# Supersonic Commercial Flight May Make Comeback with United Airlines

startup boom supersonic sells 15 planes to united airlines

## United Airlines is buying 15 jets that can fly Mach 1.7, cutting some flight times in half. The startup Boom Supersonic will sell the commercial airliner the jets, which are slated to take off in 2029. Supersonic flight requires great ingenuity.

In 2029, United Airlines flights will break the sound barrier—and a 26-year hiatus of commercial supersonic flight. The company has bought 15 jets from a startup called Boom Supersonic, whose airplanes break the sound barrier and fly as fast as Mach 1.7. Additionally, the supersonic jets will use sustainable fuel and result in net-zero carbon emission flight.

Chuck Yeager broke the sound barrier more than 70 years ago, but it was no simple task. As an aircraft approaches Mach 1, lift and control both decrease, while drag increases. In his video series The Science of Flight, Dr. James W. Gregory, Professor of Mechanical and Aerospace Engineering at The Ohio State University, explained the speed of sound in detail.

#### Something to Be Learned from a Rainstorm

“At a simple level, the speed of sound, and how it’s different from the speed of light, is familiar with thunderstorms,” Dr. Gregory said. “If you see a flash of lightning and then count, once per second, until you hear the clap of thunder corresponding to the flash, you can tell how far away that particular lightning strike was.

“The general rule of thumb is one mile of separation will give a five-second delay between the lightning strike and when we hear it.”

If sound travels one mile—or 5,280 feet—in five seconds, that means it travels 1,056 feet per second. On a cold day of 5 degrees Fahrenheit, this is exactly the speed of sound. According to Dr. Gregory, on a 70-degree day, the speed of sound is 1,128 feet per second, or 770 miles per hour.

“As the atmosphere gets hotter, the sound speed also increases—slightly,” he said. “The Mach number is simply the ratio of velocity to the local speed of sound. A Mach number of one is a flight speed equal to the speed of sound, while any Mach number less than that is subsonic, and any value above one is supersonic.”

#### X Marks the Spot

During World War II, improvements in propeller engines brought airplane speeds up by several hundred miles per hour. However, the engine that broke the sound barrier was a rocket engine built into an aircraft, piloted by Chuck Yeager.

“On October 14, 1947, over Rogers Dry Lake bed in the Mojave Desert, Chuck Yeager strapped into the Bell X-1 for that record attempt,” Dr. Gregory said. “The X-1, simply put, was a sleek rocket with wings that were solely designed for speed. With a fixed supply of fuel on board, and the inefficiency associated with rocket propulsion, flight for long periods could not be sustained.

“But it was long enough.”

According to Dr. Gregory, a B-29 Superfortress carried the X-1 up to 20,000 feet in the air and released it. Yeager initiated the rocket engine and, slowly but surely, increased his velocity nearer and nearer to Mach 1, taking care to check for control lock-up or loss of control. At 42,000 feet, it reached Mach 0.98 before jumping to a value “off the scale.”

“Post-flight data analysis indicated that he had accelerated as fast as Mach 1.06—he had done it,” Dr. Gregory said. “In this moment, Yeager became the first human being to pilot a flight vehicle faster than the speed of sound, and he lived to tell about it. Yeager’s flight cleared the way for a continued push toward ever-faster flight speeds in the quest for scientific understanding and higher performance flight.”

If United’s supersonic flights take off as planned, anyone aboard the planes would match Chuck Yeager’s flight speed.

Edited by Angela Shoemaker, Wondrium Daily