Falcon Heavy Launch

av8tr1

"Never tell me the odds!"
No I get that it happens at altitude. Here is my 7th grade geometry attempt. Z is where the sonic transition happens. X is the landing spot and Y is the spectators.
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After the transition happens the lander is still falling very fast if not right below the speed of sound so the actual boom sound isn't to much faster than it is falling up until almost a 1000 feet above the pad when they do the landing burn and slow way down. Those thing where moving! Since the crowd was farther away than being right under the transition is took longer for the boom to reach them then if they were standing on the pad. So that extra distance the sound had to travel to reach the spectators gave the landers (visually) time to almost be on the pad when they hear the booms.
Fingers and head hurt.
Yep, I think you are right. But I think the transition is much higher than 1000 feet. And they are actual slowing down to Mach 1 not speeding up.

The second part of the equation I think (as @z987k pointed out) is the direction the sound wave is traveling is generally downward so it likely hits the ground first and then expands outward.

It's been 30 years since I studied this stuff.....brain hurts just trying to think about it.
 

z987k

Well-Known Member
Yep, I think you are right. But I think the transition is much higher than 1000 feet. And they are actual slowing down to Mach 1 not speeding up.

The second part of the equation I think (as @z987k pointed out) is the direction the sound wave is traveling is generally downward so it likely hits the ground first and then expands outward.

It's been 30 years since I studied this stuff.....brain hurts just trying to think about it.
So sound actually refracts upwards in our atmosphere, in general. A jet at altitude for example. The colder the air the slower the speed of sound, and with a normal lapse rate, the air gets warmer as you go down, where the speed of sound is faster. The sound waves that initially propagate downwards are moving from a slower medium to a faster medium. This will refract up. If you took a cross section of it, especially at and angle, the bottom of the wave would hit the faster medium first, speeding up first and the top of the wave last. This also stretches the wave and increases the wavelength. Which makes sense because wave velocity = frequency * wavelength. If the velocity is increasing, and frequency is set by the source(a constant in our example), the length must also increase.
I tried to make a simple drawing real quick to hopefully better illustrate my point. Also note, that at some point on the ground there will be a shadow where you cannot hear the sound due to refraction, even though you may be able to see the source.
 

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