Trent_800
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Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 12:06 am

Just wondering if any mach 2+ aircraft cause a second sonic boom when passing through mach 2. Also is the Boom just when the aircraft is pushing through the sound barrier or constant whilst the aircraft is supersonic?
Cheers
Trent
 
Bellerophon
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 2:27 am

Trent_800

1) No.

Put very simply, the shock wave caused as the aircraft passed through the speed of sound still remains, but changed its position relative to the aircraft as the Mach number increased.

Try to imagine a vertical straight line, with the nose of the aircraft, flying horizontally, just touching it at the line's mid-point. Let's assume that this line represents the shock wave that forms at Mach 1.0. The angle between the top half of this line and the aircraft is 90°.

As the aircraft Mach number increases, the line remains touching the aircraft nose, but, instead of remaining vertical, both halves of the line now start to fold backwards towards the tail of the aircraft. The angle the shock wave makes with the aircraft has decreased, or become more acute.

This angle is called the Mach Angle, and is directly proportional to the Mach number of the aircraft.


2) Constant.

At very low supersonic speeds the boom may not reach the ground, and occasionally during turning or decelerating flight the boom can appear stronger at some points on the ground than others, but, unfortunately, it is always there.

This is the main reason that most countries have banned overflight by aircraft at supersonic speeds.

Regards

Bellerophon
 
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B737-112
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 2:42 am

If an aircraft traveling Mach 2 only causes one sonic boom can you explain this? When the Space Shuttle lands at Edwards AFB, CA it overflies the LA area (where I'm located) and clearly two seperate booms are heard.
 
PPGMD
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 3:27 am

Yep two booms for the shuttle, don't know why but I always hear two booms. I could ask NASA.
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broke
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 3:33 am

If you can handle a little Trig, the sine of the shock wave angle is equal to 1/M where M is the Mach number. At Mach = 1, the sine of the angle is 1, so the shock wave angle is 90 degrees or "Normal". The term "Normal" relates to something being 90 degrees to something else; therefore a "Normal" shock is one that is 90 degrees to the direction of flight.

When you hear multiple sonic booms, as from the space shuttle, you are hearing the sound of the shock waves from both the fuselage and the wing.
 
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 3:41 am

A sonic boom is an N shaped signature. It is a double pulse really. There is an intial, stronger pulse, with a secondary pulse that is kind of like an echo of the first. I am at work and do not have the info on this, but i remember very clearly a few years back seeing a discovery channel show describing this. I'll do some more research on it tonight when i get a chance- but every sonic boom is a double pulse.
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prebennorholm
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 3:52 am

The sonic boom is always two booms divided by a split second, probably just one tenth of a second. I don't know why, but I have heard it on various military planes flying at supersonic speed.

It always scared the hell out of me since there was of course no sound at all prior to the booms.
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Bellerophon
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 4:36 am

B737-112

Yes, it's quite simple, and in fact Concorde also causes a double boom, but it has nothing to do with it being at Mach 2.0.

First, however, let me just make a point about answering questions like these.

Supersonic flight aerodynamics is an extremely complex subject, likewise the study of shock waves and sonic booms. Trying to answer questions about them on a general forum, asked by people whose level of knowledge is not known to me, is not always easy. I could be answering a fifth grader whose knowledge is very scant, an aerodynamics professor who knows lots more than I ever will, or any level of education in between .

I try to reply at what I think is an appropriate level, and often simplifications and generalisations are necessary, just to keep the answer to a reasonable length. However, until you get to know someone, the only real way of trying to assess what level is appropriate is to consider the language the questioner used.

In this case, Trent_800 referred to "the Boom", and so I left it at that, as the double boom doesn't have anything to do with going at Mach 2.

To answer your question, and again simplifying somewhat, there are two shock waves associated with a body in supersonic flight. The bow wave and the tail wave, which are attached, oddly enough, to the front and the back of the body. When these shock waves strike the ears of an observer at ground level, the ear detects them as two distinct sounds.

Those, like me, who have heard Concorde pass overhead at Mach 2.0, will tell you that they do indeed hear a double boom, as do people who are under the Space Shuttle whilst it is supersonic, but the double boom is not related to the fact that Concorde or the Shuttle is at Mach 2.0.

If an aircraft is going really fast, it is quite possible for both shock waves to arrive at the ear so close together that the ear cannot distinguish between them, and therefore “hears” them as a single boom.

