What are the best ways to minimize rocket exhaust emissions?

While liquid fuels generally burn cleaner than their solid counterparts (hard to beat pure water coming out the tailpipe of a LOX/hydrogen engine), sometimes performance, storage, and safety requirements don't allow for a liquid propellant to be swapped in. That said, there is work being done in industry and academia to develop "green" solid propellants that utilize ingredients with reduced toxicity ingredients and produce cleaner exhaust while not sacrificing performance.

One of the many things that space programs proceeding with rocket launches could do to make them more sustainable is seek to use photovoltaics, otherwise known as the energy taken from solar panels. Another solution to develop more sustainable rocket launches is to create kerosene-burning rockets, as kerosene is a more affordable, stable, and less hazardous type of fuel that could mitigate the harmful environmental effects that typical rocket fuel is subject to create. The problem with using kerosene rocket fuel for rocket launches is that when used with other hydrocarbon propellants – it could still result in the same amount of black carbon. Therefore, mitigating the environmental impact of rocket fuel is still a work in progress.

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The number one way to reduce pollution in the exhaust is using a LOX/LH combination which produces 100% water vapor products. NASA did use this for the Space Shuttle Program and SLS (both use versions of the RS-25 engine). However, this is not applicable in all situations, as we see heavy and super heavy launch vehicles being designed with methane engines.

In my opinion, the concept presented highlights the significant role of rocket fuel type in determining exhaust emissions. It underscores the environmental impact of solid fuels, particularly those containing chlorine, and emphasizes the relatively cleaner emissions of liquid fuels, especially hydrogen-based ones. Opting for liquid fuels, particularly hydrogen-based options, appears as a viable strategy to mitigate rocket exhaust emissions and reduce environmental harm.

Increasing engine efficiency would decrease the amount of pollution produced during operations. It would increase the amount of energy produced from combustion, increasing engine performance and decreasing the amount of fuel and therefore pollution. This is why there are always engineers working on improving engine efficiency!

In my view, the concept underscores the critical role of engine efficiency in influencing rocket exhaust emissions. It highlights various factors that contribute to engine efficiency, such as combustion chamber pressure, nozzle design, expansion ratio, and cooling systems. Improving engine efficiency can lead to reduced fuel consumption and emissions, benefiting both environmental sustainability and operational efficiency in space missions. The proposed methods for enhancing engine efficiency, including increasing combustion chamber pressure and optimizing nozzle design, offer practical approaches to minimize emissions and optimize rocket performance.

While alternative fuels and approaches to propulsion do seem to decrease the amount of pollution, this is currently only true on paper. Both electric and nuclear propulsion systems do not produce anywhere near the required thrust to put a rocket in orbit. They do have high ISP which helps over long duration (interplanetary) missions but not for the short duration of liftoff to orbit.

The nozzle shape influences the outlet expansion losses in the case of underexpansion or overexpansion. There is a theoretically optimal nozzle shape for maximal exhaust speed. However, a shorter bell shape is typically used, which gives better overall performance due to its much lower weight, shorter length, lower drag losses, and only very marginally lower exhaust speed.