The Case for the Value of Time: Why the Supersonic Business Jet is Inevitable for the Future

(So here’s a little change of pace.  If you’ve ever wondered what an article for a scholarly journal looks like, here’s an example.   I’ll provide several excepts from the paper over the next few days – stay tuned!)

Part II: Technology – Extant or Within Reach

The world’s first attempt into civilian supersonic flight was obviously the British-French Concorde, a miracle of 1960’s era technology that flew for 27 years and transported thousands of passengers back and forth across the Atlantic Ocean (Candel, 2004).  The Concorde was retired in late 2003, ostensibly due to its poor economic performance, but its design and the designs which followed it highlighted the three main technological issues that face any supersonic aircraft, namely the mitigation or elimination of the sonic boom, compliance with ever more restrictive airport noise abatement policies and the minimization of emissions into the atmosphere (Candel, 2004).  With advances in noise reduction and emission control now common in the design of modern jet engines, the mitigation of the sonic boom is probably the most important technological issue facing a civilian supersonic aircraft.

US Navy Breaking the Sound Barrier

Dealing with the Sonic Boom          

Sonic booms are created by the shock waves produced at altitude by an aircraft traveling at supersonic speeds.  These waves then propagate to the ground, creating a change in pressure and generating a considerable disturbance (Candel, 2004). A graph of the pressure wave over time resembles the letter “N” with a nearly instantaneous initial shock, spiking pressure upward above ambient pressure, followed by a nearly linear decrease to less than ambient pressure over the next several milliseconds, followed by a tail shock that recovers to ambient pressure (Aronstein & Schueler, 2005).  The noise level generated by the Concorde’s boom was 105 PLdb (perceived loudness in decibels), louder than a jack hammer (Warwick, 2011b). Industry is aiming for a reduction to about 70 PLdb which is closer to a conversational noise level (Warwick, 2011b).   The most prevalent design theory to mitigate sonic booms was originated in a series of papers in the 1960’s and 1970’s and hypothesizes that the upward and lower spikes on a supersonic shock wave can be significantly reduced merely by shaping the fuselage and lift-producing surfaces on a supersonic aircraft  to do just that (Morgenstern, Arslan, Lyman, & Vadyak, 2005).  In 2003, the theory was conclusively proven through a series of tests funded by the Defense Advanced Research Projects Agency which used two F-5 aircraft, one left in production configuration and the other specially designed to soften the N-wave and reduce the impact at ground level (Morgenstern et al., 2005).  The tests showed that the shaped aircraft produced a consistent and significant reduction in the propagation of overpressure and sound to ground level, even in a turbulent atmosphere (Morgenstern et al., 2005).  Gulfstream and other aircraft manufacturers performed wind tunnel tests that also confirmed that shaped aircraft designs can reduce the sonic boom to lower levels (Henne, 2005).  Researchers at NASA have produced feasible designs which reduce the sonic boom to the range of 65-75 PLdb  (Welge, Nelson, & Bonet, 2010).  NASA’s N+3 studies have indicated a low boom supersonic business jet could be technologically viable as early as 2015 (Warwick, 2010).   In May of 2012, researchers from Japan further confirmed the theory when they dropped two asymmetric aerodynamic bodies from high-altitude over Sweden and noted that the specially shaped body reduced the sonic boom by 50% (Warwick, 2011b).

The Regulatory Language

Yet as promising as this technology may be, it faces a severe limitation in the form of federal regulations that prohibit civilian supersonic flight over land in the United States.  Specifically, 14 C.F.R 91.817 and its accompanying Appendix B to 14 C.F.R Part 91 (2012) state, in part, that flight of a civilian aircraft above Mach 1 is permitted only if “(T)he flight will not cause a measurable sonic boom overpressure to reach the surface.”  Interestingly, no specifications or limitations on acceptable levels of overpressure are specified but some manufacturers are focusing on the 70 PLdb range as their target, per the NASA research targets mentioned previously and making the assumption that once technology can prove lower levels of noise at the service can be attained, the FAA will change its regulations accordingly (Garvey, 2010; Aronstein & Schueler, 2005).  But as late as 2008, the FAA maintained that supersonic aircraft can have no greater noise or sonic boom impact than subsonic aircraft although the agency concedes that “(N)oise standards for supersonic operation will be developed as the unique operational flight characteristics of supersonic designs become known and the noise impacts of supersonic flight are shown to be acceptable” (Federal Aviation Administration, 2008, p. 3).  One might perceive a “which-came-first-the-chicken-or-the-egg” paradox here.

The Aerion SSBJ – Bypassing the Regulations through Mach Cutoff   

The FAA restrictions haven’t stopped Aerion from pursuing its design, largely because the company believes it has found a way to increase the speed of its aircraft over land without having to deal with the restrictions, using a phenomenon called Mach Cutoff (Warwick, 2008).  In words of Aerion’s technology team:

Boomless flight is not restricted to subsonic speeds. Because of the temperature and sound speed gradients in the atmosphere, there is a cutoff Mach number below which the boom from a supersonic aircraft will not propagate to the ground. For stratospheric flight in the U.S. Standard Atmosphere, the cutoff Mach number is 1.15. This represents a speed 35% faster than typical subsonic civil cruise speeds of Mach 0.85 or less. (Plotkin, Matisheck, & Tracy, 2008)

Using a cutoff altitude of 5,000 feet for the sonic boom and depending on atmospheric conditions, Aerion’s jet could cruise at an indicated Mach number between 1.03 and 1.3 at an altitude between 45,000 and 50,000 feet and realize an average ground speed of 764 mph for eastbound trips and 754 mph for westbound trips  (Plotkin et al., 2008),  These speeds are 29% and 47% higher than the average speeds for subsonic aircraft cruising at Mach .85 of 594 mph and 512 mph for eastbound and westbound trips respectively  (Plotkin et al., 2008).  As a side note, most airliners and business jets cruise at Mach .80, making the speed advantage for the Aerion jet even greater.

