Speed of Sound at Kerbin.

[Dispatcher]

I did a forum search on speed of sound and gotten a lot of results. I didn't spot any thread which seemed to directly address my question (although it may exist). Forgive me if I missed any.

At earth, the speed of sound is in the neighborhood of 345 meters per second. Since Kerbin is in some ways an earth analog (seemingly as to gravity and atmosphere anyway), does this hold true at Kerbin in KSP regarding the speed of sound?

I have seen what may be sonic transition effects (white streaking from surfaces instead of fire) near the time of atmospheric friction burn, but such speeds tend to be well more than 345 m/s.

Your knowledge would be appreciated. Thanks.

Edited August 29, 2013 by Dispatcher
Ah, this is where I show it as answered.

[Vanamonde]

In KSP, both light and sound travel instantaneously. The vaporous-looking atmospheric effect appears in response to, I believe, some proportion of speed to air pressure. That is to say, the lower you are in altitude, the lower the speed at which the vapor effects appear.

[tavert]

The re-entry / high dynamic pressure effects appear white in the highest-pressure (lowest-altitude) portion of the altitude bar, orange for lower pressures (higher altitudes). I don't remember off the top of my head what altitude the boundary's at, but should be easy enough to see in-game

There are no compressibility effects modeled in stock KSP right now, other than the visuals. The atmospheric density and pressure are directly proportional as if the atmosphere is isothermal, but you can measure values from a temperature sensor that change with altitude (and don't seem to be related to much of anything, except maybe engine overheating). You could go ahead and try to calculate a speed of sound based on some of the known properties of KSP's atmosphere and assuming it follows realistic physics, but that assumption is clearly incorrect so the result of the calculation would be fairly meaningless.

Edited August 29, 2013 by tavert

[Dispatcher]

Judging from Vanamonde's & "tav"'s reply, I'm of the opinion that there is no pat answer to my question. This is partly due to another question which arrises: what is the speed of light? But that is probably best for another thread and has probably been answered there. I can now say with some certainty that Kerbin flight below 344 m/s is probably less than the speed of sound. I've seen You Tube videos where the speed of sound was purportedly exceeded, but those were simply manifesting the compression effects mentioned by you both. Thanks for your replies! Vanamonde, Forbidden Planet is my favorite movie. "My poor Krell!"

Edited August 29, 2013 by Dispatcher
I wanted to change this to "answered" here.

[Vanamonde]

Yep, Forbidden Planet rocks.

[Motokid600]

It'd be great to see condensation clouds form around the rockets one of these days

http://upload.wikimedia.org/wikipedia/commons/thumb/a/ac/Apollo_11_launch.jpg/270px-Apollo_11_launch.jpg

[Starlionblue]

I did a forum search on speed of sound and gotten a lot of results. I didn't spot any thread which seemed to directly address my question (although it may exist). Forgive me if I missed any.

At earth, the speed of sound is in the neighborhood of 345 meters per second. Since Kerbin is in some ways an earth analog (seemingly as to gravity and atmosphere anyway), does this hold true at Kerbin in KSP regarding the speed of sound?

I have seen what may be sonic transition effects (white streaking from surfaces instead of fire) near the time of atmospheric friction burn, but such speeds tend to be well more than 345 m/s.

Your knowledge would be appreciated. Thanks.

The speed of sound is not an absolute. In Earth atmosphere, it is dependent on temperature. The formula is 38.94 times the square root of the absolute temperature (temperature in degrees Kelvin). Thus in a standard atmosphere (15 degrees C at sea level), the speed of sound is 661 knots at sea level, or 340m/s.

In a standard atmosphere, temperature decreases by 1.98C/1000ft, and the local speed of sound thus decreases with altitude. However in a real atmosphere the lapse rate (change of temperature with increasing altitude) is not constant, so the local speed of sound varies not only due to altitude.

[OhioBob]

The given properties of Kerbin's atmosphere are inconsistent and couldn't exist together in real life. This makes it impossible to derive a definitive value for the speed of sound. For example, below are two different methods for computing speed of sound. Both methods should yield the same answer but do not.

The first method uses the given sea level values of density, pressure and temperature.

ÃÂ = 1.2230948554874 kg/m²

P = 101325 Pa

T = 293.15 K

From ideal gas law the average molecular weight is

M = ÃÂRT/P = 1.2230948554874 * 8314.4621 * 293.15 / 101325 = 29.422 kg/kmol

where R is the universal gas constant.

Given this molecular weight the atmosphere is almost certainly nitrogen-oxygen, giving us a specific heat ratio of 1.40. Therefore the speed of sound is

C = (γRT/M)^1/2 = (1.40 * 8314.4621 * 293.15 / 29.422)^1/2 = 340.56 m/s

The second method uses the scale height of the atmosphere, which for Kerbin is 5000 m. Scale height is given by the equation

H = RT/(Mg)

Therefore, by substitution we get

C = (γHg)^1/2

Again assuming γ = 1.40, we obtain a speed of sound at sea level of

C = (1.40 * 5000 * 9.81)^1/2 = 262.05 m/s

The problem appears to be that the game developers selected the scale height as needed to produce the desired pressure-height profile without regard to internal consistency. If we are to believe the sea level values of P, T and ÃÂ, then Kerbin's sea level scale height would have to be 8,445 meters, not 5,000 m as given. We run into the same inconsistency at other altitudes as well. KSP atmospheres are simply not real-world possible.

Edited March 29, 2015 by OhioBob

[Pecan]

August 2013 thread about atmospherics due to be completely replaced in 1.0

[Claw]

Yes, this is a bit of a necro, although the information is not (yet?) out of date. Although the answers are all over the place in this thread, they are all still correct.

Basically, Kerbin at sea level is modeled to be about the same as a standard day at Earth sea level. Which means that yes, the speed of sound is roughly 340 m/s. However, KSP does not actually model anything with that speed. There are no "mach effects" and sound does indeed travel instantly. So right now, the speed of sound is relatively meaningless in stock.

I doubt they will do anything to instant sound/light in 1.0, but aerodynamic things happening at Mach are supposed to be included. I would also suspect that the Kerbin sea level ~ Earth sea level will stick around, but I guess we'll have to wait and see.

Cheers,

~Claw