NASA’s attempts to resuscitate supersonic commercial air travel will involve the use of technology that did not exist in Concorde’s day, with the aim of minimizing its negative environmental impact in terms of both emissions and noise.
The prospect of once again being able to fly between London and New York in under three hours will no doubt be a boon to frequent flyers, ravaged by the effects of regular long-haul travel on their bodies and minds.
However, with future supersonic passenger jets expected to burn about a quarter more fuel than equivalent subsonic aircraft, does the resurrection of this type of flight make sense at a time when the aviation industry is working hard to reduce its impact on climate change?
Unsurprisingly, environmental campaigners believe not – unless the new breed of supersonic aircraft can dramatically reduce the emissions and noise traditionally associated with breaking through the sound barrier.
“We thought supersonic air travel had largely gone away but we’ve seen it re-emerge,” says Tim Johnson, director of the UK-based Aviation Environment Federation (AEF). “It doesn’t make any sense at all, unless [supersonic aircraft] can be designed more efficiently, like their subsonic counterparts.”
Johnson adds that any future supersonic aircraft should be forced to comply with the recent aircraft CO2 efficiency standard agreed by ICAO. Supersonic aircraft are built “largely for convenience”, he argues, but this “shouldn’t come at the expense of environmental protection”.
NASA is well aware of the challenges it faces in developing a supersonic aircraft that is palatable to the public and can comply with ever-stricter noise regulations and emissions reduction targets.
“Our over-arching goal is to break down the barriers to commercial supersonic flight, and one of those is the potential environmental impact of supersonic transport,” NASA’s commercial supersonic technology project manager, Peter Coen, tells Runway Girl Network.
“From NASA’s perspective, we are seeking to bring back supersonic travel as an option to save time and improve the quality of life for people who travel frequently, but we’re not going to do it in a way that has an imbalanced effect on the environment.”
NASA announced earlier this year that it had awarded a contract to Lockheed Martin to design a low-boom supersonic demonstration aircraft – the first in a series of “X-planes” to be developed under its ‘New Aviation Horizons’ initiative – which NASA says brings “the return of supersonic passenger air travel one step closer to reality”. These X-planes will be half the scale of production aircraft and are expected to begin their flight-test campaign around 2020, depending on funding.
But there are many hurdles to cross before commercial supersonic flight becomes a reality again.
On fuel consumption, Coen concedes that “you’re not going to get as low as subsonic because you’re pushing through the air much faster”. For comparison purposes, Concorde burned around 6,770gal of fuel per hour and carried 100 passengers, while the Boeing 747 burns roughly 2,000gal of fuel less per hour but carries four times as many passengers.
NASA envisions several generations of future supersonic aircraft, with increasing capability. The first generation will be a business jet capable of supersonic flight without sonic boom. The second will be “something of a Concorde replacement” with 100 seats, says Coen, while the third generation is envisioned as being a fully capable airliner with capacity for up to 200 passengers and a range of 5,000nm.
Coen points out that in the 40 years since Concorde first took to the skies, airframe and engine technologies have come a long way, and future supersonic aircraft will be far less gas-guzzling than their predecessors. But there will still be a significant penalty.
“Our target is to get down to no more than 25% more fuel for a typical flight,” he says, referring to the third-generation aircraft described above.
The amount of fuel burned is not the only issue to consider. Supersonic aircraft operate at a much higher altitude than their subsonic counterparts and, as a result, it is expected that they would have a greater impact on radiative forcing.
In addition to carbon dioxide, aircraft emit nitrogen oxides (NOx), water vapor and soot into the atmosphere, which climate researchers predict could have a more significant impact on ozone depletion and global warming than ground-based emissions. Because supersonic aircraft fly so much higher than subsonic jets, their impact is feared to be even greater.
According to the United Nations Intergovernmental Panel on Climate Change (IPCC): “The radiative forcing of civil supersonic aircraft is estimated to be about a factor of five larger than that of the displaced subsonic aircraft [in a 2050 scenario which assumes supersonic emissions have replaced 11% of subsonic fleet emissions].”