Regards

Bellerophon
 
lehpron
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 6:55 am

The two booms you guys heard are there; there will always be two booms on anything traveling pasted the speed of sound, even bullets (just because you hear one doesn't mean there's only one)

Think of a plane (or anything in the atmosphere) like a boat, there will be a bow wave and a stern wave. When called the N-wave, the bow pushes the air pressure away from plane and the stern wave pulls it back to normal. It's called 'N’ by its shape, first down then up then normal. The one forward is more visible while the tail takes a while to collect.

Granted that engineers are trying to make quiet supersonics that don't make the N-shaped wave, rather something like a curvy half-moon wave, the idea is to make the boom less of a crack of air and more of a soft poof. Supposedly, residents would approve of it.
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timz
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Waitaminnit!

Fri Nov 08, 2002 1:01 pm

Speaking as an ignoramus, it seems like the cotangent (or maybe the tangent) of the shock-wave angle should equal the Mach number. How can sound reach the ground as the aircraft is passing directly overhead, without traveling faster than the aircraft?

At Mach 2 Concorde is traveling maybe ten times its length each second, so seems like shock waves from nose and tail should arrive a tenth of a second apart. Do they? How about the Shuttle booms?
 
timz
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On Second Thought...

Fri Nov 08, 2002 2:10 pm

...I guess Broke is right, but we should clarify that at exactly Mach 1 there isn't actually a planar shock wave accompanying the sircraft-- that's the theoretical limiting case. But at Mach 2 it does seem like the shock wave should be 30 degrees from horizontal and not 26.57 degrees like I said.
 
POSITIVE RATE
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 3:02 pm

In regards to the space shuttle- does it also make a sonic boom as it is climbing out after liftoff and passes Mach 1? How come this boom is never heard by the spectarors watching it liftoff??
 
Trent_800
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RE: Is There A Sonic Boom When Breaking Mach 2?

Fri Nov 08, 2002 3:51 pm

Positive rate,
I'm going to be brave and try and answer but ill probabally be slated for being wrong. When the shuttle passes Mach 1 on liftoff the direction of travel is upwards.(obviously) so the 90 degree line explanation that Bellerophon gave me suggests that the sound wave that we here as the boom (or double boom)is traveling upwards and outwards in relation to the shuttle. So only people above or to the side of the shuttle as it is supersonic will hear it. I think  Smile/happy/getting dizzy
Trent
 
Bellerophon
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RE: Is There A Sonic Boom When Breaking Mach 2?

Sat Nov 09, 2002 12:18 am

Timz

Sin α = 1/M, Broke is correct.

At Mach 2.0, the Mach angle is 30°.

Your estimate of 0.10 seconds for the time interval between the Bow and Tail shock waves on Concorde is a very good one, and corresponds with the measured pressure signature.

The shock waves cannot arrive on the ground when the aircraft is overhead, because, by definition, they travel at the speed of sound, whilst the aircraft is going much faster. If you hear Concorde or the Space Shuttle, it will have long passed the overhead by the time you look up.

The over-pressure caused by the shock waves, and the time interval between them, vary with the distance from the body causing them, and also with ambient meteorological conditions. This means that the characteristic double boom is actually heard only in a much narrower zone than the zone calculated from sound ray theory. Between this narrow zone and the theoretical edge of audibility, only a dull rumble, which often cannot be identified as a sonic boom, is heard.


Trent_800

Well thought out. The shuttle is also climbing rapidly and disappearing down range quickly, which assists in shielding the boom from spectators on the ground. The military pilots in the chase and camera planes, have reported hearing the boom as it passes, but they are a lot closer than anyone else.


Regards

Bellerophon
 
flightsimfreak
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RE: Is There A Sonic Boom When Breaking Mach 2?

Sat Nov 09, 2002 1:38 pm

Another reason why I would guess you could not hear the boom as the shuttle takes off is that the engines are incredibly loud, and directing their sound toward the spectators, thus they would drown out the boom.
 
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RE: Is There A Sonic Boom When Breaking Mach 2?

Sun Nov 10, 2002 1:39 pm

Imagine a SR-71 at Mach 3- it would be long gone by the time you heard the boom. Or an X-15 at Mach 6 that would be extraordinary seeing that fly over the top in silence.

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