Supersonic Commercial Aircraft
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Supersonic Commercial Aircraft J. Gabriel Funtanilla1 University of Colorado - Boulder, Boulder, CO, 80302 This paper describes the birth of supersonic flight and its subsequent rise, fall, and revival in commercial transport through the past 50 years. Supersonic flight owes its beginnings to a joint effort between NACA and the U.S. Air Force in the second half of the twentieth century. Since then NACA’s successor, NASA, has spearheaded the effort to understand and improve the flight of such aircraft. In order to survive today’s market, a viable supersonic aircraft must effectively maneuver itself beyond various noise, emission, and cost constraints. The following describes supersonic transport design, with an emphasis on engine related issues, by examining what has been done and what is to come. Nomenclature BPR = bypass ratio CAEP = Committee on Aviation Environmental Protection CD = convergent-divergent nozzle CO = carbon monoxide CO2 = carbon dioxide FAA = Federal Aviation Administration g0 = acceleration of gravity on Earth HSCT = high speed civil transport HSR = High Speed Research (Program) L/D = lift-to-drag ratio LPP = lean premixed, prevaporized mf, mi = aircraft takeoff and landing mass NOx = nitrogen oxide PMC = polymeric matrix composites QueSST = Quiet Supersonic Technology RQL = rich burn, quick mix, lean burn s = range of aircraft, as determined by the Breguet Range Equation SBJ = supersonic business jet SFC = specific fuel consumption SOx = sulfur oxides SST = supersonic transport TAPS = twin-annular pre-mixing swirler V = cruise velocity I. Introduction N 1947, NACA broke the sound barrier with its Bell X-1 experimental aircraft, ushering in a new era of flight. I Only a decade after the dawn of supersonic flight were proposals made for a supersonic transport (SST). The less heralded of the two SSTs to ever operate is the Soviet Union’s Tupolev Tu-1441, which was first flown in late 1968, a few months before its more nostalgic counterpart, the Concorde. 1 Graduate Student, Aerospace Engineering Sciences, University of Colorado - Boulder 1 American Institute of Aeronautics and Astronautics Figure 1. Bell X-1. The first supersonic aircraft. Early development of the Tupolev project was positive, with the Tu-144 becoming the first commercial aircraft to exceed Mach 2. However, due to a crash in the 1973 Paris Air Show and budget restrictions, the Tu-144 was delayed into passenger flight until 1977 (well after the Concorde). Its lifetime as a SST was cut short by another test flight crash and the project was permanently cancelled in 1983. In the early 1990’s, the Tu-144 was resurrected by NASA for use in supersonic research with its final flight occurring in 1999. The Anglo-French Concorde2 outlived the Tu-144 and was flown commercially until 2003. The Concorde entered service in 1976 and continued to carry passengers for the next 27 years with few hiccups. A fatal crash in July 2000 halted operation, but after careful investigation of the cause, modifications were set in place and the Concorde resumed flight in November 2001. Its eventual retirement was due to the many challenges SST faces, in this case namely noise and economic profitability. Figure 2. Tu-144 and Concorde. left: Tu-144, right: Concorde. Although enthusiasm for supersonic commercial aircraft was initially high after the demonstration of supersonic flight, the only cases of SST today remain the Concorde and Tu-144. This can be attributed to the overwhelming challenges engineers face in providing a supersonic aircraft design acceptable to federal regulation agencies like the FAA, airlines, and passengers alike. Supersonic flight is dramatically different than subsonic flight. To this day the fundamental problem of supersonic flight remains noise. Noise includes the so called “community noise”—noise during takeoff, approach, and landing due to the fan speed and high exhaust velocities of engines optimized for supersonic cruise—and of course the inevitable sonic boom emitted by vehicles traveling at supersonic speeds. Since the initial flight of the X- 1 and the original sonic boom, restrictions have limited supersonic commercial flight to trans-oceanic routes. This is the reason the Concorde was relegated to flying over the Atlanic. This restriction then, is related to eligible routes of SST. Routes traveled must consist mainly of over-water flight and have enough passenger traffic demand to be profitable. Hence, the Concorde’s trips from New York to London. Range, then, becomes an issue for supersonic vehicles. The lift-to-drag ratio (L/D) of supersonical vehicles is much lower than the standard 18-20 of subsonic commercial aircraft, such as the Airbus-320. Therefore the range is negatively impacted, as shown by the Breguet Range Equation: (1) 2 American Institute of Aeronautics and Astronautics where s is the range, V is the cruising velocity, SFC is the specific fuel consumption, g0 is