Hypersonic Vehicle Analyses: The Needs and Challenges of Multidisciplinary Simulations

Welcome to the AIA journal, Similar Theories. I am Tom Xi, Professor of aeronautics and astronautics at Purdue University and the editor in chief of the AIA journal. The AIA journal Similar Theories series was established to thank our authors, readers, reviewers, and editors. It has two tracks: author seminars and keynote speeches. Author seminars involve talks linked to articles published in the AIA journal to connect authors and readers. Keynote speeches cover emerging topics in our field by leaders to inspire research. Today, we are excited to have Professor Ian Boyd from the University of Colorado as our keynote speaker.

I want to thank the AIA journal Similar Theories committee, chaired by Professor Paul Tucker of the University of Cambridge and co-chaired by Professor Earl DAO of Duke University, for organizing the seminars. I also want to thank AAA and the AIA publications committee for their support. Now, I invite Dr. Kevin Boco to introduce today's keynote speaker.

Dr. Kevin Boco is a senior tech fellow and chief scientist of Hypersonics at the B company. He is well-known in the Hypersonics community and has made significant contributions. Dr. Boco will introduce Professor Boyd, who is the HT Sears Memorial Professor of Aerospace Engineering Sciences at the University of Colorado Boulder. He has a PhD in aeronautics and astronautics and has authored numerous publications. Dr. Boyd's research focuses on nonequilibrium gas and plasma dynamics processes in aerospace systems. He is a Fellow of several prestigious organizations and leads research projects in hypersonic entry systems.

I will now turn it over to Professor Boyd for his presentation on Hypersonic Vehicle Analysis. Thank you for the opportunity to share our research on this topic.

One focus area for N SI is Applied Research, aiming to build on the fundamental research capabilities of faculty and students on campuses nationwide and apply them. A key area of activity for N SI is Hypersonics. Today, I will focus on some of our Applied Research in the field of Hypersonics.

Hypersonics refers to vehicles that fly faster than five times the speed of sound, typically in the air. This equates to about 3500 MPH at sea level, meaning these vehicles cover almost one mile every second. This speed is incredibly fast for an air vehicle.

Hypersonics encompasses various types of missions and vehicles, leading to complications and making simplifications difficult. In the US, over $4 billion is being spent on Hypersonic systems for national security, including Boost glide vehicles and Scramjet powered systems to defend against adversaries' Hypersonic weapons. Hypersonics also plays a crucial role in space exploration, such as bringing astronauts back from the space station or landing rovers on Mars.

There are also discussions and plans for Hypersonic commercial aviation in the future, surpassing the speed of the old supersonic Concorde. Unique phenomena and challenges in Hypersonic vehicles necessitate a separate term from supersonics. The incredible speed at which Hypersonic vehicles fly is just one aspect of their uniqueness.

Strong shockwaves form around hypersonic vehicles, leading to high temperatures in the gas surrounding the vehicle. The primary concern with hypersonic vehicles is protecting them from high heating loads. Each subsystem of a hypersonic vehicle is unique due to its high speed. For example, a thermal protection system is necessary for hypersonic vehicles but not for slower-moving aircraft. The complexity arises from the tight coupling of subsystems within hypersonic vehicles, where aerothermodynamics affect structures, structures impact propulsion, and all of this influences guidance, navigation, and control. The focus of discussion will be on developing computational models that consider the interconnectivity of these complex subsystems within hypersonic vehicles.

Here are two examples to consider. On the left side, there is an example from space exploration. The black outline with the red cursor represents a heat shield on a capsule entering a planet's atmosphere. As the capsule experiences high temperatures, a large portion of the heat shield is intentionally ablated away. The gold area on the right side shows a prediction of the amount of heat shield removed. During hypersonic entry, the vehicle encounters high temperatures, causing ablation of the material surface and a change in the vehicle's shape, affecting aerodynamic coefficients.

On the right side is a Scramjet example. A Scramjet is an engine used for Hypersonic vehicles. The diagram shows the tight coupling between fluid mechanics, aerothermodynamics, and vehicle shape. The front of the vehicle is designed to create shock waves that condition the air for combustion in the Scramjet. The heated air affects the structure's integrity, potentially changing the vehicle's shape and performance. Understanding these processes requires modeling flow, material response, structure response, and propulsion system simultaneously. This integration is essential for optimizing vehicle performance. Today, I will discuss the progress we have made in this area at our Colorado facility.

