It was six years ago that I first wrote about NASA’s X-59 QueSST project, contracted to Lockheed Martin. Now the plane has finally been built and is ready for testing. At the time it was reported that NASA “had a design” for a quiet supersonic jet, one that would not create a sonic boom, just more of a “thump”. But having a design is not the same thing as having an actual jet – and it took six years of further research and development for Lockheed Martin to produce a prototype. Now we enter the testing phase, and it will likely be years more before there is any jet produced from this design.

One ultimate goal of NASA’s project is to develop the technology for quiet commercial supersonic passenger jets. The idea is that the knowledge gained from testing the X-59 will lead to designs for such commercial aircraft. Why is this a big deal?

As I wrote in that 2017 article, commercial flight times have been relatively flat for the last 50 years. There are several reasons for this, but the bottom line is that fuel efficiency, and therefore cost-effectiveness, is optimal around Mach 0.85, so that is the speed that commercial jets fly. The only practical way to make jet travel significantly faster is to develop supersonic technology. We had this, with the Concorde, but that jet service was ended in 2003. There is debate over exactly why this happened, which I won’t get into, but one factor was that the potential flight paths for the Concorde were limited, because it is generally illegal to fly greater than Mach 1 over land. The Concorde was a New York to Paris flight, mostly over water.

A sonic boom is created when a craft is moving faster than the speed of sound through air. A pressure wave builds up in front of the craft because the propagating sound wave are slower than the craft. They cannot radiate away in front of the craft, so instead a wake of sound waves is created, much like the wake of a fast-moving boat in water. This wake of sound waves is the sonic boom. Therefore the “boom” is not an event – it is a continuous trail of sound that travels to the ground beneath the craft. A typical sonic boom would be loud enough to potentially cause damage, breaking glass windows, affecting wild life,  and damaging hearing. Several factors affect the loudness of a sonic boom at the ground, but altitude is a main factor. For this reason the Airforce typically will only travel supersonic above 30,000 ft over populated areas.

In order for supersonic air travel to become a commercially viable venture again, many believe we need to develop the technology to significantly reduce the magnitude of the sonic boom generated by the aircraft. This would potentially open up many more routes to supersonic travel, and make it worth the cost of developing such commercial craft. That is one of the goals of NASA’s QueSST project.

If all goes well, what can we expect? At the very least, testing of the X-59 will significantly advance our knowledge of quiet supersonic travel. I suspect this testing will take at least several years, but let’s say within another 5 years or so we have reasonably mature quiet supersonic technology, enough that we can translate it to other craft, including commercial aircraft if desired. Will this happen?

There are already companies developing private supersonic jets, such as Boom Technologies out of Colorado. They are in the testing phase of a small supersonic jet, the XB-1, and they are also developing commercial supersonic jets. It could still be a decade before any of their jets are in commercial use, and that will be a big test of the market. It will ultimately come down to how many people will be willing to pay the premium for the reduced travel time. One of the factors that allegedly killed the Concorde was that other airlines could compete by providing luxury first class travel. It turns out that wealthy travelers were more likely to pay for a longer but more luxurious flight than a faster flight.

The Boom jets are also not using quiet supersonic technology. It may be 2 decades or more before the technology being developed by QueSST is translated into a commercial supersonic jet on which you can book travel. If successful this will open up many more routes for supersonic travel. I certainly love the idea of flying from NY to LA in 3 hours, or even 2 hours, rather than 6. Also, if I ever have to travel to Sydney Australia again, that 15 hour LA to Sydney flight time was a killer. Imagine the same flight in 5 hours. At that speed I would even consider going directly from Newark to Sydney, now 21 hours, but could be a doable 7 hours. New York to Tokyo is 14 hours, which could be reduced to less than 5 hours.

It is these very long flights that would be most attractive for supersonic travel, and depending on the exact price point, I would consider paying it. Will it cost 50% more, 100% more, or 300% more? That will make all the difference.

There is another factor here that will become increasingly important over the next 20 years – fuel expenditure. The airline industry, especially for long-haul flights, will likely be the last sector to decarbonize. Hydrogen will never have the energy density, even liquid hydrogen. Electric jets will be good for short hauls only. Biofuels is the only option, so it depends on whether or not we can develop an infrastructure for large volumes of cost-effective biofuels. If not, burning the extra fuel to get there faster may not be socially acceptable for routine travel.