Extreme Speed + Extreme Heat = Extreme Engineering




When a vehicle travels faster than the speed of sound, its exterior surface becomes extremely hot — risking the safety of the people and cargo inside. Molecules in the flow around the vehicle collide and change, creating a shockwave. This hypersonic environment is difficult to understand and even more difficult to study.

We asked Marco Panesi to clarify some of the mysteries of hypersonics and tell us what he and his colleagues are working on to better understand the phenomenon. He is a professor in the Department of Aerospace Engineering, a Caterpillar Faculty Scholar, and director of the Center for Hypersonics and Entry Systems Studies.

Marco, how did you first get interested in hypersonics?

PANESI - First of all, I have to say that my university in Italy did not offer any class or course on this topic. So, the first time I started studying and working with hypersonic flows was during my Diploma Course and Ph.D. at the Von Karman Institute in Belgium, where I became part of the plasma team. The focus of the plasma team research is the study of extreme physics, which is a characteristic of high-speed flows, like hypersonics. That’s where I fell in love with it.

Marco Panesi
Marco Panesi, professor in the Department of Aerospace Engineering, a Caterpillar Faculty Scholar, and director of the Center for Hypersonics and Entry Systems Studies

How would you define hypersonics?

PANESI - Conventionally in fluid mechanics, we define three different flow regimes. The one we are most familiar with is the subsonic flow regime. That’s what we see and observe every day, when, for example, we drive our own car.

In subsonic conditions, the flow moves at low speed with respect to the speed of sound. The speed of sound indicates the speed at which acoustic perturbations propagate, which is about 760 miles per hour. In these conditions, the gas is well behaved and the flow pattern is rather smooth. As the flow reaches the speed of sound — we define this as supersonic flow — new physics come into play, which manifests itself in the form of shock waves. Shock waves are interesting and understanding their behavior is important, especially in aerospace engineering as they change dramatically the dynamic of the flow around the vehicles, for instance, a fighter jet.

Now, as we move at higher and higher speeds — think of it as five times the speed of sound or about 4,000 miles per hour — we encounter hypersonic flows. In hypersonic flows, the shock waves induce very high temperatures, so that the molecules in the gas break apart and the gas becomes chemical reacting. In the harshest conditions, electrons are striped off their atoms and become free to wander in space. In this case, the gas becomes a plasma.

If this very hot gas impinges on your vehicle, it can easily burn it and consume it, as in the case of meteorites. Therefore, we have to design thermal protection systems. 

And that’s so a vehicle during entry into a planetary atmosphere, like Mars, is protected from the extreme heat?

PANESI - Yes. During entry into a planetary atmosphere, the temperature of the gas is twice the temperature of the surface of the sun. To protect the vehicles and their payloads, we have to provide some heat release. We do that by designing the thermal protection system that slowly burns, becoming a gas. We refer to these systems as ablators.

So, the ablation of the thermal protection system provides release of the energy and protects the interior payloads.

The layers on the exterior of the vehicle sort of slough off?

PANESI - Yes. In the case of Apollo, half of the heat shield was consumed, and predicting the rate at which the material decomposes constitutes a challenging problem. A few years ago, I was actually having a discussion with a scientist involved in that mission, Professor Chul Park, and he mentioned that the design was such that they expected to have one-eighth of the TPS [thermal protection system] be consumed. In reality, after the flight, they noticed that almost half was completely gone. So, it’s very hard to make predictions.

Can you explain some of the bigger questions about these entry systems that you’re working to answer in your research

PANESI - There are still many questions to be answered, and one thing that I’m interested in and that’s been at the core of my research is the understanding and the characterization of the chemical processes that take place in shock heated flows or in the flow near the wall of the vehicle, the so-called boundary layer region. It’s a very hard problem because of the complexity of the chemical processes grows exponentially. It is a very hard multi-scale, multi-physics problem.

You can never resolve all the chemical processes that take place. You must make assumptions, and we never have the exact answer. We always have to model something. This research is really a combination of engineering, physics, computational mathematics, and chemistry. My research is the intersection between these areas.

You’ve received a number of awards recently. One is a multidisciplinary university research initiative award from the Department of Defense. What are the specific challenges you’ll address in this MURI project?

PANESI - This project, led by the University of Michigan, involves myself and Prof. Tonghun Lee in MechSE at U of I, and colleagues from other universities. This MURI deals with the characterization of chemistry in hypersonic flows with the additional complexity given by turbulence. We are trying to solve the combined problem in which you have chemical reactions and turbulence present at the same time. This is an ambitious goal.

So now, for many aspects, turbulence is very similar to chemistry, in the sense that it is characterized by a wide range of spatial and time scales that cannot be explicitly computed. In addition, however, turbulence is chaotic, which is an additional complexity.

We are trying to construct a model that works for both and is able to describe their combined effects. We will work with the applied mathematicians, with experimentalists, to leverage observations and computational simulations to extract a robust model from the data that can be safely applied for predictions. We want to come up with a reduced-order model that captures the essential features of the flow but at significantly reduced computational cost.

You also have major funding from NASA through a Space and Technology Research Initiative called the Advanced Computational Center for Entry Systems Simulation, or ACCESS. What are your goals for this new initiative?

PANESI - That’s a huge effort, a $16 million institute led by the University of Colorado that includes UIUC and two other partner universities. What we are going to do here at UIUC is the modeling of the flow physics — anything related from the chemistry, gas-surface interaction, material response modeling all the way down to the fluid dynamics. We’ll study the detailed chemical processes using quantum mechanics to understand the reactivity of some molecules, will investigate in detail how these molecules react with the surface, performing molecular dynamic simulations, and also study the dynamics of turbulence on different geometries.

