Sonic booms generated by the X-15 airplane flying at Mach numbers of 3.5 and 4.8 were measured. The experimental results agreed within 12 percent with results obtained from theoretical methods. No unusual phenomena related to overpressure were encountered. Scaled data from the X-15 airplane for Mach 4.8 agreed with data for an SR-71 airplane operating at lower Mach numbers and similar altitudes. The simple technique used to scale the data on the basis of airplane lift was satisfactory for comparing X-15 and SR-71 sonic boom signatures

Green, K. S.

Putnam, T. W.

English

NASA Technical Reports Server

N A S A  T E C H N I C A L  NASA TM X-3126 
M E M O R A N D U M  
* 
PY 
Pw 
MEASUREMENTS OF SONIC BOOMS 
GENERATED BY AN AIRPLANE 
FLYING AT MACH 3.5 AND 4.8 
by Karen S. Gwm and Terrill W. Putrtant 
Flight Research Center 
Edwards, Calif: 93523 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. OCTOBER 1974 
https://ntrs.nasa.gov/search.jsp?R=19740026373 2020-03-23T02:30:39+00:00Z
MEASUREMENTS OF SONIC BOOMS GENERATED BY AN 
AIRPLANE FLYING AT MACH 3.5 AND 4.8 
Karen S . Green and Terrill W .  Putnam 
Flight Research Center 
INTRODUCTION 
Since the advent of supersonic flight, sonic booms have become an increasingly 
serious problem. Studies of a community's tolerance to frequent sonic booms 
(refs. 1 and 2) revealed that lower overpressures than expected elicited strong ob- 
jections from the populace. 
The proposed space shuttle will reenter the earth's atmosphere at high super- 
sonic speeds and will generate a sonic boom while passing over populated areas. 
Sonic booms generated by aircraft flying at speeds up to Mach 3 have been meas- 
ured and evaluated (ref. 3), but no sonic boom data are available for airplanes fly- 
ing at higher speeds. Although measurements were made of sonic booms generated 
at Mach numbers up to 16 during lift-off and reentry of the Apollo spacecraft (ref. 4) , 
the vehicle had a blunt-shaped body quite unlike that of the proposed space shuttle. 
Consequently, sonic boom data are needed for a slender-body vehicle that operates 
at Mach numbers greater than 3 to increase confidence in the predicted sonic boom 
characteristics of the space shuttle for altitudes below 30,000 meters. 
This report examines sonic boom overpressure signatures generated by the 
X-15 rocket-powered airplane during one flight at high altitudes and Mach numbers 
of 3.5 and 4.8. The measured sonic boom overpressures and the results obtained 
from theoretical methods of estimating overpressure are compared. 
SYMBOLS 
Physical quantities in this report are given in the International System of 
LTnits (81) . Measurements were taken in U .S . Customary Units. Factors relating 
the two systems are given in reference 5. 
d diameter, meters 
ground reflection factor 
airplane volume-shape factor 
airplane lift-shape factor 
airplane length, meters 
airplane wing mot-chord length, meters 
Mach number 
ambient pressure at altitude, pascals* 
ambient pressure at ground level, pascals 
calculated pressure rise across shock wave at ground level based on air- 
plane lift, pascals 
calculated pressure rise across shock wave at ground level based on air- 
plane lift and volume, pascals 
measured pressure rise, paacala 
measured pressure rise scaled to SR-71 airplane, pascals 
calcukted pressure rise across shock wave at ground level based on air- 
plane vdume , pascals 
distance from measuring station perpendicular to flightpath, meters 
airplane lift force, newtone 
TEST AIRPLANE 
The X-15 airplane was a rocket-powered research vehicle designed to attain 
hypersonic speed and altitudes in excess of 76,200 meters. Figure 1 is a three-view 
drawing of the X- 15-3 airplane, which was used in this study. The overall length 
of the airplane was 15.1 meters, and the wingspan was 6.8 meters, The total cross- 
sectional area distribution is shown in figure 2.  The empty gross weight was 
6666 kilograms, and the launch weight was 14,965 kilograms. The airplane is 
described in detail in reference 6. 
*Unit of pressure equivalent to newton per metera (ref. 5).  
TEST CONDITIONS 
Test Area 
The test flight was made over the Edwards Air  Force Base test range. The 
ground track is shown in figure 3. The terrain in which measurements were taken 
