Tuesday, March 31, 2015

Techy Tuesday - Designing Out The Sonic Boom

By the mid-60s, the "jet age" was well established.  The commercial jet business was well established after the advent of the British De Havilland Comet entering commercial service in 1952, followed by the Boeing 707 first flying in 1957, and its the three-engined sister, the 727 following five years later.  Douglas responded with the DC-8 in 1958 and the DC-9 by 1965.   Other aircraft makers joined in for the heyday of jet setting

Always looking to design the next big thing, aircraft designers and airlines were facing a great fork in the road.  On one side lay continually bigger aircraft, like the 747, the DC-10 and L-1011, reducing the passenger cost per mile and spreading jet flying to larger groups.   On the other side lay taking the currently served (smaller) number of passengers but going faster: supersonic with the Concorde, and the US' SST project.  Obviously, the world went in the direction of jumbo jet.  

If you recall that period, you'll know this is when large sections of the population first heard the term "sonic boom".  When an aircraft passes through the air it creates a series of pressure waves in front of it and behind it, similar to the bow and stern waves created by a boat. These waves travel at the speed of sound, and as the speed of the aircraft increases, the waves are forced together, or compressed, because they can't get out each other's way. Eventually they merge into a single shock wave, which travels at the speed of sound.  This expands in a cone behind the plane and creates a loud sound, sometimes close to the volume of an explosion.  On a small scale, virtually all rifle rounds are supersonic and much of the sound of a passing bullet is the sonic boom it generates. 

Sonic booms weren't a problem because they were annoying; they damaged real property, too.  When the military was first flying at supersonic speeds across the country, the booms would knock things over in people's homes and cause damages.  It's said that between 1956 and 1968, over 38,000 claims were filed against the U.S. Air Force for damage wrought by sonic booms.  This led to the banning of supersonic travel across the US. 

But there's still interest in transporting people and cargo faster.  I've always thought that if time travel is really ever invented, it will be by a company like FedEx, but probably a competitor of theirs.  Businesses will pay to get delivery as soon as possible (so they can work longer on the project).  How much would people pay to deliver it yesterday? 

Engineers being engineers, it was natural someone would ask, "can we design out the boom"? 
In 2001, NASA started the Shaped Sonic Boom Experiment to analyze sonic booms. They modified the fuselage of a Northrop F-5E Tiger II in an attempt to lower the effects of sonic booms during test flights. The nose of the F-5E was removed and replaced with a larger, longer version. The fairing under the fuselage was also lengthened and deepened.

The modified F-5E or Shaped Sonic Boom Demonstrator flew in 2003, and from that point, NASA took 1,300 sonic-boom measurements from various ground sensors. NASA engineers confirmed that the modifications led to an 18% reduction in initial pressure impulse and booms were an average of 4.7 decibels quieter compared to an unmodified F-5E.
Now 4.7 dB isn't a very large decrease in the sound level, but putting a pointy boom on an airplane isn't a big change and is a solid first step.  NASA repeated the experiments in 2007, this time on an F-15B, adding a 24 foot long spike on the nose with stepped diameters.  These steps each produced smaller shock waves and confirmed reduction of the sonic booms at speeds even farther beyond the speed of sound, Mach 1, up to Mach 1.8.
Aircraft manufacturing giants Boeing and Lockheed got interested in getting some NASA money, too, with an eye toward not just adding things to the aircraft's nose, but designing the planes to push the sonic boom predominantly up (and away from the ground).  Boeing proposed a design with top mounted engines.  Lockheed proposed a design with two conventionally mounted engines (below the wings) and one on top of the fuselage.  Almost the same layout as the engines on the 1968-design L-1011. Boeing's design is on the top in this pair of images. 
This is a hot and interesting field of research. The American Institute of Aeronautics and Astronautics, held its first Sonic Boom Prediction Workshop in January 2014. The workshop’s purpose was to assess the current technology for predicting sonic-boom propagation. The presentations consisted of the latest techniques in computational fluid dynamic (CFD) software to help design and build better aircraft bodies and wings. Research indicates that keeping airflow over the wings from becoming turbulent can reduce the amplitude of the shock waves created.  Improved CFD modeling software will improve the ability to predict the effects.

In the high tech world, it's almost guaranteed that the real engines of innovation are not on the Boeing/Lockheed/NASA axis, it's in the small companies.  Aerion Corp. has been working on reducing sonic booms since 2002, when it introduced its Supersonic Natural Laminar Flow technology. Aerion designed a wing with supersonic laminar flow control, LFC, which reduces drag by 50% over the wing. Laminar flow wings are thin and smooth so as to not trip the boundary layer and enter turbulent flow. Aerion’s wing design is unswept but tapered with a relatively sharp leading edge, and features a modified bi-convex airfoil with the upper and lower surfaces slightly curved. Aerion will use this wing on its Supersonic Aerion AS2. 

Aerion's approach is to combine both its aircraft design with the use of atmospheric properties sensed by onboard electronics.  The property being exploited is the Mach cutoff, a region of the atmosphere that blocks sonic booms from being heard on the ground.  According to Doug Nichols, Aerion CEO, normal atmospheric phenomena, mainly temperature and wind, bounce or refract sonic booms upward at 5,000 feet above the ground. Exploiting the Mach cutoff should be more effective than any low-boom features on the aircraft.  Avionics onboard the AS2 will use atmospheric data to calculate the supersonic speeds that create sonic booms that would get refracted.

Lots more details and a video at Machine Design magazine.  In my "formative years", I came to the conclusion that sonic booms were inevitable.  It turns out that shock waves may be inevitable, but the loud boom may well be something that can be designed around or possibly even reduced to insignificance.   


  1. 3 decibels is twice the power.

    So 4.7 db reduction is a very significant reduction, at least in my opinion.

  2. Seems like the drag reduction side effect would be worth the research efforts for the fuel savings.

    I'd agree that the boom is probably inevitable until we find an Einstein for fluid mechanics. But making the whole thing more efficient is what engineers do. First you break some barrier, then you make the break more efficient.

    Posts like this are one of the many things that keep me coming back here. Thanks for writing stuff like this.

  3. Weetabix - thanks for the compliment. Getting comments like that helps me keep going.

    AM - quite right that -3dB is half the sound power; -4.7 dB is 1/3 the power. Unfortunately, pyschoacoustics isn't quite so straightforward. If you give people a volume control to a stereo amplifier and tell them to reduce the volume until it sounds "half as loud" and measure the power, you'll find they don't reduce it by 3 dB. They reduce the power closer to 10 dB!

    If you asked a random group of people if the reduced sonic boom sounds quieter, it's possible they wouldn't think it was much lower volume at all. I'd bet some wouldn't have noticed it being quieter at all, without careful comparison. That linked website says the volume would be approximately 75% of an unmodified plane.

    A lot of interesting information there at http://www.sengpielaudio.com/TableOfSoundPressureLevels.htm