JOURNAL OF AIRCRAFT Vol.42.No.3.May-June 2005 Case for Small Supersonic Civil Aircraft Preston A.Henne* Gulfstream Aerospace Corporation,Savannah,Georgia 31402 Civil aviation progress in the last 40 years has included a significant expansion of the small civil aircraft market involving regional jets,business jets,and the emerging personal jets.A significant factor in the growth of the small civil aircraft market is the value of time.Recognition of the ever-increasing value of time has lead to increased interest in the feasibility of a small supersonic civil aircraft.The step to supersonic speeds offers the potential of a dramatic decrease in travel time.Feasibility studies of a small quiet supersonic jet (QSJ)have been conducted. Market research,environmental concerns,program and design requirements,and vehicle characteristics are sum- marized.Areas for concentrated future supersonic aeronautics research and development efforts are highlighted. Introduction York City heading westbound or eastbound and,with one stop,be HE remarkable progress and growth in civil aviation in the essentially anywhere in the world in 10 h. last 40 years has been fueled by advancements in a broad History range of aeronautical technologies,combined with strong economic growth.Continued advancements in aerodynamics,structures,ma- As shown in Fig.3,supersonic aircraft progress from the first su- terials,avionics,and engine technology have provided the techni- personic flight in 1947 has been impressive.In less than 15 years,the cal basis for the development of many different predominantly jet- B58 was setting records for flights at Mach 2 between New York and powered civil aircraft models.These aircraft range from the very London in less than 4 h.However,progress was almost completely large transports used by scheduled air carriers to the very small limited to military aircraft.In the 1960s,three projects were initiated personal jets emerging currently.Technical advancements coupled to bring supersonic civil transportation to market.The three differ- with economic growth have produced an exceptional record of avi- ent projects were nationally oriented.The U.S.supersonic transport ation market growth,as shown in Fig.1.This growth has included (SST)program was stopped before an aircraft was even built.The a significant expansion of the small civil aircraft market involving Russian Tu144 was first to flight but ultimately was removed from regional jets,business jets,and,now,personal jets. service.Only the British/French Concorde continued service until One of the market factors that has proven important in the growth 2003. of small civil aircraft market is the value of time in transportation Continuation of the Concorde service was largely related to na- needs.Air travel provides the highest travel speeds for trips of sig- tionalistic pride in the remarkable aeronautical achievement em- nificant distance.Scheduled carrier use of the regional jet satisfies bodied in the Concorde.Concorde economics and environmental the need to bring the time value of jet transport to low-density routes impacts limited its utility and represent a challenge in the contem- and markets.Business jet use by charter operators,fractional own- plation of future advancements in supersonic civil transportation. ers,public companies,private companies,and individuals is strongly The lesson of Concorde,as well as the other two less successful at- tied to the value of time.Business jet use and the emerging personal tempts,is that aeronautical prowess is a necessary but not sufficient jet use provide time value through jet aircraft speed and through condition for the creation of successful programs in supersonic civil broad destination flexibility.Exceptional field length performance aviation.Economic justification and environmental compatibility enables the small civil aircraft to operate into a remarkable number are also required.Subsequent to the original SST Program,the U.S.. of locations.Combine destination flexibility and jet aircraft speed in on at least two occasions-supersonic cruise research(SCAR)and a competitive business environment and one has a highly valuable high speed civil transport(HSCT)7-attempted to promote and de- means to satisfy transportation needs. velop a large supersonic civil transport,only to see the projects fail. In these later cases.the technical.environmental.and economic Attraction equations still could not be satisfied.Quite coincidentally,the an- Recognition of the ever-increasing value of time has lead to nouncement of Concorde service termination in 2003 coincides with increased interest in the feasibility of a small supersonic civil the first flight centennial celebration year. aircraft.