An Introduction to Supersonic Flight

Estimated Reading Time: 7 minutes

TL;DR

Supersonic flight and travel have long remained the province of governments and the wealthy. As we learned about in our subscriber post last Tuesday, new startups are working to bring supersonic aircraft back into commercial service. Today we learn more about the science of supersonic flight, including sonic booms and some new technological approaches to reduce the impact of sonic booms.

What is a Sonic Boom?

A sonic boom is a very loud noise created when an aircraft travels faster than the speed of sound. An aircraft traveling at supersonic speeds creates a conic shockwave, as illustrated in the figure below, that trails the aircraft and creates a hyperbolic contact region on the earth. As the second diagram below shows, the sound barrier is due to the buildup of pressure waves as an aircraft travels at the same speed as the pressure waves it emits.

A supersonic aircraft creates a conic shockwave which creates a hyperbolic contact region (highlighted in yellow) as it contacts the ground (source).

The Mach number (Ma) is the ratio between the velocity of the aircraft and the velocity of sound. If Ma < 1, the aircraft is travelling below the speed of sound and creates no shockwave. At Ma = 1, the sound created by the aircraft travels as fast as the aircraft, leading to a buildup of pressure waves (the sound barrier). If Ma > 1, the aircraft travels faster than the pressure waves it emits leading to a cone shape. The angle of the cone is determined by the value of Ma (source).

Sonic booms are typically associated with aircraft, but some terrestrial objects can create sonic booms as well. Notably, bullwhips emit small sonic booms at their tips when cracked by an expert (source).

Low Boom Technologies

Sonic booms are loud and disruptive and can shatter glass in the path of the shockwave, but in general do not pose a direct threat to human health. However, sonic booms are disruptive enough for the public that supersonic aircraft are not generally permitted to travel overland. The Concorde (see image below), was the only supersonic passenger aircraft in the West between 1976 and 2003 (the Soviet TU-144) ran limited routes between 1968 to 1999). The Concorde was limited to travel primarily over the Atlantic for wealthy travelers. The tragic crash of Air France Flight 4590 in 2000, the first Concorde crash, prompted the removal of the Concorde from active service.

The Concorde was the world’s most famous commercial supersonic plane and operated until 2003 (source).

NASA has performed extensive research into understanding the behavior of sonic booms, notably in its Shaped Sonic Boom Study), which demonstrated that it was possible to shape the sonic boom by modifying the shape of the aircraft fuselage. NASA demonstrated in 2003 that it was possible to reduce the sonic boom of a particular aircraft by a third by shaping the fuselage of the aircraft appropriately. Advances in simulation have enabled even more exotic designs, such as that of the experimental X59-QueSST pictured below, which may lower sonic booms to acceptable levels for overland travel.

A modified F-5E aircraft was able to reduce boom by a third in NASA’s Shaped Sonic Boom Demonstration in 2003. Further modifications and more advanced simulation techniques may be able to reduce sonic booms even further (source).

Concept art from NASA for its X59-QueSST aircraft which could achieve even lower boom and potentially enable overland commercial supersonic travel. Notice the distinctive shape (source).

As we learned in last week’s issue about supersonic startups, a number of companies hope to leverage the emerging technology of low sonic boom aircraft. For now, research into low boom technologies is very early stage, with first flights of the X59-QueSST planned for 2023 (source).

Discussion

Considerable research has gone into developing computational methods for fluid modeling, driven by needs from aerospace modeling. One of the recent evolutions in fluid modeling has been the increasing maturity of machine learning methods for fluid modeling (for example, see this paper from Google). Advanced machine learning enabled modeling may perhaps allow for designs even more exotic than the X59-QueSST to reach the market. We will continue tracking trends in low boom technology in future issues!

Highlights for the Week

• https://github.com/google/jax-cfd: An interesting continuous fluid dynamics solver implemented in Jax

• https://www.defensenews.com/naval/2021/06/01/if-congress-can-find-the-money-the-us-navy-would-like-another-new-destroyer-this-year/: The Navy is requesting money for a new destroyer

• https://www.wsj.com/articles/united-plans-to-buy-15-supersonic-planes-11622724910: United has made a deal with Boom Supersonic to purchase 15 of their supersonic planes. Recall that Boom has not yet manufactured a functional supersonic prototype plane yet so some skepticism is warranted.

Subscription, Feedback and Comments

If you liked this post, please consider subscribing! We have weekly subscriber-only posts on Tuesdays.

Please feel free to email me directly (bharath@deepforestsci.com) with your feedback and comments!

About

Deep Into the Forest is a newsletter by Deep Forest Sciences, Inc. We’re a deep tech R&D company specializing in the use of AI for deep tech development. We do technical consulting and joint development partnerships with deep tech firms. Get in touch with us at partnerships@deepforestsci.com! We’re always welcome to new ideas!

Credits

Author: Bharath Ramsundar, Ph.D.
Editor: Sandya Subramanian