Sonic Booms

The following description of sonic booms comes from the NASA Dryden website.  The NASA Dryden Research Center is located on Edwards AFB, CA where manned level supersonic flight was first achieved over 60 years ago.  Since then, they have continued supersonic testing of many flight vehicles.

Sonic Booms

A sonic boom is the thunder-like noise a person on the ground hears when an aircraft or other type of aerospace vehicle flies overhead faster than the speed of sound or supersonic.

Air reacts like a fluid to supersonic objects. As objects travel through the air, the air molecules are pushed aside with great force and this forms a shock wave much like a boat creates a bow wave. The bigger and heavier the aircraft, the more air it displaces.

The Cause

The shock wave forms a cone of pressurized air molecules which move outward and rearward in all directions and extend to the ground. As the cone spreads across the landscape along the flight path, 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 change in air pressure associated with a sonic boom is only a few pounds per square foot -- about the same pressure change experienced riding an elevator down two or three floors. It is the rate of change, the sudden onset of the pressure change, that makes the sonic boom audible.

All aircraft generate two cones, at the nose and at the tail. They are usually of similar strength and the time interval between the two as they reach the ground 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. Sonic booms created by vehicles the size and mass of the space shuttle are very distinguishable and two distinct booms are easily heard.

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 sonic booms stronger and louder than those of 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, and 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 feet of altitude. An aircraft, for example, flying supersonic at 50,000 feet can 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 flight path 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 one pound overpressure, no damage to structures would be expected.

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

Rare minor damage may occur with 2 to 5 pounds 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 pounds.

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

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

Typical overpressure of aircraft types are:

• SR-71: 0.9 pounds, speed of Mach 3, 80,000 feet
• Concorde SST: 1.94 pounds, speed of Mach 2, 52,000 feet
• F-104: 0.8 pounds, speed of Mach 1.93, 48,000 feet
• Space Shuttle: 1.25 pounds, speed of Mach 1.5, 60,000 feet, landing approach

(Note: The F-22 Raptor falls someplace between the F-104 and the SR-71)

 

 

The following letter, written by a member of the Alamogordo Forum in response to a letter complaining about sonic booms, was printed in the Alamogordo News in the Spring of 2009.  It has been reprinted in numerous areas on the internet.

I'm writing in response to Dory Schuster's letters about her bitter dislike of sonic booms created by military aircraft flying out of Holloman AFB.  I want to emphasize that this is in response to her letters - I'm not attempting to change her mind or comment on her definition of patriotism.  It is extremely obvious to the most casual observer that her mind is already made up.  However, I'd like to clarify a few points where she seems a little confused.

I'm a retired fighter and test pilot with many hours in a variety of fighters including the F-4, F-15, F-15E and F-16.  I have flown a considerable amount out of Holloman AFB, on White Sands Missile Range and the surrounding airspace.  I've been associated with the F-22 and many of its related systems and weapons since 1987 when it was known as the Advanced Tactical Fighter.  A group of friends convinced me to write this letter because they know that I am somewhat familiar with the subject. 

As most Americans know, the F-22 is an incredibly capable fighter.  Unlike its predecessor, the F-15, it isn't called an "air superiority fighter", the term used for decades to describe the most capable air-to-air fighters.  Instead, the F-22 is an "air dominance" fighter.  It is designed to go well beyond temporary control of the air in the battle area and rapidly destroy all enemy air capabilities.  You've probably heard about many of its superior qualities such as its advanced radar and related sensors, high agility, supercruise and stealth.  The basic approach to using all these capabilities in an air-to-air engagement is to enter the fight at high altitude (above 50,000 feet - beyond the historic altitude limit of past fighters) and very fast (supersonic - typically above Mach 1.5) to provide the energy required to the air-to-air missiles to give them dramatically increased capability to destroy enemy aircraft at previously unheard of launch ranges.

The F-22 is also replacing the F-117 in the air-to-ground role to destroy high value targets in high threat areas.  At night, either plane could perform the job.  But during the day, where the subsonic F-117 can't operate because it would be acquired, tracked and destroyed using visual sensors, the F-22 can still operate as long as it enters the target area at supersonic speeds.  The high speeds coupled with the relatively short visual acquisition ranges, force the visually guided surface-to-air weapons into flight profiles that they simply lack the energy to accomplish.  It brings a new meaning to the age-old fighter pilot adage "Speed is life."

The United States military has long been considered the most capable military force in the world.  Many people assume that this is only due to the advanced weapons.  But there are many more factors than simply having the right equipment.  Unquestionably, one of the most critical factors is how the US military trains.  The approach to training can be summed up by the expression, "Train the way you fight; fight the way you train."

The F-22 either is, or will be stationed at 5 Air Force bases - one in Alaska, one in Hawaii and three in the Lower 48.  Hickam AFB, Hawaii, Langley AFB, Virginia and Tyndall AFB, Florida all use overwater ranges that keep sonic booms away from most humans but provide a limited environment to tax the various F-22 sensors.  Only Elmendorf AFB, Alaska and Holloman AFB have the huge amount of overland airspace that will provide the opportunity for advanced F-22 training.  Holloman AFB will be the only base in the Continental United States where the F-22 aircrews can conduct flight training representative of how they plan to fly in combat. 

So, the answer to Dory's question of "Why is this going on here?" becomes clear.  To prepare for war, the crews must train in a similar manner to the way they plan to fight in combat, meaning, they must be able to train at supersonic speeds in overland airspace.  The supersonic overland airspace in the Alamogordo area provides the unique critical component of what the F-22 requires to truly achieve its capability of "air dominance" as well as to replace the F-117.  It's that simple!

On a side note, as to living around sonic booms - it's fantastic!!!  When your friends and family come to visit after the F-22 becomes operational, hearing sonic booms will be one of the highlights of their trip.  As a child, I lived in Chicago when B-58 Hustlers would fly over creating distinctive double booms on training runs against Nike sites.  We all loved it.  Twenty five years later, my family spent 4 years at Edwards where sonic booms were commonplace, including the then new distinctive double boom created by the return of the space shuttle.  I never heard of any negative effect to people, cats, dogs, horses, birds, fish, etc.  Since the strength of the sonic boom dissipates exponentially with distance, damage to surface structures (houses, cars, etc.) is highly unlikely at the altitudes the aircraft will be flying, literally over 10 miles above the ground. 

As part of testing Naval surface combatant's self-defense systems, I've flown numerous times within a few hundred feet of a crowd of people (sailors standing on deck with their cameras flashing) at deck level and Mach 1.3.  Didn't break anything - except the speed of sound.  Dispersed no crowds and harassed nobody.  Instead, I put huge smiles on every face.  They all absolutely loved it!!!

But, of course, Dory wasn't aboard ship.   

 

Lance Grace, Lt Col (Retired), USAF

Alamogordo, NM