-The step to supersonic speeds offers the potential of Whereas the stunning attraction of Fig.2 is clear to most air trav- a dramatic decrease in travel time.As shown in Fig.2,when speed elers,the supersonic stagnation reflected in Fig.3 demands a new is doubled from today's subsonic 0.8-0.85 Mach number to a 1.8 approach.The studies conducted in the last few yearsprovide Mach number,global transportation paradigms transforms.Leaving compelling evidence that the new approach should be focused on a New York City at 0700 hrs in a quiet supersonic jet(QSJ),one can much smaller vehicle as the first commercially and environmentally be anywhere in the indicated circle for at least 2 h and be back in successful step in supersonic civil aviation.The history of advances New York City by 1900 hrs.Alternatively,one could depart New in transportation has been more often defined by the introduction of a paradigm shift,first on a modest scale rather than a giant scale These advances in transportation technology were usually aimed at the more affluent as a means of introductory affordability.Once Presented as Paper 2003-2555 at the AlAA/ICAS International Air and the concept has been proven on a small scale for the more afflu- Space Symposium and Exposition:The Next 100 Years,Dayton,OH. ent,then technical advances and commercial competition have typ- 14-17 July 2003;received 10 September 2003;revision received 11 January ically led to larger-scale vehicles that appeal to a broader segment 2004;accepted for publication 14 January 2004.Copyright C 2004 by of the population.This small-to-large stepping stone approach has Preston A.Henne.Published by the American Institute of Aeronautics and clearly occurred throughout ship and aircraft transportation system Astronautics,Inc.,with permission.Copies of this paper may be made for personal or internal use,on condition that the copier pay the $10.00 per-copy history and reflects an intuitive risk management.In hindsight,it fee to the Copyright Clearance Center,Inc.,222 Rosewood Drive,Danvers. would seem that the aeronautical community has been trying to run MA 01923:include the code 0021-8669/05 $10.00 in correspondence with supersonically before it can walk supersonically.It has been try- the CCC. ing to create the giant supersonic transport before it developed the *Senior Vice-President,Programs,Engineering,and Test.Fellow AlAA supersonic DC-3.Supersonic long-term vision took priority over 765
JOURNAL OF AIRCRAFT Vol. 42, No. 3, May–June 2005 Case for Small Supersonic Civil Aircraft Preston A. Henne∗ Gulfstream Aerospace Corporation, Savannah, Georgia 31402 Civil aviation progress in the last 40 years has included a significant expansion of the small civil aircraft market involving regional jets, business jets, and the emerging personal jets. A significant factor in the growth of the small civil aircraft market is the value of time. Recognition of the ever-increasing value of time has lead to increased interest in the feasibility of a small supersonic civil aircraft. The step to supersonic speeds offers the potential of a dramatic decrease in travel time. Feasibility studies of a small quiet supersonic jet (QSJ) have been conducted. Market research, environmental concerns, program and design requirements, and vehicle characteristics are summarized. Areas for concentrated future supersonic aeronautics research and development efforts are highlighted. Introduction T HE remarkable progress and growth in civil aviation in the last 40 years has been fueled by advancements in a broad range of aeronautical technologies, combined with strong economic growth. Continued advancements in aerodynamics, structures, materials, avionics, and engine technology have provided the technical basis for the development of many different predominantly jetpowered civil aircraft models. These aircraft range from the very large transports used by scheduled air carriers to the very small personal jets emerging currently. Technical advancements coupled with economic growth have produced an exceptional record of aviation market growth, as shown in Fig. 1. This growth has included a significant expansion of the small civil aircraft market involving regional jets, business jets, and, now, personal jets. One of the market factors that has proven important in the growth of small civil aircraft market is the value of time in transportation needs. Air travel provides the highest travel speeds for trips of significant distance. Scheduled carrier use of the regional jet satisfies the need to bring the time value of jet transport to low-density routes and markets. Business jet use by charter operators, fractional owners, public companies, private companies, and individuals is strongly tied to the value of time. Business jet use and the emerging personal jet use provide time value through jet aircraft speed and through broad destination flexibility. Exceptional field length performance enables the small civil