Sonic Boom Question

[TrinidadGT20]

What exactly is a Sonic Boom?

[jdflight]

This is best described with a diagram, so try to picture this in your head. Think of an airplane pushing into a flexible piece of plastic. That plastic is the sound barrier. It is actually the movement of air over the craft, but for your purposes, that´s not extremely important. As the craft´s thrust increases, the plastic sheet flexes more and more, bending down over the craft until you have basically a cone around the craft. When the craft breaks through the plastic sheet and that cone is behind it, you hear a pop. That´s the sonic boom.

Basically, the faster you move, the denser the air around you gets (someone please correct me here if I´m wrong; I´m going purely on old memory). This pocket of air, though, stays in front of you, as does the sound you make pushing against it. At some point, you exceed the speed at which this airpocket can withhold you, and you pass through the pocket. That passing creates the boom you hear as the pocket moves behind you.

I don´t think that was a very good explanation, but I tried. We hear them in LA when the Shuttle lands at Edwards.

[TrinidadGT20]

That was a great explanation...thanks!

One thing I don't understand is why the space shuttle coming in for a landing would make a sonic boom. It seems that it is already going faster than the speed of sound upon re-entry...

[**DONOTDELETE**]

That's a good question. It's easier to imagine a boom from a breaking piece of plastic than one reforming as the shuttle comes OUT of supersonic travel. Also, I was watching the last shuttle landing on the web (and just missed seeing it overhead in the Houston night sky! So mad!!). When it finaly came out of mach, it made two booms. Why?

jtda

[jdflight]

Both good questions, and I don´t know the answers. The only thing I´ve heard with the shuttle is that it happens upon reentering the atmosphere. Not really sure what the effect of that is, though.

[reaperman]

bo-GUS [img]images/icons/shocked.gif[/img]

[John2375]

Hi everyone -
I can't give a very good explanation as to *how* sonic booms happen as I'm not too clear myself! BUT -i can give a couple observations...
First to answer a question from above: The reason for 2 booms (this according to NASA itself) is that the shuttle orbiter is a very blunt object, for one. There's actually 3: The air snaps over the nose, for the first, then snaps over the wings, for the second, and over the tail for the third, but the 2nd and 3rd sounds as one (since the delta-wings are so close to the tail) that's louder than the first.
Now for 2 observations - When I was living in Tampa last year, the landing track of the shuttle returning in March took it directly over Tampa...I was watching NASA TV at the time, waiting to see if I'd hear anything at all. At that point, they were 7 minutes to touchdown and still travelling Mach 7 at 140,000 feet (I guess a 17,000 foot per minute drop, or 280 feet per second, will get you down quick!) Well anyway, the NASA-TV animation of Discovery's position showed it pretty much over Tampa, and then Boom **BOOM** !!!! The windows pulsated, the things on the shelves rattled and moved, and the dogs went crazy barking...car alarms in the neighborhood started going off - it was incredible and great!!!
Second observation - I was lucky enough in 1997 to be able to attend a shuttle landing, in person *at* the runway. In that case, the sonic booms (equally impressive, btw) rang out 3 minutes 20 seconds to touchdown, at the point they went subsonic...they'd passed over the area still supersonic but didn't create a boom, but as they were turning around the Heading Alignment Cone (an imaginary cone around the runway they line up on) is when they were audible...
anyway- just my observations - sorry it doesn't really answer your question -
maybe someone can answer (I've heard NASA say but not really too clearly) why the shuttle doesn't make any sonic booms during the acceleration of launch into space...

John

[Mike Lewis]

As for why it doesn't make any booms when going up into space, I think that it is because it is traveling away from the earth, and so the shock wave is spread uniformly around from it in all directions and has lower intensity the farther back it gets.

A picture would best describe it, but imagine this: A sonic boom looks like a funnel. If you could slice off part of the funnel, that would represent where the shock wave intersects with the ground. Everyone encountering the shockwave would hear the boom. With the shuttle taking off, though, the funnel is on its end. And for anyone on the ground at the very end of the shockwave, the shockwave has very little energy and intesity by that point. Yes, the space shuttle reaches a top speed of 17,500 mph on it's ascent, but it's also so much higher than anything else at that point, that there is minimal disturbance to the atmosphere that can be felt at ground level.

[dtoften]

I would like to add my 2 cents on to the sonic boom principle. My understanding of the sonic boom, from my college class in Fluids, is that at subsonic speeds, you can hear the sound of the airplane before the plane is actually there. At supersonic speeds, the plane is traveling faster than the sound is going forward. This is similar to a pebble being throw into a pond. It creates circular waves that travel away from the place were the pebble hit. Think of a boat that travels in water, it creates circular waves that travel in all directions from displacing water while it is moving. But it is traveling faster than the waves can travel. This creates a cone in a V shaped called a wake. The faster it goes, the more of a slim V it is.