Back in 2012, NASA thought it had reached a breakthrough in a decades-long quest to design a “low-boom” supersonic aircraft. With leftover funding from the 2009 financial stimulus act, the agency commissioned Lockheed Martin and Boeing to separately develop preliminary designs and wind tunnel models of aircraft concepts.
Lockheed aimed its efforts at a 100-seat-class trijet design similar in scale to the four-engined Aerospatiale-BAC Concorde, while Boeing focused on a smaller, 70-seater with uniquely-configured over-the-wing mounted engines. Four years later, Lockheed and Boeing have reported back to NASA, with wind tunnel runs and simulation data appearing to have validated the agency’s calculations. NASA may have reached a pivotal moment in the quest to revive commercial supersonic air travel, more than 12 years after Concorde was retired from service by Air France and British Airways.
Indeed, the data served to persuade the superiors of Peter Coen – head of the high-speed project in NASA’s aeronautics research directorate – to authorise a feasibility study for a new supersonic X-plane that could verify the wind tunnel results in real flight. Critically, data from flight testing could, finally, force international regulators to consider overturning a 47-year-old ban on breaking the sound barrier over land.
The Lockheed concept is a 100-seat-class trijet design
NASA
Coen says: “We’re very pleased with [Lockheed’s and Boeing’s] results.
“Lo and behold, we feel there is a solution using a scaled X-plane – a 100ft-long aircraft that weighs about 25,000lb, which is capable of replicating the acoustic characteristics of a boom of a larger airplane up to 300,000lb or so.”
A final funding decision may come in February, with the release of the Obama administration’s fiscal 2017 budget request. If the programme is launched, Coen expects to have a detailed design study complete by the end of FY2017, followed by a first flight of the supersonic X-plane in FY2019.
LOWER THE BOOM
This low-boom experimental vehicle (LEBV) would be used in an effort to rewrite the rulebook on flying over land at supersonic speed. Of course, not everyone agrees such an effort is necessary. Aerion, which has partnered with Airbus, is developing the AS2 business jet to be optimised to fly at supersonic speeds over water but cruise at high-subsonic speeds over land in the USA and potentially up to Mach 1.1 in Europe, where regulations are slightly less restrictive.
But NASA officials and others in the industry, including Gulfstream, have said a commercial supersonic aircraft is only viable economically if it can fly at top speed over land.
To make that happen, someone has to give the regulators a reason to change the rules. The Lockheed and Boeing study results indicate new technology can reduce sonic boom noise from 105PLdB to as low as 75PLdB, Coen says. But while ICAO’s committee on aviation environmental protection (CAEP) has established a supersonic transport task group, it will need more than NASA-funded wind tunnel studies – no rules can be changed without real flight data from a representative aircraft and a public response.
“CAEP has essentially said there is no possibility of having a standard without community overflight data to validate the metrics that would be used for response, and to develop the procedures that would be used for certification,” he says. “There’s clearly a need for a flight demonstration. That’s why the NASA work has been moving in that direction for the past three years.”
If NASA headquarters approves, Coen intends his agency will deliver that data over several years. The X-plane will need all of 2020 to clear the flight envelope for supersonic testing over a carefully instrumented range spread over Edwards AFB in the California desert. Then, beginning in 2021, NASA plans to carry out a demographically broad community noise survey, beginning in Southern California, before deploying the X-plane to other US locations and, ideally, overseas.
This next step requires reviving memories of Operation Bongo II, a 1964 experiment during which the US Federal Aviation Administration selected Oklahoma City to perform a series of supersonic acoustic surveys. In fact, the agency carried out more than 1,200 sonic booms over the city over six months to measure how the population would react. After breaking hundreds of windows of downtown buildings, the agency was flooded with complaints and halted the tests prematurely.
The outcry came against the backdrop of a wider public backlash in the USA against the damage and annoyance caused by sonic booms as supersonic fighters became the backbone of the US Air Force fleet. The military received nearly 39,000 claims for damages caused by sonic booms between 1956 and 1968, according to a NASA book published in 2013 called Quieting the Boom.
The public backlash prompted the FAA to issue a regulation in 1969 prohibiting supersonic flight over populated areas, a stricter regulation than adopted in Europe, where only making an audible sonic boom is outlawed. Public anger would also play a role in a decision by Congress to cancel the Boeing 2707 supersonic transport (SST) programme in 1971.
