Project ATLAS Hybrid Rocket Engine Final Thesis Report; A Technical Analysis of Low-Cost Approaches to Improving Hybrid Rocket Motor Performance; Environmental Impact of Rocket Engines in the Modern Space Age

Introduction

Two hundred fifty-three—that is the total number of successful orbital-class rockets launched in 2024 [5]. Of this total, 158 were launched by the United States and 134 by SpaceX alone [13]. These tallies represent an all-time high for the industry, and the trend is only expected to continue at an exponentially growing rate as the New Glenn and Starship heavy-lift launch vehicles transition from development to the mass manufacturing phase.
While the greenhouse emissions of the launch vehicle industry are on the order of less than 0.2% [2] of the total emissions of the aviation industry, rocket combustion byproducts bring their own slew of unique disadvantages. Solid rocket motors, frequently used in heavy orbital boosters and missile systems due to their simplicity and lack of throttling, emit harmful aluminum oxide (Al2O3) and hydrogen chloride (HCl) [11]. The kerosene (RP1)-based liquid-propellant engines used by SpaceX, NASA, and most other launch providers produce primarily black carbon (soot) in addition to greenhouse gases and trace amounts of other byproducts. Black carbon is deceptively harmful from a long-term standpoint as it exhibits a much higher heat capacity than any gaseous byproducts, meaning that particles suspended in the atmosphere will capture solar radiation, retain it, and re-emit it into the atmosphere. An order of magnitude increase over the current norm of atmospheric black carbon could lead to significant climate change effects. A 2022 study done by the National Oceanic and Atmospheric Administration predicted that the annual amounts of black carbon products will increase tenfold in the next several decades [12].

Preventative Environmental Measures

Measures are already being taken to address these concerns. Organizations such as the Federal Aviation Administration (FAA) and Environmental Protection Agency (EPA) are implementing countermeasures. As of this year, the FAA began conducting environmental assessments (EAs) of launch sites to help quantify the emissions data. The EPA continues to operate under the Clean Air Act, and NASA is pushing for the development of cleaner aviation technologies through the Sustainable Flight National Partnership (SFNP).

Comparison of Engine Types

There is still an impetus on the industry itself to tackle the emissions problem, and in fact the greatest preventative opportunities lie here. Much attention has been given to reducing greenhouse gas and particle emissions on liquid engines, the current launch vehicle standard due to their tunability, reliability, and high performance. These characteristics make the liquid engine a popular choice for medium and heavy launch vehicles (i.e., the largest contributors to emissions) despite inherent complexity and cost, both in manufacturing and R&D. A perfect example of the strides being taken within industry is Blue Origin’s BE-3, a painstakingly-developed engine fueled by liquid oxygen (LOX) and hydrogen (LH2) emits only water vapor. SpaceX’s Super Heavy booster, the most powerful launch vehicle to date, is fueled by LOX/methane (CH4) and produces less than 10% the amount of black carbon per kilogram of propellant as compared to LOX/RP-1 engines, as well as insignificant amounts of greenhouse gases [1,6,8].
Solid rocket motors are currently a necessary evil. Their simplicity (no moving parts) makes them extremely easy from a cost and manufacturing standpoint. The industry has culled propellant selections down to essentially one combination, aluminum fuel with ammonium perchlorate oxidizer (NH4ClO4), which produces byproducts that exhibit behavior like that of black carbon and in addition are corrosive. A cleaner propellant combination must be devised, or a better overall alternative to solid motors suggested.

The Hybrid Alternative

Enter the hybrid rocket engine. The technology has been in use since the 1960s but experienced a resurgence in the 1990s as it was initially sidelined during the first Space Age in favor of solid and liquid propulsion systems [3]. Hybrid rockets combine the best of the solid and liquid worlds—by using a solid fuel grain and liquid oxidizer, the hybrid engine can simultaneously be throttled like a liquid engine (meaning the system is inherently more controllable and safer) while having almost no moving parts, making it almost as simple and inexpensive as solid motors. Hybrid engines have also demonstrated theoretical and empirical performance on par with that of the industry standard in solid motors, while falling short of modern liquid engine technology. However, hybrid engines offer a vast array of candidate propellant combinations, many of which produce a burn that is just as clean as that of LOX/CH4 and LOX/LH2 engines [1,4].

Hybrid Disadvantages

Why then has the aerospace industry not adopted the hybrid rocket engine with open arms? There are two primary reasons for this. The first is that the hybrid rocket engine is difficult to scale. The performance of a hybrid engine scales with the amount of surface area available for burning within the fuel grain. As the size of an engine is increased, the amount of exposed surface area increases proportionally with the square of length, that is,
A_s∝L^2
while the amount of fuel grain volume that must be burned to lift the engine increases with the cube of length:
A_s∝L^3
This means that as the size of the engine is increased, more fuel mass is required to lift the launch vehicle. There is then a limit at which the mass of fuel grain is so great that there is not enough exposed surface area to burn it all, meaning there is not enough energy released to produce the high launch velocities needed to leave the pad. Aerospace engineers have developed clever patch-based solutions to this problem, such as inserting mass cutouts to the fuel grain to expose more surface area internally. This problem can only be solved up to a point however, and then physics takes over. This means that the hybrid rocket engine will likely never be a strong candidate for medium or heavy-lift launch vehicles [9, 11].
The second problem is that hybrid rocket engine performance is quite difficult to predict. The combustion process, which amalgamates multiple branches of mechanical and aerospace engineering including heat transfer, fluid mechanics, and reaction physics, is quite complex and difficult to model. Most engines must simply be tested and the data recorded. Even then, performance fluctuates throughout the engine burn and the empirical data produced can only be extrapolated to hybrids of a comparable size. If the performance characteristics can be easily predicted, requiring hundreds of experiments with repeatable laboratory setups, the hybrid rocket engine could become an excellent long-term replacement for solid motors used in missile and smaller booster systems. They also present a simpler and safer alternative to liquid engines for small launch providers and student-led teams. Hybrid rockets are of great interest to these parties as their development times are on the order of 50-80% shorter than that of liquid engines. They also cost considerably less on a component level.

