Understanding the Color of Rocket Engine Flames

The color of a rocket engine's flame can reveal important information about the type of fuel being used, the temperature, and the completeness of the combustion. This article explores the various colors of rocket engine flames and the underlying reasons behind each color.

The Fundamentals of Rocket Flame Color

The color of a rocket engine's exhaust flame depends primarily on the fuel and oxidizer being used, as well as the combustion conditions. The vast majority of rocket fuels can be grouped into two main categories: solid and liquid. Each type has a unique characteristic that can affect the appearance of the exhaust flame.

Solid Rockets: White-hot Exhaust

Many solid rockets utilize aluminum-based propellants, such as those used in the Space Shuttle's Solid Rocket Boosters and in smaller solid-fueled rockets. When these propellants burn, they produce tiny droplets of aluminum oxide, which, as they cool and solidify, glow due to their heated state. This glow is from the hot particles glows like an 'black body', appearing 'white hot'. Due to the opaque nature of the exhaust, it blocks most of the visible light, making it difficult to see through the plume even in smaller rockets.

Liquid Rockets: Clear and Colored Exhaust

Liquid-fueled engines, such as the RS-25 used on the Space Shuttle and the RS-68 engine for the Space Launch System (SLS), often produce a mostly clear exhaust consisting mainly of water vapor (H2O). However, fuel-rich combustion can produce transient radicals, such as OH, which emit bluish light. Sodium, often present in some hydrogen sources, can also emit a faint red light. Methane engines have similar exhaust emissions but may produce more blue light due to OH and HC radicals. Near the walls, the fuel-rich regions can emit a yellow glow due to carbon particles (soot) and would produce a yellow plume in engines like the RP-1 engines in Saturn V, F-1 engines, and SpaceX's Merlin engines. N2O4-hydrazine engines, like those used on NASA Gemini and Long March, and in small reaction control systems, can emit a faint light orange color.

The Science Behind the Colors

The color of a flame is a result of the emission spectrum of the elements present in the exhaust gases. Different elements emit light at specific wavelengths, which corresponds to their color. For example, a blue flame indicates the hottest part of the flame, followed by a yellow flame, then orange, and finally red. Each color can indicate the temperature of that part of the flame or the type of fuel being used. The hotter parts of the flame, closer to the burning fuel, will appear whitish-blue, while cooler parts further away will be orange or red.

Emission Spectra and Analysis

Flame color can also be used to identify the composition of the exhaust gases, much like how emission spectroscopy is used to determine the composition of stars or materials on Earth. A table of the emission spectra for various elements can provide a detailed analysis of the exhaust gases. For example, modern liquid-fueled rockets, such as those burning hydrogen, will have a flame with a specific spectrum. Solid-fueled rockets, such as those using hydrazine, have more complex spectra, indicating the variety of elements present in the exhaust. This knowledge is invaluable for both understanding and optimizing rocket engine performance.

Conclusion

The color of a rocket engine's flame is not just an aesthetic feature but a critical piece of information that can reveal crucial details about the engine's operation. By understanding the principles behind flame color, engineers and scientists can make informed decisions about fuel types, combustion chambers, and exhaust design. This knowledge is fundamental to the continuous improvement of rocket technology and the advancement of space exploration.