PEN00057

D7. ASSESSMENT OF SONIC-BOOM PROBLEM FOR

FUTURE AIR TRANSPORT VEHICLES

By Donald D. Baals and Willard E. Foss, Jr.

NASA Langley Research Center Langley Station, Hampton, Va.

Presented at the Seventieth Meeting of the Acoustical Society of America

St. Louis, Missouri November 3-6, 1965

D7. ASSESSMENT OF SONIC-BOOM PROBLEM FOR

FUTURE AIR TRANSPORT VEHICLES

By Donald D. Baals and Willard E. Foss, Jr. NASA Langley Research Center

ABSTRACT

The economic feasibility of the supersonic transport is critically

dependent upon specification of the sonic-boom requirements. Aircraft

design techniques and modifications to the mission profile are evaluated

relative to the sonic-boom intensity. An intermediate-range, domestic,

supersonic transport optimized from considerations of the sonic boom is

analyzed relative to potential boom levels. Future air vehicles, such as

the hypersonic transport and ballistic transport, are discussed relative

to their sonic-boom characteristics.

INTRODUCTION

The introduction of the supersonic transport into commercial air

service promises a major technical advance as great as that provided by

the advent of the subsonic jet transport less than a decade ago. The

commercial feasibility of the supersonic transport, however, is closely

associated with the factor of the sonic boom in its relation to the design

and operation of the aircraft and in the field of public acceptance. An

intensive research effort over the last several years has provided a

basic understanding of the generation of the sonic-boom pressure field

and its propagation through the atmosphere, and some knowledge of the

associated structurai and community response. The sonic-boom problem is

a very broad systems problem covering many sociological as well as

technological considerations. The purpose of this report is to assess

our understanding of the sonic-boom effects in relation to the national

program of the supersonic transport and relative to future air transport

vehicles.

I. AIRCRAFT SPEED SPECTRUM

The place of the supersonic transport in the overall transport speed

spectrum is illustrated in Fig. 1, which shows the cruise speed in

relation to the date of introduction of the aircraft. The major milestones

along the way include the IX:-3 airplane in the 1930's; the IX:-6 and the

Constellation in the 1940's; and the initiation in 1958 of U.S. commercial

jet transports by the DC-8 and the Boeing 707. The next milestone on the

horizon will be the supersonic transport. Later, hypersonic-cruise air­

craft are envisioned as flying in the Mach 6 to 12 range, to be followed

perhaps by recoverable boosters and ballistic and orbital transports.

II. FLIGHT EFFICIENCY

One of the major elements in assessing the economic feasibility of

transport aircraft is the so-called "range factor," which is the product

of Mach number and lift-drag ratio divided by the engine specific fuel

consumption, or M(L/D)/SFC. For a given fuel fraction, aircraft range is

directly proportional to this range factor. Figure 2 is a plot representing

2

the range factor as a function of Mach number for the speed spectrum from

the subsonic jet to the supersonic transport. Note the high level of the

present subsonic jets. The operational efficiency of our present super-

sonic fighters and bombers is seen to be but a fraction of that for current

subsonic transports. It will be noted that the flight efficiency of the

proposed supersonic transport represents a tremendous increase over

present operational supersonic aircraft and at the higher Mach numbers

approaches the level of present subsonic jets. This increase in flight

efficiency at supersonic speeds has been brought about by significant

advances in the aerodynamics of high-speed flows and the development of

lightweight high-temperature gas turbines. These largely unrecognized

major technical advances at supersonic speeds result in a new level of

dtRJ.l.zr flight efficiency, with sF nt £18 £ tn aircraf~operating costs as Sv.? I?R\c.R.. l::o i I .m ~ those of present subsonic jets at ranges of 1000 miles and

beyond. These technical advances provide a firm basis for the supersonic

transport and portend an economically sound venture.

III. MISSION PROFJLE

To assess the supersonic transport from the standpoint of the sonic

boom one must comprehend the varied requirements of the mission profile.

