By: Kineo Wallace, Vaya Space
For decades, rocket scientists have been pushing the boundaries of propulsion technology, seeking to harness the power of combustion to propel humanity to the stars. Amidst this relentless pursuit of innovation, a transformative technology has emerged from the realm of hobbyist experimentation to revolutionize the future of hybrid rocket fuel: 3D printing.
Once confined to the niche corners of the maker movement, 3D printing has transcended its humble origins to become a cornerstone of the aerospace industry. Its ability to fabricate intricate structures with unparalleled precision has unlocked a new frontier in rocket fuel design and manufacturing, paving the way for a paradigm shift in performance, cost-effectiveness, and sustainability.
This white paper delves into the captivating journey of 3D-printed rocket fuel, tracing its evolution from a fledgling concept to a game-changer in the aerospace landscape. We will explore the intricate methods of printing rocket fuel grains, unraveling the impact of material selection, printing techniques, and geometric design on the performance of these high-performance components.
The Origin of Additively Manufactured Hybrid Rocket Engines: A Promising Concept Significant Shortcomings
Since the development of hybrid rocket engines in the 1930s, hybrid rocket fuel grain production has largely relied on traditional casting techniques. However, these methods face several limitations that hinder the advancement of hybrid rocket engine technology. These limitations include:
Lack of Control over Fuel Grain Composition: Traditional casting methods utilize pre-mixed fuel components, restricting the ability to precisely control the distribution of additives and modifiers within the fuel grain. This lack of fine-grained control over the fuel grain's composition impedes the optimization of burn characteristics, such as burn rate and specific impulse, which are crucial for achieving the desired thrust profile and overall engine performance.
Susceptibility to Crack Formation: During the cooling and curing process associated with traditional casting, inherent stresses can develop within the solidifying fuel grain, leading to the formation of microscopic cracks. These cracks can compromise the structural integrity of the fuel grain, posing potential safety hazards during combustion. Additionally, cracks can disrupt the flow of oxidizer through the fuel grain, affecting the combustion process and introducing inconsistencies in thrust and overall engine performance.
High Cost due to Specialized Tooling: Traditional casting techniques necessitate the use of custom-made tooling and molds, often designed specifically for the unique geometry of the fuel grain. These specialized tools can be expensive to fabricate and require a significant lead time, increasing the overall cost of manufacturing hybrid rocket fuel grains. The high cost of tooling can also limit the flexibility to experiment with different fuel grain designs and geometries, hindering the development process.
Lengthy Production Times: The casting process itself is inherently time-consuming, involving steps such as preparing the mold, mixing fuel components, casting the grain, and allowing it to cure. Additionally, traditional methods often require post-casting processes like machining and curing to achieve the desired fuel grain dimensions and properties. These time-consuming steps significantly extend the overall production timeline, slowing down the development process and hindering the ability to respond quickly to changing requirements or design iterations.
Inability for Rapid Prototyping: Traditional manufacturing methods are not well-suited for rapid prototyping of hybrid rocket fuel grains. The need for custom tooling and the time-consuming nature of casting makes it impractical to quickly produce prototypes for testing and evaluation. This lack of rapid prototyping capabilities hinders the development process, as it limits the ability to test new fuel grain designs iteratively and refine them based on experimental results.
These limitations can be addressed by adopting 3D printing techniques, which offer a transformative approach to hybrid rocket fuel grain research, development, and production. Vaya Space (then Rocket Crafters) was one of the first pioneers to explore the use of 3D printing as a solution to these issues, beginning their efforts in the early 2010s.
The original methodology that Vaya was pursuing for manufacturing a rocket fuel grain using 3D printing involved building up the grain in layers, each layer consisting of a concentric ring of material. The rings were printed sequentially, one on top of the other, until the desired grain size was achieved. This method was initially considered advantageous because it allowed for the precise control of the grain's geometry, which could be optimized for improved performance. Additionally, the stacked ring design was thought to help reduce the risk of stress fractures and other defects that can occur in solid rocket grains.   
However, extensive testing revealed several shortcomings with this original methodology:
Void Formation: While the original methodology aimed to produce a fuel grain with no voids, the presence of microvoids between the concentric ring bead structures would be required to allow the pattern to persist throughout the burn.
Bead-Level Ablation: Contrary to the original methodology's assumption, the regression rate of the fuel occurs at the microscopic level, where the fuel is thermally decomposing at the molecular level as the chain structure of the molecule breaks down. Therefore, each concentric ring would not be 'instantly' exposed as one is consumed.
Spiral Port Pattern Persistence: The original methodology claimed that the spiral-shaped port pattern would persist throughout the burn due to the beads maintaining the consistent geometry. However, with the understanding of the incorrectness of the previous assumptions, this claim is also invalidated. Additionally, studies have demonstrated that the pattern degrades throughout the duration of the burn, as evidenced in the publication "Effect of 3D Printing Parameters on the Internal Ballistics of Hybrid Rocket Fuel Grains" 
Vortex Flow Field Maintenance: The original methodology suggested a method to maintain a vortex flow down the entire length of the port. However, while this method may have achieved some degree of vortex flow, it did not provide the consistent and controlled vortex flow field required for optimal combustion efficiency. This limitation was addressed through the development of a novel vortex flow field injection system, as described in the Vaya Space patent, "Linear throttling high regression rate vortex flow field injection system within a hybrid rocket engine" (US11506147B2). This patent provides a more effective and proven method for achieving a persistent and controlled vortex flow throughout the entire length of the port, resulting in significantly improved combustion efficiency.
