Why two sonic booms?

[t'aint natural]

Perhaps a Concorde pilot, or a schoolboy with an interest, could clear up a confusing matter for a humble rotary jockey.
My father-in-law, an English Channel yachtsman, questions why he always hears a double sonic boom when Concorde accelerates through the speed of sound. I muttered something about trailing edge going supersonic marginally after nose, but had my fingers crossed behind my back.
He also says that he never sees the aircraft, to which I replied that it was "too high".
What is the cause of the double boom, and at what pressure altitude is Concorde normally flying when it occurs?

[chiglet]

It'a all to do with the "pressure waves". ALL a/c have a leading and trailing wave. [Front and Back of the airframe]. Depending on temperatues and pressures, the waves can either "meet" above ground [Single Boom] or not [Double Boom] I might be talking utter Bolldocks:rolleyes: , but I think that that is what happens:D
we aim to please, it keeps the cleaners happy

[WOK]

Basically true.

As the airflow becomes totally supersonic (as opposed to locally s/s, which occurs at a speed below Mach 1 in relatively small areas) the classic conical shockwave is formed. A similar shock wave is formed as the air around the boundary layer becomes subsonic again. The overpressure caused by the passing of the shockwave is what creates the 'boom', hence a double boom.

Each type will have a ditsinct pattern of shockwave formation which will also be affected by a/c mass, speed and atmospheric conditions. SSC is very long and heavy for a SS a/c so the double boom is both pronounced and quite loud. It is at its strongest during the accel owing to the high mass of the a/c at this point and the focussing effects caused by acceleration (which are exacerbated by turning-which is avoided). The focussed boom is generally expected to be 15nm long and 20nm wide. It is reckoned that the boom doesn't reach ground level 'til M1.1 is achieved.

During the decel, the boom is less strong (lower weight) but focussed by deceleration and additional problems can arise caused by secondary booms bouncing along the stratosphere so we aim to be subsonic some distance from land.

You won't normally see SSC at the accel phase because M1.1 occurs at about 30000' - too high to see the a/c and often too low to contrail. Look out for a rapidly lengthening contrail.

[WOK]

PS

Just to be be quite clear - the boom doesn't just occur AS the aircraft becomes supersonic, but ALL the time it is in supersonic flight - hence the en-route restrictions.

[t'aint natural]

WOK:
Many thanks for this succinct yet full post, which will restore my reputation as an aviation expert.

[twistedenginestarter]

Wok

What has weight got to do with it? This is an aerodynamic phenomenon and the only aerodynamic link to weight is angle of attack...?

[exeng]

Twistedenginestarter,

Surely within your question is its answer??!!??!!


Regards
Exeng

[Bellerophon]

twistedenginestarter

The heavier Concorde is, the more lift it must develop, and thus the greater the pressure disturbance caused by its supersonic overflight.

Although this pressure change is relatively small, it is a very fast change, which is why the ear detects it as a sound.

t’aint natural

Your father-in-law’s ears, which are probably between one and three metres above the sea (unless he is up aloft or down below), are actually subject to two sounds from each of the two shock waves.

The sound of the shock wave on its way to the surface of the sea (the incident shock), and the sound of the almost instantaneous reflection of the shock wave from the sea surface (the reflected shock). The time interval between the two is of course so small that they are perceived as a single sound, nevertheless, predictions of sonic boom intensity take this reflection factor into account.

Over the years there have been a few reports from yachtsmen that they have heard a long succession of booms. If we dismiss the lunchtime curry or sundowner gin theories, this can also occur when Concorde is accelerating or turning, as this affects the ground track of successive shock waves.

This could result in a rapid succession of booms passing over a given point on the sea, (boom focusing), or, should they all happen to arrive at the same place at exactly the same time, a rather larger boom than usual!

Regards

Bellerophon