Prologue
At the end of my previous piece on the BOOM Symphony, more than a few questions were asked about the Sonic Boom, Sound Barrier, Boomless Cruise etc. This piece attempts to answer the questions. For that we need to go right to the very beginning.
History
Prehistoric Man might have taken the concept of sound for granted and not given it much thought. The first inflection point was the realization there was a relation between lighting and thunder i.e how thunder always followed lightning. Some intuition later they realized that both travelled through the space around and above them.
The Norse God Thor was attributed by the the Vikings for Lightning as his hammer, Mjölnir travelled across the sky followed by Thunder as Mjölnir struck his enemies.
Similarly the Indian God Indra’s Vajra representing a diamond (indestructible) and thunderbolt (irresistible force)caused lightning as it travelled through the sky . Indra’s domain included Lightning and Thunder among many others.
History is replete with such allusions to Lightning and Thunder. From these allusions it is clear that lightning was faster than Thunder.
The concept of speed is arrived at.
Science Timeline
Scientific rigor made its entry. Below is a timeline some of the milestones in understanding sound.
- By 500 BC Pythagoras discovered that musical pitch depended on the percussion instruments string length
- Aristotle around 350 BC believed that an object striking air caused sound, he still thought sound was instantaneous
- Vitruvius in 50 BC Discovered the concept of the echo was in manmade architecture
- Between the 9-12th centuries several Islamic scholars studied vibration & resonance and the concept of sound moving through air as a medium
The Middle Ages & Renaissance periods made strides in understanding sound as a wave.
- In the early 1600s Galileo observed the concepts of vibration
- Marin Mersenne in 1636 measured the speed of sound for the first time at 1380 ft/s (the first measurement of the speed of sound)
- Issac Newton around 1687 applied his laws of motion to sound in air and calculated the speed of sound at 979 ft/s
- During the 1740s Euler and Bernouli created the wave equation for sound
There were several further increments to Man’s understanding of the phenomenon of Sound and then came Ernst Mach.
Ernst Mach
Mach was the first person to describe and photograph shockwaves.
By the 1800s when bullets or projectiles were fired at high speeds a loud cracking sound was observed. While piquing curiosity nobody understood what was taking place.
Mach who prided himself as a multi dimensional scientist merged physics , photography and optics in a fresh new manner.
Mach’s explorations into supersonic fluid dynamics led him to team up with physics photographer Peter Salcher.
Together in 1887 they presented a paper that correctly described the sound effects of a supersonic bullet in motion. They determined the existence of a shockwave with the bullet as the apex of the shockwave.
The photography technique used was Schlieren Photography, a relatively new technique. The principle of this technique is light bends when it passes through air with a slightly different density.
Schlieren photos use mirrors, a lens, a bright light and a sharp edge to detect light refractions through the air. He then fired bullets through this set up and captured the shockwave! A first…
It was August Toepler who developed this technique in the 1860s to capture heat and density differences in air. This was developed specifically for thermodynamics and gas flow. Mach innovated on this technique to capture bullets in supersonic flight.
Man finally began to grasp what the sound barrier was about….pictorially!
The speed of sound or Mach is named after Ernst Mach.
Transonic Adventures
The term transonic was coined around 1945 by NACA director (the NASA precursor) Hugh Dryden while referring to speeds that were close to the speed of sound. The speed range is between Mach 0.8 – 1.2.
In dogfights during WW2 P-38 Lightnings, Spitfires & Thunderbolts accidentally approached Mach 0.8 – 1 in steep power dives.
As these speeds were hit pilots experienced locked controls and violent shaking of the aircraft also known as compressibility buffeting. In some cases the aircraft broke apart or just stopped responding to control surface inputs.
The phenomenon was experienced for the first time and terrified pilots, who were the guys in the hot seat. They felt like they hit a wall and described it as a barrier. The term sound barrier originated here.
