NASA Chat: Taking the "Boom" Out of Booms
Sonic booms usually mean something cool. The space shuttle is coming in for a landing or a jet fighter is flying overhead. We don't hear them very often, so when we do it's an event.
But imagine if aircraft manufacturers designed and built a vehicle that carried passengers or cargo at supersonic speeds over land. Sonic booms would be happening all the time; and they're loud and annoying. That's why the Concorde flew over the ocean. Noise regulations in most countries wouldn't allow it to fly over land because of the sonic booms it generated.
Sonic booms are keeping a new era of supersonic cruise flight from happening.
For us to ever be able to enjoy the benefits of flying people or cargo over land at super-fast speeds, we have to figure out how to turn down the volume on sonic booms.
NASA has been doing flight tests and simulations and ground experiments -- with cool names like "Quiet Spike," "SonicBOBS," "SonicBREW," "LaNCETS," "House VIBES," "Low Boom/No Boom" – to help find answers.
What is a sonic boom? How is it created?
Do sonic booms cause damage? To people? Structures?
Does changing the noise level of a sonic boom affect aircraft speed?
On Tuesday, January 25, at 3:00 p.m. ET, NASA aerospace engineer and "sonic boom guru" Ed Haering answered your questions about what it's like to try to tame a sonic boom. See Chat Transcript below.
More About Ed Haering
Ed is an aerospace engineer at NASA's Dryden Flight Research Center in Edwards, Ca. He really likes the variety of his work, which can involve starting up a new research program, doing computer simulations and analysis, troubleshooting instrumentation on exotic
aircraft, and conducting field measurements in remote locations.
› Video of Dryden Sonic Boom Big Structure Test
› Video of Future Supersonic Design Wind Tunnel Test
› Feature: SonicBREWS
› Feature: Quiet Spike
› Feature: Shaped Sonic Boom Demonstration Project
› NASA's Supersonics Research Project
Karen (Moderator): We've received a bunch of questions for Ed Haering ("Ed." in the chat room), so we're going to go ahead and start posting responses. Here we go! (But quietly, of course.)
Steve: Hey - Gonna cut right to the chase, how does throwing a 'needle' like cone help dissipate the pressure of supersonic flight? I understand it cuts through the air, but is not the problem with the pressure that builds up on the wings?
Ed.: There is a shock wave that is generated around each part of the aircraft, nose, wing, inlets, and tail. Current supersonic aircraft generate booms that combine into one big shock at the front and one at the back of the aircraft. The next generation of low-boom aircraft needs to keep these shocks separated so they won't combine to create big booms, and then they will fade as they travel down to the ground. A "needle nose" may be one part of a low boom aircraft, but other shaping is needed too. The short answer is the whole aircraft needs to be reshaped to be quieter.
nasa5: Could you make a sonic boom on the moon with current technology?
Ed.: No, no air, no boom.
Quietlike: Do sonic booms really have the potential to bother people that much? I mean, does the aviation community believe there might someday be enough supersonic aircraft in the air that it would be a pain to people?
Ed.: Right now there are sonic booms around military bases, and some of them are very loud. They can be startling. If the airplane is very low to the ground, it can break windows. The very low altitude supersonic flights are done in very remote areas. These future low boom aircraft should be very quiet and be able to fly anywhere around the world.
Quietlike: Can you tell us a little bit about the airplane that's behind you in the picture? It's got a nose that kind of looks like a duck boat.
Ed.: That was the Shaped Sonic Boom Demonstrator, SSBD, an F-5E that was given a new nose by Northrop Grumman. This new shape reduced the boom from the nose of the aircraft. On the aircraft you can see two stripes. The red stripe represents the 'pressure versus time' of the regular F-5E, and the blue line shows the SSBD pressure, with a lower pressure at the nose region. It was very successful, flying in 2003-2004.
chetman1020: Would fixing this problem lead to more supersonic commercial flights?
Ed.: We hope so. Wouldn't it be great to be able to fly coast to coast in half the time? Business could be conducted on another continent and then you could be back home the same day.
richapril5: How are sonic booms created?
Ed.: You can think of booms on aircraft like the wake of a motorboat. The boat moves faster than the ripples can move out of the way, so you get a "V" shaped wake on the water that travels outward, maybe rocking other boats or washing on shore. On an aircraft we have the 3-dimensional version of this, with a cone-shaped wake behind the aircraft. When that bottom of the cone hits the ground, you generally hear two booms. In some cases the cone is warped by winds and temperature changes in the atmosphere and the cone never reaches the ground at all. This is called the Mach-cutoff condition. We have done flights at 1.2 times the speed of sound (Mach 1.2) overhead and we do not hear any boom at all. Some companies are taking advantage of this technique to keep their supersonic aircraft quiet.