aircraft to operate into a remarkable number of locations. Combine destination flexibility and jet aircraft speed in a competitive business environment and one has a highly valuable means to satisfy transportation needs. Attraction Recognition of the ever-increasing value of time has lead to increased interest in the feasibility of a small supersonic civil aircraft.1−5 The step to supersonic speeds offers the potential of a dramatic decrease in travel time. As shown in Fig. 2, when speed is doubled from today’s subsonic 0.8–0.85 Mach number to a 1.8 Mach number, global transportation paradigms transforms. Leaving New York City at 0700 hrs in a quiet supersonic jet (QSJ), one can be anywhere in the indicated circle for at least 2 h and be back in New York City by 1900 hrs. Alternatively, one could depart New Presented as Paper 2003-2555 at the AIAA/ICAS International Air and Space Symposium and Exposition: The Next 100 Years, Dayton, OH, 14–17 July 2003; received 10 September 2003; revision received 11 January 2004; accepted for publication 14 January 2004. Copyright c 2004 by Preston A. Henne. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/05 $10.00 in correspondence with the CCC. ∗Senior Vice-President, Programs, Engineering, and Test. Fellow AIAA. York City heading westbound or eastbound and, with one stop, be essentially anywhere in the world in 10 h. History As shown in Fig. 3, supersonic aircraft progress from the first supersonic flight in 1947 has been impressive. In less than 15 years, the B58 was setting records for flights at Mach 2 between New York and London in less than 4 h. However, progress was almost completely limited to military aircraft. In the 1960s, three projects were initiated to bring supersonic civil transportation to market. The three different projects were nationally oriented. The U.S. supersonic transport (SST) program was stopped before an aircraft was even built. The Russian Tu144 was first to flight but ultimately was removed from service. Only the British/French Concorde continued service until 2003. Continuation of the Concorde service was largely related to nationalistic pride in the remarkable aeronautical achievement embodied in the Concorde. Concorde economics and environmental impacts limited its utility and represent a challenge in the contemplation of future advancements in supersonic civil transportation. The lesson of Concorde, as well as the other two less successful attempts, is that aeronautical prowess is a necessary but not sufficient condition for the creation of successful programs in supersonic civil aviation. Economic justification and environmental compatibility are also required. Subsequent to the original SST Program, the U.S., on at least two occasions—supersonic cruise research (SCAR)6 and high speed civil transport (HSCT)7—attempted to promote and develop a large supersonic civil transport, only to see the projects fail. In these later cases, the technical, environmental, and economic equations still could not be satisfied. Quite coincidentally, the announcement of Concorde service termination in 2003 coincides with the first flight centennial celebration year. Whereas the stunning attraction of Fig. 2 is clear to most air travelers, the supersonic stagnation reflected in Fig. 3 demands a new approach. The studies conducted in the last few years1,2,8 provide compelling evidence that the new approach should be focused on a much smaller vehicle as the first commercially and environmentally successful step in supersonic civil aviation. The history of advances in transportation has been more often defined by the introduction of a paradigm shift, first on a modest scale rather than a giant scale. These advances in transportation technology were usually aimed at the more affluent as a means of introductory affordability. Once the concept has been proven on a small scale for the more affluent, then technical advances and commercial competition have typically led to larger-scale vehicles that appeal to a broader segment of the population. This small-to-large stepping stone approach has clearly occurred throughout ship and aircraft transportation system history and reflects an intuitive risk management. In hindsight, it would seem that the aeronautical community has been trying to run supersonically before it can walk supersonically. It has been trying to create the giant supersonic transport before it developed the supersonic DC-3. Supersonic long-term vision took priority over 765