The CENTER for National Security Initiatives focuses on Applied Research, with Hypersonics being a main research area. The CENTER develops partnerships with various organizations, including large aerospace companies, non-profit organizations, research labs, and small companies, to address customer needs in Hypersonics. Customers include the US Department of Defense, NASA, and the Department of Energy. Projects range in size, with a research staff of people with PhDs and talented doctoral students. The activities presented are just a subset of the overall work being done.

To build models of hypersonic vehicles and understand the coupling between systems, the first step is modeling the hypersonic flow. This is essential as it forms the basis for everything else, such as special materials and structures. In our work, we use a hypersonic flow code called LeMANS, which solves the three-dimensional Navier-Stokes equations and includes nonequilibrium thermochemistry, vibrational energy relaxation, chemical reactions, and plasma considerations. The code is efficient on high-performance computing systems and is coupled with surface chemistry for material response.

LeMANS has been benchmarked and validated against other similar codes using laboratory and flight data, providing a solid foundation for our analysis. This chart illustrates the range of activities involved in developing a reliable hypersonic code for analysis and design.

Examples of our research focus on the interaction of individual molecules and atoms under conditions relevant to hypersonic flow. These calculations, known as trajectory calculations, help us understand processes like molecule dissociation and energy transfer. By performing billions of these calculations and then reducing the data, we can evaluate rates of processes like vibrational excitation and dissociation. This information is then used in our CFD code, Lamont, to analyze conditions with ground test data.

One specific calculation we have done is on the double cone experiment, showing good agreement between CFD predictions and heat transfer measurements. This progression from molecules to rates to CFD code builds confidence in our predictive capabilities. Once we have a reliable CFD code, we can use it for design and analysis, as well as answer hypothetical questions.

Our research serves as a numerical laboratory for various analyses and experiments. For example, we can use our CFD code to study the aerodynamics of hypersonic vehicles, including lift and drag. This approach allows us to gain insights into the properties of prototype hypersonic vehicles.

When firing a divert thruster to control a hypersonic vehicle in flight, complications may arise such as plasma formation leading to a communications blackout. Observable phenomenology is important to consider in these situations. We have a good understanding of hypersonic flow dynamics, but need to think about how to integrate these processes together.

Material response is an important factor to consider when dealing with hot gases in contact with a material's outer surface. Various processes, both chemical and physical, can affect heat transfer and balance. Ablation, where material is removed from the surface, can lead to heat re-radiation and conduction into the solid material. More complex processes like pyrolysis, involving resin breakdown within the material, can also occur.

Our code, Mopar, is designed to model these processes by solving conservation equations. It is integrated with our hypersonic flow code, LeMANS, through surface chemistry options. This allows us to model surface chemistry, including ablation, using thermodynamic Act Fibri approaches and finite rates of surface chemistry.

The tight coupling between hypersonic flow and material response is illustrated in the image below, showing temperatures of the external hypersonic flow field and the material in a Mach five simulation. The gas temperature behind the shock wave reaches 1200 Kelvin, while the material temperature peaks at 800 Kelvin. This integrated approach allows us to analyze flow and material response simultaneously.

Once we have the capability to analyze hypersonic flow, there are numerous applications and ways to use these codes to analyze different systems. For example, in the top left corner, we are examining material response in a complex multi-layered material including zirconium diorite, silicon carbide, and carbon carbon substrate. We can study how multi-component materials interact with hypersonic flow.

Additionally, we can explore ablation and different types of materials like ultra-high temperature ceramics for passive thermal management in hypersonic systems. Choosing materials for survivability under various flight conditions is crucial. Active thermal management approaches, such as Electron Transpiration Cooling, can also be investigated. This concept involves certain materials emitting electrons when heated, which may help carry heat away from the vehicle's surface.

There are numerous opportunities to explore different approaches to hypersonic vehicle thermal protection and management using these capabilities.

I would like to switch gears to discuss three projects involving different types of coupling. The first project involves an ablating Hypersonic vehicle and explores the coupling between hypersonic flow, material response, and radiation from the gas around the vehicle. The motivation for this work is to develop rapid detection approaches for hypersonic vehicles, which present new challenges for protection.