Experiential research later scientists replicated these effects and understood that as airflow over the wings and airframe approached the speed of sound, shock waves formed and disrupted known flight rules. This was the compressibility effect.
The fix was dive recovery flaps under the wings to be deployed early in the dive. The flaps helped bring the nose up and recover the aircraft. Engineers had managed to fix an issue they had scant understanding of.
The swept wings courtesy the Germans were coming.
Glamorous Glennis
Post WW2 a large number of documents and scientists on aeronautical research were seized from the Germans and transported to the United States as part of Operation Paperclip.. The documents were on wing design, shockwave research from the Me-262 (the first Jet aircraft) and airframe design among many others.
Glamorous Glennis was piloted by Gen Chuck Yeager(had the privilege of interacting with him a couple of times on X)and named after his wife. The Bell X-1 was an all American aircraft purpose built aircraft designed to break the sound barrier. Among the many innovations it had was an airframe shaped like a .50 Calibre bullet (a shape known to handle supersonic speeds).
The wings were unswept (straight) and extremely thin with a thickness to wing chord (distance between the leading and trailing edges of the wing) ratio of 8%.
The tail plane was all moving to handle speeds in excess of Mach 1 and powered by a four chamber XLR-11 rocket engine that developed 6000lbs of thrust.
On 14 October 1947, Glamorous Glennis piloted by Gen Chuck Yeager after dropping from a B-29 Superfortress at 25,000 feet, ignited its rocket and flew into history as the first aircraft to go through the sound barrier in level flight.
The Bell X-1 flew over eighty flights and contributed greatly to our understanding of Supersonic flight.
The Golden Decade
The next decade and half would be the golden age of Supersonic flight.
Using the German WW2 research along with their own the US and Russia developed several firsts for aircraft.
Swept back wings (typically about 35 degrees) became the norm for the next generation of transonic/supersonic aircraft.
The US F-86 Sabre and Russian MIG-15 were the first aircraft to maintain high transonic level flight with stable handling.
The F-100 Supersabre and the MIG-19 Farmer the first aircraft to consistently maintain Mach 1+ speeds in level flight.
The F104 Starfighter the first fighter to hit Mach 2 and the big boy, the B-58 Hustler the first strategic bomber to hit Mach 2+.
The Human Facor
The decade of the 1960s ushered in a SST competition between the Concorde(UK & France), the TU-144(Soviet Union) and the Boeing 2707(USA). Supersonic transport stood for national superiority and attention turned to transporting people at Supersonic speeds across continents.
Supersonic speed always has the shadow of the sonic boom following it. The Boom itself is a sudden sharp booming sound or a double boom that follows the aircraft and the sound decibels range of 110 – 140 dB. The best analogy is the waves off the prow of a boat as ploughs through water.
The Oklahoma City Experiment
Starting Feb’64 for eight months Oklahoma City was subjected to 1253 sonic booms @ eight booms a day.
The FAA wanted to study the effects of sonic booms on the local population and chose Oklahoma City as the test subject.
Initially the population was enthusiastic about these tests, but soon realized the booms were everything and more than they were cut out to be.
The people of Oklahoma experienced the following:
- Disrupted sleep cycles that induced anxiety
- Physical damage to property
- Pets were frightened and stressed
- Sensitive instruments being disrupted
Oklahoma City felt like a guinea pig and a $25 compensation per household for damage caused, only added insult to injury. The experiments drew over 15000 complaints.
The Oklahoma experience snowballed into a national anti supersonic travel movement. Congress cut funding to the B-2707 program and by 1973 supersonic travel over land was banned in the United States, the rest of the World followed suit.
This was a body blow to the Concorde and TU-144 which were in advanced stages of development. They would fly Supersonic only over water (Concorde) and the TU-144 with its limited range only over the Soviet Union.
Supersonic Travel would be on the backburner for the next 25 years.