was generally flat with no vegetation, as shown in figure 4. Goldstone Dry Lake, 
elevation 934.5 meters, and Cvddeback Dry Lake, elevation 792.5 meters, were 
choeen as measuring sites because of their large, flat, reflecting surfaces of hard- 
packed, sandy cley and their accessibility from the NASA Flight Research Center. 
Goldatone Dry Lake is 100.9 kilometers and Cuddeback Dry  Lake is 54.7 kilometers 
northeast of Edwards Air  Force Base. 
Environmental Conditions 
Meteorological facilities were not available at the measuring sites, so data on the 
environmental conditions for the test area were obtained from the Edwards A i r  Force 
Base weather facility. Atmospheric pressure, temperature, wind speed, and wind 
direction are plotted as functions of geopotential altitude in figures 5(a) to 5(c). 
These soundings were taken near the time of the flight. 
Flight Conditions 
The X-15 airplane was launched from a B -52 airplane at an altitude of approxi- 
mately 13,700 meters and a Mach number of 0.8. The engine burned for about 
97 seconds and was shut down at a Mach number of 4.8 and an altitude of 
21,671 meters. Partial time histories for selected airplane parameters are shown in 
figure 6. The times the sonic booms were generated over the Goldstone rind 
Cuddeback measuring sites are indicated. The times were computed from the air- 
plane Mach number, the geometric relationship between the airplane and the meas- 
urement locations, and the temperature profile between the ground and the flight 
altitude. The sharp decrease in longitudinal acceleration 97 seconds after launch 
indicates engine shutdown and corresponds to the maximum Mach number and alti- 
tude. Angle of attack was maintained at less than 3'. 
The primary flight objective required that a low level of longitudinal accel- 
eration be maintained; thus the speed brakes were extended (fig. 7) and the engine 
was throttled to 50 percent of maximum thrust. The boom received at Goldstone Dry 
Lake was gene1 ated while the airplane was in this configuration. The boom received 
at Cuddeback Dry Lake was generated while the engine was not operating and the 
speed brakes were retracted. 
Pertinent flight conditions at the times the booms were gellarated are summa- 
rized in the following table: 
Condition 
Time after launch, see 
Mach number 
Altitude, rn 
Longitudinal acceleration, g 
Normal aoceferation , g 
Speed broker 
Rocket engine 
C3oldrtone D r y  Lake I Cuddeback D r y  Lake 
85 
4.8 
21,450 
0.1 
1.1 
Extended 40" 
5Opercent thrurt 
129 
3.5 
20,950 
-1.2 
1.0 
Retracted 
Shut down 
The X-15 ground tracks relative to the microphone arrays at Goldstone Dry 
Lake and Cuddeback Dry Lake are shown in figures 8(a) and 8(b) , respectively. 
Although the flight plan called for the airplane to fly directly over the microphone 
arrays, it can be seen that it actually passed 2740 meters south of Goldstone Dry 
i Lake and 12,649 meters south of Cuddeback Dry Lake. 
j 
1 INSTRUMENTATION 
The main elements of the ground systems used for these sonic boom measure- 
ments are described in detail in reference 2 ,  The basic equipment included a 
2.54-centimeterdiameter condenser microphone modified by partially plugging the 
vent hole to extend the low frequency response; an oscillograph recorder; a direct 
current amplifier; a tuning circuit; and a magnetic tape recorder. Frequency 
response was calibrated in the laboratory from 0.02 hertz to 10,000 hertz 22 decibels 
(ref. 0.00002 Pa), and the microphones had a dynamic range from approximately 
70 dedbels to 150 dedbels. Sound pressure level calibrations were made in the 
field with a discrete frequency calibrator. 
The microphone arrays were positioned on the lakebeds as shown in fig- 
ures 9 (a) and 9m). At each site one microphone was mounted on a 6.1-meter high 
pole at the head of the T-shaped array with the diaphregm parallel to the ground. 
The remaining seven microphones were ground plane instruments mounted as shown 
in the figure in~ets .  The microphones at Goldstone Dry Lake were shock-mounted 
(fig. 4) 0.15 meter above the ground with the microphone diaphragm parallel to the 
lakebed surface. The microphones at Cuddeback Dry Lake were mounted with the 
diaphragm at ground level. The two types of mounting arrangements resulted in 