Figure 3 represents a typical transoceanic mission profile of altitude as

a function of range. The supersonic transport must take off from existing

commercial airports and climb to an altitude of approximately 40 000 feet

at subsonic speeds. At this altitude, which is determined by considerations

of the sonic boom and engine thrust capability, the aircraft will start

3

its acceleration and climb to supersonic speeds. It is in this transonic

speed range that one of the most critical sonic-boom problems is found, for

this region represents the lowest altitude at which supersonic ~light occurs.

An alleviating factor, however, is the relatively localized ground area

affected by the sonic boom. The second sonic-boom problem area is found

at the start of cruise (about 65 OOO-foot altitude), for this flight period

represents the lowest altitude and heaviest weight for the supersonic

transport during the cruise Fortion of its flight. Aircraft descent is

initiated with subsonic speed being attained at an altitude of 55 000 to

60 000 feet. The aircra~t then continues its descent to its destination

airport, where for design purposes the aircraft is required to carry

suf~icient fuel to be diverted to a 300-mile alternate airport, hold for

1/2 hour at subsonic speeds, and then land. For a typical 4000-mile

mission the payload of the aircra~t will represent about 8 to 10 percent

of the take-off gross weight; the fuel will comprise about 50 percent.

It is within the restraints of this typical supersonic-transport mission

pro~ile that the ~actors of the sonic boom must be weighed and considered.

IV. INFLUENCE OF SONIC BOOM ON DESIGN AND OPERATION

The influence of the sonic boom on the design and operation of the

supersonic transport is shown in Fig. 4, which represents the sonic-boom

problem in climb or in cruise, depending on the scales assigned. In the

plot at the left the shaded area illustrates the variation in airplane

weight as a function of design overpressure for various airframe-engine

co~igurations. The large increases in weight necessary to meet lower

4

levels of sonic boom are the result of the increased engine and wing size

required for flight at higher altitudes plus the additional fuel for non­

optimum flight. Because increased aircraft weight of itself tends to

increase the sonic-boom intensity, such an approach quickly becomes self­

defeating. Since the current goals of the national SST program fall within

the vertical leg of the shaded area, careful design is required to achieve

overpressures equal to or less than the goals without encountering

excessive weight penalties. The most efficient design will tend to fall

in the lower left bound. Note that if the required overpressure ~ is

set unnecessarily low, such a specification could effectively preclude

the development of any supersonic transport.

When an airplane is developed and delivery is made to the airlines,

the operator is faced with the problem illustrated at the right of Fig. 4.

The airplane range (or payload, since the two are related) is very sensitive

to operational overpressure limitations. The maximum sensitivity again

falls near the current overpressure design goals. If adverse community

reaction encountered in initial operation proves to be greater than

anticipated, altering operational procedures to reduce sonic-boom levels

could result in serious or even prohibitive reductions in supersonic

range. On the other hand, if higher overpressures proved tolerable, then

the aircraft will have been unnecessarily compromised by the size and

weight increases required to meet arbitrary sonic-boom limitations.

The establishment of a definitive boom limit for design purposes is

complicated by various unknowns in prediction of the ground overpressures

from the supersonic transport. Such factors as aircraft maneuvers,

5

atmospheric variables, and local reflections are not under control of the

aircraft designer; hence, their design effects can be considered only on

a statistical basis.

Further, it might be argued that maximum overpressure is not in itself

the basic criterion and that the shape of the sonic-boom signature, or its

impulse, or the time between peaks, should be considered. The setting of

definitive levels of sonic boom has a parallel in the current problem of

jet-engine noise. After almost a decade of experience with commercial jet

aircraft, there is still no agreement as to tolerable levels of noise nor

how the noise levels should be defined. Under such circumstances it must

be recognized that any design constraints on the level of sonic boom

produced at the ground are to some extent arbitrary. The present sonic­

boom goals for domestic aircraft of ground-reflected overpressure of

2 Ib/sq ft during transonic acceleration and 1.5 Ib/sq ft at the start of

supersonic cruise were set more than 2 years ago. They represented at

that time about the lowest levels which could be specified without major

compromise in aircraft performance. Although it is believed that these

specifications are adequate from the standpoint of community reaction and

structural response, as of today this is not assured. Neither are there

Gufficient definitive data currently available to warrant a change.