766 HENNE 1000 Actual Forecast Recessionary Periods 气 Commercial Jets Business Jets 400 200 Regional Jets 92 1982 1951988 1991 1994 1997 20002003 200e Year Fig.1 Aviation market growth measured in units delivered per year. Speed that redefines a 12 hour work day --there and back with 2 hours minimum on location Assuming Mach 1.8, 4,500 nm range Hono capability Westbound from New York Eastbound from New York Worldwide Coverage in 10 hours Fig.2 Doubling speed brings remarkable transportation value. 3.5 Military O Commercial XB70 3.0 B2707U.S. 104 Never Built 2.5 Tu144 Russia Ops Stopped 爱 Fr/UK 0558 JaqwnN yoeW 2.0 "In Service" F100 No B58 Ops Stopped 2003 Supersonic 30 Years with No New Civil Supersonic Civil Transport Transport 1.0 45 Years of Subsonic Civil Jet Transports 0.5 0.0 1940 1950 1960 1970 1980 1990 2000 2010 2020 Year Fig.3 Supersonic aircraft progress
766 HENNE Fig. 1 Aviation market growth measured in units delivered per year. Fig. 2 Doubling speed brings remarkable transportation value. Fig. 3 Supersonic aircraft progress
HENNE 767 supersonic foundations.An effective supersonic vehicle needs to well as business aviation use.These studies have concluded that be introduced on a small scale first.The technical,economic.and a small supersonic civil aircraft market of 250-450 units exists at environmental equations need to be solved on a lower-risk,small a price between $50M and S100M per unit over a 10-20 year pe- scale before the giant,supersonic transport solution is once again riod.Additionally,a major fractional ownership company has con- attempted. ducted sufficient studies to conclude that a small supersonic civil aircraft has considerable appeal and seeks to participate in any such QS,J Market Research endeavor. At least four different market research studies have been con- The current perspective indicates there is a significant market ducted on the small QSJ in recent years.Gulfstream internal market potential for a small supersonic civil aircraft.Market assessments research has consisted of both a bottom-up approach and an analyt- have identified the following significant points: ical projection based on historical regression analysis.Independent 1)Two Gulfstream market assessments identify conservative market research studies utilizing direct customer contact and ques sales forecasts of 180-350 aircraft. tionnaires have also been reported.3> 2)Two independent market assessments identify sales forecasts The bottom-up approach was based on an assessment of the large- of 250-450 aircraft. cabin business aircraft known customer base.This approach essen- 3)Supersonic overland flight capability is a requirement. tially amounted to a count of the individual customers who would 4)Range beyond 4000 n mile is a requirement. step up to the next level of transportation productivity if it was avail- The supersonic overland flight requirement is key to the market able.The results of this study indicated a minimum market potential feasibility.Sonic boom suppression is the key technology required of 180 unitsover 10years.This result did not include special mission to make supersonic overland flight acceptable.All of the market or government sales and did not account for fractional ownership studies have confirmed significant value for this capability.A limi- needs.The 180 unit number is a good threshold volume.A model tation to subsonic overland flight,as a result of unacceptable sonic run of 200 units is typical for programs throughout the 40-plus year boom characteristics,represents a severe blow to the value of a small history of Gulfstream Aerospace Corporation. supersonic civil aircraft.Production costs,both nonrecurring and re- The analytical projection method extrapolated historic annual de- curring,for the supersonic vehicle are present whether or not the livery data for large cabin business jet deliveries and inferred the vehicle is limited to subsonic overland flight.The market price or potential for a QSJ product serving the same customer group.The value that the vehicle can command is strongly driven by its ability historic annual delivery data were approximated by determination of to maximize time savings.As shown in Fig.5,much of small civil a best-fit constant growth rate.This growth rate applied year-after- aircraft flight is over land.In this case,a random sample of flights year yields a fleet size equivalent to the actual historic data.Once was taken from actual flight records for the in-service Gulfstream determined,this growth rate was used to extrapolate anticipated GIV and Gulfstream GV aircraft for one year.The sample indicates future deliveries.Historically,a conservative 10%capture is recog- that only 25%of the flight time was over water.Consequently,max- nized for new-capability product introductions.This future delivery imum time savings and maximum market value dictate supersonic estimate,coupled with the 10%market capture,provides a means overland flight requirements.If this higher market price(supersonic to estimate QSJ potential.The historic data,equivalent growth line overland capability)is not achieved,the business case is weak,and and projected QSJ deliveries are illustrated in Fig.4.It