Civil supersonics – as difficult as ever

Walking into a London office around lunchtime, and announcing that I had enjoyed breakfast in Singapore, was a memorable experience. That was at the end of the 1977 International Air Transport Association AGM, as Concorde F-BTSC completed a 30,000 mile sales tour of the Far East, including a landing at tricky Kai Tak and a stop in Manila to see whether Ferdinand Marcos might back his interest with hard cash. He didn’t, and Sierra Charlie would, many years later, crash after take-off from CDG, the beginning of the end of the project.

A year or so later I stopped in Long Beach to interview Dick FitzSimons, who was running McDonnell Douglas’ Advanced Supersonic Transport project. The AST design was much nicer looking than Boeing’s rather graceless final, fixed-wing SST, canceled seven years earlier. The staggering swing-wing 2707-200 that had preceded it was much more impressive, although if you ever want to put yourself to sleep, you can blow up the patent drawings and count all the moving wing and tail surfaces. (Concorde had 14, which normally operated as seven pairs.)

MacDAC’s NASA-derived arrow wing would later show up on the F-16XL. Their AST was to be made largely from superplastically-formed, diffusion-bonded titanium, based on work done at St. Louis in the 1960s on the Culinary Institute of America’s wonderfully insane ISINGLASS boost-glide project. But not much more happened with SSTs until 1986, when the US Department of Commerce engaged the Battelle Memorial Institute to establish a High Speed Research Center and organized a conference in Columbus. A big focus was on whether technology from the Orient Express project – launched months earlier, after the Challenger disaster – could be exploited commercially, to give U.S. industry the same kind of advantage that they enjoyed in the 1950s with the first jets.

Northwest Airlines and Federal Express had been persuaded by McAir in St. Louis that a Mach 5 intercontinental transport was within reach, drawing a furious tirade from Ben Rich as to why it wasn’t. Instead, the Battelle research was enough to persuade NASA to to start a High Speed Research program – and despite some interruptions, that work has never stopped, although some of it slid over to DARPA in the early 2000s.

Ben Rich telling someone he’s wrong about hypersonics

I was reminded of all that when NASA announced this week that it had awarded two study contracts under its Advanced Air Vehicles Program (AAVP), to develop conceptual designs and technology roadmaps for future supersonic transports, with cruise speeds between Mach 2 and Mach 4. The work is in parallel with the NASA/Lockheed Martin X-59 low-sonic-boom demonstrator, now complete and moving towards flight test.

One team is led by Boeing, with partners including Exosonic, GE Aerospace, and Georgia Tech Aerospace Systems Design Laboratory. Northrop Grumman Aeronautics Systems lead the second team, with partners Blue Ridge Research and Consulting and Boom Supersonic – which is separately preparing its own XB-1 demonstrator, now cleared for flight test,

Rolls-Royce North American Technologies is on both teams. Surprisingly, perhaps, since the Rolls mothership in Derby walked away from Boom last year and Indianapolis isn’t known for big engines; but there are some interesting studies in the works.

But is there any hope, in the era of flight-shaming, climate change, and NetZero, of getting an SST off the ground? Two things I have learned covering SST developments over the years: first, how supersonic shares the market with subsonic airplanes is a very complex question, and second, the goalposts move with goat-like agility.

A very basic feature of the airline business since longer than anyone, professionally, can remember is that most aircraft go at the same speed. This rule was broken on four routes for just over 25 years in the Concorde era. Airlines always had trouble coping with the idea that they were competing with themselves and offering different products.

As for the goalposts: for the first half of Concorde’s career, there were few if any truly intercontinental business jets. Only in the last few years of service was Concorde competing against anything beyond premium cabins with reclining seats, lie-flat beds being introduced in 2000. And the other argument for speed in 2000 was that when you were in the airplane, you were out of touch with the world (and your underlings were probably wrecking everything). That no longer applies. More recently, the idea that the passenger might want to speed to an urgent meeting has taken a hit. (Zoom over boom.)

Those moving goalposts have also made a supersonic business jet a very doubtful idea. Ultrahigh-net-worth individuals (oonwees?) value their jets for convenience on the ground, which means being able to use low-volume airfields with 6,000-foot runways; and for privacy, security, and productivity en route, in an age where everyone can be the paparazzi, including the proles crammed into the back. I wasn’t surprised in the least when Aerion folded, although I’m a bit regretful that the SAI QSST, the most utterly Thunderbirds design to emerge from a real design organization, never made it.

In the airline world, the question of which passengers will consistently pay more for speed remains complex and dynamic. Where I find NASA’s approach very confusing, however, is in its speed range. I’m not a great fan of Boom Supersonic (I find their schedules and engine plans… aspirational) but I think they have a point in saying that Mach 1.7 is as fast as you can go with a fixed-cycle engine – that is the speed limit with a bypass ratio that, combined with an aerodynamic design that balances take-off/landing performance with supersonic efficiency, squeaks you in with current noise rules.

I have not seen any good ideas about variable-cycle engines that will break that lock, without the massive complexity of mid-tandem-fan layouts or other bizarre ideas that have been floated over the decades. Certainly not for Mach 4.

On the demand side, it’s also true that for any given range, each added Mach point has a lesser impact on gate-to-gate speed than the one before it, because it takes longer to accelerate and to slow down. On still-important Atlantic routes, even reaching Mach 4 may be a problem. As for longer routes – even if you could build a Mach 3 or Mach 4 airplane with a 6000 nm range, it’s going to carry so much fuel that it will be overdesigned and oversized for the Atlantic.

The AAVP market studies have been premised on supersonic flight remaining banned over populated land, which seems a mismatch with the X-59 low-boom demonstrator. That aircraft is complete, and is intended to make supersonic flights over selected areas in 2026. But you have to wonder whether time has passed it by, and how such a long, slender airplane (which it will be, scaled to full size) will avoid the fate of the 1990s High Speed Commercial Transport study.

Lockheed Martin

The end of HSCT was not fully explained at the time. One factor was that McDonnell Douglas was far more interested in supersonics than Boeing, which dominated the subsonic market and had no interest in competing with itself, and that once Boeing acquired MDC the HSCT lost an industry sponsor. Another was that the space program was, as always, looking for money.

But what wasn’t reported at the time was that a National Academy of Sciences report had unearthed a monster of a problem, called the APSE (aero/propulsive/servo/elastic) system. The 300-foot-long HSCT, with big engines and massive noise suppressors on a relatively thin wing, was both flexible and unstable, with low mode frequencies. An inlet unstart would have started every part of the airplane dancing. APSE effects could be controlled, in theory, with a tightly integrated flight-management/flight-control/propulsion-control system, but, the scientists stressed, “developing and certifying such a system is completely outside industry’s experience base.”

Getting through the APSE problem, the report advised, would require a full-sized X-plane. And exactly nobody thought there was a chance of getting the money for that. But an X-59-shaped SST – as opposed to a business jet – will face the same challenge.

But between the scope of the AAVP program and the dubious relevance of the X-59, you start to wonder about the direction of NASA aeronautics. It has long been the red-headed stepchild of the astronaut-fixated agency, which is a great shame considering what NACA used to accomplish; but there it is, occupying a space in the government structure and forming an uneasy, and not very productive, triad with DARPA and the Air Force Research Laboratories. It’s not the way to get things done.

Finally: None of the above suggests that there isn’t a defense application for supersonic cruise, or that it isn’t near-term practicable. But that’s material for another blog. And in retrospect, Concorde was bloody amazing, crushing the record (and holding it today) for unrefueled range at supersonic speed, and doing it several times a day for more than a quarter-century.

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