v. BOOM MINThITZATION

Several methods for sonic-boom minimization are available to the

aircraft designer and the operator. From an operational standpoint one

of the most obvious procedures might be to limit the transport to

6

supersonic speeds over water only. Under such limitations the supersonic

transport would accelerate, cruise, and decelerate over water and fly at

subsonic speeds inland. There are serious objections, however, to such an

approach. The supersonic transport would have to forego the potential of

the long-range domestic traffic market, which might represent from 50 to

75 percent of the total SST market. In addition, there would be increased

flight time and a resulting increase in direct operating costs.

Another approach to minimizing the effect of the sonic boom would be

to consider bypassing populated areas in domestic overland operation.

Recent studies indicate that up to a point community complaints and damage

claims can be reduced substantially by circuitous routing with but

relatively small increases in direct operating costs. However, drastic

rerouting will result in sharp increases in the operating costs and in

flight times. Also, from an operational standpoint, one would expect

commercial operational procedures to provide for a minimum of maneuvers

to prevent the occurrence of superbooms from accelerated flight.

From a design standpoint one of the obvious steps to be taken to

minimize the sonic boom would be optimization of the volume and lift

distribution of the aircraft configuration. Previous papers have

discussed the potential of this approach relative to ttneartt and ttfar"

field conditions. Studies of supersonic transport configurations have

indicated a spread of as much as 25 percent for configurations designed

without consideration of sonic boom as compared with near-optimum designs.

Since theoretical procedures for estimating the level of sonic boom for

7

generally quiescent atmospheric flight conditions are well in hand, the

aircraft designer has the necessary tools to optimize the compromises in

sonic boom and aircraft performance.

One question closely related to configuration optimization is whether

or not some "far out" concepts might be devised for complete elimination

of the sonic boom. The shock wave, however, appears to be fundamental to

supersonic flight, and attempts toward shock-wave elimination to date have

been discouraging. Fig. 5 illustrates some interesting, but to date

impracticable, approaches to the sonic-boom problem. Configuration A is

a modification of the well-known Busemann biplane concept where for one

specified Mach number at zero lift the wave drag can be made zero with no

external shock waves. No satisfactory solution for the lifting case

attempted here, however, has yet be_~~ devised. The ring wing of configu- =

ration B is essentially the Busemann wing wra.pped around a contoured

fuselage. It, too, is ineffective for the lifting case.

Configuration C is an intriguing approach. Envisioned here is a

large supersonic transport flying at altitudes low enough for the ground

to be within the "near field. II The volume and lift distribution have

been optimized so that the shape of the sonic-boom signature would approach

that of a sine wave. Bioacoustic experience suggests that such a signature

should greatly decrease the apparent loudness. A preliminary analysis,

however, indicates that the aircraft would have to be approximately 500 feet

long and fly at an altitude of about 40 000 feet, and it would be limited to

relatively low supersonic Mach numbers. There would be no significant

reduction in impulse relative to far-field N-wave signature.

8

A point to be made from this discussion is that any solutions to the

sonic-boom problem, if they are to be effective, must fall within the

constraints of a practicable supersonic-transport configuration. In the

forms considered here the three approaches of Fig. 5 may be impracticable,

but perhaps elements of these concepts may be applicable to the supersonic

transport in some form. For example, the major acoustic benefits of the

so-called "sine wave concept II might be realized by making a more modest

configuration change, designed to produce a rise time intermediate between

that of the far-field N-signature and the sine wave. Studies of such

"way out" concepts should be continued, but for the present the only

apparent course of action available to the designer is to refine the

supersonic transport in the light of our established technology.

Another approach to boom minimization would be to design the super­

sonic transport to operate at high altitudes. The potential from this

source has been discussed in the paper by F. E. McLean and B. L. Shrout

and will be considered subsequently.

