Shushing Sonic Booms: NASA’s Supersonic X-Plane to Take Flight in 2021

The history of aviation is full of notable X-Planes, a number of which heralded in new generations of flight. The Bell X-1 became the first aircraft to break the speed of sound during level flight in 1947 with the legendary Charles “Chuck” Yeager at the controls. A few years later the X-2 would push man up to Mach 3, refining our understanding of supersonic flight. In the 1960’s, the North American built X-15 would not only take us to the edge of space, but set a world speed record which remains unbroken.

Compared to the heady post-war days when it seemed the sky was quite literally the limit, X-Planes in the modern era have become more utilitarian in nature. They are often proposed but never built, and if they do get built, the trend has been towards unmanned subscale vehicles due to their lower cost and risk. The few full-scale piloted X-Planes of the 21st century have largely been prototypes for new military fighter jets rather than scientific research aircraft.

But thanks to a commitment from NASA, the Lockheed Martin X-59 might finally break that trend and become another historic vehicle worthy of the X-Plane lineage. Construction has already begun on the X-59, and the program has recently passed a rigorous design and timeline overview by NASA officials which confirmed the agency’s intent to financially and logistically support the development of the aircraft through their Low Boom Flight Demonstrator initiative. If successful, the X-59 will not only help refine the technology for the next generation of commercial supersonic aircraft, but potentially help change the laws which have prevented such aircraft from operating over land in the United States since 1973.

Lockheed Martin X-59 QueSST

The X-59 QueSST (a rather loose acronym for “Quiet Supersonic Transport”) is something of a return to the classic X-Planes of the past; a pure research vehicle that has no commercial or military application. It exists purely to explore new supersonic technology. But it’s not to study controlling an aircraft flying faster than the speed of sound, we’ve had that knowledge since the 1950’s. Nor is it testing new engine technology, General Electric is already well on their way to producing their own supersonic engines for the commercial sector. The X-59 is designed not to make supersonic flight safer or faster, but to make it quieter.

When an object travels faster than the speed of sound, pressure waves which are formed in the front and rear become compressed together. This compression forms a shock wave, the energy of which is directly proportional to the amount of air being displaced. For a small supersonic object such as a bullet, the shock wave is too small to do any damage and simply sounds like a loud crack. But a large aircraft can produce a shock wave strong enough to break windows and startle people on the ground.

To mitigate this effect the X-59 utilizes a long and narrow fuselage designed to put more space between the front and rear pressure waves, thus preventing their compression. This doesn’t make crossing the sound barrier completely silent, but it does go a long way towards eliminating the traditional “sonic boom”.

On the ground, instead of one loud bang, observers will hear the two separate (but much less energetic) waves as dull thumps. Ultimately, the sound produced by the X-59 at sustained supersonic speeds is expected to be about as loud as a car door closing; quiet enough that most people in urban environments probably won’t even be consciously aware of it.

Beyond its unique shape, the X-59 is not unlike a contemporary jet fighter. In fact, principal components for the new X-Plane such as the cockpit and landing gear came from a Northrop T-38 Talon and General Dynamics F-16, and it will be powered by the same General Electric F414 engine as the McDonnell Douglas F/A-18. But as the X-59 is designed specifically to test the shape of the aircraft and not new engines or control techniques, this isn’t particularly surprising.

Though that doesn’t mean the design lacks special considerations. For example, because of the X-59’s long nose and inline cockpit, the pilot has extremely limited visibility. To overcome this there’s a 4K camera with a 33° by 19° angle of view in the nose, which the pilot will view through a monitor in the cockpit. A similar problem was faced on the Concorde, where its long slender nose made it impossible for the pilots to see the runway during landing. In that case, the solution was to lower the entire nose as far as 12.5 degrees via hydraulic actuators during landing; giving the craft its infamous “droop nose” look.

Real-World Testing

With today’s computer modeling, it might seem strange that the X-59 needs to be built at all. The science of supersonic shock waves is well understood at this point, and neither NASA nor Lockheed Martin really need a physical test object to tell them what they already know. Whether or not the X-59’s unique shape will cut down on the intensity of sonic booms isn’t really a question; the reason they’re building it is to prove it to the average American taxpayer.

Once the X-59 has completed its initial flight testing, it will immediately begin its primary mission: flying over cities in the United States to see how people react. Will they be annoyed by what NASA is calling the X-59’s “sonic thump”? Will they even hear it if they don’t know what time to expect it? These are the sort of questions the agency is looking to answer by building a real-world test article, something no computer model will ever be able to do. The hope is that the testing will prove advanced supersonic aircraft like the X-59 can travel over populated areas without producing the aggravating and possibly damaging shock waves which caused the FAA to ban overland supersonic flight in the first place. In time, this data could be used to get this decades old regulation overturned or amended; opening the door to a new generation of civil supersonic aircraft.

