Supersonic’s not-so-super emissions

Those who want to revive supersonic passenger flight will need to do more than build Mach 1 aircraft. They’ll need to convince a climate-change-rattled world that their comfort won’t make the greenhouse gas problem a whole lot worse. The industry has some creative ideas for addressing the problem.

Life turned hard for supersonic enthusiasts at British Airways and Air France not long after they started flying Concordes in 1976 on regularly scheduled routes from London to Bahrain and Paris to Rio de Janeiro. Orders for the pioneering aircraft failed to materialize; instead of an anticipated fleet of hundreds, only 14 Concordes ever flew commercially. A ban on overland sonic booms in the United States, a major aviation market, limited the sound-barrier-busting aircraft to servicing a few American cities relatively close to coastlines.

Maintenance and fuel costs continued to spiral far higher than expected. Capping a fraught 27-year run, the last Concorde took off from JFK airport and touched down at Heathrow in 2003, completing a final trans-Atlantic crossing.

Flash forward to 2019, and a new class of supersonic aircraft entrepreneurs — led by Aerion Supersonic of Nevada, Boom Supersonic of Colorado and Spike Aerospace of Massachusetts — has taken up the torch. All must deal with the overland supersonic ban and now also a hot issue that was not on the agenda in the Concorde’s heyday: carbon emissions. The U.S., at least at the moment, has not made regulating carbon emissions from aircraft (or anything, really) a priority. But elsewhere, the aviation industry is under scrutiny for its small yet sizeable and growing greenhouse emissions, uniquely damaging because of their deposition at altitude.

Supersonic planes add to this challenge by flying higher and faster than their subsonic cousins. Cruising above 50,000 feet, exclusively in the stratosphere’s thinner air, cuts down on heat-generating, airframe-stressing, and fuel-efficiency-eating resistance. Yet such heights pose new climate-warming emissions complications. Fundamentally, flying faster requires more energy and fuel burn, translating to greater emissions.

“We fully recognize that we are going to pump out more greenhouse gases than a subsonic aircraft will. That’s just the laws of physics,” says Gene Holloway, chief sustainability officer at Reno, Nevada-based Aerion.

The three companies plan to address the emissions challenge in several ways. The most straightforward — applying to subsonic aircraft too — is improving aerodynamics and developing more efficient engines. Switching from fossil-fuel-based, conventional jet fuel to alternative fuels with lower net life-cycle carbon emissions is another tactic. Aerion has further emphasized how considering its aircrafts’ entire lifecycles, not just flight operations, will reduce carbon footprints. Meanwhile, both Aerion and Boom are angling for new, supersonic-specific environmental standards that are more permissive than subsonic standards, acknowledging the supersonic sector’s emissions potency.

Emissions could prove to be one of the harder nuts to crack for supersonics. Ongoing research into so-called low-boom technology by NASA and industry players, focusing on aerodynamic design, looks like it could downgrade a boom from a disruptive thunderclap to the soft thump of a car door closing. As for the root business case, all three supersonic companies also claim the demand is there for their product. Boom plans on a roughly 55-seater and says that ticket fares will be competitive with subsonic business-class seating. Aerion and Spike, meanwhile, are targeting deep-pocketed business jet owners willing to pay more to slash travel times. All companies are aiming for deliveries by the mid-2020s.

“What Concorde showed is there is a market for customers who want to reach destinations faster,” says Vik Kachoria, CEO of Boston-based Spike. “What we need to figure out is how to do the Concorde, but we need to do it safer, more efficiently and more sustainably.”

A growing impact

At present, aviation accounts (depending on methodology) for 2 to 3% of humanity’s total greenhouse gas emissions. That share is expected to triple at a minimum by mid-century, according to ICAO, the United Nation’s International Civil Aviation Organization, due to ever-increasing demand for air travel and more planes plying the skies. In just 20 years, the current worldwide fleet of circa 22,000 commercial aircraft is projected to double.

