Few people today realize that supersonic research is actually represented in the familiar NASA insignia. The red chevron element on the NASA “meatball” logo is actually an artistic rendering of an arrow wing model that was developed for supersonic applications by Clinton E. Brown and F. Edward McLean, and wind-tunnel-tested in the late-1950s. A NASA historical monograph prepared by Joseph R. Chambers (NASA SP-2005-4539) notes that the model had been observed on display by James J. Modarelli from the NACA Lewis Laboratory, and Modarelli and his graphic artists later included it in the adopted NASA insignia.

The problems the SST had to overcome included pollution of the upper atmosphere and depletion of the ozone layer through jet emissions; sonic booms; and the noise levels generated by the types of engines needed for supersonic cruising speeds.

The arrow wing element was entirely appropriate for the agency that had been a key contributor to the breaking of the sound barrier by the Bell X-1 more than a decade earlier. In fact, at the time that new NASA insignia was adopted, the nation’s first satellites were orbiting the Earth, and the agency had a fleet of experimental aircraft that were pushing the envelope beyond Mach 3 and investigating myriad designs and concepts. The X-15, approaching its first powered flight, would soon fly four times faster than sound at the edge of space. A sky full of supersonic transports seemed inevitable.

“In the late-’50s, it’s just amazing how fast we got to thinking about ‘commercial aircraft flying supersonic,” said Project Manager for Commercial Supersonic Technology Peter Coen, offering the example of the Super Sonic Transport (SST) Study Group “that NACA and then NASA were participants in, eventually leading to the creation of the U.S. SST program, which was actually run by the FAA [Federal Aviation Administration].”

The SST program coincided with “an explosion of research and development related to bringing that U.S. SST aircraft program to flight,” Coen said.

Following the creation of that program, he said that NASA became “heavily involved in all of the development and evaluation of the SST concepts” from Lockheed, McDonnell Douglas, and Boeing.

“At that time, they were all contributing designs, and eventually Boeing was selected,” he explained, noting that some of the early research looked at sonic boom noise reduction for supersonic aircraft. In fact, some of NASA’s original sonic boom reduction research was conducted by Harry Carlson and none other than F. Edward McLean, whose wing it was on the NASA logo.

“By that point in time, NASA had a series of concept configurations that got the name SCAT – Supersonic Commercial Air Transport – that explored the application of high-performance wing configurations to supersonic air transport,” he added.

Dreams of an American supersonic transport were ultimately grounded, however, when the SST was cancelled in 1972, chiefly due to environmental and economic issues. The resources and effort needed to develop technologies to resolve those issues simply weren’t available at the time. The problems the SST had to overcome included pollution of the upper atmosphere and depletion of the ozone layer through jet emissions; sonic booms; and the noise levels generated by the types of engines needed for supersonic cruising speeds.

“But recognizing that part of the reason the U.S. SST program failed was lack of technology, NASA actually continued supersonic research through the Supersonic Cruise Aircraft Research [SCAR] program, and the later Supersonic Cruise Research [SCR] program,” Coen said. “And those programs lasted until about 1980, with, again, lots of work on things like a fundamental understanding of supersonic flow, improvements for aerodynamics, improvements to the low-speed performance of supersonic airplanes, high-performance inlets, and nozzles – just a wealth of research activities.”

The early ’80s marked something of a brief hiatus in large-scale supersonic research, with no major identified supersonic cruise research program.

“Then in the mid- to late-’80s, pushed by the Executive Branch from the Office of Science and Technology Policy, there were significant activities to reinvest in aeronautics,” he said. “The policy recommended that the United States pursue leadership in subsonic aircraft, supersonic aircraft, and hypersonic aircraft. That eventually led to the Advanced Subsonic Technology [AST] program, High Speed Civil Transport [HSCT] study contract – which became the High Speed Research [HSR] program – and also ‘The Orient Express,’ as President [Ronald] Reagan called it, that eventually led to the National Aero Space Plane [NASP].”

If supersonic flights are to be allowed over the United States, the sonic booms generated by supersonic aircraft will have to at least be mitigated.

The NASP concept, actually a hypersonic vehicle, highlights the significant contributory research overlaps between areas like supersonic and hypersonic performance. As another example of overlapping technology, Coen pointed to the X-15 program, which was originally tied to supersonic research “but eventually touched the early space program and hypersonic speeds.”

However, in terms of supersonic commercial transport, it was the HSCT study contract that really focused on defining configurations and technology needs for future supersonic commercial transportation.

“That HSCT effort led to the High Speed Research program in 1990, with the first phase of that program focused on overcoming the three main environmental challenges for supersonic aircraft: sonic boom; high-altitude emissions; and takeoff and landing noise,” Coen said.

He described the activities as “a NASA funded/industry contracted effort that involved all of the major players in the airframe and propulsion area,” with the program eventually “deciding that they couldn’t make a viable ‘low boom’ airplane at the size they were talking about, which was a Mach 2.4/300 passenger/5,000-nautical-mile-range aircraft.”