By then the aeronautics industry was well aware of what needed to be done. Two NASA scientists, Richard Seabass and Albert George, had by 1969 developed the basic mathematics for relating aircraft size and shape to sonic boom noise. It would still take decades to transfer the mathematical theory into even experimental flying aircraft, but their work gave aerodynamicists tools for shaping an aircraft with sonic boom noise as a predictable and primary requirement at the design stage.
As Concorde was heading towards a retirement date in 2003, NASA finally began applying those formulae to a flying vehicle. The shaped sonic boom demonstration (SSBD) in 2003 used a Northrop F-5E with a heavily modified forward fuselage, sculpted to muffle the double-thumb signature of a sonic boom.
The SSBD programme succeeded in demonstrating aircraft shaping can reduce the pressure rise of the supersonic shockwave, thus muffling the boom signature. But it also proved, as expected, that more than the forward fuselage would have to be sculpted. In fact, new software-based design tools were needed to optimise the shaping of the forward and aft structures to achieve the lowest boom signature.
At the same time, NASA’s aerodynamicists also needed to solve what was then called the “low-boom, low-drag paradox”; shapes that reduce the sonic boom also increase drag. Achieving a “low-boom” noise profile might, then, gain regulatory approval – but result in an aircraft so inefficient as to be unmarketable.
Another consideration is the human ear’s tolerance for sonic boom noise. NASA used a Boeing F-15A test aircraft to break the sound barrier over a range and measure human responses to the noise. The agency also created a ground-based acoustic chamber to replicate the same noise signature in a controlled environment, leading to a determination of the range of tolerable noise for the human ear.
“It’s somewhere between 70-80PLdB,” Coen says. That roughly compares to the noise made by a passing car as heard from inside a building, versus the explosive cracks of the Concorde’s unmuffled double-boom.
To justify the investment of an X-plane programme, NASA wanted proof modern airframe design tools and propulsion systems could achieve those goals. So-called Phase 2 reports submitted by Lockheed and Boeing recently helped Coen make that case.
More than 45 years after the first flight of Concorde, aviation technology has clearly progressed. Lockheed’s Model 1044-2 is almost a direct comparison to the payload of that 100-seat pioneer. As Lockheed reports, the 1044-2 should fly about 40% farther than the 3,900nm-range Concorde, but its maximum take-off weight is expected to be 14% less than the 185,000kg (408,000lb) mass of the Anglo-French jet. NASA targeted a 300% improvement in fuel efficiency per passenger mile per gallon, but achieved a 250% improvement with the Lockheed and Boeing designs, Coen says. Most important, the Lockheed and Boeing designs fall within the perceived noise range NASA is targeting.
NASA's 2003 activity with a modified Northrop F-5E showed that an aircraft's shape could muffle its sonic boom
NASA
Both companies were allowed to use technologies that are likely to become available in the next decade. For Lockheed’s 1044-2 model, this allowed designers to switch to a relaxed compression and bleedless engine inlet that improved fuel performance, especially compared to the heavy and complex bleed inlet designs developed for Concorde’s Rolls-Royce Olympus engines.
Moreover, the 1044-2 also would benefit greatly from two major advances in propulsion technology. The conceptual supersonic airliner would be powered by three engines, each using a GE Aviation-supplied variable cycle engine. Developed under the US Air Force’s versatile advanced aircraft turbine engine (VAATE) programme, the engine opens a third stream of airflow during take-off to increase bypass airflow, thus reducing noise. At the same time, the other two flow streams would be reversed compared to a typical turbine; rather than bypass flow surrounding the air flowing through the engine core, the 1044-2’s engines would have their core section on the exterior, with bypass flow streaming through the centre.
“Essentially, that milks every decibel out of the jet that you possibly can,” Coen says. “You wind up with a fairly low speed subsonic jet where essentially you bring the jet noise down to where it’s substantially below the requirement.”
There are still many hurdles to realising quiet supersonic propulsion. One of Coen’s team’s biggest concerns remains meeting community noise levels for take-offs at airports, which are scheduled to grow tighter after 2021. Supersonic aircraft generally require low-bypass ratio engines, which lack the built-in noise mufflers provided by high-bypass systems. Opening a third stream of airflow certainly helps, but may not solve the problem.
“If I could point to anything that was a challenge it was community noise,” notes Coen. Pushing propulsion and nozzle technology very hard has achieved a great amount, he says, but going to such technical extremes “kind of says to us that that’s an area where more effort is needed to try and come up with simpler solutions that can give us more margin.”
Source: FlightGlobal.com