There is a lot of interest in Hypersonics, especially in the military sector, due to the unique challenges they pose for existing radar systems. One approach to detecting hypersonic vehicles is to observe radiation in different parts of the electromagnetic spectrum. The project focuses on a specific vehicle, the IRV two, and examines the radiation generated during its trajectory, particularly at a high altitude of 55 kilometers and a high speed of just under seven kilometers per second.

The challenge lies in determining the confidence level in the results obtained from complex calculations involving 20 different chemical species and 195 chemical reactions. Uncertainties also arise in material and radiation modeling. The goal of the project is to identify key processes affecting radiation and quantify uncertainties in the calculations.

The approach involves multidisciplinary analysis, with tight coupling between the Flow code LeMANS and the material response code Mo Park. The radiation modeling is weakly coupled for specific reasons. The project aims to assess uncertainties in chemistry and quantify overall uncertainty in the results. This work involves a thorough analysis and understanding of the complex interactions between flow, material response, and radiation.

When considering fluid mechanics and Aerothermodynamics in Hypersonic flight conditions, the timescales involved are much smaller than those associated with material response. The material response is slow while the flow response is fast. To address this, we first fully converge the flow solution through computational fluid dynamics (CFD). We then transfer information from the CFD to the material response code and time resolve the material response. This process continues in a cycle until both the flow and material solutions are fully converged.

This iterative process is automated in our simulations. The meshes used for the solid material (carbon carbon) are unstructured, while a shock-aligned structured mesh is used for the Navier-Stokes equations in the CFD. The CFD and material responses interact along the interface between the two domains. The example shown includes the carbon monoxide mass fraction in the flow field, illustrating the effects of ablation on the vehicle's nose over time.

The changing outer mold line of the vehicle throughout the flight underscores the need for coupling between flow and material responses.

The flow of material analysis for the Radiation Analysis is conducted weakly coupled, meaning it is a one-way flow of information. The results from the combination of the flow and material response are fed into a radiation solver called NECA, a NASA code. The information fed into the radiation solver includes number densities of different species and temperatures, from which radiation emissions are calculated. There is no feedback from the radiation to the flow due to the systems being optically thin.

After completing computational fluid dynamics and material response analysis, the information is fed into NECA for Radiation Analysis. Radiation is evaluated along hundreds of lines of sight perpendicular to the vehicle's surface. This provides a picture of the radiation, allowing for consideration of uncertainties and sensitivities. To focus on understanding sensitivities and uncertainties, total radiation in three different infrared bands (Ira, B, and C) is chosen as quantities of interest.

The chart shows the total radiance in these three bands as a function of distance along the vehicle's surface. It helps determine the location in the vehicle and the part of the infrared spectrum being observed. This information serves as the quantities of interest for further sensitivity analysis.

A key aspect of Sensitivity Analysis is the soba index, which measures the level of influence of a particular process on determining a quantity of interest. In this case, the quantity of interest is the total infrared radiation generated at different locations along the vehicle surface. The SOOL index identifies the chemical reactions that most affect the infrared radiation among the 195 reactions being studied.

One important reaction identified by the SOOL index is the First Zeldovich reaction, where an N two molecule interacts with an oxygen atom to form an nano molecule and a nitrogen atom. This reaction is crucial because NO is one of the strongest radiating molecular species in this portion of the electromagnetic spectrum. The result indicates that this reaction is the most important across the vehicle surface under the given conditions.

This information can be used to prioritize certain reactions for further study, such as through shop troop studies or Computational Chemistry studies. By focusing on reactions identified as important by the soba index, researchers can decrease uncertainty and improve understanding of the processes influencing infrared radiation generation. Additional reactions of interest can also be identified by analyzing the data presented in the figures, allowing for a more targeted approach to research and analysis.

The final part of this analysis involves uncertainty quantification. We conducted hundreds of simulations by varying the chemistry rates of 195 reactions. In total, we have 500 coupled simulations with a range of results. By varying the chemistry rates, we can determine the range of results obtained from these simulations. The level of confidence in predicting radiation changes varies depending on the location around the vehicle. The area with the most uncertainty is at the stagnation point, where there is a wide range of radiation results. As we move along the side and back of the vehicle, the level of uncertainty decreases. This information helps us identify where we need to focus our efforts to reduce uncertainty.