Airlines have gotten on board with emissions reductions, with all major carriers in the U.S. and most worldwide voluntarily committing to ICAO’s Carbon Offsetting and Reduction Scheme for International Aviation resolution, known as CORSIA. Established in 2016, CORSIA caps emissions at 2020 levels for international flights, which accounts for about two-thirds of aviation’s carbon dioxide emissions; flights within a nation’s borders are covered under the Paris Agreement, the U.N. climate accord reached in 2016, and national commitments. Starting in 2027, when CORSIA becomes mandatory for all 192 countries that are part of ICAO, airlines can stay under their cap on their country-to-country flight routes by buying offsets through carbon marketplaces. An example offset would be buying a carbon credit, equivalent to a 1,000 kilograms of carbon dioxide production, that helps pay for, say, installing a solar power array for a rural electrification project. Airlines are thus incentivized to fly lower-emissions aircraft to avoid the expense of exceeding the cap.

Supersonics could prove a challenge in that regard. Estimates widely vary, but faster-than-sound planes could generate between two-and-a-half to seven times the carbon emissions of comparable subsonic craft. The International Council on Clean Transportation, a Washington, D.C.-based nonprofit research group, put out a study in January 2019 that arrived at the higher end of that range. Dan Rutherford, director of aviation programs for the council, says the estimate stems from supersonics’ higher emissions per passenger kilometer figures. Smaller passenger capacity planes mean more fuel burn per passenger, and a supersonic airliner probably would need to stop to refuel on trans-Pacific flights.

Plus, the svelte supersonic jets could not carry belly cargo, as today’s conventional airliners do along with passenger luggage, likely requiring additional subsonic flights to meet this cargo demand. “Just because of their speed, you’d expect supersonics to burn about three times as much fuel as a comparable subsonic, and then from there you start adding other multipliers,” says Rutherford.

Although the three supersonic companies plan to deliver only several hundred aircraft apiece, that’s still a tremendous leap from those 14 Concordes that ever entered commercial service. For Rutherford, the exploding demand for conventional flights is bad enough without exacerbating those emissions with supersonic aircraft. “I’ve described this as ‘when you’re in a hole, stop digging,’” says Rutherford.

Making the Mach

When it comes to their part in aviation’s climatic reckoning, Aerion, Boom and Spike each have unique supersonic speed targets, which in turn determine their aircraft development and emissions mitigation outlook.

Boom has based its business model on achieving Mach 2.2 cruise, about 2,700 kph (1,700 mph), or more than three times faster than a typical commercial plane flies today. That celerity would slash a typical seven-hour flight between New York and London to around three hours.

So far, Virgin Atlantic and Japan Airlines have preordered a combined 2½ dozen of Boom’s proposed 55-seat, $200 million airliner, called Overture. To help refine the aircraft’s design and engineering, Boom is building a one-third-scale, two-seat demonstrator, the XB-1, nicknamed “Baby Boom.” The first test flights are slated for next year, powered by three General Electric J85-15 military jet engines, the progenitors of the CJ610, a popular business jet engine.

Boom is working with undisclosed engine manufacturers to develop medium-bypass turbofan engines with greater efficiency than the afterburning turbojets that powered the first generation of supersonic airliners. “While turbofans have been powering subsonic airliners for years, only recently have they advanced enough to be viable on supersonic airplanes,” says Steve Ogg, chief aerodynamicist for Boom.

Materials-wise, Boom wants to build airframes with carbon composites, in line with the current state-of-the-art. The Boeing 787 and Airbus A350 XWB both make extensive use of the material, which provides necessary strength but with about 20% less weight than conventional aluminum.

Other aerodynamical, fuel-saving efficiencies include wing extensions, called chines, that stretch toward an aircraft’s nose. Chines boost lift and thus reduce the speed (and fuel burn) necessary during takeoff and landing, while contributing additional lift at cruise. Overture also tapers toward its aft cabin where the wings are thickest, tamping down air disturbances incurred by significant changes in cross-sectional area. Finally, the wings are designed with a mild camber and a swept trailing edge, reducing drag and helping quiet the sonic boom.

Aerion’s approach markedly differs from Boom’s. The Aerion jet, dubbed the AS2 and with a sales price of $120 million, will seat 12 and have a top speed of Mach 1.4. Where Aerion feels it has a technology edge over competitors is in engine design. Powering the AS2 will be three twin-shaft, twin-fan turbofan Affinity engines. Developed by GE Aviation and derived from the core found in F-16 fighter jets and the Boeing 737, it is undergoing final design for a prototype build in 2020. “We have the first supersonic engine in 55 years,” says Aerion’s Holloway.

Also in Aerion’s corner is Boeing, which inked a deal (funds undisclosed) in February to accelerate design and technology development. Lockheed Martin and Airbus had both previously worked with Aerion, going back to 2014.

