Why the fuss over a sonic boom? Why don't we hear them more often?

[MAddis]

The media in the UK has given a lot of air time or column inches over the last couple of days to a story about a Typhoon fighter aircraft whose sonic boom was heard by a large number of people across England and which, initially, caused a lot of confusion about what it might have been.
http://www.bbc.co.uk/news/uk-england-17699357

The thing that surprised me was why a sonic boom might be so unusual and unfamiliar to people. Surely, I thought, the Royal Air Force has dozens of supersonic aircraft and they must be going supersonic all the time as part of their training regime? But apparently not. The Ministry of Defence says that they need special permission.

So, my questions are, is it really so rare for military aircraft to go supersonic? Or do they merely do it out to sea or at a high altitude where no one can hear them? Are sonic booms more commonly heard in other countries?

I grew up in the Forest of Dean and we would frequently have fighter jets (Tornados and Jaguars, plus also Harriers - although I am aware that the latter are not supersonic) screaming low and loud over our school. Was I hearing a sonic boom then, or just subsonic flight? Is a sonic boom much worse, hence the surprise and confusion across the UK on Thursday?

I'm sure that many BAUTers have the aerospace knowledge to answer me, so thanks in advance for the enlightenment. I hope I am posting these questions in the right place.

[PetersCreek]

In the US, military supersonic flight is normally restricted to airspace reserved for training, so long as that area is cleared for Mach+ flight...meaning over un- or sparsely-populated areas.

Call it being a good neighbor or good public relations. Sonic booms can be annoying and even frightening to adults, children, and animals and sufficiently close by, sonic booms can cause property damage. People who are upset by these things, often complain to their elected representatives. In turn, those representatives (who may have sway over military funding) may complain to the military. When other Mach+ training areas are available, it's kind of hard to justify a necessity for doing so near a complaining populace.

[IsaacKuo]

Fighter jets like the Typhoon can't sustain supersonic flight for very long, and do not regularly fly supersonic even during combat operations.

You did not hear a sonic boom, unless it was one or two loud sharp cracks.

In contrast, the Concorde was designed to sustain supersonic flight for hours.

[redshifter]

I used to hear them a lot in my hometown in eastern Washington (state). They'd frequently rattle windows and would be quite loud sometimes. Over here in Seattle, I've heard them maybe twice in 20 + years. The last time we heard them here were under interesting circumstances. Apparantly an unknowing pilot flew his small craft too near the airspace around Air Force One. F-15's were scrambled from Portland and apparantly the booms were from when they decelerated out of supersonic flight. Big news item that day. IIRC the pilot was questioned and found to be no threat, he just didn't realize what was going on. Not sure if his license was revoked or not...

http://blogs.seattleweekly.com/daily...eattle_dur.php

[Gillianren]

Yeah, I was going to mention that second one. I was standing outside the grocery store at the time, and the windows shook. A bunch of us were speculating about what was going on, and when the second boom came, we all said, "Oh." But here in Olympia, it was loud.

[slang]

So, my questions are, is it really so rare for military aircraft to go supersonic? Or do they merely do it out to sea [...]

Yes. For air to air combat training there are several socalled 'Temporary Reserved Airspaces' (TRA) over the North Sea.

(caveat: IIRC!) There were/are about 7 of them controlled by Dutch airforce, and I'm pretty sure the RAF has several available as well. Supersonic flight was allowed only in the northern most of those. I can't recall whether it was the northernmost 2, or more. These areas are used for live firing practices as well (using towed targets, sophisticated hit counters and flags).

I grew up in the Forest of Dean and we would frequently have fighter jets (Tornados and Jaguars, plus also Harriers - although I am aware that the latter are not supersonic) screaming low and loud over our school. Was I hearing a sonic boom then, or just subsonic flight? Is a sonic boom much worse, hence the surprise and confusion across the UK on Thursday?

You were just hearing subsonic jet noise. The Tornado is very loud at high power settings and low altitude. Try to visit some flight demonstration days, or somewhere the Red Arrows perform. Even those can be quite loud, and they're just trainers!

[PetersCreek]

You were just hearing subsonic jet noise. The Tornado is very loud at high power settings and low altitude. Try to visit some flight demonstration days, or somewhere the Red Arrows perform. Even those can be quite loud, and they're just trainers!

To that I'll add that if the Tornado is like any other fighter I worked on/around, the sound level can vary greatly depending on aspect relative to the listener. Take the F4 Phantom, for instance. They were loudest at a point approximately 45° to either side of the exhaust nozzles, while at a distance, the aircraft could seem virtually silent at certain aspects. This means a hard turning aircraft could exhibit a rapidly rising and falling rumble...kind of boomish, if you will.

