The Tu-144 was both the first to go supersonic (June 5, 1969) and the first commercial transport to exceed Mach 2 (July 15, 1969). But two crashes (neither during passenger service) shortly after it went into service permanently grounded the fleet after only 55 scheduled flights.
Concorde, however, went on to record 4,358 revenue flights in 27 years with only one accident – for which the SST itself was found blameless – making it one of the safest passenger aircraft in history. But the Tu-144 crashes and governments banning supersonic flight over land severely limited Concorde’s market, which, for the bulk of its history, comprised only British Airways and Air France, both of which ended their SST service in 2003.
In the decade since, new technologies and research have opened the door to what many experts believe will be a certain revival of supersonic commercial flight. Peter Coen, supersonics project manager with the Aeronautics Research Directorate’s Fundamental Aeronautics Program (FAP) at NASA, recently spoke with senior writer J.R. Wilson about the status and future of supersonic passenger aircraft.
J.R. Wilson: What supersonic programs does NASA currently have going?
Peter Coen: Starting in FY13 there will be restructuring within the FAP, and a lot of the work in supersonics will become part of the High Speed Project. The primary reason was a decision for NASA to ramp down its hypersonics research and create projects with more procurement available for testing and higher TRL [Technology Readiness Level] effort.
The challenges we’re working in supersonics are sonic boom mitigation, take-off and landing noise, high altitude emissions, lightweight durable structures and materials for engines, and aeroelasticity for long, slender SSTs. Our two primary efforts to address those are N+2 and N+3.
N+2 is a 2025 TRL capability for a small supersonic airliner in the transatlantic range, meeting environmental goals that will enable it to operate without any impact larger than current subsonic aircraft.
N+3 is a generation beyond that – a 2035 technology availability date and a larger airliner, in the 100-to-200 passenger class, with transpacific range, again meeting all environmental restraints in place at that time. That obviously requires more technology.
N+2 system validation activity is in its second phase right now. We’re working on technologies such as shaping the aircraft to reduce sonic booms, nozzle concepts for low take-off and landing noise, and some 3-D modeling and design methodology that would allow us to simulate boom reduction and efficiency enhancement.
The goal [is] a design optimization study focused primarily at creating a shaped sonic boom ground signature with a perceived loudness of less than 80 PNdB, which we think is the threshold for supersonic overland flight.
Will tighter budgets limit those efforts?
Under this restructuring, we have consolidated some efforts in the aeronautic sciences program. Work in materials and structures and high altitude emission reduction have been consolidated because of their cross-cutting impact. They will be moving forward at a slower rate, allowing other projects to move toward higher TRL and larger scale testing.
The boom must be minimized for the entire boom ‘carpet’ in level flight, which propagates off to the side rather than straight down. You can’t minimize in just one location, so Phase 2 is addressing how to improve full carpet boom optimization.
How important is collaboration with major airframers – both large commercial passenger jets and business jets?
Very – they are the ones who eventually will produce the products. They won’t tell us everything they have in mind for the next generation of aircraft, but can give us a perspective on how the technologies we are working on need to perform in an integrated design in order to be applicable to future products.
Within FAP, we have a range of ways we interact with industry – funded research, such as the N+2 studies, where we get more of the industry perspective without being proprietary; collaboration on some of the more fundamental technologies, such as with Gulfstream and Boeing in modeling low noise sonic booms in a recent series of experiments looking at how people perceive it on the ground, how it interacts with structures, what kind of indoor noise it creates, etc.
We are building a database of information that eventually will be useful in developing a low sonic boom certification standard as opposed to the current prohibition against supersonic flight over the U.S.
Do NASA efforts on next-generation SST technology include any non-U.S. partners or research?
Our most active international research collaboration is with the Japan Aerospace Exploration Agency, primarily understanding the impact on the civil community of low sonic boom noise, addressing transmission of low noise sonic boom signatures indoors, the resulting noise heard and human response. We’re also working the fundamentals of supersonic boundary layer transition with them.
Concorde was expensive and limited in its flight operations – how is NASA looking to resolve those issues through the use of technologies at least half a century newer?
Pretty phenomenally. Our N+2 concept is nominally an 80-passenger vehicle that achieves the range of Concorde, with take-off and landing noise compatible with subsonic aircraft, does not produce high levels of NOx [nitrogen oxides] at altitude and weighs less than two-thirds of Concorde’s take-off weight. Of course, that’s a conceptual design and they actually built and flew Concorde.
NASA sees overland flight as the key barrier to future supersonic aircraft, which is why our focus is on developing the technologies to reduce sonic boom concerns and developing data to determine if there is an acceptable level of sonic boom noise and the ability to fly overland without creating a boom.
What are the enabling technologies that could make a next-generation supersonic business jet viable?
It’s really sonic boom mitigation, design optimization for low sonic boom and a combination of engine cycles and nozzle technologies that will enable it to meet take-off and landing noise requirements for the diverse airports from which general aviation operates. A third might be high altitude emissions – regulations on that are being considered for the future. But they can put up with more weight and fuel burn because their clientele is more accepting of that to get the time savings supersonic flight offers.
And a supersonic commercial jetliner?
Efficiency – some sort of propulsion system that enables us to operate efficiently from take-off to super-cruise, combing low-boom technology with shaping and drag reduction.
What technologies are still needed that have not yet left the lab?
From my perspective, which is shared by many of my colleagues, if we can’t solve the boom problem there is no sense working the other issues because the airlines won’t buy an aircraft they can’t fly wherever they want to fly.
It is tough to compare the perceived level of a sonic boom with current subsonic aircraft noise. There is another scale used to measure noise in general – dBA-weighted, which integrates sound pressure levels with a function that weights the noise. The dBA we’re trying to achieve is in the high 60s, which is equivalent to the noise inside a modern luxury car.
If and when we start flying a low-boom demonstrator, I believe the boom noise in urban environments won’t be a problem, but it will be more so in rural environments and especially in the extreme quiet environments, such as the Grand Canyon and national parks. Which is where the regulators will step in and determine if there is sufficient value to override any of that.
When might a next-generation supersonic civilian passenger aircraft – business or commercial – actually go into production?
From my perspective, the N+2 timeframe , at the rate we’re going, doesn’t give us all the technologies we need to achieve a completely acceptable level of boom, efficiency and affordability. But I don’t think we really have to wait until 2035 [N+3]; if we continue at the current rate, 2030 is possible. And the business jet market, even with technology available in the 2025 timeframe – provided we can resolve the overland boom – could see a product built.
The boom is key. We’ve made progress and are getting to the point of a flight demonstrator to develop the data for a regulatory process. That is the goal of the project now and, funding willing, we’ll get there.