ANNOUNCER: The Sonic Boom: Our greatest obstacle for super sonic flight. Can commercial aircraft exceed the speed of sound without creating the thunderous shock wave? Is it possible to reduce the sonic impact and boost aircraft performance? Find out on NASA EDGE.
BLAIR: We’re here with Peter Coen. Peter, your job is to actually explore technologies related to supersonic flight. Can you tell me a little bit about the research you’re doing in that area?
PETER: Sure, you bet. What NASA is trying to do is put the technology in place that will enable a U.S. company to build a supersonic airline sometime in the near future. We’re primarily working on technologies that help reduce the environmental impact of a supersonic airplane, reducing sonic boom, take off and landing noise, high altitude emissions. We’re also working on some technologies that help improve the efficiency and reduce the weight and reduce the drag of a supersonic aircraft. Our primary focus right now is on what we consider the toughest barrier to future supersonic aircraft, the sonic boom.
BLAIR: Take me through how they actually occur, why they occur, or why they’re difficult to mitigate.
PETER: Yeah. The simplest answer is the sonic boom happens because the air doesn’t know the plane is coming.
BLAIR: It sneaks up on it.
PETER: Well, when anything that is traveling through the air, it’s pushing the air out of the way. It’s creating a pressure change. What’s a pressure change? A sound. Okay? So, in effect, the pressure wave in front of a subsonic airplane travels out in front of the airplane allows the air to get ready for the fact the airplane is coming to start to move out of its way. Airplanes flying supersonic, it’s going faster than sound. So, it’s going faster than its pressure wave. When it arrives the pressure changes instantaneously and creates what we call a shock wave. The shock waves come off the nose of the airplane, the wings, the canopy, the engine inlets and they go out in three dimensions. They travel out in all directions from the airplane. The ones we’re most interested in are ones that go down towards the ground.
BLAIR: That’s why the cone is so important in the design because that’s the first point of contact with that pressure.
PETER: Exactly. What we’re trying to do is make that initial shock wave be the right strength and then the waves that follow it be of similar strength and position so that they don’t merge when they hit the ground. Because in an ordinary supersonic airplane as those waves travel towards the ground, they merge, so at a very short distance from the airplane, actually, you are left with an acoustic phenomenon of two loud pressure impulses.[Boom Boom]
PETER: A bang, bang sound. That occurs under the entire flight path of the airplane. A lot of people mistakenly think that a sonic boom is the sound of the airplane breaking the sound barrier. Well, it doesn’t actually start until you get to above Mach 1, then when you’re cruising along all under the flight path of the airplane to a distance of about 25 miles on either side of the flight path is affected by the sonic boom. That’s why it is such a problem. That’s why you really have to reduce it because so many people would be affected by this loud sound if you were to try to fly a F-18 or a Concord across the country. One of the most important things to figure out in the sonic boom research is how quiet is quiet enough. We have done research using simulators, basically large speaker arrays. We’ve given people a variety of sounds to listen to and say is this sound acceptable, what’s annoying about this sound, etc. But the original simulator is just a little cement block booth with a chair in it and a wall of speakers on the door. We have a bigger one now but it’s not really an environment that’s typical to what people experience everyday. We’re always looking for ways to make the experience more realistic. We found out something real interesting about sonic boom is that the further it travels the atmosphere affects it. So, that even a normal boom, if we make it travel far enough, it starts to sound like one of our idealized shape boom waveforms. What we found is if we dive the F-18 almost straight down, just about Mach 1, it produces a sonic boom that comes off the top of the airplane, which is a weaker set of shock waves, and then travels a very long distance before it reaches the ground. We also found that we could control the amount of sound that’s hit at a certain location by controlling the dive point of the airplane; where the pilot starts his dive.
BLAIR: Whose idea was that to make the plane dive to do this research?
PETER: A researcher out of NASA Dryden by the name of Ed Haering, who has made a lot of contributions to sonic boom. When SpaceShipOne was flying he went up and measured the sonic boom. When he saw the characteristics of that boom he said boy, we could use something like this in our research. His first thought was we’ll use a sounding rocket. That comes almost straight down. That’s a pretty good idea. Then we thought well, that sounding rocket is going to go right into the community that we want to listen to. That’s probably not such a good idea. Within a few weeks, he had come up with the idea of diving the airplane. Then the idea was to create a maneuver that is repeatable, in other words, fairly easy for the pilot to do but would produce the characteristic that we did. So we tried that a number of times. We worked on it in the desert. Since then we’ve done research with it. We’ve primarily used the Edwards Air Force Base housing community as our research subjects. They have to be there anyway.