Although not able to develop a low boom transport, Coen emphasized that the program, which had a total investment of approximately $1.5 billion, continued to study supersonic designs and did come up with solutions for the noise and emissions challenges that could be applied to an airplane designed for supersonic “over-water” operations.

“It also addressed a lot of propulsion materials and performance technologies,” he added. “And it’s important to realize that all of these efforts had valuable spinoffs as well.”

Although shifting markets caused industry to move away from any commitment to bring a large supersonic aircraft to market in the time frame targeted by HSR, Coen noted the program still “left a legacy of improved computational fluid dynamics tools, improved composite materials, design techniques, ceramic materials for propulsion components, and low emissions combustors for supersonic and subsonic engines. So there were a lot of spinoffs that we see on subsonic aircraft today, but no supersonic aircraft product.”

These low emissions combustor technologies helped to solve the high-altitude emissions challenge dealt with during the HSR program, said Jay E. Dryer, Director of the Fundamental Aeronautics Program Office in the Aeronautics Research Mission Directorate (ARMD).

“Among the challenges that faced supersonic aircraft were the high-altitude emissions and NOx [nitrogen oxide] issues. Designing low-NOx combustors is very important for these kinds of aircraft. The combustor work that was done in the Environmentally Responsible Aviation [ERA] project benefitted from concepts started in our supersonic project. There really is a link between these areas. And because ERA has been able to advance that technology, that will in turn benefit potential future supersonic turbine engines that might be developed for a commercial market.”

The time for that commercial supersonic aircraft market may be coming.

The Case for Commercial Supersonic Aircraft

Across the globe, there is an ongoing pattern of urbanization. Today, 54 percent of the world’s population lives in cities, and that figure is expected to increase to 66 percent by 2050, according to a United Nations report. More than 90 percent of this demographic movement is occurring in Asia and Africa. These future centers of population and trade are in many cases separated by great distances.

“So connecting these city pairs becomes even more important, especially with the growth that we see in the East and in Asia,” said Dryer. “There are large distances to travel to connect some of these points and the people that want to be connected.

“Speed makes a big difference in the equation. Even moving across the United States, if you were able to do that in half the time, that might change the way you look at a workday, or how you would attend a meeting. It might change the way that we even think of work in some cases. So this really presents a potential growth area for a new market to expand where aviation can have an impact.”

But overland supersonic flight is currently banned in the United States and Europe, and because of that, the Concorde, 14 of which operated for decades, was limited to flying for the most part over water. If supersonic flights are to be allowed over the United States, the sonic booms generated by supersonic aircraft will have to at least be mitigated.

“I would say that ‘low boom’ research is the primary emphasis of what we’re working on, because it’s the key to unlocking that more flexible use of these kinds of aircraft,” said Dryer.

“Why we think that low boom is such a key aspect is that it offers that flexibility to fly multiple routes connecting those city pairs. Granted, many of those flight routes of course would be over the water. But without the corresponding overland segments, the aircraft will not be viable for typical airline operations. Therefore, low sonic boom is such a key element to enable economical commercial supersonic flight.”

Lowering the Boom

The relatively abrupt end of HSR (the program concluded at the end of FY 1998) meant another hiatus in large-scale supersonic research. However, within a year or two of that program end, significant interest began to materialize in smaller supersonic aircraft designs, with an associated emphasis on low boom research.

“DARPA [the Defense Advanced Research Projects Agency] got interested in the potential for a small supersonic military aircraft,” Coen said. “And some other players – including Gulfstream, Lockheed and their partner at the time, Supersonic Aerospace International – got interested in low boom supersonic business aircraft. And with NASA being kind of the repository of all of this supersonic technical data, everybody started looking to NASA to provide information. So, in about the 2001 time frame, we began to get back into supersonics.”

“The next most significant activity was what became known as the Shaped Sonic Boom Demonstration [SSBD],” Coen said, reiterating that “sonic boom reduction technology and understanding the details and perception of sonic booms had been a part of the SST program, part of SCAR, and part of HSR. There had been a lot of analysis, theory, and wind tunnel experiments, but nobody had ever really done a flight experiment related to sonic boom reduction.”

When the aerospace industry builds that first commercial supersonic aircraft, it will probably start small.

DARPA’s activity, the Quiet Supersonic Platform (QSP) program, prompted the idea of trying to demonstrate that all of the theories related to “‘shaping’ a sonic boom waveform to reduce its annoyance actually worked in flight. So eventually Northrop Grumman was selected as the lead contractor in a team effort to modify the nose of an F-5 fighter to incorporate ‘low boom shaping.’” The SSBD was successfully flown in the Edwards Air Force Base Supersonic Test Range in 2003.

Coen said that at about the same time, “Gulfstream Aircraft was very active in the supersonic business jet area,” with their concepts, resulting in a proposal for a boom reduction technology called “Quiet Spike,” which he described as “a telescoping nose extension which breaks up the shock wave into weaker components”

The Quiet Spike nose extension was subsequently tested in partnership with NASA on the nose of NASA’s F-15 at Edwards Air Force Base.

