Supersonic is Safe for the Environment

Emissions from supersonic transport have raised concerns due to its high cruising altitude. The Concorde, for example, flew at 60,000 feet, placing it in the lower layers of the stratosphere roughly where the ozone layer begins. Further study is warranted to assess the environmental impact of larger, high altitude SST. Nonetheless, the evidence to date, including 27 years of Concorde operations, indicate that passenger SST ozone risk is well within the realm of acceptability. 

Simulations conducted by NASA Glenn Research Center have considered the effects of a fleet of supersonic business jets over a ten year period within the atmospheric conditions projected for 2020. A total of 24 scenarios were evaluated by varying parameters like fuel burn, cruise altitude and a nitrogen oxide emissions index. The most probable scenario considered a fleet of supersonic business jets burning 18 million pounds of fuel per day at a height of 15-17km, or roughly 50,000 to 56,000 feet, resulting in a maximum local ozone depletion of only 0.038% and a rate of global ozone depletion orders of magnitude smaller. For comparison, concern in the 1990s that pollution was creating a growing “hole” in the ozone layer stemmed from observations of ozone depletion on the order of 20-60%.

In fact, emissions in the lower altitude range of 13-15km may actually create ozone on net, although at a similarly small rate, through a process related to the phenomenon of urban ozone. Supersonic business jets are thus considered essentially ozone neutral within their range of likely cruising altitudes.


MYTH: Supersonic Transport Will Destroy the Ozone Layer

Researchers in the 1970s generated a polarized debate after it was posited that the nitrous oxide emissions from a fleet of Concordes might cause reactions contributing to catastrophic ozone loss. Several governments and Academies of Science set up committees to further investigate the claims. They ultimately rejected them as unfounded, given limited and uncertain data. The view that SST emissions pose little threat to the ozone layer was cemented after subsequent researchers realized nuclear tests conducted by the US and Soviet Union had injected large magnitudes of nitrogen oxide and other particulates without causing significant ozone problems, despite being comparable to, as one author put it, “the flying of 500 Concordes seven hours a day for some five years.”

That did not stop anti-Concorde activists from propagating false and exaggerated claims of ozone risk. As supersonic researcher Preston A. Henne put it, “The difficulty in early programs was lack of credible understanding of atmospheric science. The absence of such knowledge left the door open for wild and exaggerated claims of atmospheric trauma based on speculation, misinformation, and political agendas.” Atmospheric science has advanced significantly since the 1970s, and today, it is widely accepted that emissions from supersonic aircraft in the lower stratosphere pose minimal risk to the ozone layer. 

MYTH: We Need New Environmental Regulations Just for Supersonic

As supersonic transport overland becomes liberalized it will come to fall under at least six distinct sources of existing environmental regulation, including local air quality certification standards, airport noise rules, and the Montreal Protocol on ozone pollution. In general, these regulations were not designed with SST in mind, meaning they may require updating with appropriate consultation from federal agencies like the EPA and international bodies like ICAO. Clarifying the environmental status of overland SST under existing regulation is an essential step for creating the conditions of policy certainty the industry needs to thrive, however this does not mean any new regulations are required.

Governments have a role in requiring consumers to internalize the social cost of their behavior, including nitrogen oxide and carbon emissions. But as long as the cost is accounted for the law should remain neutral with respect to how the emissions are created. This is the reason economists favor policies like carbon taxes to ad hoc prohibitions on carbon-intense goods or services.

 Jets reduce noise by by-passing air around the engine.

Jets reduce noise by by-passing air around the engine.

MYTH: There is No Trade-off Between Airport Noise and Fuel Efficiency

Modern supersonic passenger jets will be both quieter and dramatically more fuel efficient than the Concorde. Indeed, the Concorde's turbo jets were very noisy independent of the sonic boom, and its use of afterburners on take-off was incredibly wasteful and dirty. Modern aircraft engines are capable of reaching supersonic speeds without afterburners, and by-pass air around the jet in order to insulate noise.

Subsonic passenger aircraft have met airport noise standards by increasing their bypass ratio, at the same time producing a huge gain in fuel efficiency. The more air the engine lets bypass the jet, the higher its bypass ratio, and the quieter the aircraft. However, the relationship between air bypass and fuel efficiency runs in the opposite direction for supersonic aircraft. Supersonic engines have not experienced the decades of incremental improvement that subsonic engines have—thanks in part to the supersonic ban over land—and therefore it makes sense to set a slightly less stringent airport noise rule for supersonic transport, at least at first. It may therefore make sense to set a slightly less stringent airport noise rule for supersonic transport, at least at first. We currently tolerate Stage 3 airport noise standards, as they are known, but newly certified aircraft are required to meet more stringent Stage 4 noise standards. Ideally, a new supersonic passenger jet would be certifiable under the current Stage 3 standards until later designs are able to climb the noise abatement learning curve. Future solutions include variable by-pass engines that reduce the jet's by-pass ratio once the aircraft is beyond the ears of airport residents.