What is the air speed in ksp?

[paul23]

Well we are given two velocities when moving around: the ground speed and orbital velocity.

 

However during lift off and while getting into orbit neither is particularly important. What is important is the amount of drag, which is mostly based on air speed (or indicated air speed). What is the air speed on kerbin (and other planets). Is it equal to the ground speed? Based on altitude?

[bewing]

Yes, the air is always still above the ground. So your airspeed is always equal to the ground speed.

[paul23]

That seems illogical though, it would mean that at high orbit the air speed is actually above the speed of sound. I can't believe this to be the case to be frank?

[bewing]

Above the speed of sound with respect to what? It's perfectly stationary with respect to the ground. The air ends at 70km altitude. If you are flying suborbital, with your Pe below 70km, then of course your speed with respect to the air and the ground is hypersonic.

Edited November 2, 2019 by bewing

[paul23]

6 hours ago, bewing said:

Above the speed of sound with respect to what? It's perfectly stationary with respect to the ground. The air ends at 70km altitude. If you are flying suborbital, with your Pe below 70km, then of course your speed with respect to the air and the ground is hypersonic.

Compared to the actual ground velocity: if the air is "on the same spot above the ground" it means it has the same *angular* velocity. The ground track/speed is actually a measurement for this angular position/velocity.

[Laie]

11 hours ago, paul23 said:

That seems illogical though, it would mean that at high orbit the air speed is actually above the speed of sound. I can't believe this to be the case to be frank?

Yet it's true.

Strictly speaking, KSP provides "surface" and "orbit " speeds. Both give the magnitude of your velocity relative to Kerbin, the sole difference being that "surface speed" takes Kerbin's rotation into account. In Kerbin-stationary orbit, your surface speed is zero. And if you're in a Minmus-like orbit, it's 13km/s against the spin.

While still in the atmosphere, "surface speed" equals "airspeed". By definition. All drag is computed based on your surface speed.

If you wonder how air can be supersonic while standing still, consider how fast things go around the Earth's equator.