is interest- we have not solved the economic equation.The supersonic-over- ing that this long-term regression analysis discounts the blistering water-only assumption made in previous large supersonic transport delivery levels of the late 1990s/early 2000s.As has been noted re- studies certainly hindered,if not fatally restricted,the aircraft po- cently,delivery levels have retreated back to levels actually closer to tential in the marketplace.Consequently,definition of an acceptable the projection line.This projection method results in a QSJ market solution for sonic boom suppression is key to the success of a small potential over a 10-year period to be over 350 units.Independent supersonic civil aircraft. market research studies have also been conducted.3-5 These studies have used extensive customer surveys and considered a number of Environmental Concerns questions to clarify product requirements.They have also introduced The viability of a supersonic civil aircraft hinges on the ability the concept of a small supersonic aircraft for scheduled service,as to design and manufacture a configuration that is environmentally 1000 Projection Assuming a 6.2%AAGR seleAllea er sseulsng History 2.277 Units Over 36 Years 100 QSJ Projection 190 to 570 Units (2013-2022) 10 1960 1970 1980 1990 2000 2010 2020 2030 Year Projected QSJ Market Equal to 350+ Airplanes over 10 Year Period Fig.4 Analytical projection approach for QSJ market forecast
HENNE 767 supersonic foundations. An effective supersonic vehicle needs to be introduced on a small scale first. The technical, economic, and environmental equations need to be solved on a lower-risk, small scale before the giant, supersonic transport solution is once again attempted. QSJ Market Research At least four different market research studies have been conducted on the small QSJ in recent years. Gulfstream internal market research has consisted of both a bottom-up approach and an analytical projection based on historical regression analysis. Independent market research studies utilizing direct customer contact and questionnaires have also been reported.3−5 The bottom-up approach was based on an assessment of the largecabin business aircraft known customer base. This approach essentially amounted to a count of the individual customers who would step up to the next level of transportation productivity if it was available. The results of this study indicated a minimum market potential of 180 units over 10 years. This result did not include special mission or government sales and did not account for fractional ownership needs. The 180 unit number is a good threshold volume. A model run of 200 units is typical for programs throughout the 40-plus year history of Gulfstream Aerospace Corporation. The analytical projection method extrapolated historic annual delivery data for large cabin business jet deliveries and inferred the potential for a QSJ product serving the same customer group. The historic annual delivery data were approximated by determination of a best-fit constant growth rate. This growth rate applied year-afteryear yields a fleet size equivalent to the actual historic data. Once determined, this growth rate was used to extrapolate anticipated future deliveries. Historically, a conservative 10% capture is recognized for new-capability product introductions. This future delivery estimate, coupled with the 10% market capture, provides a means to estimate QSJ potential. The historic data, equivalent growth line, and projected QSJ deliveries are illustrated in Fig. 4. It is interesting that this long-term regression analysis discounts the blistering delivery levels of the late 1990s/early 2000s. As has been noted recently, delivery levels have retreated back to levels actually closer to the projection line. This projection method results in a QSJ market potential over a 10-year period to be over 350 units. Independent market research studies have also been conducted.3−5 These studies have used extensive customer surveys and considered a number of questions to clarify product requirements. They have also introduced the concept of a small supersonic aircraft for scheduled service, as Fig. 4 Analytical projection approach for QSJ market forecast. well as business aviation use. These studies have concluded that a small supersonic civil aircraft market of 250–450 units exists at a price between $50M and $100M per unit over a 10–20 year period. Additionally, a major fractional ownership company has conducted sufficient studies to conclude that a small supersonic civil aircraft has considerable appeal and seeks to participate in any such endeavor. The current perspective indicates there is a significant market potential for a small supersonic civil aircraft. Market assessments have identified the following significant points: 1) Two Gulfstream market assessments identify conservative sales forecasts of 180–350 aircraft. 