When rockets fire into space, the inside of their engines becomes an extreme environment where temperatures soar and tiny particles are thrown around at hypersonic speeds.
These particles behave in ways that break long-held assumptions, according to new research that could help improve the durability, safety and performance of future space and defence technologies.
The study shows that particles travelling at hypersonic speeds do not remain spherical, instead melting and deforming mid-flight in ways that change how heat, drag and energy move through rocket systems.
The findings, published in Physics of Fluids, led researchers to develop a new drag model that more accurately predicts particle behaviour under extreme conditions.
Co-author Associate Professor Qijun Zheng from Monash Mechanical and Aerospace Engineering said the work provides new insight into how particles interact with air under some of the harshest conditions engineers encounter.
"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 investigated microscopic alumina particles formed when aluminium fuel burns inside solid rocket motors. Although thousands of times smaller than the width of a human hair, the particles can travel at speeds of up to 10 kilometres per second through rocket nozzles.
Using molecular dynamics simulations, a type of atom-by-atom computer modelling, the research team tracked how the nanoparticles behaved in high-temperature, high-pressure air.
The findings could also apply to atmospheric re-entry, energy systems and other high-temperature industrial processes involving nanoparticles.
The study found slower-moving particles remained relatively stable, while particles travelling at extreme speeds experienced intense collisions with surrounding air molecules that caused rapid heating and melting.
Researchers discovered smaller particles heated faster because more of their surface area was exposed relative to their size.
The team also discovered that molten particles could stretch into thin "bag-like" structures before collapsing back into new forms 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 study found molten particles disturbed surrounding airflow more strongly than solid particles, generating larger regions of turbulence and energy transfer.
Associate Professor Zheng said the improved modelling could help engineers design more reliable propulsion systems and better predict material wear inside rocket engines.
"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 led by researchers from the Southeast University–Monash University Joint Research Institute, Monash University and Shanghai University.
Read the research paper: https://doi.org/10.1063/5.0321480
