Simulating the expansive conditions of a rocket plume

One of the most iconic parts of a rocket launch is the plume, which can often stretch for miles behind an ascending rocket. This plume contains a mixture of gases —including ionized particles — that can interact with their environment in important ways. For instance, these plumes can generate strong radar and infrared signals that can make tracking rockets in the upper atmosphere easier.

Clout et al. used a series of direct simulation Monte Carlo (DSMC) models to simulate the expansion of rocket plumes over large distances, a task that is traditionally too complex to perform numerically.

“The main challenge with this approach is that most DSMC solvers are not designed to generate, at the boundary, fluid particles far from equilibrium,” said author Antoine Clout. “To solve this, we developed a technique that breaks down a non-equilibrium fluid into a set of equilibrium sub-species, which can then be easily injected into the next domain regardless of the solver being used.”

To implement this method, the authors created a chain of DSMC domains with increasing time-steps. The researchers coupled the domains together with a sum of Gaussian functions to represent the particle velocity distributions. This method preserved both the velocity distribution functions and macroscopic flow quantities.

With this technique, the authors simulated the third stage of the Japan Aerospace Exploration Agency’s Mu-5 rocket. They found a significant separation of gas species in the exhaust, with lighter species first segregating towards the edges of the plume and then recompressing towards the axis of symmetry closer to the rocket.

The authors point out that this approach can have broader uses.

“While this method was developed specifically for rocket plume expansion, its application is not limited to this field,” said Clout. “We believe it can be adapted for any simulation involving high-density gradients and strong non-equilibrium effects.”

Source: “One-way hybrid computational fluid dynamics to direct simulation Monte Carlo to direct simulation Monte Carlo long-range simulation of a solid rocket motor plume at high altitudes,” by Antoine Clout, Pierre Bernigaud, Adrien Langenais, Yann Dauvois, Luc Mieussens, and Julien Labaune, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0353453 .