2) Two independent market assessments identify sales forecasts of 250–450 aircraft. 3) Supersonic overland flight capability is a requirement. 4) Range beyond 4000 n mile is a requirement.1 The supersonic overland flight requirement is key to the market feasibility. Sonic boom suppression is the key technology required to make supersonic overland flight acceptable. All of the market studies have confirmed significant value for this capability. A limitation to subsonic overland flight, as a result of unacceptable sonic boom characteristics, represents a severe blow to the value of a small supersonic civil aircraft. Production costs, both nonrecurring and recurring, for the supersonic vehicle are present whether or not the vehicle is limited to subsonic overland flight. The market price or value that the vehicle can command is strongly driven by its ability to maximize time savings. As shown in Fig. 5, much of small civil aircraft flight is over land. In this case, a random sample of flights was taken from actual flight records for the in-service Gulfstream GIV and Gulfstream GV aircraft for one year. The sample indicates that only 25% of the flight time was over water. Consequently, maximum time savings and maximum market value dictate supersonic overland flight requirements. If this higher market price (supersonic overland capability) is not achieved, the business case is weak, and we have not solved the economic equation. The supersonic-overwater-only assumption made in previous large supersonic transport studies certainly hindered, if not fatally restricted, the aircraft potential in the marketplace. Consequently, definition of an acceptable solution for sonic boom suppression is key to the success of a small supersonic civil aircraft. Environmental Concerns The viability of a supersonic civil aircraft hinges on the ability to design and manufacture a configuration that is environmentally
768 HENNE 3500 目Over Water 3000 Over Land 2500 2000 1500 1000 500 Average range:1290 n.m. Percent of miles flown over water:25% Fig.5 Random sample of small civil aircraft operation compatible.6 In this context,environmentally compatible means effectively addressing the following:1)sonic boom,2)engine ex- Initial Cruise Sonic Boom Strength haust emissions,and 3)airport noise.A feasible design must reduce 3.0 the configuration's sonic boom signature such that supersonic flight HSCT over land is acceptable to the public.As discussed earlier,the market 2.5 research studies have repeatedly confirmed that supersonic overland Concorde flight is of critical importance to the value of the vehicle..3.5 The 2.0 design must also minimize adverse atmospheric effects due to en- gine exhaust emissions throughout all operations.Emissions must SBJ be minimized during both low-speed,low-altitude airport operations 1.5 as well as high-altitude supersonic cruise operations.Furthermore, a successful QSJ must be a quiet,good neighbor to the airport en- 1.0 vironment.It should make no more noise than today's quiet small QSJ Shaped DARPA QSP Goal subsonic civil jets. QSJ Internal Concepts Sonic Boom The effort to suppress sonic boom successfully has several ma- 200000 400000 600000 800000 jor aspects.First,the vehicle aerodynamic design is used to shape Initial Cruise Weight-lb the sonic boom signature.Shaped signatures are defined by the prevention of coalescence of the signature into an annoying N- Fig.6 Sonic boom initial overpressure strongly affected by vehicle shaped pressure wave.Second,psychoacoustic testing is being used weight. to establish signature levels and desired signature shapes that are considered environmentally acceptable.Third,a flight demonstra- Gulfstream has defined several unique airplane configuration tion will be required to prove boom suppression acceptability.A details that reduce the initial overpressure and enhance the vehi- flight demonstration will provide a foundation of scientific data cle designer's ability to control the shape of the ground signature necessary for regulation of supersonic overland flight,as well as produced.Results for several of these configurations are in- risk reduction for the business decision to launch a production dicated in Figs.6 and 7.Several of these design concepts have program. been validated in recent wind-tunnel testing conducted at NASA When the physics of sonic boom are considered,it is very clear Langley's 4 Foot Unitary Wind Tunnel.As indicated in Fig.8 that a small aircraft has a profound advantage over large aircraft.The the data for the advanced boom suppression concepts show very size and weight of a vehicle has a first-order effect on the strength of promising signature characteristics.Based on these computational the sonic boom signature.As the vehicle weight decreases,the