How fast is "faster than sound/burn the air" speed?

[Bathawk]

Hey, I've seen this a few times in fiction, and I think it's a real thing. But you know as you approach the speed of sound a "mach cone" forms? (you see it in anime and video games all the time)...but apparently you go even faster and the air can't "get out of the way" fast enough and it's pulverized into plasma...the effect is the object looks like a laser streaking?

short version: How fast would an object have to be traveling?

[Refplace]

Originally Posted by Bathawk

Hey, I've seen this a few times in fiction, and I think it's a real thing. But you know as you approach the speed of sound a "mach cone" forms? (you see it in anime and video games all the time)...but apparently you go even faster and the air can't "get out of the way" fast enough and it's pulverized into plasma...the effect is the object looks like a laser streaking?

short version: How fast would an object have to be traveling?

Well Move 381 will get you the speed of sound most of the time. The required speed actually varies a bit based on temperature, so its lower at higher altitudes and higher in the warmer climates.
Pretty much just hitting it means the air molecules can't get out of the way fast enough and start slamming into each other as you push through. So you get the effect you are looking for soon as you hit Mach 1. However its visibility increases based on the volume of air affected and how fast your hitting them so larger and blocker objects will excite more air molecules than smaller ones - say bullets.

[Varyon]

A quick bit of Googling indicates the plasma-shock boundary (that is, when the air around the object gets superheated into plasma, causing the "air on fire" effect) is around Mach 10, or roughly Move 3750.

[Fred Brackin]

To add to what Refplace said the speed of sound also goes down as the air gets thinner. At jet cruising altitudes it may be more 660 mph instead of the 760 mph at sea level.

The sorts of vapor clouds you see form around fast moving aircraft are water vapor trying to change phase. Just like in exhaust contrails.

"Sonic booms" do occur at speeds over Mach 1 but nothing's on fire.

Varyon was generally right about the air becoming incandescent plasma. That takes meteoric speeds or at least returning spacecraft or a specially equipped X-15.

[Agemegos]

Originally Posted by Fred Brackin

To add to what Refplace said the speed of sound also goes down as the air gets thinner.

No it doesn’t. Except at extreme pressure, the speed of sound in any gas depends on its temperature and composition (molar mass, adiabatic index), and is independent of pressure. The variation of the speed of sound with altitude is overwhelming the result of the variation of temperature with altitude.

[Witchking]

Further about the Speed of Sound:

IIRC

The Sound Barrier (or the perception of it) developed in the early 1940s, wherein several aircraft approaching Mach 1 (speed of sound) had difficulties leading to aircraft damage and a number of crashes.

There were a number of issues (mostly not directly related):

I remember...

The 'compression' of the air made the manipulation of the controls more difficult as the airspeed increased, until said controls were effectively locked. Since one of the easiest ways for aircraft (particularly early 1940's prop birds) to achieve speed is in a DIVE...well controls locked or slow to respond lead to several Move Through attempts on Mother Earth.

Also the wing thickness and shape needed changes. The thick wing projecting from the body of the aircraft at a right angle became thinner and swept back at an angle.

Hell I am a historian not an engineer...but they teach historians how to research!! (GDAMN...Research and Bibliography (pre internet) was required for the major, was only offered every other semester, only had 1 instructor and was a M-W-F 8AM...ugh bad memories).

https://en.wikipedia.org/wiki/Sound_...d%20of%20sound.

If you can travel fast enough to 'set the air on fire' and are not limited to running...well you appear to be halfway to orbit.

[RyanW]

Originally Posted by Agemegos

No it doesn’t. Except at extreme pressure, the speed of sound in any gas depends on its temperature and composition (molar mass, adiabatic index), and is independent of pressure. The variation of the speed of sound with altitude is overwhelming the result of the variation of temperature with altitude.

There may be a bit of illusory effect because an aircraft flying at 40k feet will be going a lot slower than one at 10k feet at the same mach number, if you go by indicated airspeed.