only very small differences in waveform (ref. 1) . 
At Cuddeback Dry Lake the data were recorded directly on the recording oscillo- 
graph. The Goldstone Dry Lake data were recorded first on magnetic tape at 
0.762 meter per second. Then, after the flight, the data were played back from the 
tape and recorded on an oscillograph recorder to obtain a graphic copy of the sonic 
boom signatures. 
The inatrumentation system used in this study was identical to that used in the 
investigation of reference 7,  which established the overall accuracy for this type of 
inatrumentation, considering instrument calibration and measuring and reading 
procedures, as 215 percent. 
RESULTS AND DISCUSSION 
The sonic boom overpressure signatures from Goldstone Dry Lake and Cuddeback 
Dry Lake are shown in figures 10 and 11, respectively, in time sequential order. The 
waveforms are of the N-wave type. The pressure signatures are not directly com- 
parable because of differences in the sensitivity of various channels of the recording 
equipment. The signatures from Cuddeback Dry Lake are more rounded than those 
from Goldstone Dry Lake. This is consistent with other sonic boom data (ref. 3) 
which show that sonic boom signatures become rounder with increasing lateral die- 
tame from the ground track. The sonic boom signatures measured by the pole micro- 
phones at both measuring sites show a step in the initial overpressure rise. The 
first rise in pressure is caused by the pressure from the incident shock wave, and 
the second pressure rise results from the addition of the reflected shock wave to the 
incident shock wave. The overpressures range from 27.8 pascals to 42.1 pascals at 
Goldstone Dry Lake and from 20.1 pascals to 27.3 pascals at Cuddeback Dry Lake. 
.{ 
Theoretical analysis of sonic boom phenomena has indicated that the signature f overpressures can be attributed to airplane volume and lift (ref. 8). Volume has 
been shown to be dominant at low altitudes, and lift, which is dependent on the type 
of aircraft, becomes dominant at altitudes of 14,900 meters and above for vehicles 
similar to the X-15 airplane (ref. 9). The effects of lift and volume on the over- 
pressures were calculated for the present investigation by using equations (1) and 
(2) in the appendix. 
\ The average overpressure measured by the seven ground plane microphones at i 
Goldstone Dry Lake was 34.4 pascals; the theoretical overpressure was 31.4 pascals. 
k I The average overprevsure measured at Cuddeback Dry Lake by the ground plane i 
i microphones was 25.0 pascals; the theoretical overpressure was 28.5 pascals. Thus i 
P I the theoretical and measured overpressures agreed within 12 percent. i I i 
j The Mach 4.8 sonic boom data from the ground plane microphones at Goldstone 
Dry Lake are compared in figure 12 with sonic boom data for an SR-71 airplane 
(ref. 3) . The X- 15 and SR- 'I1 peak overpressures were not directly comparable 
because of differences in the sizes and ;wights of the airplanes; tk erefore, the X-15 
j data were scaled to the weight and wing root-chord length of the SR-71 by using 
, equation (1). The X-15 data are in good agreement with the SR-71 data when only 
' the lift equation was used to male the data. As noted previously, ~t altitudes of 
14,900 meters and above, overpressure8 due to lift dominate in the total ovtr- 1, pnssures for vehicle. with slender bodies, such as the X-15 and the SR-71 air- 
planes. The Mach 3.5 sonic boom data from Cuddeiack Dry Lake were not plotted 
because they were obtained at a greater lateral distance from the ground track than I the SR-71 data. 
CONCLUDING REMARKS 
Sonic boom meamrements were mcade for the X-15 airplane flying at Mach num- 
be- of 9. S and 4.8. The experimental rc~llul!a greed within 12 percent with 
results obtained by using theoretical methods. No unusual phenomena related to 
overprmsure were encountered. Soaled data hu#n the X-15 airplane flying at 
Maoh 4.8 agreed with sonio boom data gemrated by an 8R-71 airplane at lower Mach 
numbera and similar altitudes, The simple technique used to scale the data on the 
bmb of airplane lift waa satisfaotory for comparing X-15 and SR-71 sonic boom 
dgnaturee . 
Flfght Research Center 
Natfonaf Aeronautfccl and Space Acfittfnfstmtion 
Edwomb, Cafff, , September 5, 1974 
APPENDIX 