sonic fluid dynamics(CFD)and wind-tunnel test results,initial overpres- boom disturbance is decreased.Careful attention to the physical sures of less than 0.2 psf and peak overpressures of 0.5-0.6 psf shape of the vehicle can result in a shaped ground signature rather seem quite achievable.Clearly,additional model and flight test- than a coalesced N wave.Shaped ground signatures avoid the large ing are needed to develop,validate,and fully exploit sonic boom abrupt pressure increases associated with beginning and end of a suppression technology.This aerodynamic technology need is a typical sonic boom N wave.Shaped signatures have lower initial long-standing aeronautical barrier that has not yet been adequately and final pressure increases and are perceived as substantially quieter addressed. due to their more-sinusoidal,very-low-frequency character.Shaped Sonic boom strength for configurations with signatures such as signatures can be thought of as precoalesced signatures designed to those in Figs.7 and 8 have been analyzed in terms of perceived a special wave shape. level of noise in decibels (PLdB)and A-weighted noise in decibels Figures 6 and 7 show the impact of vehicle size and shaping [dB(A)].The results shown in Fig.9 show a remarkable improve- on the sonic boom ground signature.These charts show data for a ment in the cruise signature acoustic strength.Results for configu- 300-passenger high-speed civil transport(HSCT),the Concorde,a rations shown in Fig.9 indicate that the noise levels can be reduced generic supersonic business jet(SBJ),and two QS]configurations. more than 35 dB below that of the Concorde sonic boom.These The effect of weight alone is shown in Fig.6 by the line connecting levels are at or below conversation-level acoustics.Such signatures three N-wave vehicles:HSCT,Concorde,and SBJ. are better characterized as sonic puffs rather than as sonic booms
768 HENNE Fig. 5 Random sample of small civil aircraft operation. compatible.1,6 In this context, environmentally compatible means effectively addressing the following: 1) sonic boom, 2) engine exhaust emissions, and 3) airport noise. A feasible design must reduce the configuration’s sonic boom signature such that supersonic flight over land is acceptable to the public. As discussed earlier, the market research studies have repeatedly confirmed that supersonic overland flight is of critical importance to the value of the vehicle.1,3,5 The design must also minimize adverse atmospheric effects due to engine exhaust emissions throughout all operations. Emissions must be minimized during both low-speed, low-altitude airport operations as well as high-altitude supersonic cruise operations. Furthermore, a successful QSJ must be a quiet, good neighbor to the airport environment. It should make no more noise than today’s quiet small subsonic civil jets. Sonic Boom The effort to suppress sonic boom successfully has several major aspects. First, the vehicle aerodynamic design is used to shape the sonic boom signature. Shaped signatures are defined by the prevention of coalescence of the signature into an annoying Nshaped pressure wave. Second, psychoacoustic testing is being used to establish signature levels and desired signature shapes that are considered environmentally acceptable. Third, a flight demonstration will be required to prove boom suppression acceptability. A flight demonstration will provide a foundation of scientific data necessary for regulation of supersonic overland flight, as well as risk reduction for the business decision to launch a production program. When the physics of sonic boom are considered,9 it is very clear that a small aircraft has a profound advantage over large aircraft. The size and weight of a vehicle has a first-order effect on the strength of the sonic boom signature. As the vehicle weight decreases, the sonic boom disturbance is decreased. Careful attention to the physical shape of the vehicle can result in a shaped ground signature rather than a coalesced N wave. Shaped ground signatures avoid the large abrupt pressure increases associated with beginning and end of a typical sonic boom N wave. Shaped signatures have lower initial and final pressure increases and are perceived as substantially quieter due to their more-sinusoidal, very-low-frequency character. Shaped signatures can be thought of as precoalesced signatures designed to a special wave shape. Figures 6 and 7 show the impact of vehicle size and shaping on the sonic boom ground signature. These charts show data for a 300-passenger high-speed civil transport (HSCT), the Concorde, a generic supersonic business jet (SBJ), and two QSJ configurations. The effect of weight alone is shown in Fig. 6 by the line connecting three N-wave vehicles: HSCT, Concorde, and SBJ. Fig. 6 Sonic boom initial overpressure strongly affected by