Note: While deployed to Nellis AFB, Nevada one year, a squadron each of Tornadoes and Jaguars were parked down the ramp from us. I didn't get to spend any time around the jets, really but their maintenance crews were billeted in the same hotel we were. The were...um...colorful characters. We had a lot of fun off duty.

[swampyankee]

Fighter jets like the Typhoon can't sustain supersonic flight for very long, and do not regularly fly supersonic even during combat operations.

You did not hear a sonic boom, unless it was one or two loud sharp cracks.

In contrast, the Concorde was designed to sustain supersonic flight for hours.

The Eurofighter Typhoon, Rafale, F-22, and possibly the Gripen can all cruise supersonically. So could, historically, the F-104, the F-14, and probably the F-15. I believe the last three could not go from M<1 to M>1 without using their afterburners; the F-22 can, and the Typhoon may be able to do so, also.

Most aircraft can't operate on afterburner very long; as a reasonable rule of thumb thrust at 35,000 ft and M=2 is the same as sea level, static thrust. Fuel consumption in afterburner is about 2 - 2.5 lbm/hr/lbf (the higher the bypass ratio, the worse the sfc with afterburner), so an aircraft with about 60,000 lbf thrust in afterburner (sea level) will burn about 120,000 to 150,000 lbm of fuel per hour. Since the internal fuel capacity of a Typhoon is about 9000 lbm, it will run its internal tanks dry in about 4 minutes of afterburner. It's dry thrust is about 25,000 lbf (it will depend on Mach number, altitude, and the intake's efficiency) and the engine's sfc (dry) is about 0.7, so it would run its tanks dry in about 30 minutes.

[Tensor]

The Eurofighter Typhoon, Rafale, F-22, and possibly the Gripen can all cruise supersonically. So could, historically, the F-104, the F-14, and probably the F-15. I believe the last three could not go from M<1 to M>1 without using their afterburners;

The F-15A (B,C and D without conformal tanks) could not cruise above mach 1 and, yes, it needed AB to go above Mach 1 in level flight or in a climb. The higher weight of the F-15E models slows them down a bit, but in level flight, they can go above mach 1 in AB.

[slang]

The Eurofighter Typhoon, Rafale, F-22, and possibly the Gripen can all cruise supersonically.

Ref; wiki supercruise

Most aircraft can't operate on afterburner very long

So to add as much time available for training in one of the TRA's our F-16's are usually carrying external tanks. Pilots prefer a clean configuration tho, because external stores put limits to the amount of G's they can pull. So if (as planners) we had one or more clean aircraft available, they'd ask to be assigned to the local airfield TMA instead of having to travel to the TRA's first.

[swampyankee]

The F-15A (B,C and D without conformal tanks) could not cruise above mach 1 and, yes, it needed AB to go above Mach 1 in level flight or in a climb. The higher weight of the F-15E models slows them down a bit, but in level flight, they can go above mach 1 in AB.

I was referring specifically to the transition through the transonic drag rise. In general, combat aircraft are optimized for subsonic performance and manage to get supersonic through brute force. Some, between their aerodynamic design and engine properties, can cruise supersonically. I believe that, except for the F-22 it's never been a requirement.

[profloater]

The Concord could fly supersonic for long periods and unfortunately for some, supersonic flying over land was banned after Boeing decided not to pursue their version. It doomed Concord to two basic routes and limited production. It used to go supersonic over the coast of Wales on the outward trip to NY and just occasionally when I lived in West Wales I caught the bang. It was very high by then and not especially frightening. I worked on that amazing aircraft and it was a pity from the technology point of view that it's routes were curtailed. Now the energy use would also be a concern.

[Tensor]

I was referring specifically to the transition through the transonic drag rise. In general, combat aircraft are optimized for subsonic performance and manage to get supersonic through brute force. Some, between their aerodynamic design and engine properties, can cruise supersonically. I believe that, except for the F-22 it's never been a requirement.

Ahhhhhh, Ok. If that is the case, then in a clean configuration (no conformals, pylons, rails, tanks, missiles) then, yes, you are correct. However, the minute you start putting anything on the aircraft (either tanks, missiles or the gear needed to load them), to increase range or to make it combat capable, that ability goes away rather quickly. I think it can get away with having pylons and maybe rails.

[Nicolas]

Just for clarity: an aircraft going supersonic is boomin' all the time. It doesn't go boom only when going from subsonic to supersonic or vice versa, but also as long as it is supersonic. The booms you hear are the shock waves. And a plane going supersonic has a constant shockwave, usually one at the nose and one at the tail. Hence the twin booms. You don't always hear twin booms though; sometimes it sounds more like a single boom.

