5 ms·

"Pilot Lt Col Ed Yielding and Reconnaissance Systems Officer Lt Col Joseph Vida took off at Palmdale, California and landed at Washington-Dulles a scant hour, f

"Pilot Lt Col Ed Yielding and Reconnaissance Systems Officer Lt Col Joseph Vida took off at Palmdale, California and landed at Washington-Dulles a scant hour, four minutes, and 20 seconds later."[1] I don't remember any sonic boom complaints when this or other record setting SR-71 flights over the US occurred. If you are flying in thin atmosphere the magnitude of the sonic boom at the surface should be much reduced. So, maybe it would be ok to fly supersonic at 70,000+ feet. Of course, it might be disconcerting if you were flying subsonic at 38,000 feet. [1] https://www.google.com/amp/jalopnik.com/the-sr-71-blackbird-retired-by-flying-coast-to-coast-in-1689846454/amp https://www.google.com/amp/jalopnik.com/the-sr-71-blackbird-...

vilhelm_s 9y ago
The SR-71 definitely did produce sonic booms, there is a Stackexchange thread about it here: https://aviation.stackexchange.com/questions/17661/can-a-sonic-boom-produced-at-60-000-be-heard-on-the-ground https://aviation.stackexchange.com/questions/17661/can-a-son... They also point out that the space shuttle flew even higher and still made booms. I think the lack of complaints for the SR-71 may be more due to the fact that it didn't fly very often, and that people had more understanding for the need for military aviation to be supersonic.
- c517402 9y ago
  Thank you for the link to the stackexchange discussion. I read it and the Wikipedia and NASA pages it links to. Based on the Wikipedia link the calculated second table by the stackexchange author is incorrect. The Wikipedia article points out that the path of the N-wave will generally be curved upwards due to the temperature gradient from altitude to the ground making the distance longer. The path of the N-wave is not going to be the cone normally depicted. But, there are two things not discussed in the Wikipedia article that I think will also play a role in the magnitude of the sonic boom. 1) As altitude increases, air density decreases; meaning that for a given speed a smaller number of air molecules will be displaced thus reducing the magnitude of the sonic boom at the ground. 2) The sharp N-wave close to the supersonic source is not a soliton and will undergo dispersion reducing the over pressure. That is, as altitude increases and the distance to the ground increases, the N-wave is going to spread out into more of a layed-over-S wave reducing the magnitude of the sonic boom. With regard to the SR-71 making record setting flights and flying at the limits of its flight envelope to do so(100,000+ feet or "the edge of space"), I think the sonic boom at that altitude may not be noticeable to human hearing. I know the that the SR-71 and the Space Shuttle have produced very noticeable sonic booms, but what isn't answered is at what altitude did the sonic booms become noticeable or annoying. It seems to me that with all the things that reduce the the magnitude of a sonic boom that it may be possible to fly supersonic without being annoying and slow down to subsonic to takeoff and land. It might also be possible to design the aircraft body to promote dispersion of the N-wave. Typo edit
  - vilhelm_s 9y ago
    Yeah, there's lots of potential issues, I don't know. It's true that the air is much thinner higher up, but I'm not sure how much that will help--I think a large part of the sonic boom comes from air displaced by the plane to create lift, and of course the momentum imparted by the plane to the air is constant (= the weight of the plane). Also, I'm not sure the fact that the rays that forms the sonic boom are curved makes the sound itself weaker, it just takes longer to propagate down the the ground. But the fact that the sound spreads out towards the sides of the plane should help, the circumference of the shock cone at the point where it touches ground is proportional to the height of the airplane, so I'd expect things to scale down because of that? I found and skimmed this document, "Review of Sonic Boom Theory" by K.J. Plotking (http://adl.stanford.edu/aa210b/Lecture_Notes_files/AIAA-1989-1105-891.pdf http://adl.stanford.edu/aa210b/Lecture_Notes_files/AIAA-1989...), which has some helpful information. First, it notes that the shape of the wave does change while it propagates, but it turns out that the effect is in the opposite direction from what you suggested. That is, close to the aircraft the pressure forms a smooth S (the "mid-field" shape), but then as it moves further away, nonlinear effects causes the wave to bunch up to create the discontinuous N-shape. (See fig 1 and 2 on page 30.) So it seems, in this respect flying higher is actually worse---most research about reducing sonic booms tries to shape the aircraft to be long and thin, in order to make the waveform smoother, so we'd want there to less time for the wave to bunch up again. Second, the document actually has a diagram of how the over-pressure depends on altitude! In fig 10 on page 34 it plots the calculated overpressure from a hypothetical Mach 2.7 supersonic transport. As I eyeball it, the over-pressure goes from 212 pounds/ft^2 at 40,000 feet (airliner level) to 129 pounds/ft^2 at 80,000 feet (SR-71 level), so it went down by a factor of 0.6. (In a cute coincidence, that 60% is almost exactly what's predicted by the scaling law eq. 25 on page 16, even though that equation is only intended for hypersonic aircraft.)
    - c517402 9y ago
      For an Advanced SST the quote near the end is, "Advances in aircraft technology suggest that sonic boom amplitudes for this type of aircraft, also referred to as a High Speed Civil Transport (HSCTJ or the "Orient Express". could be made substantially lower than first generation SSTs. potentially to the point that overland flight could be acceptable." I'm not sure what to make of Fig. 10. I'm sure they are accurately plotting their equations, but the impulse for the SST is a minimum at 50,000 feet and increases for higher altitudes altitudes. The impulse for the HST generally increases with altitude. OTOH they discuss the N-wave forming in the far field and maintaining its shape, but also broadening which seems like it would reduce the impulse just like the overpressure. The references 31-33 have alot of data. Here is a link to the first one: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19640014910.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/196400... PS - If you are getting three digits of accuracy eyeballing Fig. 10, you must have gotten a much cleaner copy than I did. ;)