We do have to heavily rely on computations. In hypersonics, we cannot rely on full-scale testing. We can only test a few subcomponents of the entire vehicle.

We will perform a large number of quantum mechanical calculations to understand how the molecules interact with each other, and then we will compile data sets that we can put into our CFD [computational fluid dynamics] solvers.

For the NASA STRI project, we have a collaboration with Professor Hua Guo at the University of New Mexico, who is a chemist, and of course, we will leverage our standing collaboration with the quantum chemistry group at NASA Ames. I’ve been working with them since 2008.

One of the goals of the STRI project is to collaborate very closely with NASA to create new relationships. So, I think we can leverage a lot of what they have to offer.

Marco, you were also awarded a very prestigious Department of Defense fellowship. What is the Vannevar Bush Faculty Fellowship and what does it mean to you to have received it?

PANESI - It is a five-year fellowship. I love this award because it is unconstrained and targets fundamental research. There is no limit. It is high-risk and high-reward. It pushes you to invent new things, new theories, new ideas, and you can take as much risk as you feel comfortable with. The only limit is your imagination. They accept the wildest idea. It has to be something that has the potential to transform a field.

What I proposed, of course, deals with hypersonic plasmas and aims to substitute some of the standard governing equations in fluid mechanics with something which is much more powerful and has a broader application. It is a very bold goal. The new equations have to work for plasmas and for chemical reacting gases in general, but also for conventional flows, such as the flow over a car. If it works, the same model could be applied to parallel fields of study, like, for example, astrophysics.

Another very important goal is to generate the future researcher and scientist but given the kind of research it would be faculty or researchers in national labs. This is probably the most interesting project I’ve ever written. It’s nice to be unconstrained — to propose something without being worried that it might be rejected because it’s too out there. This is about finding new ideas.

Of course, you’re also the director of the Center for Hypersonics and Entry Systems Studies here at the University of Illinois. Can you tell me a little bit about what the center does?

PANESI - Yes, but before I do that, I would like to thank The Grainger College of Engineering, Dean Bashir, and the entire leadership in the college, because they actually provide us with the support and advice and the funding necessary to make this a reality. They’ve been very supportive. They just have been amazing.

The center deals with hypersonics, but it has a much broader scope. We care about the fluid mechanics, aerodynamics, aerothermal dynamics, and chemistry, but we also care about everything else that goes into hypersonic systems from detection, material science, detection control, sensors, you name it. There are many different aspects. We are strong in our aerothermodynamics. And in the next few months, we’ll be trying to broaden to include other disciplines, such as computer science, control automation, GNC — guidance, navigation, and control — and a lot more about ECE [Electrical & Computer Engineering] in terms of sensors.

We are trying to take advantage of all the disciplines in The Grainger College of Engineering to solve the entire problem. I think it’s very ambitious, but that’s what the goal is. That’s the vision for CHESS — to study the entire problem, in all its aspects, from the fundamental point of view to the more applied. We want to have an impact. That’s key.

Can you talk a little bit about hypersonics facilities at Illinois.

PANESI - The Grainger College of Engineering has been investing a lot of funding into purchasing two one-of-a-kind facilities. One is a smaller facility for more fundamental work, which is mostly operated by Professor Panarai and his group. So, this is a reduced-scale, about 100-kilowatt facility.

The second one is an ICP — inductively copper plasma facility — the largest in the United States. It is much more powerful, about 400 kilowatt, so we can test bigger objects. It can go from supersonic to Mach 4, so four times the speed of sound, and a temperature of several thousand degrees — so like 5,000 to 6,000 degrees Kelvin.

The way it works is fairly simple. You have an antenna, which is like a solenoid [to convert electrical energy into heat]. It consists of a coil, which is wrapped around a pipe. We run very high-power and high-frequency current in this coil, generating plasma into the torch. This plasma gets flushed into a test section where we put the material sample to be tested. This is unique technology, because especially in the United States, there are not many of these. The benefit is that there is no contact between the electrodes and the plasma, so they are basically free of any contaminants.

Other facilities have electrodes, which are in contact with the gas and so the electrodes, as they erode, contaminate the plasma, changing the chemistry. They’re not ideal for the kinds of studies we would like to do, which is very fundamental material science quality testing and thermochemistry.

Finally, what are some of your hopes and dreams for the future of research in hypersonics at Illinois?

PANESI - Two things. Broaden the area of research in CHESS to cover other disciplines that conventionally are not dealt with in hypersonics. The Department of Defense also recognizes this need, and recently I have been invited by the DoD to discuss future research in parallel areas of critical importance to hypersonics.

Also, at Illinois we are very good at doing fundamental research, but there is also some need for transitional research — to move a lot of this research toward the application so that we can have more impact. 

Speedy Success

Members of Grainger Engineering’s Center for Hypersonics and Entry Systems have notched several major funding wins in the previous year, including:

Professor Kelly Stephani (a Kritzer Faculty Fellow in Mechanical Science & Engineering) is deputy director of the University Consortium for Applied Hypersonics. This 10-institution consortium is funded by a five-year, $100 million award from the Department of Defense.

Stephani and Professors Harley Johnson (a Kritzer Faculty Scholar in Mechanical Science & Engineering and Marco Panesi (Aerospace Engineering) are part of the multi-institutional leadership team for the $15 million Advanced Computational Center for Entry System Simulation, supported by the NASA Space and Technology Research Initiative.

Panesi and Professor Tonghun Lee (a Kritzer Faculty Scholar in Mechanical Science & Engineering) are part of a Department of Defense Multidisciplinary University Research Initiative award led by the University of Michigan, totaling $7.5 million.

Panesi is a Department of Defense Vannevar Bush Faculty Fellow, supported by a five-year, $3 million award.