Rocket engine exhaust pollution extends high into Earth's atmosphere

Understanding rocket emissions in the atmosphere by modelling fluid dynamics of rocket exhaust gases

Reusable space technology has led to a rise in space transportation at a lower cost as popularised by commercial spaceflights of companies like SpaceX and Virgin Galactic. What is poorly understood, however, is rockets' propulsion emissions creating significant heating and compositional changes in the atmosphere.
In 'Physics of Fluids', researchers from the University of Nicosia in Cyprus assessed the potential impact of a rocket launch on atmospheric pollution by investigating the heat and mass transfer and rapid mixing of the combustion by-products for altitudes up to 67 km into the atmosphere.
"Improved understanding of rocket emissions requires modelling and simulation of fluid dynamics of rocket exhaust gases into the atmosphere," said co-author Dimitris Drikakis.
The team modelled the exhaust gases and developing plume at several altitudes along a typical trajectory of a standard present-day rocket. They did this as a prototypical example of a two-stage rocket to transport people and payloads into Earth's orbit and beyond.
"We show that pollution from rockets should not be underestimated, as frequent future rocket launches could have a significant cumulative effect on the Earth's climate," said co-author Ioannis Kokkinakis.
The researchers also found the production of thermal nitrogen oxides (NOx) — components of the combustion exhaust — can remain high up to altitudes with an ambient atmospheric pressure above or even slightly below the nozzles' exit pressure, i.e., below an altitude of approximately 10 km.
At the same time, the emitted mass of CO2 as the rocket climbs 1 km in altitude in the mesosphere is equivalent to that contained in 26 cubic km of atmospheric air at the same altitude.
Furthermore, they found the impact on the atmosphere locally and momentarily in the mesosphere can be significant. While air currents will gradually transport and mix the exhaust CO2 throughout the atmosphere, eventually bringing the CO2 back down to its naturally occurring levels; the time scale over which this happens is not clear.
The scientists believe a certain number of rocket launches might still exist above which mesospheric CO2 could accumulate over time, thus increasing the naturally occurring levels and affecting our climate.
Their results suggest that in the worst-case scenario, sufficient NOx could be produced over the time it takes the rocket to reach an altitude of 10 km to pollute over 2 cubic km of atmospheric air with a NOx concentration that — according to the World Health Organisation — would be at a level hazardous to human health.
"We hope that commercial flight companies — such as SpaceX, Virgin Galactic, and the New Shepard, and their associated engine manufacturers — will consider these effects in future designs," said Drikakis.