In fact, the first phase of these tests have already started. Earlier this month, NASA started performing simulated “sonic thumps” over Galveston, Texas using an F/A-18. By performing a series of special dives, the aircraft was able to approximate what the X-59 should sound like to observers on the ground. A fleet of microphone stations, as well as hundreds of Galveston citizens, collected sound data which will be used as a baseline when the X-59 flights start.

Pushing Ahead

The X-59 is the latest move in a concerted effort from commercial players and NASA to return supersonic flight to civilian operators. In an era when the world has gotten smaller and the pace of life faster, the speed of our passenger and cargo aircraft simply hasn’t kept up. One could even argue that its slowed down, as during the Concorde’s years of service consumers at least had the option of riding a supersonic liner if their trip matched up with one of the destinations it serviced.

Assuming they could afford it, that is. Critics will claim that no matter what NASA is able to prove with the X-59, the cost of supersonic flight will simply be too high for the average consumer. Or that the amount of fuel consumed by a supersonic aircraft will be off-putting in an increasingly ecologically aware society. Those points may well be valid, but unless the laws which currently shackle commercial supersonic flight are lifted, we’ll simply never get the chance to find out.

35 thoughts on “Shushing Sonic Booms: NASA’s Supersonic X-Plane to Take Flight in 2021

  1. “With today’s computer modeling, it might seem strange that the X-59 needs to be built at all. The science of supersonic shock waves is well understood at this point, and neither NASA nor Lockheed Martin really need a physical test object to tell them what they already know. Whether or not the X-59’s unique shape will cut down on the intensity of sonic booms isn’t really a question; the reason they’re building it is to prove it to the average American taxpayer.”

    Test vehicles and prototypes have an amazing ability to point out holes in theory and flaws in simulation. The more complex the system (and this is to be a very complex system) the more issues come to light, but this is still necessary with simple systems.

    True, they don’t need the physical test object to tell them what they know. They need it to tell them what they don’t know, and more importantly, what they think they know, but is wrong.

    1. Well said and in relation to atmospheric effects of which the permutations are huge, not just the shock wave spectra and energy but, its psychological complexities for ground-huggers such as at various altitudes further complicated by weather effects. Also the landing takeoff sounds below supersonic etc etc yada yada…

      1. It has nothing to do with the engines of the plane, on which a “muffler” would be of no use in preventing a sonic boom. It has to do with the shape of the aircraft and the flow of air over and around it. The noise is produced not by the plane, but by the air, and you can’t muffle that.

      2. The primary concern isn’t engine noise; that’s been addressed pretty well by engine manufacturers. The big problem is shock waves from the airframe itself compressing air as it flies through the air.

  2. Hey, hey. Galveston, TX, checking in. I was part of the observation survey group. Penn State would give us surveys to complete to log when and how low the sonic thumps were.

    Some were quite loud, others we must not have noticed. The loudest sounded like an empty dumpster crashing down. I remember being in a local retail store one afternoon when we heard one boom. Other shoppers also paused and looked outside. Our neighbors (who had recently moved here a couple weeks prior) were unaware of the test and said they kept hearing thunder when there wasn’t even a cloud in the sky.
    I’m glad I got to hear some of the test booms. I don’t know I’d want to hear them all the time if I lived along an air way or major flight path – although I’m sure there were some thumps that were indistinguishable from your everyday urban noises.

    1. Low Earth Orbit is 90 minutes. “1 hour flight around the world” would be powered all the way, with the thrusters pointed away from the Earth, at right angles to the direction of travel. Not even remotely reasonable.

      1. “Pointed away from the earth?” the thrust vector needs to be point to space, so would that not be pointing the thrusters pointing towards earth? (the way I’ve known thrusters is flame/force out the back, pushing forward.. so pointing away from the earth, engines are pointing thrust to the ground)

        anywho, lest see, to lap the world 40,075 km, over the hour, vs escape velocity of 40,270 km/h, i think you’ll still need to point horizontally, but you will have to be in the high atmosphere, since that about mach 33, and gonna be hot.. once you get upto speed, you need less energy to cruse at that speed, but once you stop, its reentry all the way baby!

        1. let me think a little more, that Escape velocity.. not orbital entry velocity.. hmm, so you’re right, you will need to pull it down some to hold it into the atmosphere, and i my mind is telling me you’ll need more thrust at 90 degrees then what it took to get it upto that speed, Oh boy

          1. You would not want to stay in the atmosphere anyway at that speed. So in reality it would be a more ballistic suborbital trajectory. Similar to an ICBM but without the big boom at the end. :-) The fuel you saved by not “holding it in the atmosphere” you can put to good use to brake at reentry. A problem at this kind of flight will be serving lunch or using the toilet – difficult without gravity. But in micro-gravity you probably need more sick bags instead of lunch anyway.

    2. So we should cut down on the security theater at the airport. This is already annoying, not every flight is long distance “around the earth”. But this security time waste at the airport stays the same also for flights less than 1h.

