAPID Prototyping (RP), often called Additive Manufacturing (AM), is a family of technologies used to generate 3-dimensional shapes by computer control. Several diverse technologies fall under this broad term. In the typical Rapid Prototyping process, thin 2-dimensional layers of material are laid down, one on top of the other to form 3-dimensional shapes. A wide range of materials from plastics to foodstuffs can be literally printed in three dimensions. 1 The obvious benefits of RP techniques apply to a great many fields and products. These include the elimination of expensive tooling, decreased design cycle-time, and the de-coupling of design complexity from fabrication cost. 1 Additionally, RP offers some advantages that are more subtle and more profound for the specific applications of hybrid and composite solid rocket motor fabrication. Many RP processes deposit organic materials and can be employed to directly produce fuel grains. These grains, free from the constraints of physical tooling, may incorporate shapes and features that increase mixing and surface area or help balance the oxidizer-to-fuel ratio over the course of a burn. Another benefit of RP is that structural, plumbing, ignition, propellant management, and even diagnostic features can be "written" directly into the fuel grain design and then fabricated as a monolithic part. 2 The current disadvantages of rapid prototyping, as applied to fuel grain fabrication, are a paucity of appropriate fuel materials, slow fabrication speed, size limitations, and a lack of flight heritage. Also, most providers of RP fabrication machines and materials show little interest in working with unusual additives that could potentially increase regression rate or specific impulse . Many of these problems will naturally be alleviated as the use of this rapidly evolving set of technologies continues to expand.Hybrid rocket motors suffer from several performance and reliability problems that might be alleviated by taking advantage of the design freedoms afforded by fabrication techniques that are not limited by physical tooling such as molds, mandrels, and dies. Candidates for ...
Rapid Prototyping of polymers has evolved to the point where it can be applied to fabricate hybrid rocket fuel grains. Intricate grain designs with complex port geometry can be made with increased surface area and with no additional machining cost. Several samples of either printed pure acrylic, printed heterogeneous paraffin/acrylic matrix, or cast paraffin grains were provided by collaborators at the Aerospace Corporation and have been tested in the Long Grain Center Perforated (LGCP) hybrid rocket motor at the Pennsylvania State University's High Pressure Combustion Laboratory (HPCL). These solid fuel grains exhibit either a star-swirl or protruding vane turbulator center port design. Due to the complexity of these shapes, post-burn regression analysis for star-swirl type geometry was performed using SolidWorks CAD software to match a computer model to the actual burn profile. Regression rate increases of over 270% were noted in 1 / 2 -tpi star-swirl acrylic samples. Cast paraffin containing 20% aluminum with a printed 1 / 4 -tpi turbulator insert has been shown to increase regression rate by over 70% and demonstrates the ability to incorporate energetic additives while still applying swirl-inducing geometry. An oxidizer swirl injector system for the LGCP hybrid rocket motor has been designed and fabricated; the 45-degree injector head provided over 180 % increase in regression rate for straight-port cast paraffin fuel grains in comparison to straight-port axial injection. Using the same swirl injector configuration with straight-port printed paraffin/acrylic samples provides minimal improvement, likely due to the flow-straightening effect and boundary layer disruption of the acrylic support material. Overall, these tests provide a better understanding of the capabilities of rapid prototyping for application to future hybrid fuel grains. They also demonstrate the potential of hybrid rockets for small-scale thrusters by overcoming some of the limitations associated with solid-fuel regression rate. NomenclatureABS = Acrylonitrile Butadiene Styrene HPCL = High Pressure Combustion Laboratory LGCP = Long Grain Center-Perforated hybrid rocket motor PMMA = Polymethyl Methacrylate SP = Straight Port ST-SW = Star-Swirl TRB = Turbulator tpi = Turns Per Inch XTC = X-Ray Translucent Center-Perforated
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. Nomenclature HC= Honeycomb HPCL = High Pressure Combustion Laboratory LGCP = Long Grain Center-Perforated hybrid rocket motor SP = Straight Port tpi = Turns Per Inch
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