PETER: But it’s also under the supersonic flight card out at NASA Dryden. We’ve done experiments that have tested people’s reactions to sonic booms, tested structure’s reactions to sonic booms, and actually tested the procedures that we might use if we were going to do a test somewhere else and try and judge reaction to sonic boom. It’s really become a key element of our research to be able to produce these simulated low sonic booms using the F-18. Ultimately, we have to get out, get off the range, so to speak. We have to get out over an average community that does not hear sonic booms on a regular basis and their livelihood is not related to supersonic flight. What we need is a demonstrator. An airplane that is small, relatively affordable but still produces that desired shape sonic boom characteristic. We can go out and fly it over any community and have our microphones and stuff like that on the ground and begin to develop the data that says these booms are acceptable. Then, we turn it over to the regulators and say, please make us a law that says supersonic flight is allowable over land.
BLAIR: Now, is that demonstrator a plane, what you’re developing in the wind tunnel or what other research are you doing in the wind tunnel?
PETER: At the moment, one of our key, most recent successes was we looked at the design of a small, low-boom airliner. Based on those designs we created some wind tunnel models and verified that our design tools enabled us to produce a design that meets our low-boom target. We’re fairly confident we can do this now. We need to demonstrate it in a realistic atmosphere. Our work has been focused on an actual potential product airplane. Now, in order to do the final verification, we’d like to get an opportunity, in the future, to do a flight demonstration.
BLAIR: And is that a unique aircraft that you’ve created for that demonstration?
PETER: Well, we haven’t created it yet but it would be a unique airplane. Again, shape is king and paramount. So, we’d probably use an existing engine, maybe existing landing gear from a current aircraft but the shape would basically have to be a new design shape.
BLAIR: And if successful, you could go on to produce planes that would fly commercially possibly with low sonic booms, meaning I could get to Los Angeles for lunch and…
PETER: Back home for dinner.
BLAIR: If luggage and everything else wasn’t an issue.[Laughing]
PETER: That technology still needs to be worked.
BLAIR: Awesome. Looking at some of the images in the wind tunnel, it looks like the plane looks very similar to Firefox that Clint Eastwood flew in the movie back in the 80s. Did you guys borrow from that technology?
PETER: Not really, no but it is kind of interesting that the airplane is not really all that different from other supersonic aircraft designs, in that it still has a delta wing, and sonic inlets. The key thing that we’re really going after is the detailed, three-dimensional shape of the airplane, particularly the shape of the nose, and particularly how the engine is integrated.
BLAIR: Have you seen some results in your research where you’ve actually been able to mitigate the level of the sonic boom?
PETER: Actually, we have recently been quite successful in demonstrating in an integrated sense a technology for sonic boom that’s actually been around for a long time, since the early 1970s. The idea of creating a shaped sonic boom has been studied. In other words, instead of having two large shock waves that create that boom, boom sound that you hear.[Boom, Boom]
PETER: You’re creating a series of smaller shock waves that are equal in strength and positioned carefully along the length of the airplane. What happens is they don’t merge. They stay as individual shocks and then you get the advantages of atmospheric continuation, each shock wave gets reduced in volume. By the time it gets to the ground, you’ve got something that sounds more like a thump than a boom. It turns out though if you’re trying to create a balanced design, you’re best off flying at the altitude of the Mach number that matched the performance of your engine and give you the best overall efficiency. And then set your sonic boom target based on that. We’re pretty much going to fly at the same altitude as the Concord, 15,000 feet or so. One thing that’s a little bit different is we’re targeting a lower Mach number.
PETER: Concord flew at Mach 2, twice the speed of sound. These future airplanes will be slightly slower, maybe Mach 1.6 to Mach 1.8. There are several reasons for that. One is the shape of the sonic boom solution is easier to achieve in a practical design at those lower Mach numbers, but then there’s also things like the noise performance. We can make a better engine for take off and landing noise by flying at Mach 1.8 than we can at Mach 2.
BLAIR: Not many people are going to quibble over 1.6 and over 2, I think in the long run because of the increased efficiency.
PETER: I think in the long run if you’re reducing your cruise flight time by nearly a factor of 2 that’s going to be a benefit worth going for. But our goal in reducing the sonic boom is to change the law. The U.S. law currently says you shall not fly greater than Mach 1 over the U.S. unless you have a permit from the FAA. We would like to replace that with “you shall not produce a sonic boom louder than x dBs as you fly over land.” That’s primarily the goal of our current research.
BLAIR: I’d like to work something in that law that means I can get an aisle seat not matter when I fly if I could.[Laughing]
PETER: Now you’re really asking for some tough stuff.