  3. if you want to make people not hear the sonic boom. all you have to do is make another sound that is way louder, in order to drown out the boom. ie, someone stepping on lego bricks, or the screams of a university student who clicked delete instead of save on their 10,000 word essay.

  4. Interesting that if you push a body through a fluid at high speed close to the speed of sound you will still get pressure waves just as if you’re putting the boat through water you see a band waveform off the front of the back so we’re going to be sonic you will still get these pressure ways for me I’ve heard one idea of trying to support them that either different thermoclines in the atmosphere which will differ from one side of a continent to another meaning that some people’s will still experience a sonic crack or a bang the reason that aircraft have side by side cop it is for safety reasons in terms of control and airworthiness however this will take a change of in legislation from the ICAO as well as the FAA to allow this aircraft to fly with passengers over civilian areas very interesting that the Americans will move the goalposts to suit their own industry yeah when the British and French showed up with Concorde it wasn’t allowed because of these very reasons.

  5. So maybe an odd question, but why not have an antenna/probe that points way out front of the aircraft, rather than a large nose section? Does the cross section of the protrusion need to be a certain width relative to the rest of the body of the aircraft in order to lessen the compression behind it?

    1. The sharper the better. The biggest issues are the thermal and structural forces involved. We do not have materials that can survive that. Also, the body has to have smooth transitions rather than abrupt changes in area (for similar reasons).

      The pointed tip has an attached shock with massive heat transfer into the small tip. A blunt tip will create more drag, but has a separated shock which will produce more heat overall, but lower peaks.

      Here’s an earlier, but impractical iteration of the Quesst:

    2. Any subsequent widening creates another shock so it you had a super thin needle on the front of the existing nose you’d get a near undetectable shock off the needle (if it points perfectly into the airflow) but the nose would give the compression wave it always does. Adding lots of spread out shocks would help close to, but there is a problem… leading shocks heat up the air a little, increasing the speed of sound for subsequent waves to catch up. This means compression waves always try to intensify into big booms. The key is to be far enough away that they die away in intensity. This leads to the elephant in the room… when people hear sonic booms they are from military aircraft and not at 55,000 feet, where supersonic airliners like Concorde would operate. The sonic boom from this altitude of a pointy plane has never been bad, no damage just possibly an annoying noise. Otherwise there would be a lot of complaints from shipping if their windows blew out as Concorde flew over. I don’t doubt with modern modelling and without the need to carry a large number of passengers efficiently, this X59 will be a fair bit quieter. But as has been pointed out, modelling can tell you how loud it is, and calibrated speakers can do research. This plane is the cost of overturning protectionist laws built on misleading comparisons, by pretending things have advanced to a game changing degree, when the biggest change is that US industry is now likely to lead instead of losing market share. There should always be an altitude limit, and beyond that the decision is the same arbitrary decision about aircraft noise. A house under a supersonic flightpath won’t enjoy unbroken silence, but neither will a house near an airport of subsonic airliners, and the disturbance is comparable even with Concorde… but also, I guess, politicians rarely live next to the airport.

  6. Thank you for info and im very happy that they building, but from my experience this Quote is not true: “the reason they’re building it is to prove it to the average American taxpayer.”

    When and who asked taxpayers whenever? rhetoric question!

    1. Good point. They are probably building it to prove to the chuckleheads in Washington that it works. Maths and technology aren’t really their strong suit. Heck their are a few of them that don’t or haven’t used email or a computer.

    2. They are just giving the American tax payer a voice louder than the echoes of the politicians who pretended to be speaking for the American taxpayers when Concorde was banned. Back then they scared some people with the damage that military aircraft had caused on occasion by going supersonic at lower altitudes and extrapolated to ban a foreign aircraft from taking market share from US industry.
      It is last century’s political games taking money out of this century’s limited NASA budget.

  7. Another NASA X-plane that probably won’t fly. Remember how many “space planes” we had. Meanwhile NASA has no capability to put a human in orbit. I am so aggravated with NASA lately, this is not the same team that put humans on the moon. Seems to me like the place is run by professors rather than engineers these days, they study and study and never deliver a functional item that they tried to wow us with.

    1. You are right, they are nothing like NASA of the 60s. In the 60s the US government plowed what has been argued as an unsustainable amount into the space race. Many technological advances were made but not much more science. As soon as the US and USSR stopped measuring trouser snakes in terms of achievements in space, the budget went away. In order to survive NASA went from pushing for tech advances to minimising need for development (minimising tech advances). To make matters worse, industrial lobbying and government control of programs and contracts places further challenges on doing things sensibly. With the SLS, there is almost a case of providers saying “if we do this efficiently we won’t need the tens of thousands of people we employ, you wouldn’t want to be the government that made all these voters redundant would you?” NASA end up having to work out how to put together lots of ready developed equipment that should already have the development cost amortised but somehow seems to cost far more than SpaceX’s kit which has been developed from scratch. And yet they still manage to achieve incredible things for science (e.g. Mars Insight). Don’t be annoyed at NASA, feel sorry for them.

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