New research from Monash University and its collaborators has revealed that microscopic particles inside rocket engines behave very differently from what engineers have long assumed, potentially influencing the design of future aerospace and defence technologies.
The study, published in Physics of Fluids, found that nanoparticles travelling at hypersonic speeds can melt and deform during flight rather than remaining spherical, affecting how heat, drag and energy move through rocket propulsion systems.
Researchers developed a new drag model to better predict particle behaviour under these extreme conditions.
Co-author Associate Professor Qijun Zheng from Monash Mechanical and Aerospace Engineering said the findings shed light on processes occurring inside some of the most demanding engineering environments.
“Inside rocket motors, these nanoparticles are exposed to enormous temperatures, pressures and speeds,” Associate Professor Zheng said.
“Our simulations show that once particles reach hypersonic speeds, they can rapidly heat up, melt and even dramatically change shape while travelling through the airflow.”
The research focused on alumina nanoparticles produced when aluminium fuel burns inside solid rocket motors. Although thousands of times smaller than a human hair, the particles can travel at speeds of up to 10 kilometres per second through rocket nozzles.
Using molecular dynamics simulations, which model interactions at the atomic level, the team examined how particles responded to high-temperature, high-pressure airflow.
The study found that slower-moving particles remained largely stable, while those travelling at extreme speeds experienced intense collisions with surrounding air molecules. These collisions generated rapid heating, melting and significant deformation.
Researchers also discovered that smaller particles heated more quickly because of their higher surface-area-to-volume ratio.
One of the study’s most striking observations was that molten particles could stretch into thin “bag-like” structures before collapsing and reforming during flight.
“These changing particle shapes affect how heat and energy move through the flow, which is important for predicting wear and performance inside rocket systems,” Associate Professor Zheng said.
“Current engineering models often assume particles remain perfectly spherical, but our work shows that assumption no longer holds under these extreme conditions.”
The researchers found that molten particles generated stronger disturbances in surrounding airflow than solid particles, creating larger zones of turbulence and energy transfer.
According to Associate Professor Zheng, the improved understanding of particle dynamics could help engineers better predict material wear and improve propulsion system reliability.
“Understanding how these particles behave under extreme conditions is essential for improving the accuracy of future aerospace simulations and developing more resilient high-speed technologies,” he said.
The study was conducted by researchers from the Southeast University–Monash University Joint Research Institute, Monash University and Shanghai University.
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