Coupled with what Coen termed “the reformulation of NASA Aeronautics” in the 2005/2006 time frame, the flight test results and other related research fed into a new supersonic project that “wasn’t as large an investment as some of the previous programs but had a focus on continuing to explore technology advances that could bring about more efficient, quieter supersonic flight.”

Coen, who became the Project Manager for the resulting Commercial Supersonic Technology Project, identified one of the key breakthroughs as “trying not to make a very large supersonic aircraft, but rather something smaller; recognizing that something about the size of the Concorde probably could be commercially successful, but also environmentally compatible, with low emissions, low noise, and high efficiency. So that’s been part of our primary focus for the past half dozen years.”

“If you actually look over the history of these activities, it’s very interesting to note the steady progress that has been made in the face of the continuing challenges,” he said. “In fact, we now feel that we have got the technology that would allow us to have an efficient supersonic airliner that could fly over land without creating a disturbing sonic boom – flying Mach 1.6 to Mach 1.8 with up to 200 passengers. We’ve wind tunnel tested configurations that meet those performance levels.

“That would reflect the limits of the technology at this point in time,” he added. “But who knows? Based on history we will eventually be able to apply those technologies in finding solutions for larger aircraft.”

“Has technology advanced to the point that construction and operation of a commercial supersonic aircraft is economically feasible? Absolutely,” said Dryer. “Highly efficient engines and composite materials that allow us more flexibility in construction of the airframe have made this possible.”

One area of recent flight research at NASA’s Armstrong Flight Research Center relied on the geographic advantage of flights permitted at supersonic speeds to “assess the public’s response to sonic booms in a real-world setting.”

Parallel work conducted at Langley had “volunteers from the local community rate sonic booms according to how disruptive they determined the sound to be.” The aim is to find out what level of sonic boom might be tolerated by those living beneath the flight paths of supersonic aircraft.

Looking toward the next few years, Coen said that the strategic thrusts for NASA Aeronautics “include one specifically for commercial supersonic technology development, with the initial goal of doing a flight demonstration of these low boom technologies.” Specifically, the ARMD strategic vision describes developing “solutions to make commercial supersonic flight over land possible …”

“What we are going to do is create the analytical tools, some of the key technologies, and help understand the overall concepts, like how we measure sonic booms, and give industry the key missing links, if you will, that are needed to allow them to build and innovate,” said Dryer. “We’ve created design tools and capabilities that allow a designer to understand and help predict the signature, and then carry that back to understand from that signature how the designer can shape or build the aircraft. In the past, it may have taken industry months to just turn one design cycle. And we’ve turned that into a matter of days or less because of the computational advances that we’ve made in this area. So that’s been one area of tremendous success.”

When the aerospace industry builds that first commercial supersonic aircraft, it will probably start small. “Likely the first initial products will be small, business jet-class aircraft, because the physics of the problem is easier,” said Dryer. “But it’s important to note NASA is not just fixated on creating and opening a business jet market. Most of these studies that we’ve done focused on a small airliner, analogous to a regional jet in terms of passenger capability. When we’re doing our wind tunnel model testing that has shown why we believe that these low boom signatures are possible, those are the kinds of configurations that we’re using. So while we think that the business jets are the initial step, our vision is really on opening that market up to more of the flying public.”

“So beyond doing the boom reduction demonstration, there are still challenges to be met related to the design of small, supersonic transport types of aircraft, with a longer term goal – 2030 and beyond – to continue to look for solutions that will enable us to scale up those aircraft to a larger size to improve the economics and make supersonic flight more accessible to a larger portion of the population.”

Coen sees the anticipated demonstration of low boom flight technologies as a first step, adding his hope that the low boom demonstration airplane would subsequently be used “to create enough data that the FAA and the international community would be able to change the current rule that prohibits supersonic flight over land. Hopefully they would change the rule to make it effectively a ‘noise-based certification standard’ for supersonic overland flight. That’s where we’d like to go in the next five years.”

Even further into the future, he pointed to “similar progress on takeoff and landing noise solutions for supersonic aircraft.” While acknowledging that “they are not as quiet as the quietest subsonic aircraft,” he believes that NASA could help design a supersonic airplane “that would be quieter than the lower limits of current noise regulations and actually be lower than the proposed future noise regulations, which are even more stringent.”

Coen went on to identify combustion technologies as “another synergistic development between subsonic and supersonic aircraft,” expressing an organizational belief “that we could design the engine for a supersonic airplane so that there would be no negative impact on the ozone or global atmosphere.”

“But we need to continue that work,” he said. “So beyond doing the boom reduction demonstration, there are still challenges to be met related to the design of small, supersonic transport types of aircraft, with a longer term goal – 2030 and beyond – to continue to look for solutions that will enable us to scale up those aircraft to a larger size to improve the economics and make supersonic flight more accessible to a larger portion of the population.”

This article first appeared in the NACA/NASA: Celebrating a Century of Innovation, Exploration, and Discovery in Flight and Space publication.