The level of sonic boom may also be lowered by reducing aircraft

gross weight and size. This approach represents a major challenge to

the designer and is a desirable goal regardless of the effects on sonic

boom. Important reductions in the level of sonic boom can be made by

improvements in the aerodynamiC, propulsion, and structural efficiency or

by refinement of operational techniques. Thus, advances in the afore­

mentioned areas for a given mission requirement can result in reduced

gross weight or perhaps higher operational speed and altitude. These

characteristics in turn will be reflected in lower levels of sonic booms

9

on the ground. Key items in improvement of aircraft efficiency are the

lift-drag ratio, fuel reserves, structural efficiency, engine thrust-

weight ratio, and specific fuel consumption. A 5 percent improvement in

each of these individual items could add up to as much as a 25 to 30 per-

cent reduction in gross weight of the aircraft. Thus, as the design of

the aircraft improves in efficiency, there could be a corresponding

reduction in level of the sonic boom.

The ~f gross weight and size of the supersonic transport

relative to the sonic boom logically leads to the concept of a small,

lightweight domestic SST. The initial emphasis of the national program

on the long-range transoceanic aircraft is logical, for the area of

greatest initial demand will be found at the longer ranges where increased

speed offers the greatest time savings. It should be recognized, however, . --- ---------- -

that the resulting level of sonic boom for these transoceanic designs may

be undesirably high for routine domestic day and night operation.

Results of a preliminary analysis of a domestic supersonic transport

concept are illustrated in Fig. 6, wherein cruise overpressure is plotted

against range for two different types of aircraft having essentially the

same payload. The upper curve is representative of current transoceanic

designs wherein fuel is off-loaded from the aircraft for the shorter

ranges. The resulting lightening of the aircraft leads to lower sonic-

boom overpressures for the shorter ranges. If, however, an aircraft should

be optimized for a specific shorter range with sonic-boom reduction as

the overriding consideration, then the lower curve would result. It is

10

noted that for domestic ranges sonic-boom overpressures approaching

1 lb/sq ft might be attainable.

The design rules for such a special domestic transport, however, are

very restrictive. The lower curve is based on the assumption of a high

supersonic cruise efficiency to reduce the aircraft fuel consumption and

resulting gross weight. Large engines (same size as for transoceanic

operation) are incorporated to provide for cruise at high altitudes.

Further, the aircraft volume and lift distribution has been made optimum

insofar as possible from the standpoint of near- and far-field sonic-boom

characteristics. Under these design conditions, the lower curve should be

attainable. This analysis suggests that a domestic supersonic transport,

designed primarily from considerations of the sonic boom, could be an

effective solution to the sonic-boom problem. The concept of a small

domestic SST would be consistent with the composition of present subsonic

jet fleets, where we now have aircraft designed solely for transoceanic

ranges, and specially designed aircraft for purely domestic missions.

VI. FUTURE FLIGHT VEHICLES

At this time let us consider some of the future transport vehicles

beyond the currently conceived supersonic transport. Fig. 7 is a pro­

jection of aircraft flight efficiency plotted as a function of cruise

Mach number for two types of fuel - present JP and liquid hydrogen.

Note that the flight efficiency term includes the factor of lift-drag

ratio as well as propulsion efficiency. For JP fuel, cruise Mach

numbers are shown to extend up to about 4 1/2. Beyond this speed, it is

11

anticipated that major problems will occur in temperature stability of JP

fuels, and that structural and environmental cooling requirements may

dictate a heat-sink capacity beyond that available with noncryogenic fuels.