As for sonic boom versus jet noise. In case of low flying aircraft, if you're not sure whether what you heard was a supersonic boom or not, then it wasn't. Don't get me wrong: jet engines can be LOUD and afterburners LOUDER. But sonic booms, they have a sudden character and the power to break things. While low flying Starfighters never shattered any of our windows, they could send a crack throughout the house's construction.

We almost never hear them anymore because of regulations.

[swampyankee]

The Concord could fly supersonic for long periods and unfortunately for some, supersonic flying over land was banned after Boeing decided not to pursue their version. It doomed Concord to two basic routes and limited production. It used to go supersonic over the coast of Wales on the outward trip to NY and just occasionally when I lived in West Wales I caught the bang. It was very high by then and not especially frightening. I worked on that amazing aircraft and it was a pity from the technology point of view that it's routes were curtailed. Now the energy use would also be a concern.

Of course, part of the reason its routes were curtailed was because it was very closely tailored to the North Atlantic trade, which is still likely to be the most remunerative and most heavily used trans-oceanic route. Incidentally, I believe that the reason Boeing "decided not to pursue" their SST (the 2707) was because environmental concerns -- including sonic booms -- caused an end to government funding. In other words, Boeing decided not to pursue with their own money, especially since the likelihood was that commercial supersonic flight would not be allowed over the US.

[MAddis]

Thanks everyone.

[profloater]

I worked in the sales office during part of my apprenticeship and there were routes planned all over the world, not just the Atlantic and until supersonic flying over land was outlawed, they looked very promising. Many airstrips were already being extended or approved for extension for the much longer run Concord needed when the axe fell. Just one example was to Australia but the hops all involved overland flying. It is true the range planned was always for the New York run and all other routes used that limitation. When flying over America was assumed it was routed to avoid flying over the large cities.

[publiusr]

Mystery booms/skyquakes recent...

http://news.lalate.com/2012/04/13/sa...as-sonic-boom/
http://hosted2.ap.org/ALOPE/d0066651...7e61ab303e3d59,

[BigDon]

Also, let's not forget that ~ 45% of the regular customers of the Concord were killed in the 9/11 attacks. Can't tell me that doesn't effect your bottom line.

[Delvo]

That attack happened about 8 years after the end of Concorde flights.

The book "Skunk Works" by Ben Rich, about the secret aircraft development section of Lockheed-Martin when it made the U-2, Blackbird, and F-117, tells a story about sonic booms. They had a policy of notifying the few little communities in the area where they did their tests, whenever they had supersonic test flight scheduled. But the complaints and hostility they got over the booms got so extreme that they started doing a funny little test: they'd occasionally put out a notification of upcoming supersonic flights, not actually make any such flights at those times, and sit back and watch the boom complaints roll in anyway.

[cjl]

That attack happened about 8 years after the end of Concorde flights.

Concorde retired in November of 2003, more than 2 years after 9/11.


I do agree about the book Skunk Works though - it's an excellent read, and a must have for anyone interested in the history of military and high-performance aviation.

[swampyankee]

Ahhhhhh, Ok. If that is the case, then in a clean configuration (no conformals, pylons, rails, tanks, missiles) then, yes, you are correct. However, the minute you start putting anything on the aircraft (either tanks, missiles or the gear needed to load them), to increase range or to make it combat capable, that ability goes away rather quickly. I think it can get away with having pylons and maybe rails.

It's not the aerodynamicist's fault that air forces want to turn their aircraft into flying airbrakes.

[Tensor]

It's not the aerodynamicist's fault that air forces want to turn their aircraft into flying airbrakes.

Hehehehe, yeah, with as much stuff as they put on the E model, I'm surprised the thing got over 650 knots.

[Tog]

I just want to say (somewhat tongue-in-cheek) that the F-22 doesn't really cruise at supersonic speeds as much is it desperately tries to avoid the horrendous noise it makes.

I used to live about 1 mile off the final approach for an air base. I got to the point I could identify F-16's and A-10's from the sound alone. The only thing that was really obnoxious was the KC-135 or 10's that took off to the north. They'd bank away and we'd get a tail-on shot of them for about 3 minutes as they climbed away. Any one of them made it hard to hear the TV when they went by.

Then the F-22's came to visit.

In my car, at freeway speed, with the windows up and the radio on loud enough to drown out the road noise, I can hear F-22s from miles off when they take off. When they land it's nearly as bad. It doesn't just wash out the TV, it washes out the yelling across the living room.

I've heard that the US has noise restrictions that apply to military craft, and that the Tornado is an exception since it's not a US plane. I've read where people complain about the noise Tornados make, but I can't imagine it could be much worse than an F-22.