Ltpinter: Hi Ed. I hope NASA is keeping you busy on really cool stuff. I would like to know if sonic booms can be reduced to a low rumble?
Ed.: Yes, we can make sonic booms that are very quiet, and can't be heard over normal conversation. It sometimes sounds like distant thunder. And referring to my last comment sometimes you can make the boom totally quiet if the aircraft is slow enough or high enough in altitude.
Nir_A: How are you planning to reduce the level of the sonic booms?
Ed.: There are two parts to reducing the booms -- the shape of the vehicle and the vehicle size. Things like nose spikes and reshaped bodies and wings will help keep the individual shock waves from each part of the aircraft from combining to be too large. A smaller aircraft is easier to keep quiet than a large aircraft. We will start with small civil supersonic aircraft first. Large passenger supersonic aircraft that are quiet will be much harder.
Nir_A: Are you planning on using this as a weapon
Ed.: No, we are working on making supersonic civilian aircraft quiet.
Dell_Conagher: Is there much work with private companies, to improve the possibility of commercial super-sonic flights?
Ed.: Yes, we have partnered with several private companies and universities in this area.
Spacegirl: Has the spike on the nose of the plane significantly affected the noise levels of sonic booms?
Ed.: With the Gulfstream QuietSpike we showed that the shock waves didn't combine, which is what it was supposed to do. That spike was put on a standard F-15 which makes a loud boom, so it was still loud on the ground. We need to have a complete aircraft that is made for quiet supersonic flight from front to back to be quiet on the ground.
bob124: So what are some of the more promising theories/approaches to cancelling out the boom in actual flight that you've seen so far?
Ed.: Every cooperative sonic boom program we have done has shown promise, including QuietSpike, the Shaped Sonic Boom Demonstrator, and others.
Burin: Is there any chance a laser or plasma beam could be appended to the nosecone of a plane to help pierce the atmosphere and prevent the pressure wave from ever forming?
Ed.: Yes, plasma would change the gas constant of the air, potentially reducing the sonic boom. Unfortunately a small powerplant would be needed on the aircraft to generate that plasma. Maybe after the "Mr. Fusion" (from the Back to the Future movie) is ready...
Karen (Moderator): Thanks for your questions so far! Ed is reviewing a bunch of new ones right now (I can hear him talking). So keep submitting your questions. Try to skim the q's that have already been asked before asking yours in case we've addressed your topic already.
BillD104: We hear sonic booms in the Tampa Bay area on a regular basis. I just attribute them to military training in the GoM but the training areas are fairly far offshore. Do sonic booms propagate and travel better over water than land?
Ed.: Yes, that would probably be military aircraft doing training over the ocean, and sometimes their booms unintentionally reach land. The loudness of the booms is affected by humidity, humid air making the booms louder than dry air.
Hush: Do people in the planes hear them?
Ed.: No, not a boom sound, unless another supersonic aircraft flies by. We have flown supersonic aircraft in formation to measure the sonic booms. We had an F-16XL within 100 ft below an SR-71. The F-16XL pilot feels slow pressure changes on his helmet, but does not hear a boom. The normal booms we hear at NASA Dryden and Edwards Air Force Base are about 1-2 pounds per square foot. This is the pressure change you experience in going down a flight or two of stairs, but with a boom it happens in a few thousandths of a second. It is the rapid change of pressure in time that makes a boom. The pressure changes aren't very large. You need about 25 pounds per square foot of a sonic boom to break a house window.
chetman1020: Do you see this area as having high growth in the future, i.e. a possible career field?
Ed.: Yes, please ask your guidance counselor and ask how you too can join the illustrious ranks of engineers at NASA ...
John_R.: Whoa, 25 pounds per sq foot is a lot, can you make a person deaf with this loud noise?
Ed.: No, you need to get upwards of 144 pounds per square foot before we hurt anyone. We are aiming for booms around 0.1 to 0.3 pounds per square foot for the quiet supersonic aircraft.
Markster: Would a rocket have a less significant boom than a winged aircraft since it has less profile area, considering same speed?
Ed.: Booms have been measured from rockets launched from Florida and California, and some are loud. The difference is the large rocket plume itself makes a boom.
chetman1020: Do sonic booms ever disrupt other things in the air?
