# Sonic Boom in Aircraft and Spacecraft

I would like to know why aircraft, and spacecraft produce a double sonic boom on breaking the sound barrier.

A while ago, I thought I got it, as there’s a start and finish point to every vehicle.

But is that it?

I already knew that condensation on wingtips and tail fins is caused by localised hypersonar and the pressure induced by the shockwave.

I researched a few articles and watched a few flights (not like I ever need an excuse! 😁), but still don’t understand why it isn’t a smooth lengthwise transition once the first shock has been achieved.

Any explanations most welcome, with thanks in advance!

Post scriptum; I don’t have 1000 rep, and so can’t tag this with max Q.

PPS Or where it generally happens either, apparently.

I would like to know why aircraft, and spacecraft produce a double sonic boom on breaking the sound barrier.

One clarification here is that a sonic boom is produced, continuously, when an object moves at speeds exceeding the speed of sound - not just "on breaking the sound barrier".

The effect on the air in front of an object moving at supersonic speeds is different than the effect on the air behind it: the air in front of an object is compressed, the air behind it is rarefied. Both compression and rarefaction at supersonic speeds produce shock waves: positive for compression, negative for rarefaction.

The N-shaped graph below (copied from this Wikipedia article), referred to as an N-wave, is a pressure profile characteristic for a double boom.

We can see here that the first boom, associated with the nose of an aircraft, is produced by an overpressure (positive shock wave), while the second boom, associated with the tail, is produced by an underpressure (negative shock wave).

As mentioned earlier, these shock waves or wavefronts, closely following each other, are continuous (making up a cone behind a supersonic aircraft), but, for an observer at a given location, they will appear as two booms in a quick succession.

• Are the compression and rarefaction mathematically symmetric processes here? Does rarefaction increase entropy as well? Any simple equations highlighting the basic physics of why both compression and rarefaction result in shocks would be greatly appreciated. Oct 30, 2018 at 13:54
• @Mathews24 This is my interpretation/understanding of the mechanism behind the negative wave, so I would not rely on it without confirmation from other sources. I'd like to help, but I have no further insights on this matter.
– V.F.
Oct 30, 2018 at 14:41

I would like to know why aircraft, and spacecraft produce a double sonic boom on breaking the sound barrier.

Look at the image in the following link https://physics.stackexchange.com/a/306184/59023 and see the discussion at https://physics.stackexchange.com/a/281767/59023.

You will notice that there are multiple places where the index of refraction changes. This is because each location with a different cross-section generates its own density wave (some of which generate shock waves). The larger the cross-sectional change, the stronger the density wave. Thus, the nose and tail tend to generate the largest density waves, which can lead to shocks, if the object is supersonic.