Project ATLAS

As the Mechanical and Aerospace Engineering Department at the University of Virginia (UVA) is interested in furthering our competitive edge in the Student Researched and Developed (SRAD) category at the International Rocket Engineering Competition (IREC), as well as contributing to the furtherance of hybrid engine technology for the benefit of the propulsion industry as a whole, my capstone team has developed ATLAS, the first advanced rocket propulsion system developed at the University.
ATLAS serves as a laboratory-scale engine prototype that is scalable to a full-scale (16-24 foot) competition rocket. We designed the engine to be rapidly reusable, safe, automatically controllable, and environmentally friendly. The engine produces no toxic or black carbon byproducts but rather trace amounts of greenhouse gases and nitrous oxide (N2O), which is harmless to humans.
The engine implements a N2O/ABS plastic propellant system. The ABS fuel grains are all 3D-printed in the University’s advanced manufacturing facilities, allowing for limitless geometric possibilities within the design. We designed 15 different fuel grain geometries, each with a specific performance-boosting design intention. Each fuel grain will be tested and burn data logged to determine which geometries produce the most efficient combustion and highest amounts of thrust, and why. ATLAS also implements a completely original design never seen before on a hybrid engine—a 3D-printed injection system.
Injection systems are notoriously the bane of rocket engine design. Crucial for proper mixing of propellants for combustion, the performance of the engine hinges upon the design of the injector. Simple injector geometries are easily accessible to student teams and small launch providers but perform poorly compared to the metal 3D-printed complex injector geometries that larger firms can produce. We have solved this problem by designing a complex injector and 3D printing it on a dental-grade resin printer. This printer ensures maximum print fidelity while still allowing high temperature resistance thanks to the two-stage curing process conducted in the labs within our Material Science and Engineering Department. Our final injection system offered high performance that was easily correlated with predictive models due to the complex geometry, all while costing less than twenty dollars per injection system.
Project ATLAS completed a rigorous four-month analytical design process, during which it was reviewed twice and approved by a panel of engineering faculty and industry professionals. The design was then heavily analyzed and optimized using state-of-the-art modeling software such as ANSYS Fluent CFD. The team modified the design based on the theoretical models and then manufactured the entire engine system and test facility in less than six weeks. We then proof-tested the engine for strength with a hydrostatic test campaign and then for performance benchmarks using a cold-flow test (tests standard within the propulsion industry). The engine is currently on standby for static hot-fire testing as the UVA Department of Environmental Health and Safety (EHS) reviews our test plan. The engine is expected to leverage additive manufacturing technology in a manner never seen within the industry, reducing costs while improving performance for student teams and industry professionals alike. Additionally, the engine will be used to create data from dozens of tests demonstrating the effects of fuel grain geometries and oxidizer flow rates on the predictability and performance of a small-scale rocket engine. We hope to publish these results for use by the industry and academia. The results can be used to predict the performance of engines of similar scale and will be used to develop the much larger engine for UVA’s competition rocket, Project PROMETHEUS.

Conclusion

The Modern Space Age necessitates an active movement by industry, academia, governing entities, and the like to install measures for alleviating the impending environmental impact of a booming industry. Liquid engines are already being improved in preparation for tenfold operational scaling, while solid motors are still an incredibly harmful and commonly used propulsion system. Small launch providers and student teams limited by time and budgeting constraints have little else to resort to.
The hybrid rocket engine represents a clean, simple, high-performing alternative. Current hybrid technology is limited in its scope, but there is an incentive on academia and small-scale industry to research and develop this technology to improve its predictability and performance while reducing costs.
Project ATLAS, the first hybrid engine developed at the University of Virginia, accomplishes both goals. ATLAS serves as a rapidly-reusable, low-cost, high-reliability testbed capable of sustaining dozens of hot-fire tests and logging and streaming live data at multiple collection points. These data can be published for future use by industry, academia, and the University. In addition, ATLAS harnesses cutting-edge advanced manufacturing technology to produce a low-cost, optimized, high-performing design with easily replaceable components. The team is confident in Project ATLAS’s potential to push the boundaries of hybrid engine technology and make an impact within the industry as a whole.

School of Engineering and Applied Science

Bachelor of Science in Aerospace Engineering

Technical Advisor: Chloe Dedic, Daniel Quinn

STS Advisor: Richard Jacques

Technical Team Members: Harshit Dhayal, Mannix Green, Silas Agnew, Taka Suzuki, Sean Dunn, Zachary Hinz, Ved Thakare, Harrison Bobbitt, Adis Gorenca, Isaac Tisinger, Darsh Devkar, James Dalzell, Dominic Profaci, Alexander Gorodchanin, Joshua Bird, Thomas DeCanio, Jack Spinnanger, Aiden Winfield