vehicle weight. Gulfstream has defined several unique airplane configuration details that reduce the initial overpressure and enhance the vehicle designer’s ability to control the shape of the ground signature produced.10,11 Results for several of these configurations are indicated in Figs. 6 and 7. Several of these design concepts have been validated in recent wind-tunnel testing conducted at NASA Langley’s 4 Foot Unitary Wind Tunnel. As indicated in Fig. 8, the data for the advanced boom suppression concepts show very promising signature characteristics. Based on these computational fluid dynamics (CFD) and wind-tunnel test results, initial overpressures of less than 0.2 psf and peak overpressures of 0.5–0.6 psf seem quite achievable. Clearly, additional model and flight testing are needed to develop, validate, and fully exploit sonic boom suppression technology. This aerodynamic technology need is a long-standing aeronautical barrier that has not yet been adequately addressed. Sonic boom strength for configurations with signatures such as those in Figs. 7 and 8 have been analyzed in terms of perceived level of noise in decibels (PLdB) and A-weighted noise in decibels [dB(A)]. The results shown in Fig. 9 show a remarkable improvement in the cruise signature acoustic strength. Results for configurations shown in Fig. 9 indicate that the noise levels can be reduced more than 35 dB below that of the Concorde sonic boom. These levels are at or below conversation-level acoustics. Such signatures are better characterized as sonic puffs rather than as sonic booms
HENNE 769 3.0 ●HSCT/2.4W57KW190 2.5 -Concorde/2.0M/49K/243 ☐-SBJ/1.8W55K/333 01510 ■QSJ/1.8W55K/333 ●-QSJ++H1.8W55KW333 1.0 -1.5 -2.0 -2.5 50 0 50 100 150200 250300 350 400 Time(msec) Fig.7 Ground boom signature progression with vehicle size reduction and advanced shaping concepts DARPA Initial Wind Tunnel Test Data-Symbols Overpressure Goal CFD Calculation-Lines 。 0.4 1.8M 0.2 耍 0.0 0.2 Low Boom Config 1 -0.4 Low Boom Config 2 -0.6 Excellent Correlation&Validation of Boom Suppression Concepts -0.8 0.0 X/Body Length 1.0 2.0 Gulfstream Wind Tunnel Models in NASA Langley UPWT Fig.8 Advanced boom suppression concept testing results QSJ Advanced+>35dB Quieter Than Concorde Engine Exhaust Emissions Engine exhaust emissions are another environmental concern that 口PLdB must be addressed responsibly.In the airport environment there are ■dB(A) regulations that can be used as design goals.These regulations limit oxides of nitrogen(NOx),unburned hydrocarbons,carbon monox- >35dB Reduction ide,and smoke emissions.These existing limits are well under- dB(A) stood and currently considered well within the QSJ design capa- bility.Unfortunately,this clearly defined situation is not the case Traffic for engine emissions at high-altitude cruise conditions.There are no current regulations or standards for cruise emissions.This ab- Talking sence of current regulations,however,does not mean the issue can be neglected.In fact,a lack of understanding about cruise emis- sions has been a key argument against previous supersonic transport programs.6 The difficulty in early programs was lack of credible un- SCT derstanding of atmospheric science.The absence of such knowledge Configuration left the door open for wild and exaggerated claims of atmospheric trauma based on speculation,misinformation,and political agen- Fig.9 Progress in lowering sonic boom strength. das.The current state of knowledge has advanced sufficiently to
HENNE 769 Fig. 7 Ground boom signature progression with vehicle size reduction and advanced shaping concepts. Fig. 8 Advanced boom suppression concept testing results. Fig. 9 Progress in lowering sonic boom strength. Engine Exhaust Emissions Engine exhaust emissions are another environmental concern that must be addressed responsibly. In the airport environment there are regulations that can be used as design goals. These regulations limit oxides of nitrogen (NOx), unburned hydrocarbons, carbon monoxide, and smoke emissions. These existing limits are well understood and currently considered well within the QSJ design capability. Unfortunately, this clearly defined situation is not the case for engine emissions at high-altitude cruise conditions. There are no current regulations or standards for cruise emissions. This absence of current regulations, however, does not mean the issue can be neglected. In fact, a lack of understanding about cruise emissions has been a key argument against previous supersonic transport programs.6 The difficulty in early programs was lack of credible understanding of atmospheric science. The absence of such knowledge left the door open for wild and exaggerated claims of atmospheric trauma based on speculation, misinformation, and political agendas. The current state of knowledge has advanced sufficiently to