[cjl]

I just want to say (somewhat tongue-in-cheek) that the F-22 doesn't really cruise at supersonic speeds as much is it desperately tries to avoid the horrendous noise it makes.

I used to live about 1 mile off the final approach for an air base. I got to the point I could identify F-16's and A-10's from the sound alone. The only thing that was really obnoxious was the KC-135 or 10's that took off to the north. They'd bank away and we'd get a tail-on shot of them for about 3 minutes as they climbed away. Any one of them made it hard to hear the TV when they went by.

Then the F-22's came to visit.

In my car, at freeway speed, with the windows up and the radio on loud enough to drown out the road noise, I can hear F-22s from miles off when they take off. When they land it's nearly as bad. It doesn't just wash out the TV, it washes out the yelling across the living room.

I've heard that the US has noise restrictions that apply to military craft, and that the Tornado is an exception since it's not a US plane. I've read where people complain about the noise Tornados make, but I can't imagine it could be much worse than an F-22.

If you're comparing the 22 to the 16, I would imagine it would be quite a bit louder, as it is still using low bypass, afterburning turbofans, but it is twice the weight with more than twice the thrust. I doubt it's much louder than comparably sized fighters though, so an F-15 or F-14 might be a better comparison. Similarly, the A-10 shouldn't be terribly loud, as it has relatively little thrust and a higher bypass ratio. If anything, all the sources I can find on the F-22 indicate that it should generate less noise than average for a figher with its level of installed thrust and bypass ratio.

[BigDon]

Mr. Tog, I can't quite disagree as I've never heard an F-22 inflight.

But I have to cast my vote for the A-6 Intruder in loiter mode.

Holy Mother of Pearl!

When one of the dirty so and so's would wander over the flightdeck about 500 feet up in said loiter mode, with a full bomb load and their exhaust nozzles pointed down, grown men wearing full flightdeck hearing protection are driven to tears from the pain. This is while wearing hearing protectection rated for a constant 128 DB enviroment.

[cjl]

Here's a report on sound levels of several current fighters - interestingly enough, the 22 does seem to be one of the louder ones, especially far field. I'm having a hard time coming up with a reason why this might be, but it does seem to support your assertion.


http://www.f-16.net/f-16_forum_download-id-11481.html

[BigDon]

Nice link Cjl!

[swampyankee]

If you're comparing the 22 to the 16, I would imagine it would be quite a bit louder, as it is still using low bypass, afterburning turbofans, but it is twice the weight with more than twice the thrust. I doubt it's much louder than comparably sized fighters though, so an F-15 or F-14 might be a better comparison. Similarly, the A-10 shouldn't be terribly loud, as it has relatively little thrust and a higher bypass ratio. If anything, all the sources I can find on the F-22 indicate that it should generate less noise than average for a figher with its level of installed thrust and bypass ratio.

The A-10 is a fairly quiet aircraft -- those high-bypass fans, even with no special noise abatement treatments in the nacelles are quiet -- and the neighbors of the Connecticut Air National Guard were quite unhappy when it was announced that CANG was getting F-16s, which are noisy. They didn't -- the CANG's 103rd Fighter Wing became the 103rd Airlift Wing, and got Learjets (and is supposed to get G.222s) -- but I can't imagine the F-16s are as noisy as the F-100s CANG operated before the Warthogs.

Incidentally, the jet noise from an aircraft engine scales to the eighth power of the exhaust velocity. Yes, that means that doubling the jet velocity increases the radiate sound pressure level by a factor of 256.

[cjl]

The A-10 is a fairly quiet aircraft -- those high-bypass fans, even with no special noise abatement treatments in the nacelles are quiet -- and the neighbors of the Connecticut Air National Guard were quite unhappy when it was announced that CANG was getting F-16s, which are noisy. They didn't -- the CANG's 103rd Fighter Wing became the 103rd Airlift Wing, and got Learjets (and is supposed to get G.222s) -- but I can't imagine the F-16s are as noisy as the F-100s CANG operated before the Warthogs.

Incidentally, the jet noise from an aircraft engine scales to the eighth power of the exhaust velocity. Yes, that means that doubling the jet velocity increases the radiate sound pressure level by a factor of 256.

Really? The eighth power? Is that for a comparable mass flow rate, a comparable thrust, or a comparable jet diameter, because if it's for the same jet diameter or mass flow rate, the two engines won't really be comparable to each other (as the higher jet velocity engine will also have substantially more thrust). That would explain why rockets are so loud though, as they have jet velocities approximately an order of magnitude higher than commercial jets, and perhaps 4 times higher than afterburning military jets.