QueSST
Psychoacoustic , a word that pique’s interest
By the 1990s NASA and the United States after exiting the Supersonic race had data across fluid dynamics, community response and shockwave shaping. Technological and material improvements improved aircraft airframe and engine efficiencies, the time had arrived to realign Supersonic research from creating new aircraft to better understanding the shockwave and creating designer shockwaves!
Psychoacoustic is how a sound comes across to people. The factors that contribute to it are things like startle factor (suddenness and loudness), frequency (low or muffled sounds are more accepted by the human ear) and timing of the day.
NASA already knew that repeated sound over 100 dB drew complaints, while a decibel level of 75dB was more acceptable to people.
The QueSST ( Quiet Supersonic Transport)program is about laying down the guidelines on the future of Supersonic transport across airframes, flight paths, speeds, altitudes and many more parameters, hopefully overcoming the 1973 ban.
During the 1990s NASA used a T-38 Talon to fly controlled supersonic passes over microphone arrays to understand boom propagation over different terrains and study boom carpet (the impacted ground area of a sonic boom)patterns.
In 2003 a F-5E Tiger with a reshaped nose was used to demonstrate that shockwaves can be manipulated. This test proved that boom intensity can be reduced by at a third.
Between 2011-2016 there were several acoustic simulation studies conducted which arrived at the 75 dB target.
Between 2016-2021 NASA used a F-15B in conjunction with advanced ground sensor arrays to measure shockwave propagation and help tune future computational models. These tests were run over unpopulated desert.
There were several sonic boom carpet models used to test and control sonic boom impact area. Some of the tests included F/A-18 Hornets dive maneuvers to narrow the boom carpet.
The X-59
The Lockheed Skunk Works is NASA’s low boom flight demonstrator aircraft, designed to fly at Mach 1.4 over land.
The X-59 is a specially designed airframe to control, break and reduce sonic booms. The aircraft will be used to confirm the sonic boom carpet, identify shockwave interaction with different parts of the aircraft and further fine tune computational models.
The design innovations include a long tapered nose that is over a third of the length of the aircraft at thirty feet. The nose stretches the shockwave formation at the very start. Due to the nose design, the aircraft uses 4K cameras for forward vision. The design of the wings in relation to the nose need the use of canards (small wing shaped appendages forward of the wings) for stable flight. The all moving tail plane is designed to handle supersonic speeds with minimum shockwave. Lastly the GE F414 engine is mounted on top of the fuselage to direct shockwaves away from the ground.
The aircraft uses area rule shaping to reduce drag (and mitigate shockwaves) at and around the speed of sound. Simply put, the rule states the shape of the aircraft should transition as smoothly as possible, also called the coke bottle shape.
The X-59 will be flown over multiple cities and communities across the United States and surveys will be taken of these communities to study their response to the shockwave characteristics of the X-59 in multiple flight profiles.
NASA’s goal is to prove to regulators that designed booms are acceptable to communities and pave the path to overturning the Supersonic ban over the Continental United States.
The Boom Connection
BOOM with the XB-1 did achieve Boomless cruise although they used a different approach.
BOOM used atmospheric refraction from a high altitude (over 35,000 feet) to use the atmosphere to bend the waves upward and keep away from the ground. They used area rule shaping to optimize shockwave generation. Furthermore BOOM limited the speed to Mach 1.1 to control shockwaves.
BOOM is part of the ecosystem NASA is looking to create. The X-59 tests are expected to initiate a Supersonic standards conversation and BOOMs Overture will benefit from these tests.
The Final Word
Finding innovative solutions to overcome physical limitations is the path forward. At the heart of the solution is managing human behavior and attitudes towards Supersonic Shockwaves.
Disclaimer: This article has used images from multiple sources accessed through Google.
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[…] aircraft that flies supersonic is accompanied by the shadow of the sonic boom (read about it here : https://theaviationevangelist.com/2025/04/13/shockwave/ ). QueSST’s aim is to fly supersonic with no audible sonic boom. To achieve that there are two […]