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Author: Kshitij Sabnis, Lecturer in Aerospace Engineering, Queen Mary University of London
Original article: https://theconversation.com/supersonic-passenger-aircraft-may-be-returning-heres-what-that-would-mean-for-the-climate-250116
The US aerospace company Boom Supersonic recently announced it has successfully tested its latest aircraft, the XB-1. The company is developing a larger plane and aims to begin passenger flights at supersonic (faster than sound) speed within the next five years, and claims it already has orders from a handful of airlines.
However, with ever-increasing scrutiny on the environmental consequences of flying, it is important to consider how supersonic aviation may affect the industry’s ability to meet its ambitious net zero by 2050 target.
The latest test flight represents an important stage in the development of supersonic aircraft that minimise their characteristic “sonic boom” – the considerable noise generated as the shock waves travelling along with a supersonic aircraft pass over people on the ground.
Boom Supersonic has carefully designing the aircraft shape to reduce this noise pollution. It did this by exploiting a phenomenon known as “Mach cutoff”, where air temperatures in the atmosphere cause shock waves to reflect upwards rather than towards the ground.
Supersonic aircraft won’t receive certification to fly over land if they are too noisy, and overland flights are essential for their commercial viability. Indeed, failing to receive such certification limited Concorde’s routes to London-New York and Paris-New York, ultimately contributing to its demise. These recent noise improvements pave the way for Boom Supersonic to progress its larger 80-seat supersonic airliner, Overture. If all goes to plan, Overture will cruise at 1.7 times the speed of sound and could fly from London to New York in just 3.5 hours.
Other organisations working on supersonic flight are making similar progress. US firm Spike Aerospace is developing a smaller business jet, for instance, while Nasa and defence and aerospace firm Lockheed Martin plan to begin test flying their supersonic X-59 later this year. There is every indication that planes like these are on their way back, more than two decades after Concorde last took to the skies.
Andre Cros / wiki, CC BY-SA
Shock waves increase aerodynamic drag
The key to understanding the environmental effects of supersonic aircraft is that, whenever its speed exceeds the speed of sound, shock waves form around the aircraft. These shock waves significantly increase the aerodynamic drag, and so more fuel needs to be burned to compensate for the drag force. Indeed, it is estimated that up to ten times more fuel needs to be burned by a supersonic aircraft compared to the equivalent subsonic airplane for every passenger mile.
Chabacano / wiki, CC BY-SA
The cost of this extra fuel is why typical aircraft speeds have remained pretty constant at around 85% of the speed of sound for several decades. It also leads to greater greenhouse gas emissions – as much as five to seven times more than subsonic aircraft.
In fact, the situation may be even more stark. Supersonic aircraft tend to fly at high altitudes (Concorde flew at 60,000ft (18km) rather than the 40,000ft (12km) typical for most passenger jets) to take advantage of lower turbulence levels. This means their emissions tend to remain in the atmosphere for longer.
Supersonic and sustainable?
There are considerable efforts to align supersonic aircraft development with the aviation industry’s environmental ambitions. For instance, the new engines designed by Boom Supersonic are powered entirely by “sustainable aviation fuels” (Saf) which are direct replacements for traditional jet fuels that are made from renewable raw materials, often used cooking oil or crop residues. Due to its exclusive use of Saf, the Overture is advertised as having a zero-carbon footprint.
Boom Supersonic, CC BY-SA
In reality, the situation is more complex. Saf is often produced from edible crops and has been linked to deforestation – the total land required to power all of commercial aviation in this way is impractically immense.
To address this longer-term problem, it is necessary to look towards alternative fuel sources. While hydrogen or electric power is being developed for regular aircraft, for now they aren’t developed enough to ensure a plane reaches supersonic speeds. Instead, one possibility is e-kerosene, a synthetic fuel generated from hydrogen and carbon dioxide using renewable electricity.
Despite all these technological advances, a supersonic aircraft still cannot beat physics. Shock waves, and their associated drag, will still exist. So, a single supersonic aircraft will still produce considerably more carbon emissions than its subsonic counterpart.
Beyond carbon emissions, contrails also have an effect. These are thin clouds of water vapour produced by aircraft exhausts, which can trap heat in the atmosphere the same way as greenhouse gases. These contrails are thought to have twice the impact of carbon emissions, or perhaps even more, so it is essential to take their effects into account. For now, we simply don’t know enough about contrails, especially at much higher altitudes, to definitively say how supersonic aircraft will affect the environment.
Given the costs involved, supersonic aircraft will account for only a very small percentage of aircraft worldwide. The overall impact on the environment, in comparison to the tens of thousands of subsonic aircraft currently in operation, will be moderate.
There is perhaps one environmental upside. The research and development activity making supersonic aviation more environmentally friendly (such as developments in fuel and propulsion technology) will likely yield technologies that transfer to subsonic aircraft too. This should help to address the much broader problem of environmental damage caused by the aviation industry as a whole.
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