Fascination surrounding spaceflight and rockets is at an all-time high. Sites near launchpads draw crowds of spectators, eager to witness the flash of fire and feel the vibrations as the rumble of the motor becomes a roar. People, squinting and craning their necks to watch the rocket hurtle out of sight, aren’t likely thinking about the science behind the propulsion that makes it all possible. 

What are the key elements that influence the combustion process? Are there advantages to utilizing solid propellants versus liquid? Simplicity, lower cost, and ease of storage and handling make solid fuel sources ideal for military and space applications. 

To advance the fundamental knowledge of how polymeric solid fuels combust, the Department of Defense (DOD) has awarded $7.5 million to a multi-university partnership as part of the agency’s Multidisciplinary University Research Initiatives program. 

The project, led by Virginia Tech over the next three years, will bring together leading researchers and engineers from Penn State, Georgia Tech, Iowa State University, Stanford University, University of California Riverside, and North Carolina State University to conduct experiments and develop computational models that detail how a variety of solid fuels will burn in various flow conditions.

Research on the combustion of polymeric solid fuels has a long history, but high-level studies have revealed fundamental gaps in the chemistry and physics needed to predict results for new polymeric solids and combustors. This research is being sponsored by the Office of Naval Research, as the data gathered is relevant to the U.S. Navy and can be applied to developing high speed and hypersonic vehicles.

“Our goal is to develop a unified understanding of solid fuel combustion for different fuels under a diverse set of flow conditions,” said Virginia Tech’s Gregory Young, the primary investigator leading the multi-institutional research effort. “Through detailed measurements and computations, we will have a better understanding of the fundamental processes. This knowledge will allow for the future development of revolutionary solid fuels that may operate in extreme conditions such as high speeds and altitudes.”  

Young, associate professor in the Kevin T. Crofton Department of Aerospace and Ocean Engineering, is a leading expert in energetic materials, combustion, and propulsion. His research over the past decade has focused on the development, characterization, and optimization of energetic materials and propellants, specifically for applications in pyrotechnics, rocket propulsion, and high speed air breathing systems such as ramjets and scramjets.

Solid fuel combustion is crucial to hypersonic and space-propulsion systems. Fuel types and flowfields influence the combustion process – specifically heat transfer, pyrolysis, condensed phase chemistry, mixing, and gas phase chemistry.

Using a coordinated multidisciplinary approach involving novel experimental, theoretical, and numerical techniques, the research team aims to unravel complex, highly coupled combustion behavior of solid fuels over a wide range of conditions. Researchers will then integrate the obtained knowledge into a unified and reliable model for solid fuel combustion.

Top: Image from a combustion test, burning a material called polymethyl methacrylate (PMMA) on the solid fuel scramjet rig at the Advanced Propulsion and Power Laboratory. Bottom: On the solid fuel ramjet rig, Young and his students test solid fuel source hydroxyl-terminated polybutadiene (HTPB).
(Above) An image from a combustion test, burning a material called polymethyl methacrylate on the solid fuel scramjet rig at the Advanced Propulsion and Power Laboratory. (Below) On the solid fuel ramjet rig, Young and his students test solid fuel source hydroxyl-terminated polybutadiene. Images courtesy of Gregory Young.

Large scale experimentation will be conducted at research facilities at Virginia Tech and Penn State. At Virginia Tech, Young and his graduate students will investigate aspects of solid fuel combustion in both subsonic and supersonic flows at the Advanced Propulsion and Power Laboratory. The interdisciplinary research facility is equipped with several state-of-the-art experimental rigs and diagnostic instrumentation systems.

Data derived from this study will enable scientists and engineers to better understand the characteristics and behavior of physicochemical processes in solid fuel combustion. With the comprehensive knowledge on how specific fuels burn at higher altitudes and accelerated speeds, researchers will be able to utilize the model to make predictions for revolutionary solid fuel sources as they are developed. 

“These are complex issues, and we’ll be one of the first groups to tackle this problem in this level of detail,” said Young.

A community of subject matter experts

To expand the educational impact for students across institutions, the team will actively cross-train students among the various laboratories. For instance, Virginia Tech students will have the opportunity to travel to partner universities for hands-on experience with advanced diagnostics approaches, while students from other institutions will be able to participate in the large-scale experiments in Blacksburg.

Ph.D. candidate Dominic Gallegos, left, and master’s student Ethan Schlussel review the footage and data captured in an experiment on the solid fuel ramjet rig.
Ph.D. candidate Dominic Gallegos, (at left) and master’s student Ethan Schlussel review footage and data captured in an experiment on the solid fuel ramjet rig. Photo by Jama Green for Virginia Tech.

Similarly, students will interact and cross-train on continuum modeling efforts and multiscale modeling improvements.

“This cross-training will be key in demonstrating the power of multidisciplinary research,” said Young. “We hope this experience will foster a collaborative relationship that the students can build upon as they enter the workforce together.” 

The research will also involve collaborators from government labs, such as the U.S. Naval Research Laboratory; U.S. Naval Surface Warfare Center, Indian Head Division; U.S. Naval Air Warfare Center, China Lake; and the Air Force Research Laboratory. With a goal of training the next generation of scientists and engineers to lead the aerodynamics, combustion, and energetics communities, the project will introduce students to internship opportunities at DOD laboratories and facilities.

The model developed and knowledge gained will enable the DOD to develop revolutionary solid fuels to operate under extreme conditions of altitude and combustor residence times. The modeling and diagnostic tools developed will improve future studies on fundamental and applied combustion, and the resulting kinetic models for solid fuels represent the initial framework for binder chemistry necessary to develop composite solid-propellant models.


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