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If I'm understanding right#sonic boom it's the first stage rocket that is creating the Sonic boom we hear after a Falcon 9 has been launched from Vandenberg AFB. I am approximately 100 miles south of the base. I usually am outside observing the launch. I've heard that the sonic boom comes about 8 minutes after the launch. Where in proximity is the first stage rocket from the launch site when it creates the sonic boom? How big of an area should be able to hear the boom? I guess I'm having a hard time understanding why we hear it here 100 miles south of the base when it's approaching the drone ship near Baja?

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    $\begingroup$ After 8 minutes, the booster is back on the ground at the landing zone, about 450m from the launch site. $\endgroup$ Feb 16 at 23:22
  • $\begingroup$ I accidentally misread and then voted against the proposed edit but it's a good edit so I hope it's accepted. There's no such thing as " silly or a duhh" questions here :-) $\endgroup$
    – uhoh
    Feb 17 at 3:02

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A sonic boom is a shockwave. This starts at the object that causes it, and travels in a direction that depends on the speed of the object. At Mach 1 (the speed of sound), the shockwave travels in the same direction as the object. As the speed of the object gets higher, the direction changes: you get an arrow shape, and the angle gets sharper as speed increases. The angle between the long axis of the rocket/airplane and the shockwave is always below 90º.

A shockwave is created continuously, as long as the object is traveling faster than Mach 1.

Shockwave

So when a rocket is launched, its shockwave will travel upwards and you will not hear it. Once the rocket starts to pitch over, the shockwave could reach the ground, but by then the rocket is often too high for the shockwave to be noticeable on the ground.

When the Falcon 9 first stage lands, the shockwave is aimed downwards, and you can hear it on the ground.

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    $\begingroup$ @Diane It's important to realize that a sonic boom is not a single event that happens at a particular moment in flight; it's a wave passing across the land that the vehicle drags behind it as it moves. You hear it as a bang or pop (usually a double-bang because of the way the wave is generated) because the wave just passed by where you're standing. The metaphor might be a lighthouse sending out a constant beam of light, but it appears to flash as the beam sweeps past you. $\endgroup$ Feb 16 at 15:18
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    $\begingroup$ Falcon 9 specifically has a distinctive ba-bang---bang most likely caused by the engines, the landing legs, and the grid fins. $\endgroup$ Feb 16 at 23:36
  • $\begingroup$ yep. As you can see in the photo, every major change in the shape causes a shockwave. $\endgroup$
    – Hobbes
    Feb 17 at 8:51
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Assuming that by 100 miles south you mean 100 miles southeast down the coast from Vandenberg, that actually puts you less than 60 miles from the ground track of a typical 53 degree inclination Starlink Vandenberg launch. Since you didn't specify an exact location I will use as an example a point along the coast about halfway between Oxnard and Malibu.

Falcon 9 ground track

Falcon 9 would be about 85 miles downrange when it passes at its closest distance of about 58 miles. 58 miles is definitely close enough to hear a sonic boom, although it probably won't be that loud depending on atmospheric conditions and other factors. Is it something that you notice just because you are listening for it, i.e. is it loud enough that anyone else would notice it?

According to an article on the USAF website:

Ground width of the boom exposure area is approximately one mile for each 1,000 feet of altitude; that is, an aircraft flying supersonic at 30,000 feet will create a lateral boom spread of about 30 miles.

Using this calculation an aircraft (or rocket) flying at 116,000 feet would create a sonic boom that reaches 58 miles. However the distance will vary due to several factors including temperature, air density, and aircraft size and shape. It also depends on what the vehicle is doing at the time. In typical supersonic cruise, for example Concorde, the shockwave trails behind the aircraft.

Sonic boom path
Shock cone path (author Cmglee, via Wikimedia Commons, CC-BY-SA-4.0)

However if an aircraft is climbing or turning the shockwave can move ahead of the aircraft. This would be the case with Falcon 9 which is climbing quite steeply through the lower parts of the atmosphere.

You are probably aware of the fact that the speed of sound is slower at higher altitudes. Interestingly the USAF article points out that in order for a shockwave to reach the ground the aircraft must be travelling faster than the current speed of sound on the ground.

Depending on the aircraft's altitude, sonic booms reach the ground two to 60 seconds after flyover. However, not all booms are heard at ground level. The speed of sound at any altitude is a function of air temperature. A decrease or increase in temperature results in a corresponding decrease or increase in sound speed.

Under standard atmospheric conditions, air temperature decreases with increased altitude. For example, when sea-level temperature is 58 degrees Fahrenheit, the temperature at 30,000 feet drops to minus 49 degrees Fahrenheit. This temperature gradient helps bend the sound waves upward. Therefore, for a boom to reach the ground, the aircraft speed relative to the ground must be greater than the speed of sound at the ground. For example, the speed of sound at 30,000 feet is about 670 miles per hour, but an aircraft must travel at least 750 miles per hour (Mach 1.12, where Mach 1 equals the speed of sound) for a boom to be heard on the ground.

With all of these complexities, and the fact that SpaceX launch broadcasts don't show the downrange distance, it's difficult to say exactly where Falcon 9 is on its ground track when it generates the sonic boom that you hear eight minutes after launch. And the exact location likely varies from launch to launch even when following the same ground track. Also polar orbit launches head nearly due south and so it's less likely that you would hear sonic booms from those launches.

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Not an answer, but a comment with a diagram:

As an addendum to Steve’s excellent answer, the idealized sketch below illustrates the geometric relationship of sonic shock wave to ground “sonic boom” from an aircraft flying a straight, horizontal flight path at Mach3 over a flat Earth with uniform air temperature.

The ground sonic boom forms a hyperbola travelling at the aircraft’s velocity.

If the craft is climbing (as in a rocket launch) the ground intersection line will still be a conic section.

The speed of sound in a gas is the average velocity of the gas molecules which varies with the square root of the absolute temperature. Atmospheric temperature falls for the first 10 km of altitude, and begins to increase again above 20 km. Concorde’s boom was easily heard at ground level. It cruised at 18km altitude.

enter image description here

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  • $\begingroup$ Thanks Woody, although I think I mainly gave a thorough non-answer, explaining why it's difficult to know where the rocket is when it creates the sonic boom that they hear. I have often thought about the sonic boom that is heard after RTLS booster landings, wondering where the booster was during its freefall when it created the sonic boom. Of course that depends on the distance of the hearer. But unlike an airplane passing overhead at high altitude with displaced sound, where you can tell where the plane was when it made the sound you are hearing, I'm not sure you can do that with sonic booms. $\endgroup$ Feb 17 at 18:33
  • $\begingroup$ @StevePemberton ... you got me scratching my head and doodling. A craft at constant speed (flying through air at constant temperature) has a conic shock wave. A decelerating craft has a bullet-shaped shock wave. This is the type of thing that keeps me awake at night. Why don't you post a question? $\endgroup$
    – Woody
    Feb 18 at 1:26

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