Rocket Plume Analysis Ready for Takeoff

Rocket plume testing equipment. (Photo: Elisa Haber/The Aerospace Corporation)

If rocket plume analysis doesn’t sound like a familiar concept, well, it shouldn’t. Aerospace scientists are among the very few — and in some areas, the only — researchers studying and dissecting the fiery plumes that shoot from the base of launched rockets.

With all of the iconic images of glorious, fire-spewing rocket ships that have graced our televisions, computer screens, and newspapers over the years, surprisingly little has been done in the way of researching what these spectacular launches leave behind. Aerospace’s Patti Sheaffer and Dr. Martin Ross are working to change that by deeply examining the makeup and environmental impact of rocket plumes. Their groundbreaking research is laying a strong foundation for intensive rocket emission analysis for years to come.

When a rocket is launched, it typically burns off two-thirds of its propellant in the lower atmosphere and one-third of its propellant in the upper atmosphere (stratosphere and mesosphere). It’s the exhaust trapped in the upper atmosphere that concerns scientists and climatologists since it typically lingers in that region of the atmosphere for three to four years. This rocket launch exhaust usually consists of soot particles, aluminum particles, water vapor and other chemical remnants that can affect the atmosphere’s properties in a substantial way. With the growing international concern over climate change, Ross and Sheaffer’s emission analysis is becoming increasingly timely and relevant.

Sheaffer, who has worked at Aerospace for more than 30 years, handles most of the nuts and bolts experimental operations. She has built a one-of-a-kind testing facility at Aerospace for simulating rocket launches and a larger rocket simulator for testing in the field. The research that she’s carried out for more than a decade has led to a clearer understanding of the chemical composition of rocket plumes and the definitive characteristics that they exhibit. “There is a big chunk of the atmosphere that we just can’t examine to make measurements,” explains Sheaffer, “so we have to use models and that’s what started a lot of our lab.”

Patti Sheaffer with equipment used for simulating rocket launches. (Photo: Elisa Haber/The Aerospace Corporation)

Patti Sheaffer with equipment used for simulating rocket launches. (Photo: Elisa Haber/The Aerospace Corporation)

With years of controlled experimentation and data analysis, Sheaffer has developed unique computer models that are capable of analyzing the emissive qualities of rockets. Her labyrinthine, in-lab simulator reproduces the rocket-induced chemical reactions that take place in the upper atmosphere and tests the accuracy of the models. Once the integrity of the models is assured, Sheaffer will often head out to launch sites in order to collect data from the rocket plumes. The data is obtained with infrared instruments that allow for a very detailed depiction of the chemical species and aerothermodynamics contained within the plume.

“Molecular species can be excited in three different ways” says Sheaffer. “Electronic excitation, vibrational excitation, and rotational excitation. The flame is very very hot, so these things are vibrationally excited so they emit in certain characteristic wavelength bands.” By examining where a given plume fits in the wavelength spectrum, Sheaffer can diagnose its emissive qualities. This information not only adds significantly to a more well-rounded portrait of a given rocket launch, but can be useful in gauging the potential environmental impacts of a specific engine or rocket type. In essence, Sheaffer has developed a highly accurate method of analyzing rocket exhaust, which can be applied to nearly any rocket in the public and private sector.

Dr. Martin Ross has been monitoring and examining rocket exhaust at Aerospace since the early 1990s. His initial research was sparked by the international distress surrounding ozone-depleting chemicals, which eventually resulted in the signing of the Montreal Protocol in 1987. In the wake of that landmark treaty, Ross and his research team concluded that rockets were not significant contributors to overall ozone destruction. More than 20 years later, Ross is still exploring the effects of releasing rocket emissions into the Earth’s atmosphere. He currently sees rocket emissions as a small part of the global climate change picture, but believes that climate change can have significant impacts on spacecraft operations as the atmosphere evolves in the coming decades.

A rocket engine in operation during a static-firing experimental data collection. (Photo: The Aerospace Corporation.)

A rocket engine in operation during a static-firing experimental data collection. (Photo: The Aerospace Corporation.)

“As time goes on, space operations will be increasingly affected by the changing climate — and in ways that may not be obvious,” says Ross. “We think there’s a whole spectrum of things that climate change will affect when it comes to national security space. For example, when rockets deposit water vapor in the ionosphere it changes the ionosphere. And GPS signals have to go through the ionosphere and they may change a little bit. And maybe it’s just a little bit, but when it comes to dropping ordnance, a little bit may determine whether or not you are successful in your mission.” This straightforward approach to analyzing the potential hazards of climate change lends a practical appeal to an issue that is often clouded by political banter and posturing.

Overall, the chemical footprint of the rocket industry is much smaller than that of the aviation or auto industries, but seems poised for expansion as it continues to grow and enmesh with the private sector. Space tourism and other concepts that open the skyways to the masses may lead to a large increase in the amount of emissions we see in the upper atmosphere. It’s already happening on a small level, but some day soon, ordinary people may be able to travel out of our Earthly atmosphere for entertainment and personal pleasure. If so, the research of Ross and Sheaffer will undoubtedly stand among the pioneering efforts in examining the way these rocket launches will affect our environment.

—Matthew Kivel