When a vehicle travels at hypersonic speeds—typically defined as Mach 5 and above—it encounters extreme aerodynamic conditions that fundamentally alter the behavior of the surrounding air. One of the most critical phenomena in hypersonic flight is the formation of a shock layer, a thin region of highly compressed, heated gas that forms near the nose or leading edges of the vehicle due to the conversion of the kinetic energy of the air into the thermal energy.
Unlike at subsonic or even supersonic speeds, the air in a hypersonic shock layer does not behave as a simple gas. Instead, it undergoes complex chemical and physical transformations due to the extreme temperatures. Molecular dissociation and ionization, as well as a state of thermodynamic nonequilibrium, occur, creating a chemically reactive environment that significantly influences aerothermal loads, vehicle stability, and communication systems.
Understanding the chemistry of hypersonic shock layers is critical for designing efficient thermal protection systems, improving vehicle performance, and developing next-generation hypersonic aircraft and space exploration vehicles.
The Extreme Conditions of Hypersonic Shock Layers
At hypersonic speeds, air is subjected to intense compression and heating as it encounters the vehicle’s leading edge. This creates a shock wave, which drastically increases the temperature and pressure of the air.
For reference, while standard atmospheric conditions near Earth’s surface involve temperatures around 300 K (27°C), the temperatures in a hypersonic shock layer can exceed 10,000 K and be as high as 15,000 – 30,000 K for spacecraft reentry. These temperatures are hot enough to break apart air molecules into their atomic components. At these temperatures:
● Molecular dissociation occurs: Diatomic gases such as oxygen (O₂) and nitrogen (N₂) to break apart into individual atoms, and species such as nitric oxide (NO) form.
● Ionization begins: At extreme temperatures, electrons are stripped from atoms, forming a plasma – a partially ionized gas that exhibits unique electromagnetic properties.
● Vibrational and electronic excitations take place: Following the rapid heating, excitation of internal, vibrational and electronic, modes of molecular motion occurs with some delay, influencing reaction rates and heat transfer.
● Emission of infrared, visible, and ultraviolet radiation takes place due to the excitation of molecular and atomic electronic states. In fact, radiative heat flux contributes more than 50% to the total heat flux to the surface in spacecraft reentry from the orbit and up to 90% to the surface heat flux in atmospheric entry of the spacecraft returning from a moon or future Mars mission.
This complex interplay of chemical and physical processes defines the air chemistry of hypersonic shock layers, influencing both aerodynamic forces and vehicle survivability.
Key Chemical Processes in Hypersonic Shock Layers
1. Molecular Dissociation and Other Chemical Reactions
The primary chemical change in a hypersonic shock layer is the dissociation of diatomic molecules due to extreme temperatures. The oxygen and nitrogen atoms then react with non-dissociated molecules, forming such species as the nitric oxide (NO).
Additionally, modern thermal protection systems (TPS) are designed to be ablative, i.e. to handle the extreme heat fluxes by evaporation/erosion, which releases various species into the air in the shock layer. The reactions of those species with the dissociated air are quite complex.
Overall, the complex chemistry alters the gas composition in the shock layer and changes the energy balance, affecting the thermal conductivity, viscosity, and heat transfer to the vehicle surface. Downstream of the shock layer, as the gas cools, the dissociated atoms recombine to form molecules, releasing energy and influencing thermal loads on the vehicle.
2. Vibrational and Electronic Excitation: Thermodynamic Nonequilibrium
In thermodynamic equilibrium at high temperature, the temperatures of all molecular and atomic degrees of freedom are equal to each other. The situation in hypersonic shock layers, however, is dynamic. Behind the shock front, the temperature of random translation motion of molecules jumps to very high values very quickly, since this requires no more than a dozen molecular collisions. In contrast, excitation of internal molecular vibrational and electronic modes requires many (thousands or millions) of collisions. Since hypersonic flight typically occurs at high altitude where the atmosphere is rarefied, the many collisions needed for vibrational and electronic excitation take a relatively long time. Consequently, there is a pronounced region in the shock layer where the ‘translational’ temperature can be as high as 15,000-30,000 K while the vibrational temperature is only 1,000 K or so. How can molecular dissociation proceed in this state of extreme thermodynamic nonequilibrium, in a gas that is extremely hot but where molecules are ‘cold’ inside? Fundamental understanding of this problem and practical models capable of describing the thermally nonequilibrium chemistry are crucial in our ability to predict aerodynamic and heat flux to the vehicle surface.