In conjunction with the previous chart, we can determine which specific chemical reactions to focus on to reduce uncertainty. Moving on to a second example of coupled simulations, we will now discuss scramjet inlets.

Earlier, I described how the Flow heats up the material under thermal and aerodynamic loads, potentially causing deformation. To analyze this situation, we must couple the hypersonic Flow with material response, specifically Structural Response. This collaboration with Professor Kurt Ma from Colorado involves using the Morris element code to model the Structural Response. The sequence of images demonstrates how the coupling is achieved, starting with the original geometry of a Hypersonic Inlet where flow is from left to right. Initially, the CFD code Le Mans is applied.

This text describes the shockwaves and fluid mechanics around a configuration. It generates a Temperature Field, which indicates the gas temperature around the vehicle. The high temperatures near the vehicle surface cause heat flow into the vehicle structure.

Next week, we will perform the Thermal Response to show the temperatures inside the material structure. Following that, we will perform the Structural Response. In this case, the original structure is represented by the white area.

The original structure on the Kyle was pushed upwards slightly. The white area on the Kyle represents the original structure, which has been pushed down due to thermal and aerodynamic loads.

We go through a process where we have deformed geometry with a heated structure. We apply CFD again in a cycle to ensure convergence of Flow, material, and structural properties. With this capability, we are now considering how to minimize deformation and prevent deformations.

Deformation control via active thermal management is being explored in relation to the structure of the Scramjet Inlet. By controlling surface temperatures, we can reduce or manipulate deformations. Sensitivities to material, flight conditions, and surface temperatures are being observed. Deformations in the inlets can impact Scramjet engine performance, which is also being studied. Validation experiments are being considered, but are challenging due to the need for hypersonic flow and sufficient time for heating and softening of the structure. Multidisciplinary optimization is also being considered to take advantage of deformation processes.

The third and final example I want to discuss is Hypersonic Vehicle Trajectories. This involves the coupling between the Guidance Navigation Control System and the hypersonic flow. Specifically, we will examine a Boost glide trajectory constrained by aerodynamic acceleration loads and heat loads. One aspect we will focus on is Reachability, which refers to the range of achievable trajectories a Hypersonic vehicle could fly from a common initial point.

To study these concepts, we are using the Common Aero Vehicle cage, with its properties listed on the right-hand side. Additionally, we are interested in assessing the performance of Ultra High Temperature Ceramics on the Nose and leading edge of the vehicle. For UHTC, we have set a maximum heat flux of just over 600 watts per centimeter squared. Therefore, any trajectory these vehicles follow must ensure that the heating does not exceed 605 watts per centimeter squared.

We conduct our analyses within the AAA Multidisciplinary Optimization framework using a Particle Swarm Optimization Algorithm. The core of the analysis involves a three-dimensional dynamics system of equations with weakly coupled heating. We have created a database of thousands of CFD calculations with Lemon to provide high-fidelity interpolated predictions of heating for trajectory constraints. To impose constraints, we utilize different cost functions for optimizing downrange and crossrange distances.

In this study, we are examining the effects of the chemistry model on trajectory design through constraints on heat transfer and air chemistry modeling.

We examined two different chemistry models for the purpose of understanding how chemistry modeling can affect the trajectories we can fly. The first model is the Legacy two temperature model by Park, commonly used in hypersonic flow codes worldwide. The second model is the modified Marrone Trainer model (MMT), a newer model based on a decade of Computational Chemistry research by various groups globally.

Our goal was to analyze how these two models differ in predicting heat transfer for a spear cone flying at five kilometers per second at altitudes ranging from sea level to 80 kilometers. Our comparisons showed that the Park and MMT models agree at very low and very high altitudes, but there are discrepancies in heat transfer predictions at intermediate altitudes. Sometimes MMT predicts lower heat transfer, while other times it predicts higher heat transfer.

These findings have raised questions about the impact of these discrepancies on trajectory design and optimization.

Here are some results. On the left side is the Vertical Plane, showing Altitude and Downrange Distance. The Solid Lines represent predictions from the MMT Chemistry Model, while the Dash Lines are from the Park Chemistry Model, showing significant differences. These lines represent possible trajectories for the vehicle. The chemistry modeling affects the range of trajectories. This is also evident in the Horizontal Plane, which shows Reachability in terms of crossrange and downrange. The Hypersonic vehicle can fly various trajectories in between. In the interest of time, let's discuss Future Directions, focusing on multidisciplinary analysis and optimization.