Ed.: Sonic booms can temporarily dissipate or accentuate a "Sun-Dog," the small bit of a rainbow off to either side of the Sun caused by high altitude ice crystals. Aircraft are not affected by booms from other aircraft.
Brad: Do you get a sonic boom and the tail end?
Ed.: Yes, current supersonic aircraft have a large boom at the nose and another at the tail by the time the shock waves reach the ground. If you graph the pressure versus time it looks like the letter "N". Future low boom aircraft will have a series of small booms that do not combine, so maybe 6 or more little shocks, each of which are not annoying.
Kjrinca: Is it possible to change the frequency of a sonic boom and thus make it less offensive of a noise?
Ed.: Yes, the human ear is more sensitive to higher frequencies than the low frequencies of sonic booms. Fortunately when we reduce the shock wave level, the frequency naturally shifts lower (longer time to rise to that pressure), so that helps the overall noise level too.
nasa5: Do you know of any websites with photos of sonic aircraft?
Ed.: You can go to NASA Dryden web site with photos: http://www.nasa.gov/centers/dryden/multimedia/imagegallery/index.html or just search on the Internet for supersonic aircraft, shock waves, etc.
davide_joshua: After an airplane passed the sound wall, if it reduces drastically its speed do the shockwaves running over it damage it?
Ed.: No, once the shockwaves are generated, they leave the aircraft just as a wake leaves a motorboat. The boat isn't bothered by its wake, and neither is an aircraft bothered by its own shockwaves.
Arvind: @Karen - is there a URL for the chat history? My page just shows the chat from the time I've joined.
Ed.: Come back to this page; after 48 hours a transcript will be posted.
Spacegirl: What is your favorite part of being an aerospace engineer?
Ed.: I really enjoy the variety of work at NASA Dryden, from working on different programs in aerodynamics, and also on a variety of current and cutting-edge prototype aircraft. I never really know what direction the research will take. I hadn't done any sonic boom work before 1993. I have no idea what new areas I'll be working on in the future, but I know they will be interesting.
Karen (Moderator): So we're heading into the last ten minutes of the chat. Stay with us! Ed's typing fingers are holding up okay so we'll take it to the last minute. Get your question in and we'll try our best to answer it if it's a new topic.
Karen (Moderator): A clarification for Arvind ... you will be able to see back about 6-8 questions from the point you joined; not the entire chat at that moment. The entire chat will be in the transcript. Thanks!
Dell_Conagher: How much commercial interest is there in developing a boom-less aircraft?
Ed.: The Aerion Corporation is working on the boomless aircraft over land (see the Mach-cutoff comments above) which will limit them to about Mach 1.1 to 1.3, and they will be flying faster over the oceans. There are several other US and foreign companies that have active efforts in supersonic aircraft development as well. They are working on quiet boom technology that will not be as restrictive of speed over land.
Shulevski: Ok, a possible naive question... an aircraft breaks the sound barrier. We hear a sonic boom on the ground and the aircraft continues to move supersonically. If there are people on the ground over which the supersonic aircraft passes, will they hear sonic booms as the craft moves over them supersonically?
Ed.: The boom continues to make a sonic boom the entire time it is supersonic, much like a motorboat continues to make a wake even after it first starts moving in the water.
Kjrinca: I live near Edwards [Air Force Base] and have noticed that the sonic booms from aircraft differ from that of the shuttle double boom, why is that?
Ed.: Sometimes the time between the front and back shocks are so short that our ears can't hear the difference. Some aircraft make three booms if their shocks haven't completely combined. Sometimes maneuvering aircraft will give you a sound with a double boom, then a few seconds of pause, and then another double boom. You are hearing booms from two different parts of the aircraft's trajectory.
DJones: Fuel efficiency is a consideration still, as that issue impacted the Concorde. How is this being addressed?
Ed.: Yes, fuel efficiency, as well as engine life will be important issues for any new supersonic aircraft.
Rammton: At what angle or vector does the sonic boom shockwave energy typically travel towards the ground?
Ed.: The angle is about the arcsine of the reciprocal of the Mach number. You need to know trigonometry to understand the last sentence. (Hey kids, yes, you do need to learn your math!!!) Atmospheric effects can warp this further.
DJones: So, which design is most effective? I have read that designs place the engines on the top of the wing, rather than underneath the wing, hoping that the wing itself will reduce the volume of boom. I have also seen a prototype of an odd, inverted V-shaped tail that might lessen the boom through airflow control. Is one design preferred?
Ed.: The design of an aircraft is always a tradeoff of many issues. The integration of the engines is probably the most difficult part. The most effective design will be the one that very few people hear. We have to flight test any new design.