vilhelm_s 9y ago

The SR-71 definitely did produce sonic booms, there is a Stackexchange thread about it here: https://aviation.stackexchange.com/questions/17661/can-a-sonic-boom-produced-at-60-000-be-heard-on-the-ground https://aviation.stackexchange.com/questions/17661/can-a-son... They also point out that the space shuttle flew even higher and still made booms. I think the lack of complaints for the SR-71 may be more due to the fact that it didn't fly very often, and that people had more understanding for the need for military aviation to be supersonic.

c517402 9y ago
Thank you for the link to the stackexchange discussion. I read it and the Wikipedia and NASA pages it links to. Based on the Wikipedia link the calculated second table by the stackexchange author is incorrect. The Wikipedia article points out that the path of the N-wave will generally be curved upwards due to the temperature gradient from altitude to the ground making the distance longer. The path of the N-wave is not going to be the cone normally depicted. But, there are two things not discussed in the Wikipedia article that I think will also play a role in the magnitude of the sonic boom. 1) As altitude increases, air density decreases; meaning that for a given speed a smaller number of air molecules will be displaced thus reducing the magnitude of the sonic boom at the ground. 2) The sharp N-wave close to the supersonic source is not a soliton and will undergo dispersion reducing the over pressure. That is, as altitude increases and the distance to the ground increases, the N-wave is going to spread out into more of a layed-over-S wave reducing the magnitude of the sonic boom. With regard to the SR-71 making record setting flights and flying at the limits of its flight envelope to do so(100,000+ feet or "the edge of space"), I think the sonic boom at that altitude may not be noticeable to human hearing. I know the that the SR-71 and the Space Shuttle have produced very noticeable sonic booms, but what isn't answered is at what altitude did the sonic booms become noticeable or annoying. It seems to me that with all the things that reduce the the magnitude of a sonic boom that it may be possible to fly supersonic without being annoying and slow down to subsonic to takeoff and land. It might also be possible to design the aircraft body to promote dispersion of the N-wave. Typo edit
- vilhelm_s 9y ago
  Yeah, there's lots of potential issues, I don't know. It's true that the air is much thinner higher up, but I'm not sure how much that will help--I think a large part of the sonic boom comes from air displaced by the plane to create lift, and of course the momentum imparted by the plane to the air is constant (= the weight of the plane). Also, I'm not sure the fact that the rays that forms the sonic boom are curved makes the sound itself weaker, it just takes longer to propagate down the the ground. But the fact that the sound spreads out towards the sides of the plane should help, the circumference of the shock cone at the point where it touches ground is proportional to the height of the airplane, so I'd expect things to scale down because of that? I found and skimmed this document, "Review of Sonic Boom Theory" by K.J. Plotking (http://adl.stanford.edu/aa210b/Lecture_Notes_files/AIAA-1989-1105-891.pdf http://adl.stanford.edu/aa210b/Lecture_Notes_files/AIAA-1989...), which has some helpful information. First, it notes that the shape of the wave does change while it propagates, but it turns out that the effect is in the opposite direction from what you suggested. That is, close to the aircraft the pressure forms a smooth S (the "mid-field" shape), but then as it moves further away, nonlinear effects causes the wave to bunch up to create the discontinuous N-shape. (See fig 1 and 2 on page 30.) So it seems, in this respect flying higher is actually worse---most research about reducing sonic booms tries to shape the aircraft to be long and thin, in order to make the waveform smoother, so we'd want there to less time for the wave to bunch up again. Second, the document actually has a diagram of how the over-pressure depends on altitude! In fig 10 on page 34 it plots the calculated overpressure from a hypothetical Mach 2.7 supersonic transport. As I eyeball it, the over-pressure goes from 212 pounds/ft^2 at 40,000 feet (airliner level) to 129 pounds/ft^2 at 80,000 feet (SR-71 level), so it went down by a factor of 0.6. (In a cute coincidence, that 60% is almost exactly what's predicted by the scaling law eq. 25 on page 16, even though that equation is only intended for hypersonic aircraft.)
  - c517402 9y ago
    For an Advanced SST the quote near the end is, "Advances in aircraft technology suggest that sonic boom amplitudes for this type of aircraft, also referred to as a High Speed Civil Transport (HSCTJ or the "Orient Express". could be made substantially lower than first generation SSTs. potentially to the point that overland flight could be acceptable." I'm not sure what to make of Fig. 10. I'm sure they are accurately plotting their equations, but the impulse for the SST is a minimum at 50,000 feet and increases for higher altitudes altitudes. The impulse for the HST generally increases with altitude. OTOH they discuss the N-wave forming in the far field and maintaining its shape, but also broadening which seems like it would reduce the impulse just like the overpressure. The references 31-33 have alot of data. Here is a link to the first one: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19640014910.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/196400... PS - If you are getting three digits of accuracy eyeballing Fig. 10, you must have gotten a much cleaner copy than I did. ;)