Rocketing Through the Atmosphere

Hello readers! Today we’re going to be discussing rockets, more specifically, reusable rockets. As you may have heard, this past Thursday (March 30th), SpaceX marked the first launch of a reusable rocket, a historic event for aerospace. Reusable rockets could drastically reduce the cost of rocket launches and could make space travel a lot more plausible. The founder of SpaceX, Elon Musk, has dreams of sending people to Mars.
While posts here on From Physics to English may someday discuss the difficulties of setting up a colony on Mars, today we’re looking at something a little closer to home–the reusable rockets. Not the SpaceX Falcon 9, however. Today’s featured article is “Global atmospheric response to emissions from a proposed reusable  space launch system” written by Erik J. L. Larson, Robert W. Portmann, Karen H. Rosenlof, David W. Fahey, John S. Daniel ,anMartin N. Ross. Published online in 2017 in the American Geophysical Union journal Earth’s Future, this article takes a look at Rocket Engines Ltd. proposed Skylon rocket. Skylon is fueled by hydrogen and is reusable.
Instead of Mars, the Skylon vehicle is envisioned by its company as the launch vehicle used to create a space-based solar power plant. This would require thousands of launches every year for ten years, if it is to make sense economically. This might sound like an incredibly large amount of launches, but as the authors point out, the space industry is starting to look like aviation, where slowing down the growth would come with unwelcome economic damage.
Hydrogen burning engines are generally considered ‘cleaner’ than their counterparts, as the main emission is H2O–water. However, even this, in large amounts, could affect the atmosphere. Additionally, nitrogen oxides would be created, and these are known to damage the ozone layer (which protects the earth from harmful ultraviolet radiation).
Nitrogen oxides would be created both during part of the ascent (when the Skylon uses the air surrounding it in its propulsion method) and reentry. So, the authors used data on Skylon launch and reentry to model the atmospheric effects.
Two different climate models were used, and showed that if 100,000 flights were launched each year, that about three percent of the H2O levels currently in the atmosphere would be added. If it ‘lives’ in the atmosphere for three years, that increases it by nine percent overall. The models give ten percent, though it would vary by location and altitude.
A diagram of Earth’s atmospheric level. Taken from Portrait of a Planet, page 699.
Nitrogen oxide is a little bit more difficult. Calculating the emissions of these is not simple, as multiple factors are involved. The designers of Skylon haven’t yet made a nitrogen oxide emission estimate for the rocket, so our authors used estimates for other jet and rocket engines and scaled them to the Skylon, aware of the high uncertainties this creates.
With 100,000 launches per year, the ozone is noticeably and significantly damaged, with the highest effect in the polar regions. Above about 25 kilometers (15.5 miles), or 20 kilometers over the poles (12.4 miles), the ozone is depleted, destroyed by nitrogen oxide. Between 18 and 24 kilometers (11 and 14.9 miles), the ozone actually increases, due to the chemistry of smog. Simulations that included only nitrogen oxide, and those that included H2O and Has well as nitrogen oxide are nearly identical, indicating that the contributions of the latter are not significant. There are other impacts, including changes in clouds over the poles, but the ozone was the most significant.
Most likely, these impacts would last for a few years after the high volume of flights came to an end. These are not, of course, the only kind of rockets (some, for example, burn kerosene), and given the uncertainty in the calculations, should not be assumed to be the option with the least atmospheric impact. As the authors point out, few studies have been done on this topic using state-of-the-art models. There’s more to be investigated, as always in science.

 

Bibliography
Chang, Kenneth. “Recycled Rockets Could Drop Costs, Speed Space Travel“. New York Times. The New York Times, 30 March 2017. Web.
An article about the recent historic SpaceX launch.
Larson, E. J. L., R. W. Portmann, K. H. Rosenlof, D. W. Fahey, J. S. Daniel, and M. N. Ross (2017), Global atmospheric response to emissions from a proposed reusable space launch system, EarthFuture, 5, 3748, doi:10.1002/2016EF000399
Today’s main article.
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed.  New York; W.W. Norton & Company, 2008. Print.
Geology textbook used for reference on the ozone and the diagram used in today’s post.
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