An additional interesting feature of the Aerion jet is its use of natural laminar flow as a drag reduction mechanism at high speed which allows the use of a wing design that is not swept nearly as severely as that of most supersonic designs (Sturdza, 2007).  A wing that is more conventionally shaped allows for better slow-speed handling characteristics in the take-off and landing phases of flight without the use of sophisticated, heavy and cumbersome variable geometry designs (Sturdza, 2007).  Also, Aerion’s choice of engine is an adaptation of the venerable and proven Pratt and Whitney JT8D-200 engine which currently powers the McDonnell-Douglas MD-80, Boeing 737-200 and Boeing 727 (Aerion Corporation, 2012; Pratt And Whitney Corporation, 2012).  With patents applied for and received on everything from airframe to wing design, the Aerion SSBJ seems firmly rooted in today’s technology and with the validation of its supersonic natural laminar flow system, capable of flight tomorrow (Aerion Corporation, 2012).

The Hypermach SonicStar – Bypassing the Regulations through Elimination of the Boom

On the other end of the technology, speed and regulatory spectrum is Hypermach Aerospace Ltd.’s SonicStar, a jet that will enable boomless flight over land at speeds approaching Mach 4 through the use of cutting-edge technology that eliminates the sonic boom entirely (Hypermach Aerospace Ltd., 2011a).  Rather than relying solely on aerodynamic design, the SonicStar utilizes a unique solution, injecting plasma into the atmosphere in front of the aircraft as it flies thus rapidly heating an extended path ahead of the shock wave and hence creating a hot, low-density core through the rapid expansion of the plasma (Hypermach Aerospace Ltd., 2011b).  Then, according to Hypermach’s description, the “vehicle’s bow shock expands into the core, followed by the vehicle itself. The shock bows as the core provides a route for the high pressure front to escape around the vehicle, reducing the shock strength” (Hypermach Aerospace Ltd., 2011b).

The creation of the quantity of plasma energy necessary to produce this effect is made possible by the SonicStar’s engine, originally called S-MAGJET (“S” for supersonic), a five-stage electric-turbine hybrid engine, now being developed into a hypersonic derivative called H-MAGJET (“H” for hypersonic) by Portland, Maine-based SonicBlue (Trauvetter, 2011).   The S-MAGJET engine design uses a superconducting ring motor-driven fan, compressor and turbines, and a combustion chamber that converts air into plasma via the following process:

As air enters the engine, it is accelerated in the first stage of a dual counterrotating bypass fan section, where it enters an eight-stage counter-rotating, statorless compressor. The compressed air reaches about 2,250F, and then is forced into an ion plasma fuel combustor and ignited by an array of electric- and magnetic-field-generating fuel injectors. The air is converted into plasma within the combuster (sic) before exiting to drive a five-stage counter-rotating gas turbine and integrated superconducting electric generator.  (Wall & Norris, 2011)

The fan assembly which produces the engine’s thrust is driven by turbines that are magnetically levitated and not mechanically connected to the core, hence fan RPM  is completely independent of core RPM and the fan assembly can make use of larger, more aerodynamically efficient blades, improving the engine’s efficiency (HyperMach Aerospace Ltd, 2011c; Wall & Norris, 2011).  The S-MAGJET design boasts a specific fuel consumption (SFC) ratio (pounds of fuel burned / pounds of thrust) of 1.05 – 1.10 (Trauvetter, 2011).  While this SFC is roughly twice that of typically modern airliners, it is well less than the Concorde’s SFC of approximately 1.2 and renders about twice the speed in return (Steelant, 2006).  SonicBlue’s engine technology was developed in 2006 by Richard Lugg, the current CEO of Hypermach, has been granted several US patents and has been pronounced viable by at least one industry expert, Sam Wilson, the president of AVID LLC, a Virginia fan-design company that has done work for a number of aerospace companies (Wickenheiser, 2006).

So with no regulatory changes whatsoever, both the Aerion jet and Hypermach’s SonicStar could begin flying tomorrow, with technology that is in the process of validation as of this writing.  Aerion was slated to begin full supersonic wing testing of its natural laminar flow technology in late 2011 although a lack of follow-on funding may have stalled the process (Norris, 2010; “Recession Keeps SSBJ on Drawing Board,” 2010).  Hypermach has formed a partnership with Eagle Harbor Technologies and plans to continue the initial engine testing in Autumn 2012 with full scale testing planned for 2013 (Hypermach Aerospace Limited, 2011d).  So it would seem the realization of the appropriate technology has already occurred or is just a matter of time.

But when the time comes, will it matter?

Stay tuned for Part 3 – coming soon!


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Appendix B to Part 91, Authorizations To Exceed Mach 14 C.F.R. §91.817. (2012).

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