Spike Aerospace splits some of the differences between Boom and Aerion, opting for Mach 1.6 from its proposed 18-seater business jet, the S-512. Many details regarding Spike’s industry partners, let alone emissions mitigation, have not been made public, though Kachoria says announcements will be made possibly before the close of 2019.

What’s in the tank?

Fuel will, of course, factor prominently into how the supersonic sector addresses its emissions. All the companies have expressed interest in having their products fly on alternative fuels. That category includes familiar biofuels, derived from a living feedstock, as well as synthetic fuels. These are made from pulling carbon dioxide out of the air and pairing it with hydrogen, stripped from water, ultimately building up to usefully combustible hydrocarbons. Like biofuels, if manufactured with renewable energy, synthetic fuels would have an overall low carbon footprint, taking out of the air what gets put back in. Following through on that hope, Boom has partnered with a California startup, Prometheus Fuels, to produce carbon-neutral synthetic fuel for the XB-1 demonstrator.

Alternative fuels still have quite a way to go, though. Rutherford, the supersonics skeptic, says the International Air Transport Association and airframe manufacturers “have made getting renewable jet fuel their main environmental priority over the last 10 years.” A decade ago, rosy predictions put biofuel penetration, for instance, as high as 10% by 2020. Rutherford says the actual percentage has worked out to be less than a hundredth of a percent currently, due in large part to biofuels not being cost-competitive with conventional jet fuel. “Biofuels haven’t scaled, costs have not come down, and to date it hasn’t worked,” he laments.

Aerion’s Holloway acknowledges the issue but points to the deep interest in alternative fuel development internationally, improving yields with new feedstock types from algae to municipal waste.

Alternative fuels could also be engineered to have more favorable emissions profiles. Typical jet fuel, when burned, produces emissions composed of about 70% carbon dioxide and a shade under 30% water vapor — both heat-trapping greenhouse gases. Those two products are more or less inevitable. But the trace remainder of other emissions offers significant opportunities for reductions. These traces, consisting of carbon monoxide, nitrogen oxides (NOx), oxides of sulfur and volatile organic compounds, all have indirect warming effects as they interact with atmospheric gases. NOx, for instance, produces the greenhouse gas ozone in the troposphere, the lowest part of the atmosphere that ends at about 20 kilometers (65,000 feet) around the equator down to 7 k (23,000 feet) at the poles. Depending on the chemistry of their feedstocks, some biofuels produce fewer NOx emissions because they contain fewer nitrogen compounds, thus improving the overall emissions profile of the jets they power.

Specks and clouds

Particulates represent another set of nasties from fossil fuel combustion that innovation could address. One particularly potent component is black carbon, or soot, which, unlike gaseous carbon dioxide, absorbs and then re-radiates sunlight. “Per unit mass, black carbon is a million times more powerful than CO2 at warming the air,” says Mark Z. Jacobson, a professor of civil and environmental engineering at Stanford University who studies fossil fuels and air pollution. With supersonic flight, deposition of black carbon at high altitudes is especially worrisome. Most soot, produced by vehicle emissions, rains out of the atmosphere in a week, Jacobson says. In the stable stratosphere, where air does not rise or sink, particles could linger for months, even years. “You’re injecting all these particles into the stratosphere, and they’re going to stay there a long time,” says Jacobson.

Emitted particles, provided there is enough water vapor, also contribute to contrail formation. Each particle provides a mote upon which water vapor condenses and freezes, creating the picturesque cirrus clouds that trail airplanes when atmospheric conditions are right. Pretty as they can be at sunset, these high-altitude clouds pose a problem, climatically speaking. Contrails reflect infrared radiation back toward the ground, with studies suggesting they trigger as much warming as aviation’s carbon dioxide emissions.

There is disagreement about whether the altitudes supersonics would fly at would add to the jets’ environmental impact, indicative of the still-unsettled science about conditions affecting contrail formation, duration and impact. [See “Curbing Contrails” from the Aerospace America February 2016 issue]. One agreed-upon advantage, though, is the lower humidity compared to subsonic cruise altitudes. “That’s good news for keeping contrail generation and cirrus clouds down,” says Jonathan Seidel, chief engineer in the Propulsion Systems Analysis Branch at NASA’s Glenn Research Center in Ohio, who studies supersonic flight. The downside at these rarefied altitudes, though, is that there is little atmospheric mixing to dissipate contrails. “While formation should be less, they’re likelier to persist longer,” says Seidel.

