Endeavour heads into space ...

[kimmer]

http://www.nasa.gov/mission_pages/shuttle/main/index.html

Picture gallery: you can save them to your desktop:

http://www.nasa.gov/mission_pages/shuttle/...lery-index.html



God's speed to the crew, and especially to Donald R. Pettit -- an Oregonian.

[RNKIII]

And to Capt. Heidemarie Stefanyshyn-Piper of St. Paul, MN.  


Bob K.   rnkiii

[sandbox]

When it's clear I can see the launches from here. If they return to the Cape we can hear the double boom-boom, a signature reentry sound that one never forgets.

[RNKIII]

I've heard of the "double boom" reentry sound... but don't know why??  Any explanations out there???  Simple words, please.

Bob K.   rnkiii

[Xairbusdriver]

Probably because of two shock waves from the same object or maybe as simple as an echo. There are actually different waves coming from many different parts of an aircraft. With a smaller aircraft they would be so close together they would just sound like one 'big bang.' The Shuttle being one of the biggest controllable objects operating at hypersonic speeds, probably has enough large, separate waves to actually hear at least two of them. There is probably a good explanation on the NASA site. But I usually just provide my own ideas instead of finding out the facts.

Sorta close but no banana! <Here's the real story. See section [127]> Not really two different waves, after all.

[krissel]

I happened across some video the other day of planes making a sonic boom but can't find the link now. I was really surprised to see that there was visible evidence of the moment of exceeding the sound barrier.

Here is one picture and more of an explanation:

http://www.daviddarling.info/encyclopedia/S/sonic_boom.html

[sandbox]

Bob, the first one is a Bow Shock and the second one is a Stern Shock or some call it a wake shock.
You hear the double boom because the shuttle is a blunt instrument, the bow makes a larger noise than an aerodynamic jet would at the same size.

[Xairbusdriver]

SB, that's basically what I'd always thought but check the link in <this post.> Section 127 explains the phenomenon and it helps explain why we don't usually hear them with smaller and much less massive objects. It's not particularly 'fun' reading but it does present the answer to Bob's question. Glad you asked, I've actually learned something this year! And there are still a few days to go!

[sandbox]

I'm confused, what I said was pretty accurate. I had a friend who worked on the SR-71 project to mitigate sonic booms on fast moving aircraft and from our long discussions and tall beers I concluded that there is a direct correlation between the blunt nose compression of the shuttle and the loud double boom I hear.
Most planes with needle noses produce little sound going in to the sound barrier and thus the louder sound is heard in it's wake.

I was not a pilot, I was just an independent contractor for the air force that happens to live in the returning fight path of the shuttle.
" Depending on the mission, the space plane passes over Florida's west coast somewhere between Sarasota and Yankeetown and proceeds across the central part of the state, with its tell-tale sonic booms heralding its arrival." I'm in the middle.

Maybe I missed something?

[Xairbusdriver]

Well, you are correct about different ways to mitigate the booms, but that wasn't much of a priority in the Shuttle, I'd guess. any object or part of one that is moving faster than the local speed of sound will create a 'boom/noise.' That could be the tip of a whip, the tip of a propeller, even parts of an airliner, although usually not in normal operations. Of course, if the whole object is moving faster than the SOS, everything in the airstream could be creating a sonic-boom/noise. But unless you were extremely close and had super-human ears, it would sound like a single 'boom.' And the farther away you are the more 'thunderous' it would sound as the higher freqs fade faster than the longer waves. The reason for a double boom, no matter what creates it is the shear power and magnitude or the wave. Remember, air is compressible. When the shock wave passes through the air, it actually compresses the air which then rebounds back to its normal atmospheric pressure. But with a massive, very strong wave, the air compresses so much that the rebound actually lowers its pressure much below the normal pressure! Now, when that 'decompressed' air 'realizes' its condition, it immediately tries to repressurize itself, and makes a smaller but closely following, secondary 'boom.' The original is still a single, combined wave made up of any and all the mini-shocks caused by the object, they just end up munged together and 'confused.' But they are so powerful they cause the secondary shock wave.

That's my story, anyway!

[sandbox]

maybe we're just speaking past each other.

QUOTE

Aerodynamicists soon realized that the slender aircraft body suited to supersonic flight was unsuited to hypersonic flight. Rather, they found that a blunt-nose body experienced much less heating than a pointed body, which would burn up before reaching the Earth's surface. The blunt reentry body, discovered in 1951 by H. Julian Allen, an engineer with the NACA's Ames Research Center, created a stronger shock wave at the nose of the vehicle and dumped a good deal of the reentry heat into the airflow, making less heat available to heat the reentry vehicle itself. This finding was so significant, and so in contrast with intuitive thinking, that it was classified for a while. But by 1958, Allen's work became public, and all successful reentry vehicles since have used the blunt body.

http://www.centennialofflight.gov/essay/Th...sonics/TH23.htm

[Xairbusdriver]

QUOTE

maybe we're just speaking past each other.

I guess so, I thought you were wanting more info on the 'double'- sonic boom.