The fidelity and accuracy of our ability to model each individual component will determine the quality of our results. As a community, we need to focus on improving the modeling of the chemistry, ablation, radiation, and structural response individually. Additionally, we must work on developing robust, flexible, and resilient approaches to coupling. This will require extensive research due to the different timescales and nature of information exchange involved.

There are many examples of multiphysics analysis that we are interested in, and others are also exploring. It is clear that there is still a long way to go in this aspect. It is important not to forget to acknowledge the efforts of others in this field.

As a community, we are focused on improving the modeling of the chemistry of radiation ablation and structural response individually. In terms of coupling, we need to develop robust, flexible, and resilient approaches due to the different timescales and nature of information exchange. There is much research needed in this area. Many examples of multiphysics analysis exist that are of interest to us and others.

I would like to acknowledge the individuals who contributed to the results presented today, including my grad students Nick, Jenny, and Jens, as well as research staff at NSI and the AEROSPACE department. My colleagues KT and Jay also played a role in this work. Funding support from various sources made this research possible. If you have further questions beyond what we can address today, please feel free to reach out to me.

Thank you for your attention. I am now available to answer any questions. Thank you, Ian, for the excellent presentation. It generated several questions. I will go through them quickly to cover as much as possible.

We use flight data when available for validation, although there is less flight data compared to ground data. Flight testing in the hypersonic regime is challenging but essential for assessing our models.

The coupling of ablation products with the flow field, including gaseous and solid products, is a complex process. Surface heating is treated differently for the bulk of the surface and the chemistry happening instantaneously. We use thermodynamic equilibrium tables to predict the chemical species released due to surface chemistry and ablation. Most of the time, material removal occurs one atom or molecule at a time, with occasional chunks of material coming off in a process called spall. Ablation primarily occurs at the molecular level. Thermochemical equilibrium tables, known as B prime tables, are used, but an alternative approach involves finite rate surface chemistry to model surface processes. This approach considers different chemical compounds, reactions, and rates for surface processes, providing a more comprehensive method. Experimental data is crucial for determining surface rates, and progress is being made in developing models.

Turbulence chemistry interaction in hypersonics is a key question, especially in relation to ablation products and rough surfaces. While not currently under study, the broader community is exploring this area. The need for high fidelity experimental data is emphasized to improve the accuracy and confidence of results.

In analyzing constraint trajectories, the range limits are based on both drag differences and thermal constraints. The process involves iterating across subsystems for each time step of the unsteady process with ablation. Flow timescales are short, requiring convergence of computational fluid dynamics (CFD) to steady state, followed by time-discretized material response calculations. This staggered approach ensures fully coupled CFD and material response results.

Questions about inward turning inlets and fluid thermal structural interactions prompt discussions on the impact of 3D versus 2D inlets and the potential for damage evaluation from high cycle fatigue. Sensitivity analysis of quantities of interest (QOIs) to flow physics processes and the computational time required for aero thermos structural solutions are also considered. The continuum assumption is generally effective, but cases may require techniques like direct simulation Monte Carlo (DSMC) or molecular dynamics at very high altitudes. You might have to use DS MC for flying down a reentry trajectory from space. However, for most Hypersonic systems, the real action occurs in the continuum regime where there is maximum heating and aerodynamic loads. Gas-solid heterogeneous reactions can be limited to the ab blading surface or model porosity and reactions below the surface. This capability is useful for NASA and military applications with porous media and plying materials.

Regarding the old Sprint missile program data, there is ongoing work to explore old data vaults for valuable information to validate complex simulations. Digitizing and resurrecting old data could be incredibly valuable for current research efforts.

Ian's talk on the complexities of physics in Hypersonics and challenges in modeling and simulation was well received. The talk has been recorded and will be archived for those who registered. Questions can still be asked via text for the next 48 hours.

The next AIAA journal keynote will be on May 16, 2024, with Professor Marria J from the University of Texas at Austin discussing space environmentalism. It is a critical area for the future of humankind in space.

Thank you to Ian for the informative talk, Kevin for moderating, and all attendees for being part of the keynote. Have a wonderful day and thank you for joining.