Date:   Sun Jul 03 1994  14:29:00
From:   John Powell
To:     All
Subj:   NASA and Sonic Booms
BAMA                           -------------------------------

 Space Shuttles and Sonic Booms

	One of the most distinctive features associated with the return of
Space Shuttles from orbital missions are the twin sonic booms that
herald their supersonic arrival back into the atmosphere.

	Like all other vehicles traveling faster than the speed of sound, the
orbiters produce shock waves as they pass through the atmosphere. On the
ground, these shock waves are heard much like sharp thunderclaps.

	Most people expected to hear a sonic boom when the orbiter Columbia
soared back into the atmosphere for the first Space Shuttle landing Apr. 14,
l981, at NASA's Dryden Flight Research Facility, Edwards, CA. Single
booms were heard when the Mercury, Gemini, and Apollo spacecraft reentered
the atmosphere. The double sonic booms are now distinctive signatures of
each Space Shuttle landing.

       The Cause

	Sonic booms are created by air pressure.  Much like a boat pushes
up a bow wave as it travels through the water, a vehicle pushes air
molecules aside in such a way they are compressed to the point where
shock waves are formed.

	These shock waves form two cones, at the nose as well as at the
tail of the vehicle.  The shock waves move outward and rearward in all
directions and usually extend to the ground.  As the shock cones spread
across the landscape along the flightpath, they create a continuous
sonic boom along the full width of the cone's base.  The sharp release
of pressure, after the buildup by the shock wave, is heard as the sonic
boom.

	The nose and tail shock waves are usually of similar strength.  The
time interval between the nose and tail shock waves is primarily
dependent on the size of the aircraft and its altitude.  Most people on
the ground cannot distinguish between the two and they are usually
heard as a single sonic boom.  As the time interval increases, two
booms are heard.  A small fighter-type aircraft about 50 ft long will
generate nose and tail shock waves of less than a tenth of a second
(0.1 sec).  The ear usually detects these as a single sonic boom.

	The interval between nose and tail shock waves on the Space
Shuttles, which are 122 ft long, is about one-half of a second (0.50
sec), making the double boom very distinguishable.

      General Factors Associated With Sonic Booms

	There are several factors that can influence sonic booms -- weight,
size, and shape of the aircraft or vehicle, plus its altitude, attitude
and flight path, and weather or atmospheric conditions.

	A larger and heavier aircraft must displace more air and create
more lift to sustain flight, compared with small, light aircraft.
Therefore, they will create stronger and louder sonic booms than
smaller, lighter aircraft.  The larger and heavier the aircraft, the
stronger the shock waves will be.

	Altitude determines the distance shock waves travel before reaching
the ground, and this has the most significant effect on intensity.  As
the shock cone gets wider, as it moves outward and downward, its
strength is reduced.  Generally, the higher the aircraft, the greater
the distance the shock wave must travel, reducing the intensity of the
sonic boom.  Of all the factors influencing sonic booms, increasing
altitude is the most effective method of reducing sonic boom intensity.

	The width of the boom "carpet" beneath the aircraft is about one
mile for each 1000 ft of altitude.  An aircraft, for example, flying
supersonic at 50,000 ft will produce a sonic boom cone about 50 miles
wide.  The sonic boom, however, will not be uniform.  Maximum intensity
is directly beneath the aircraft, and decreases as the lateral distance
from the flightpath increases until it ceases to exist because the
shock waves refract away from the ground.  The lateral spreading of the
sonic boom depends only upon altitude, speed, and the atmosphere -- and
is independent of the vehicle's shape, size, and weight.

	The ratio of aircraft length to maximum cross sectional area also
influences the intensity of the sonic boom.  The longer and more
slender the aircraft, the weaker the shock waves.  The fatter and more
blunt the vehicle, the stronger the shock wave can be.

	Increasing speeds above Mach 1.3 results in only small changes in
shock wave strength.

	The direction of travel and strength of shock waves are influenced
by wind, speed, and direction, and by air temperature and pressure.  At
speeds slightly greater than Mach 1, their effect can be significant,
but their influence is small at speeds greater than Mach 1.3.
Distortions in the shape of the sonic boom signatures can also be
influenced by local air turbulence near the ground.  This, too, will
cause variations in the overpressure levels.

	Aircraft maneuvering can cause distortions in shock wave patterns.
Some maneuvers -- pushovers, acceleration, and "S" turns -- can amplify
the intensity of the shock wave.  Hills, valleys, and other terrain
features can create multiple reflections of the shock waves and affect
intensity.

      Overpressure

	Sonic booms are measured in pounds per square foot of overpressure.
This is the amount of the increase over the normal atmospheric pressure
which surrounds us (2,116 psf/14.7 psi).

	At 1 lb overpressure, no damage to structures would be expected.

	Overpressures of 1 to 2 lb are produced by supersonic aircraft
flying at normal operating altitudes.  Some public reaction could be
expected between 1.5 and 2 lb.

	Rare minor damage may occur with 2 to 5 lb overpressure.

	As overpressure increases, the likelihood of structural damage and
stronger public reaction also increases.  Tests, however, have shown
that structures in good condition have been undamaged by overpressures
of up to 11 lb.

	Sonic booms produced by aircraft flying supersonic at altitudes of
less than 100 ft, creating between 20 and 144 lb overpressure, have
been experienced by humans without injury.

	Damage to eardrums can be expected when overpressures reach 720 lb.
Overpressures of 2160 lb would have to be generated to produce lung
damage.

      Sonic Boom Footprints

	Overpressures recorded on the ground during the landing of the
orbiter Discovery on mission STS-26 Oct.  3, 1988 revealed that the
intensity was 1.06 lb in the Santa Barbara area as Discovery crossed
the coastline at a speed of Mach 4.37 at an altitude of 115,400 ft.
Intensity rose to 1.15 lb in the Santa Clarita Valley area, 45 miles
southwest of Edwards, as the vehicle's speed and altitude reduced.  At
Edwards, when Discovery was about 60,000 ft overhead just moments
before the landing the overpressure was 1.25 lb.  The highest reading
during the landing approach over Southern California was 1.75 lb in the
areas of Palmdale and Lancaster 20 to 30 miles southwest of Edwards.

	Typical overpressure of other aircraft types are:


	    SR-71: 0.9 lb, speed of Mach 3, 80,000 ft
	    Concorde SST: 1.94 lb, speed of Mach 2, 52,000 ft
	    F-104: 0.8 lb, speed of Mach 1.93, 48,000 ft


       =======================================================

     Don Nolan
     Dryden Public Affairs Office
     (805) 258-3447
     Don_Nolan@qmgate.dfrf.nasa.gov