3. Ionization and Plasma Formation
At the very high temperatures in hypersonic shock layers, particularly those during spacecraft reentry, electrons are liberated from atoms and molecules. This results in the formation of a weakly ionized plasma, which impacts radio communication and navigation signals due to its ability to absorb and reflect electromagnetic waves. This phenomenon, known as radio blackout, is a significant challenge for reentry vehicles and hypersonic missiles.
On the other hand, plasma can be acted upon by electric and magnetic fields, creating an opportunity to use such field for control of aerodynamics and heat fluxes.
Advances in Understanding Nonequilibrium Air Chemistry
1. Old empirical models
The need to have a simple model of molecular dissociation suitable for thermally nonequilibrium environment where the vibrational temperature Tv is lower than the translational temperature T was recognized at the dawn of the space era. In 1960s and 1980s, several empirical models and the corresponding formulas for the dissociation rate coefficient as a function of Tv and T were suggested. The empirical models worked reasonably well when inserted into the CFD (computational fluid dynamics) codes and checked against the very limited sets of experimental data. However, it was becoming increasingly clear that a more fundamental approach is necessary in order to ensure that the theoretical models can be relied upon in a wide range of conditions, including more extreme velocities and shock layer temperatures.
2. Models based on massive computing
With the explosive development of computational power, a new approach to nonequilibrium dissociation and other reactions emerged. Quantum chemists did ab initio computations, with a reasonable accuracy, the interaction energy between the colliding molecules or atoms, called the Potential Energy Surface (PES). The laws of classical mechanics were then used to compute many thousands of trajectories of the reacting system on a given PES, with each trajectory characterized by a certain energy and a randomly selected initial conditions such as angles, oscillator phases etc. Statistical analysis of the trajectories resulted in the probability of a certain outcome, e.g. dissociation, as a function of the vibrational and translational energy, which was then converted into the reaction rate coefficient as a function of Tv and T. The principal products of such massive computational effort conducted primarily at the University of Minnesota were the multi-dimensional arrays of data which, in some cases, was then represented as an interpolation formula to be inserted into a CFD code.
3. Theoretical (non-empirical) models
A physical, i.e. non-empirical, model with an accuracy comparable with that of the massive computational approach but still yielding a formula simple enough to be used in CFD was developed by me and my colleagues. The model, dubbed the Macheret-Fridman (MF) model, describes the rate coefficient for collisional dissociation reactions. In the MF dissociation model, the vibrational, rotational and translational motion is described by classical mechanics, and the chemical transformation is assumed to occur instantaneously. These assumptions are justified at high translational temperatures such as those occurring in hypersonic shock layers. The model produces an analytical expression for the rate coefficient as a function of vibrational, rotational, and translational energies or temperatures. The original MF model was developed in 1993-1994, and was later further developed and implemented in both computational fluid dynamics (CFD), or continuum, version, and in Direct Simulation Monte Carlo (DSMC), or statistical, framework. The results were in good agreement with those predicted by those obtained with ab initio calculations of potential energy surfaces using quantum chemistry and subsequent computer-based trajectory modeling of the reaction cross sections and rates. Thus, the analytical or semi-analytical formulas of the MF model can be recommended for use in hypersonic computations.
Challenges in Hypersonic Air Chemistry Research
1. Experimental Limitations
Simulating hypersonic shock layers in a laboratory setting is challenging due to the extreme temperatures and pressures involved. While shock tunnels and arc-jet facilities provide valuable data, they cannot replicate the full range of hypersonic flight conditions. Advanced numerical simulations with computational fluid dynamics (CFD) play a crucial role in filling these gaps.
2. Material Response
Understanding how air chemistry interacts with vehicle materials is an ongoing challenge. The chemical erosion of ablative heat shield materials, the formation of new compounds at high temperatures, and the effect of surface catalysis must all be considered when designing hypersonic vehicles.