THEORETICAL CALCULATION OF OVERPRESSURES 
The equation used to calculate the effect of airplane lift on overpressure was as 
foliowe: 
The ground reflection factor, K1, chosen was  1.8, which is the reflection factor for 
a sandy, flat surface (ref. lo). The airplane lift-shape factor, K3, varied for 
afferent lift distributions; however, an average value of 0.55 (ref. 9) was used. An 
drplam wing mot-chord length, Q, , of 3.35 meters was used. The rocket engine 
fuel was nearly exhausted at the time the sonic booms were generated; therefore, 
the empty aircraft weight was uved in conjunction with the normal acceleration to 
arrive at lift forces of 71,925 newtons and 65,386 newtons, which were used in the 
computations of overpressure for the Goldstone and Cuddeback measuring sites, 
reepectfvely . 
The equation used to calculate the effect of airplane volume on overpressure was 
ae follows: 
The airplane volume-shape factor, K2, for an airplane similar to the X- 15 airplane 
is given as 0.645 in reference 10, so that value was used. The overall length of the 
airplane, 2 ,  wm 15. 1 meters. The diameter, d , used in the calculation was the 
diameter of a dmle having an area equal to the maximum cross-sectional area of the 
airplane (ref. 6). The maximum cross-sectional area of the X-15 airplane was 
dependent on the speed brake position. Thus the diameters used in the calculations 
for the Goldstone and Cuddeback sonic booms were 2.38 meters and 2 . O 1  meters, 
respectively. 
The combined effects of lift and volume on the overpressures were determined 
by the procedure presented in reference 11. The equation used was m follows: 
REFERENCES 
I .  Hilton, David A .  ; Huckel , Vera; Steiner , Roy; and Maglieri , Domenic J . : 
Sonic-Boom Exposures During FAA Community-Response Studies Over a 
6-Month Period in the Oklahoma City Area. NASA TN D-2539, 1964. 
2.  Stanford Research Institute: Sonic Boom Experiments at Edwards Air Force 
Base. NSBEO- 1-67, NTIS , U .S . Dep. Com . , July 28, 1967. (Available from 
DDC as AD 655 310 .) 
3. Maglieri , Domenic J . ; Huckle , Vera; and Henderson, Herbert R . : Sonic-Boom 
Measurements for SR-7 Aircraft Operating at Mach Numbers to 3.0 and 
Altitudes to 24 384 Meters. NASA TN D-6823, 1972. 
4. Henderson, Herbert R . ; and Hilton, David A.  : Sonic-Boom Ground Pressure 
Measurements From the Launch and Reentry of Apollo 16. NASA TN D-7606, 
1974. 
5. Mechtly , E . A .: The International System of Units - Physical Constants and 
Conversion Factors. Second Revision. NASA SP- 7012, 1973. 
6. Saltzman , Edwin J . ; and Garringer, Darwin J .: Summary of Full-scale Lift 
and Drag Characteristics of the X-15 Airplane. NASA 1'N D-3343, 1966. 
7. Hubbard, Harvey H . ; Maglieri , Domenic J . ; Huckel , Vera; and Hilton, David A.  
(with appendix by Harry W .  Carlson): Ground Measurements of Sonic-Boom 
Pressures for the Altitude Range of 10,000 to 75,000 Feet. NASA TR R- 198, 
1964. (Supersedes NASA TM X-633. ) 
8. Morris, John: An Investigation of Lifting Effects on the Intensity of Sonic Booms. 
J .  Roy. Aeron. Soc., vol. 64, no. 598, Oct. 1960, pp. 610-616. 
9. Maglieri , Domenic J . ; and Hubbard, Harvey H . : Ground Measurements of the 
Shock-Wave Noise From Supersonic Bomber Airplanes in the Altitude Range 
From 30,000 to 50,000 Feet. NASA TN D-880, 1961. 
10. Maglieri , Domenic J . ; Hubbard, Harvey H . ; and Lansing, Donald L . : Ground 
Measurements of the Shock-Wave Noise From Airplanes in Level Flight at 
Mach Numbers to 1 .4  and at Altitudes to 45,000 Feet. NASA TS D-48, 1959. 
11. Carlson, Harry W .: An Investigation of the Influence of Lift on Sonic-Boom 
Intensity by Means of Wind-Tunnel Measurements of the Pressure Fields of 
Several Wing-Body Combinations at a Mach Number of 2.01. NASA TN D-881, 
1961. 
Ffgure 1 .  Three-~iaw drawing of the X-15-3 afrplane. Shaded 
area denote 8peed brake$. Dlmen8ion8 fn metera. 
Speed brakes extended 4 
Vertical fins and 
horizontal 
Side faring 
4.5 
4.0 
3.5 
3.0 
Airplane 2.5 
cross-sectional 
area, m2 2.0 
1.5 
.5 
l g O  
Figure 2.  X-15-3 cross-sectional area dfstribulion. 
- 
- 
- 
- 
- 
- 
- 
Circular fuselage 
I / I I I I I 1 A. 1 
Rogers 1 
r I W a r d s  Air  
L I I I I I .  
0 2 4 6 8 10 12 14 16 
Fuselage station, m 
Figure 3. X-15 ground trock. 