But Coen says NASA has “two things going for us” on this front: its under-development supersonic aircraft is designed to fly at a lower Mach level than Concorde and will consequently fly at a lower altitude. “[It will operate] in the lowest part of the stratosphere, which gets us out of the highest concentrations of ozone,” he notes.
The second element is the development of improved engine combustor designs, which Coen says could result in a 70-80% reduction in NOx emissions compared with a Boeing 737-800 or 777. “This emissions reduction technology is applicable to supersonic aircraft and could enable achievement of the goal of an emissions index (gms NOx/kg fuel) of less than five. This level would result in no appreciable ozone impact from supersonic cruise aircraft,” he says.
Putting emissions aside, the biggest barrier for NASA to overcome is the fact that commercial supersonic flights over land are currently banned because of the sonic boom caused when aircraft pass through the sound barrier. Coen describes sonic boom as “the market denier” when it comes to designing supersonic air transportation, adding: “We want to reduce the sonic boom to an acceptable level over land, and we’re on the cusp of doing that.”
Sonic booms are the product of shockwaves of varying strengths caused by aircraft travelling at supersonic speeds. These shockwaves travel through the atmosphere at slightly different speeds but catch up with each other as they reach the ground, creating two incredibly loud bangs which can cause distress to people and animals.
But NASA is working on a “low boom design” which controls the position of the shockwaves so that they are equally spaced and do not catch up with one another. The amplitude of each shock decreases the further it travels from the aircraft and because they are not reinforced by combining with other shocks, the final intensity is much lower.
As each shock interacts with the atmosphere it gets “thicker”, says Coen, and the time it takes the pressure to increase in each shock gets larger. “Although the rise time only changes by a fraction of a millisecond, it is enough, particularly when applied to all the shocks in the pressure signature, to eliminate the bang.”
NASA’s aim is to build a research aircraft to test this theory and then send the data to regulators, to enable them to design a noise standard for future supersonic aircraft to work towards.
“We would like to fly this aircraft over a community and essentially have nobody notice,” says Coen. “If we’re successful, we’ll take the data to the FAA, EASA and ICAO and say, ‘this is the level of sound from a supersonic aircraft.’”
Airport noise is also a concern and NASA is researching ways to make take-offs and landings for supersonic aircraft no louder than for subsonic aircraft. “We’re targeting engine technology, aircraft technology, the Mach level we choose to fly at and noise-reducing exhaust technology so we can beat the [ICAO] Chapter 4 noise standard and the newly-proposed Chapter 14 standard,” says Coen.
Another important question is whether, after all this research, there will be a market for commercial supersonic air travel.
As Dan Rutherford, program director for marine and aviation at the International Council on Clean Transportation (ICCT), points out, when Boeing began designing what became the 787 15 years ago, it gave airlines the choice between a ‘Sonic Cruiser’, which proposed a cruising speed of up to Mach 0.98, or the 7E7, which put fuel efficiency ahead of increased speed. Airlines went for the latter.
“I’d be curious to see how much demand there is now from airlines,” says Rutherford.
Coen’s expectation is that demand would initially come from the business aviation sector but would later trickle down to the premium end of the commercial market. “Airlines weren’t comfortable with the Sonic Cruiser because they didn’t have an operating model that could make the best use of it,” he says.
Timeframe-wise, Coen believes a supersonic aircraft could enter the business aviation market between 2025 and 2030, while commercial passengers will likely have to wait until at least 2030.
“Of course, these designs are NASA visions of what the application of future technology could yield. The actual aircraft will be designed and built by aircraft manufacturers to meet the market that develops in the future,” he says.
There are many unanswered questions surrounding the potential return of supersonic flight, particularly in light of increasingly stringent noise and emissions regulations. But you can’t help thinking that if NASA can put a man on the moon, a Rover on Mars and take close-up images of Pluto, designing a more environmentally-friendly supersonic airliner should not be beyond the realms of possibility.
Image: Lockheed Martin