Space Rocket Combustion, chemical reaction and air pollution

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Space rocket combustion involves rapid redox reactions between a fuel and an oxidizer, releasing high temperature gases (such as  water vapor, CO2 and others)  and particles directly into various layers of the atmosphere.  

The main types of rocket fuels are liquid hydrogen, kerosene (RP-1), liquid methane, and solid propellants, each producing different, significant air pollutants. 

Rocket launches release pollutants directly into the stratosphere (approx. 12–50 km altitude), where they can persist for several years, causing ozone depletion and contributing to climate change. While currently small compared to aviation, the environmental impact of rocket emissions is disproportionately high due to the altitude of injection and the accumulation of pollutants in the upper atmosphere.

Rocket propulsion; source: Wiki

Let's have a look on couple of examples:

1. Liquid Hydrogen (LH2) and Liquid oxygen (LOX): Used by Space Shuttle main engines, Ariane 5, the SLS upper stage. This is considered the cleanest rocket propellant, because its exhaust is primarily water vapor. However, at high altitudes, this water vapor can form reflective clouds, potentially increasing climate warming. 

2H2(l) + O2 → 2H2O(g) + energy

The LH2/LOX combination is considered environmentally friendly compared to kerosene (RP-1) or solid propellants, as it doesn't contain carbon and therefore it doesn't produce carbon dioxide (CO2), carbon monoxide (CO), or soot. The water vapor emitted directly into the stratosphere can form thin clouds, but modeling indicates its overall impact on the ozone layer is minimal.

2. Kerosene (RP-1) and Liquid Oxygen (LOX): Used by SpaceX Falcon 9 and Soyuz, these produce significant amounts of black carbon (soot) and carbon dioxide (CO2). Black carbon from rockets is estimated to be up to 500 times more efficient at warming the atmosphere than soot from other sources.

Stoichiometric Combustion Formula (Idealized): 

2C12H26 + 37O2 → 24CO2 + 26H2O

Actual Rocket Combustion (Incomplete/Fuel-Rich): 

C12H26 + LOX → CO2 + H2O + CO + C(soot) + NOx

Kerosene/LOX rockets release pollutants directly into all atmospheric layers, including the stratosphere, where they can persist for long periods. 
  • Soot/Black Carbon: The most significant concern, black carbon is a very effective absorber of solar radiation, directly heating the stratosphere. It is over 1 million times more efficient at heating the atmosphere than an equivalent amount of CO2. 
  • Ozone Layer Depletion: Pollutants from rocket exhaust, such as water vapor, NOx, and soot particles, facilitate chemical reactions that destroy stratospheric ozone.
  • Greenhouse Gases: CO2 and H2O are released, but the direct impact of rocket CO2 on global warming is considered small compared to aviation or industry.
Air pollution risk is quite high. Black carbon is a major concern because as black carbon particles are released in the stratosphere, primarily from rocket launches, they have a disproportionately large impact on climate and ozone due to their ability to absorb solar radiation and their long atmospheric lifetime, lasting over a year compared to weeks in the troposphere. When deposited in the stratosphere, these soot particles warm the surrounding air and alter atmospheric dynamics, leading to significant ozone depletion.

3. Liquid Methane (CH4) and Liquid Oxygen (LOX): A popular choice for new, reusable rockets (e.g., SpaceX Raptor). It produces less soot than kerosene but still releases water vapor and CO2.

Balanced Chemical Equation: 

CH4(l) + 2O2(l) → CO2(g) + 2H2O(g) + Heat

Actual conditions rarely operate at perfect stoichiometry. They usually operate fuel-rich, meaning excess of methane is coming to process, meaning some methane remains unburnt This results in the production of hydrogen (H2), carbon monoxide (CO), and smaller amounts of carbon dioxide (CO2). Methane engines can emit significant amounts of hydrogen oxides (HOx) into the stratosphere. These radicals, along with possible minor amounts of nitrogen oxides (NOx), can contribute to ozone layer depletion.
Because methane is a simple hydrocarbon (single carbon atom) compared to complex kerosene (RP-1), the combustion process produces significantly less soot (black carbon).

4. Solid Propellants (SRMs): Used in NASA's Space Launch System (SLS) and Europe's Ariane 5/6, these emit alumina (aluminum oxide) particles and reactive chlorine (HCl), both of which are potent ozone destroyers.

Solid rocket boosters (SRBs) use a highly exothermic combustion reaction, typically fueled by powered aluminum (Al) and oxidized by ammonium perchlorate (NH4ClO4). A common chemical reaction for these boosters is: 

3Al(s) + 3NH4ClO4(s) → Al2O3(s) + AlCl3(s) + 3NO(g) 6H2O(g)

Aluminum oxide particles (Al2O3), hydrogen chloride (HCl) which can contribute to localized acid rain, water vapor, and CO2. Alumina particles and HCl are highly destructive to the ozone layer and produce black carbon.   

Conclusion

The "best" overall rocket fuel from an air pollution perspective is Liquid Hydrogen (H2) paired with Liquid Oxygen (LOX), commonly known as Hydrolox. However, the industry is rapidly moving toward Liquid Methane (CH4) with Liquid Oxygen (LOX), known as Methalox, as the best balance of environmental cleanliness and practical usability.

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