Geopotential 
altitude, 15 
rn 
Atmospheric pressure, Pa 
( a )  Atmospheric pressure. 
Figure 5 .  Meteorological conditions for the flight. (Duta 
obtained from Edwards Air Force Base weather faci l i ty . )  
Geopotent ial 
altitude, 
m 
Temperature, "C 
(b) Temperature and wind speed. 
Figure 5. Continued. 
0 5 10 
Wind speed, mlsec 
25 - 
20 - 
Geopotent iaf 
altitude, 1s - 
m 
10 - 
5 - 
I 
0 I 
320 
Normal 
acceleration, 
9 
Longitudinal 
acceleration, 
Q 
Angle of attack, 
de9 
Altitude, 
m 
Mach number 
Goldstone boom Cuddeback boom 
r generated generated i I 
Time after launch, sec 
Figure 6. Time histories of selected airplane parameters. 



0 Microphone on 6. I-meter pole 
T-F' 0 Ground plane microphone 
t Magnetic nor th  
Side view I 
30.48 
r-30.48-t30.4 
( a )  Goldstone Dry Lake.  
0 Microphone on 6.1-meter pole 
- n 11 0 Ground plane microphone 
Side view 
t Magnetic nor th  
( b )  Cudaeback Dry Lake.  
Figure 9 .  Microphone arrays  (not drawn to s c a l e ) .  Dimensions in meters .  

1 .-C Is-U v,
0 SR-71dataenvelope, M = 1 . 3 5 t o 3 . 0 ( r e f .  3)  
o X-15 ground microphone scaled data, M = 4 .8  
Altitude, m 
Figure 12. Comparison of  scaled X-15 and S R -  71 ovetSpressur*c 
data measwed within 5 . 5  kilometem of the ground trlack. 


Measurements of sonic booms generated by an airplane flying at Mach 3.5 and 4.8

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