Other atmospheric scientists, and the supersonic companies themselves, think contrail formation will be minimal. “In my opinion, the humidity is really too low to form contrails,” says Andrew Gettelman, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado.

Cleaner burning engine combustors, which would devote more air intake to the combustion process itself and less to cooling, would further squeeze emissions improvements from choice fuels. “You’re burning in a much leaner fuel-to-air ratio to control the formation of various emission components,” says Peter Coen, the mission manager for NASA’s Low-Boom Flight Demonstration mission, geared toward aerodynamically quieting sonic booms and overall advancing supersonic commercial flight. Another attractive technology, Coen says: making the combustor from innovative high-temperature materials that would up the operating temperature for more thorough combustion.

Beyond the airframe

Sleek aerodynamics, alternative fuel and the latest technologies may not be enough for substantial, sufficient emission reductions.

Supersonic flight’s backers are therefore advocating for new emission standards tailored to their vehicles, allowing time for further advancement — not unlike the efficiencies gained over decades of conventional aircraft refinement. “Subsonic engines have enjoyed 60 years of development, going back to the [Boeing] 707’s introduction,” says Aerion’s Holloway.

Holloway’s company is working with subgroups of ICAO’s technical committee on aviation environmental protection to formulate supersonic emissions standards. Policymakers and manufacturers trying to quantify the problem have the Concorde as their starting reference point.

Flying at Mach 2, it pumped out about three times as much carbon dioxide and NOx as subsonics. Factoring in its 60,000-foot cruising altitude, the Concorde overall had about a five-fold stronger climate impact.

Today’s carbon standards rely on a “tank to tailpipe” model that specifies an allowable amount of emissions from the aircraft. One way to accommodate supersonic airplanes would be to shift to considering not just those emissions but also the entire life cycle of a plane. This “well to wake” approach wraps in everything from the origins of a plane’s fuel (like from an oil well) to how emissions chemically interact with the atmosphere in the plane’s wake and beyond, rather than just focusing on the bulk amount of carbon dioxide released. “If the world is going to judge us on a tank-to-tailpipe basis, we will never win that battle,” says Holloway. “We’re looking at it from the standpoint of what can we do in the design and manufacturing of our jet that also reduces our carbon footprint overall.”

That standards approach could extend to materials on the plane, for instance recycled (and ultimately re-recyclable) plastics for floors, seat frames and other interior elements. Aerion could opt for suppliers of parts that have environmentally friendly certifications, such as green-building ratings from LEED, for Leadership in Energy and Environmental Design. Also compensating for aviation-related emissions could be companywide initiatives in matters unrelated to aviation, such as office energy use.

“The push-and-pull right now about environmental standards is important,” says Rutherford, who is also a technical observer at ICAO on climate matters. “It will fundamentally influence where the money flows and what type of aircraft may or may not be built.” Indeed, for supersonic airplanes, the “what” does not loom so much as an issue, but the “how.”

“Flying supersonic itself is not very difficult,” says Holloway. “The real challenge is flying supersonic at scale while also meeting environmental standards.”

About Adam Hadhazy

Adam writes about astrophysics and technology. His work has appeared in Discover and New Scientist magazines.

“What Concorde showed is there is a market for customers who want to reach destinations faster. What we need to figure out is how to do the Concorde, but we need to do it safer, more efficiently and more sustainably.”

Spike CEO Vik Kachoria
An artist's rendering of a sleek, futuristic supersonic jet with a pointed nose and dual engines flying against a twilight sky backdrop.
Spike Aerospace’s proposed 18-seat business jet, the S-512. Credit: Spike Aerospace
A sleek, red and white private jet is parked on a tarmac at sunset. A black vehicle is positioned near the jet's door, and lights illuminate the scene.
Aerion's proposed 12-seater, the AS2, in an illustration. Credit: Aerion
A detailed cross-sectional view of a jet engine, showcasing its internal components including the fan, compressor, combustion chamber, turbine, and exhaust.
Three twin-shaft, twin-fan turbofan Affinity engines will power Aerion’s AS2 jet. Credit: GE Aviation
A white supersonic jet with the name
Boom's proposed jet, the Overture. Credit: Boom Supersonic

Supersonic’s not-so-super emissions