The lower band of flight efficiency for JP-fueled aircraft corresponds

to the current level shown in Fig. 2. The upper bound (advanced JP­

fueled aircraft) indicates the general level believed to be attainable on

the basis of current research results. The largest gains are noted in

the high subsonic speed range where application of newly developed

"supercritical" airfoil and aerodynamic interference technology combined

with the propulsion efficiency of the high-temperature, high-bypass-ratio

turbofan (as exemplified by the C-5A engines) portends a whole new level

of flight efficiency. It is this second generation of subsonic jet

transports cruising at speeds ~ below Mach 1 which will provide the

competition-fOr the supersonic transport. Since these hIgh:iy efficient

subsonic aircraft, of course, have no sonic boom problems, there is

little room for complacency for the SST proponents.

The marked drop in flight efficiency at transonic speeds is believed

to preclude the development of a "transonic transport" designed to

cruise at speeds just below the sonic-boom cut-off Mach number (M ~ 1.15).

Considerations of winds aloft and nonstandard atmospheric conditions

could reduce the cruise speed to a value only slightly supersonic. Under

such conditions the "transonic transport" appears to be effectively

squeezed from the spectrum of future transport vehicles.

Later in the development cycle there may be a second generation of

supersonic transports designed to cruise in the speed range of Mach 4 or

12

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slightly higher. The limiting speed will be determined mainly from

consideration of the propulsion system - its efficiency and reliability

at the high operating temperatures as well as the thermal stability of

the fuel.

As cruise flight speeds progress into the hypersonic speed range

(that is, at Mach numbers above about 5) liquid hydrogen will become the

dominant fuel because of its high heating value and heat-sink capacity.

It will be noted that the range factors at Mach 6 to 8 are superior to

all but the advanced JP-fueled subsonic aircraft. This combination of

large range factors with hypersonic speeds is of sufficient potential to

warrant continued study of hypersonic flight within the atmosphere.

Should liquid hydrogen become a practicable fuel for commercial

aircraft, one would estimate about a 2 1/2-fold increase in range factor

over their JP-fueled counterpart. This increased range factor could be

reflected in either increased range, or reduced gross weight for a given

range. If the latter direction should be followed, there would be a

favorable effect on sonic-boom overpressure by virtue of a smaller and

lighter transport.

Beyond the hypersonic cruise aircraft, the orbital vehicle - once

it is in orbit - has an essentially infinite range factor. The orbital veh;de.-

vehicle and its shorter range counterpart, the ballistic ~, operate

basically outside the sensible atmosphere and therefore present no sonic-

boom problem except during the launch and terminal phases. The resulting

local sonic-boom intensities will be governed largely by the exit and

entry trajectories employed.

13

Figure 8 swnmarizes in a qualitative 'tray the cruise sonic-boom

characteristics of the spectrum of supersonic/hypersonic cruise vehicles.

Note that there is a characteristic increase in cruise altitude with

increasing flight speed, which is associated with the necessity for

maintaining the cruise lift coefficient for maximum lift-drag ratio. The

progressive decrease of sonic-boom overpressure with increasing flight

speeds is primarily an altitude effect. As a result, cruise sonic-boom

levels appear to be less critical for hypersonic aircraft than for

currently projected supersonic cruise aircraft of the same weight. Over­

pressure levels are comparable to those projected for proposed domestic

SST. The lateral spread of sonic boom on the ground, however, will be

somewhat greater for the higher flying hypersonic aircraft, since the

lateral spread tends to be proportional to the flight altitude. The

transonic acceleration phases will still=15e I3:\!rrft1:!al~ro15lem for

hypersonic aircraft, however, with the magnitude of the problem largely

determined by the characteristics of the propulsion system selected for

the mission.

VII. CONCLUDING STATEMENT

In summary, the potential of commercial supersonic flight is rapidly

approaching reality, but the feasibility of such flight is closely

associated with the factor of the sonic boom in the design and operation

of the aircraft. The problem of sonic boom is as inherent as the shock

wave to supersonic flight, and there appears to be no simple solution.

However, the scope of technical progress holds promise of development of

14

an aircraft which will be publicly acceptable and economically viable.

O-=~L~ To attain such a goal, however, the sonic boom must be .. a1 in

the mission specification, airframe/engine design, and flight operation.

Anything less could seriously jeopardize the future of commercial

supersonic flight.

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