Sectional Bonding: The original methodology proposed printing the grain in sections and then bonding each of those sections together along the grain's primary axis. However, this approach presented challenges due to the complexity of precision machining, the increased labor required for bonding, and the risk of creating voids or weak points at the interfaces between the sections.
The initial methodology for manufacturing rocket fuel grains using 3D printing, while promising in concept, presented several challenges that limited its practical application. These shortcomings, including void formation, assumptions on bead-level ablation, spiral port pattern degradation resulting in inconsistent vortex flow, hindered the development of reliable and high-performance hybrid rocket engines.
To address these challenges, Vaya Space embarked on a path of innovation, moving away from the original methodology and developing new approaches that capitalized on the strengths of 3D printing while overcoming its limitations. This led to the creation of novel fuel grain designs, advanced manufacturing techniques, and optimized material formulations, resulting in a significant leap forward in hybrid rocket technology.
As highlighted in a UPI article, "Florida space startup Rocket Crafters pivots with new patents for 3D-printed fuel," Vaya Space's innovative approach to 3D-printed rocket fuel grains has paved the way for a new era of hybrid rocket propulsion. By overcoming the limitations of the original methodology, Vaya Space has unlocked new possibilities for more efficient, reliable, and cost-effective hybrid rocket engines. 
At the heart of this transformation were two pivotal patents: US11434180B2 and US11506147B2. These patents represent a groundbreaking advancement in 3D-printed rocket fuel grain technology, effectively resolving the shortcomings of the original methodology and paving the way for a new era of high-performance hybrid rocket engines.
Material Optimization and Efficient Production with US11434180B2
The patented method described in US11434180B2 represents a significant advancement in the manufacturing of rocket fuel grains using 3D printing. By focusing on material optimization and streamlined production, this patent enables the production of more reliable, efficient, and cost-effective fuel grains for hybrid rocket engines.
Elimination of Sectional Bonding: Unlike the original methodology, which required bonding fuel grain sections, the patented method enables the production of fuel grains in a single, seamless piece. This eliminates the need for precision machining and labor-intensive bonding, significantly reducing production time and complexity.
Reduced Production Time: By streamlining the manufacturing process and eliminating the need for sectional bonding, the patented method significantly reduces the overall production time for rocket fuel grains. This improvement in production efficiency translates into lower costs and faster turnaround times.
Elimination of Voids: By employing a specialized printing pattern and strategic nozzle placement of the extruder, the beads can be deposited in a manner that positions the subsequent bead within the gap of the preceding bead, effectively eliminating voids within the fuel grain.
Tailored Material Formulation: The patented method allows for the adjustment of the material formulation to achieve different burn properties. This flexibility enables the creation of fuel grains tailored to specific engine requirements and performance objectives.
Consistent Vortex Flow Field with US11506147B2
The patent US11506147B2 addresses the challenge of maintaining a consistent vortex flow field inside the hybrid rocket engine. This patent describes a novel vortex flow field injector system that effectively promotes a controlled and uniform flow of oxidizer through the fuel grain, leading to improved combustion efficiency.
Self-Sustaining Vortex Flow: The patented system eliminates the need for a spiral pattern in the fuel grain by creating a consistent and self-sustaining vortex flow field. This vortex flow field ensures efficient mixing of fuel and oxidizer, resulting in improved combustion efficiency.
Enhanced Combustion Efficiency: The vortex flow field injector system described in this patent enables the achievement of higher combustion efficiency compared to previous methods. This improvement in combustion efficiency translates into increased thrust-to-weight ratios and reduced fuel consumption, making hybrid rocket engines more competitive and attractive for various applications.
Elimination of Spiral Pattern: The patented system eliminates the need for a spiral pattern in the fuel grain, simplifying the manufacturing process and reducing potential inconsistencies in oxidizer flow. This simplification further contributes to improved combustion efficiency and overall engine performance.
In conclusion, the two patents, US11434180B2 and US11506147B2, have revolutionized the field of 3D-printed rocket fuel grains. By addressing both the manufacturing process and the fundamental performance characteristics, these patents have paved the way for a new generation of hybrid rocket engines that are more efficient, reliable, and cost-effective.
Following the success of the first two awarded patents, Vaya has continued to refine and mature the associated technologies with the goal of achieving further advancements in production speed and burn-to-burn performance consistency across different production batches.