Ed.: Well, we are out of time and my fingers have had enough. We've had over 135 questions, sorry I couldn't get to all of them. There were some excellent questions asked. Thanks for your interest in sonic booms and NASA's research. Ed
Karen (Moderator): Later that day, Ed took some time to answer some of the questions that were still in the queue when we had to end the chat. We weren't able to capture the Usernames in all cases, so read through and maybe your question got answered after all. Thanks Ed!
Q: Does the magnitude of a sonic boom vary depending on the aircraft size or speed at which it broke the sound barrier?
Ed.: The magnitude of the sonic boom depends on the aircraft size, shape, weight, speed, altitude, acceleration, and atmospheric conditions at every point of its supersonic flight, not just when it first breaks the sound "barrier."
Q: Does the material that the aircraft is made of have any effect on sonic booms?
Ed.: No, but it needs to be strong for structural integrity and lightweight to use less fuel.
Q: I haven't seen a lot of discussion regarding the microstructure of the surface of supersonic
Craft; is there any research into biomimicry of surfaces that prevent a pressure wave from building. Something akin to sharkskin, which creates micro-eddies and lessens drag for example?
Ed.: Yes, there is some ongoing research with certain surfaces to produce laminar flow at supersonic speeds to reduce drag, and thereby increase fuel efficiency.
Q: Have you done any other research with airplanes besides quieting sonic booms?
Ed.: Yes, quite a bit. I have 50 publications on various NASA aerodynamic research topics including sonic boom.
Q: Does changing the noise level of a sonic boom affect aircraft speed?
Ed.: No, but the aircraft speed does affect the noise level.
Q: How can I collaborate with the project? if i can do this.
Ed.: Possibly, it depends on your status. We have college intern students (co-ops) that work at NASA Dryden. [visit NASA's one-stop shop for intern opportunities: http://intern.nasa.gov]
Q: Wait -- the F-16XL pilot feels the pressure waves? Isn't the cockpit meant to be hermetically sealed?
Ed.: No, not hermetically sealed. There is a pump that keeps the cockpit pressure at a reasonable pressure for the pilot, but changes in outside pressure from the nearby supersonic aircraft make small changes inside the F-16XL cockpit that the pilot can sense.
Superstring: Hi, first of all hello from Europe! As a physics student I would like to know what's with the drag coefficient? So maybe we will overcome the issue of the sonic boom but then there is still an efficiency issue. So will we ever see commercial aircraft flying in the transonic/supersonic regime AND gaining profits?
Ed.: Yes, the current designs intend to be fuel efficient enough to generate profit. As cool as it would be to fly supersonic, a commercial venture doesn't get very far without profitability.
Q: What do you think the most important class(es) are to take in high school if you want to go into this career field?
Ed.: Math, physics, math, computer programming, and did I mention math? I would also recommend some hands on work such as robotics teams, flying aircraft and rocket models, etc.
Q: So far we been talking about noise in flight, but do you and nasa have something to decrease the noise in aircraft starting up or in the beginning of flight? (the jet engine starting up noise is way too cool but is very loud)
Ed.: Yes, NASA is also working on take-off and landing noise around airports.
Q: Is there any scope for undergrad engineering students to get involved with this (or similar research at Dryden) as interns?
Ed.: Definitely. I started at a NASA Cooperative Engineering Student (co-op) back in 1984 as a sophomore in college. I've been a mentor to over 30 students over the years. See:
http://www.nasa.gov/centers/dryden/education/coop.html for details, and ask if your college engineering department has a co-op program.
John_R: But I have that classical question, how did you get from college to NASA?
Ed.: By airplane of course. Other times I drove. :-D See the above answer about the co-op program.
Spacegirl: How did you become interested in aerospace engineering?
Ed.: From building model rockets in 4th grade. I knew then that I wanted to
be an aerospace engineer.
chetman1020: Which part of the designing process do you mostly work in?
Ed.: I don't design aircraft. I do design and perform flight research on various new designs that others have come up with, and I've figured out how to use existing loud sonic boom aircraft to make quiet sonic booms in small areas so we can test what a quiet sonic boom aircraft may sound like in the future.
Q: Why arcsine, why not just the sine?
Ed.: This is why you need to study trigonometry, to find out why. Arcsines give angles, sines do not.
Q: What is the most important subject in aerospace engineering (math-geometry algebra calculus or science-physics astronomy etc.)?
Ed.: With a three-legged stool, which leg is most important? You need them all, just like you need a mix of math, science, and practical experience.