the acceleration of gravity on Earth, mi is the initial mass at takeoff, and mf is the final landing mass. A low lift-to-drag ratio requires higher velocities, lower specific fuel consumption (SFC), and/or low structure weight to maintain equivalent range to subsonic aircraft. The previous generation of SST were heavy, gas guzzlers requiring high exhaust velocities to maintain flight range. To obtain high cruise velocities, but minimize ram drag supersonic engines are invariably turbojet engines or low bypass ratio (BPR) turbofans. These engines consume more fuel per thurst, which along with the increasing prices of fuel in the early 2000’s was another contributing factor to the Concorde’s retirement. The last big hurdle for supersonic engine design deals with atmospheric pollution. Emissions of gas turbine engines include greenhouse gases, water vapor and carbondioxide (CO2), ozone-affecting nitrogen oxides (NOx), carbon monoxide (CO), sulfur oxides (SOx), unburned hydrocarbons (UHC), and soot. The most significant of these pollutants are CO2 and NOx. Of the two, NOx has the bigger influence on supersonic engine design. Because supersonic vehicles, especially those that cruise above Mach 2.0, fly in the stratosphere, the interaction between NOx and ozone at these altitudes results in the depletion of the ozone layer. And because NOx production rate increases exponentially with temperature, with the exact amount dependent on the gas’s residence time, for the best engine performance the combustor must simultaneously raise the turbine entry temperature to the material limits and quench the hot gas as quickly as possible. If supersonic transport is going to be successful, it is paramount that the combustor technology be able to meet NOx emission regulations. Alluded to earlier, NASA began a project in the 1990’s to overcome the challenges faced by the Tu-144 and Concorde. NASA’s High Speed Research (HSR) Program3 sought to develop a new generation of commercial high speed civil transport (HSCT). The goals of the program were to diminish supersonic deficiencies in noise, emissions, service life, weight, range, and payload for an HSCT that would carry 300 passengers 5,000 nm at Mach 2.4. the scope of the study also included defining three notional SST classes: a supersonic business jet (SBJ), an overland SST, and the aforementioned HSCT. After about a decade of design study and research the following conclusions were made by the team. 1) Airframe design techniques could minimize the sonic boom 2) The use of a low bypass ratio turbofan was suggested, even though many supersonic aircraft are powered by turbojets, this is to keep SFC as low as possible 3) Need an advanced combustor that emits ultralow levels of NOx, possible designs include lean premixed, prevaporized (LPP) and rich, quick mix, lean (RQL) combustors 4) High temperature materials are needed due to the heating of the “skin” of the airframe when traveling at Mach numbers upwards of 2.0, investigated aluminum and titanium alloys as well as polymeric matrix composites (PMC); these materials need to be light weight to maintain range 5) Aerodynamic wind tunnel experiments were done with the intention of improving L/D while minimizing wave drag; possible 10-15% increase in L/D from Concorde and Tu-144 design Eventually, the technical challenges for the project were deemed to difficult to overcome and the project was ended. It concluded that the focus on achieving Mach 2.4 was too aggressive and that a smaller, lower speed, vehicle, such as a supersonic business jet cruising around Mach 1.6 would be more feasible4. It also suggested that a vehicle cruising at Mach 2.0 may have a net productivity similar to that of a Mach 2.4 SST. The inception of a supersonic aircraft into the commercial sector was still wishful thinking at the approach of the 21st century. Fast forwarding to today, there have been many advances in technology that have made SST attractive again. Techniques for mitigating the sonic boom, the supersonic commercial aircraft’s fundamental problem, using shock wave shaping have been physically demonstrated beginning with NASA’s flight of an F-5E Tiger II with a lengthened fuselage in 20035. NASA then moved on to the “Quiet Spike”5, a 24ft telescopic lance like structure attached to the nose of an F-15B. The spike created three small shock waves that remained separate rather than coalescing into one large sonic boom. Another concept to reduce the sonic boom is a biplane configuration in which the interaction between the two wings reduce or deflect skyward the shock waves6. NASA is currently teaming with Lockheed Martin’s Skunk Works in its design of a Quiet Supersonic Technology (QueSST) demonstrator aircraft7 to perform human testing and obtain data on sonic boom mitigation. Overall these techniques can reduce the effects of the boom felt on the ground and testing results may convince the Federal Aviation Administration (FAA) to lift restrictions of supersonic commercial flight over land.