Initial ABS Fuel Material Challenges
During the initial testing and validation phase, Vaya employed ABS thermoplastic as the primary base fuel material. However, this material presented several challenges. ABS is a three-part co-polymer of Acrylonitrile, Butadiene, and Styrene, and its performance was directly influenced by variations in the specific blend and brand of ABS used. This variability in the material composition led to inconsistencies in burn rate, specific impulse, and thrust profile. To mitigate these batch-to-batch variations, maintaining a consistent blend of ABS was essential. However, this requirement posed significant challenges in terms of supply chain availability and necessitated stringent quality control measures, effectively eliminating the potential for utilizing recycled plastics. These constraints resulted in increased material costs due to supply chain limitations. Additionally, ABS is like its thermoset cousin Hydroxyl Terminated Polybutadiene (the favorite of legacy hybrids) is a sooting polymer which means that when it burns it produces carbon soot during the combustion process. the ABS combustion process also produces a variety of volatile organic chemicals, including hydrogen cyanide. 
Transition to High-Density Polyethylene (HDPE) Fuel Material
In light of these considerations and other factors, Vaya embarked on an investigation into alternative materials that could better align with their goals of enabling safe, sustainable, and cost-effective access to space while leveraging the advantages of additive manufacturing. After extensive literature review and evaluation of readily available hydrocarbon-based polymers, a promising option emerged.
During NASA's Hybrid Propulsion Technology Program in the 1980s, a comprehensive assessment was conducted to evaluate the feasibility of replacing the Space Shuttle Solid Rocket Boosters. NASA and its contractors concluded that High-Density Polyethylene (HDPE) would be the superior fuel material. However, at that time, producing large-scale fuel grains using HDPE was not technically feasible.  Even today, traditional manufacturing methods are inadequate for producing large-scale HDPE grains. This is where Vaya's patented and patent-pending additive manufacturing methods shine.
Advantages of HDPE and Vaya's Additive Manufacturing Methods
Similar to ABS, HDPE presents printing challenges due to its high coefficient of thermal expansion (CTE). However, HDPE introduces additional complexities, including a significantly higher melting point and its notoriously poor adhesion properties. Despite these challenges, Vaya's fuel grain production methodologies, designed for rapid production and harnessing the material's internal stresses as an advantage rather than combating them like traditional fabrication methods, enabled the seamless integration of HDPE into their proprietary printing processes.
The adoption of HDPE offered several advantages over ABS and other Hybrid Rocket Fuels:
Non-Sooting Polymer: HDPE is a non-sooting polymer, producing minimal to no visual signature in tactical settings.
Non-Toxic Exhaust: HDPE combustion produces non-toxic exhaust gases since it and other polyolefins “thermally degrade due to homolytic chain scission, followed by inter- and intra- molecular chain transfer, resulting in the formation of volatile fragments”, enhancing environmental safety. 
Industry Leading ISP Potential: HDPE possesses a theoretical specific impulse (ISP) that surpasses RP-1, indicating improved propellant efficiency.
Readily Available and Recyclable: HDPE is readily available in the plastics industry in both virgin and recycled forms. As a homopolymer, its molecular structure is relatively simple, with variability limited to the chain length, which is determined by the melt flow index.
3D Printable: Being a thermoplastic that is used extensively in extrusion molding there is a plethora of data and knowledge on processing HDPE the material matches Vaya’s technologies well.
Result: Rapid Fuel Grain Prototyping and Scalability
The transition to HDPE proved to be a pivotal decision for the Vaya team and has had just as much of an impact on Vaya’s hybrid rocket technology as the initial adoption of using additive manufacturing in the first place. Since adopting HDPE, Vaya has further refined its printing methodology and developed an entirely new manufacturing process for producing fuel grains. Additionally, complete integration into the engine assembly has enabled rapid production, scaling, and prototyping, allowing for the implementation of new grain geometries within days.
This rapid prototyping capability was recently showcased during the Tactical Relight Vortex-Hybrid Engine demonstration. Following the first week of firing, the team utilized the test data to modify the fuel grain geometry. Within a single day, a new grain with adjusted geometry was printed and integrated into an engine to be tested the next day. This unprecedented rapid turnaround marks a significant breakthrough in hybrid engine testing and surpasses traditional rocket engine testing timelines.
Vaya's continuous advancements in printing technology have paved the way for the technology that will enable the ambitious goal of producing an entire orbital-class fuel grain in less than one day. This remarkable feat, from an empty machine to a completed grain, represents a substantial leap forward compared to traditional manufacturing techniques or even Vaya's earlier methods, which would require weeks or even months to produce a single orbital-class grain. By leveraging additive manufacturing and utilizing recycled plastics, Vaya has revolutionized hybrid rocket engine research, development, and manufacturing while simultaneously reducing costs, enhancing sustainability, and improving safety.
About the Author:
Head of Propulsion and Fluids
Kineo Wallace; the inventor of Vaya Space’s first two patents consisting of the Vortex Induced Flow-Field Injector, and the Method for Producing a Hybrid Rocket Fuel Grain Horizontally. Through these innovations, as well as more in development, Kineo and his Propulsion Team’s incurious need to defy the odds are the core reasons why Vaya’s engines perform at liquid level performance with hybrid level costs.