Kerbin circumference vs how far my plane traveled

[CrashyMcCrashFace]

The wiki says the equatorial circumference is 3,769,911 m . I took off from KSC and flew straight at 90 deg. F3 says 3,201,959 m for Ground Distance Covered but I'm only about half way around. What gives please?

Edited July 9, 2018 by CrashyMcCrashFace

[Rocket In My Pocket]

Well, I'm no math expert but it would seem to me that the distance given is at terrain height, which obviously you can't fly at.

The higher you go up, the more the distance around the sphere increases right? (Ie. it's less distance to take a corner tight rather than wide. Or to put it more simply, if you had one circle (the terrain height) that was smaller than the other ( the circle your plane forms flying some considerable distance above terrain height) obviously the total distance around the larger circle, is larger?

I guess it comes down to how the game calculates ground distance covered, which I honestly don't really know.

Edited July 9, 2018 by Rocket In My Pocket

[CrashyMcCrashFace]

That would makes sense but it's actually an aircraft (sorry I didn't state that initially) and the ground and actual distance is relatively small. F3 gives the "Total Distance Traveled" and "Ground Distance Covered" separately.

[4x4cheesecake]

15 minutes ago, Rocket In My Pocket said:

if you had one circle (the terrain height) that was smaller than the other ( the circle your plane forms flying some considerable distance above terrain height) obviously the total distance around the larger circle, is larger?

Doesn't fit either (or my maths are wrong) but this would require a flight altutide above 418,000m.

Assuming the whole circumnavigation would require twice the length of the already done flight, this would be ~6,400,000m.
Circumference = pi * radius * 2 -> 6,400,000/(2*pi) = radius = 1,018,591m
Lets subtract the 600,000m equatorial radius of kerbin -> 418,591m

I have another theory: Ground distance may include 'distance in hight' so every mountian and every valley will increase your ground distance compared to your flight distance, which is basically a straight line.

[OhioBob]

I suspect it might be measuring not from where you started in relation to the planet's topography, but from your starting point in a celestial frame of reference.  As you've been traveling eastward, the planet has also be rotating.  So you might be halfway around the planet when measured from KSC, but since you've also been moved along by the planet's rotation, you've traveled through an arc of over 300 degrees from where you started when measured in relation to the stars.  At least that's my guess.

(edit)

@CrashyMcCrashFace, let's see if the numbers work out.  You say you've traveled 3,201,959 m, and Kerbin's circumference is 3,769,911 m.  So that means you've traveled an angular distance of,

3201959 / 3769911 * 360 = 306 degrees

And you say you're about halfway around the planet from where you started, so let's call your position 180 degrees east of KSC.

So if you've moved 306 degrees in relation to the stars, but only 180 in relation to the planet's surface, then that means the planet has rotated 126 degrees during the time you've been flying.

Since Kerbin rotates 1 degree per minute, then that means you've been flying for 126 minutes.  And if you've traveled 180 degrees in relation to the surface, then that means your average ground speed has been,

3769911 * (180/360) / (126*60) = 249 m/s.

Do those number sound about right?  You've been flying for about 126 minutes at a speed of about 249 m/s?
 

Edited July 9, 2018 by OhioBob

[Greenfire32]

It's probably because KSP is a spaceflight simulation game and, as such, is using your starting point in space rather than your starting point on Kerbin.

Since you're traveling east, the planet is rotating with you. Since you're moving pretty slow compared to the planet's rotation, you are still out-pacing it, but it's also making up some of that ground you've covered.

The result is that you've traveled more distance than you think, because the planet is spinning with you. It's similar to taking 4 steps forward and 1 step back. Still a net positive gain on distance, but not at 100%.

The opposite would be true too. If you were to travel west and have the planet rotate underneath you, you would travel the entire circumference in less distance than the circumference is (think Kessler run in 12 parsecs).

This is all because KSP is tracking your movement in space and not your movement on the surface. Change the starting reference, and you'll probably get different numbers.

[CrashyMcCrashFace]

Oh yes. I like the last 2 answers. That seems to make sense.

I'm about 7/8th of the way around now and what looked like I could go around twice is starting to make me nervous about the fuel left. I'll update with the numbers once I finish. Not physics warping because I'm still doing work-work and just letting it run in the background.

Edited July 9, 2018 by CrashyMcCrashFace

[Snark]

13 minutes ago, CrashyMcCrashFace said:

Oh yes. I like the last 2 answers. That seems to make sense.

Pretty sure that @OhioBob and @Greenfire32 nailed it. 

Just now did a little experiment with a simple airplane, and verified the following:

• Barely gave it a nudge and let it roll a few meters at about 1 m/s.  F3 results:  "Highest speed" is 175 m/s, i.e. the planet's rotation is included.  "Distance traveled" appears to use only ground distance actually covered, though.
• Start to taxi, then take off.  Result:  As soon as it gets airborne (just a few meters down the runway, at only about 21 m/s; it's a small, light plane), then suddenly the "ground distance covered" and "total distance covered" goes bananas and it thinks it's covered hundreds of meters.  If I let it fly along a couple of seconds and hit F3 again, it's clear that the thing is racking up "distance traveled" at what it thinks is close to 200 m/s speed, even though it's going under 30.
• If I turn the plane around to point west before taking off, I get different numbers and it doesn't seem to think I'm going so fast.

[CrashyMcCrashFace]

Here's the results:

Time (from T+ status and F3): 3h 54m 24s

Cruise Speed (set with Mechjeb autopilot): 280m/s

Cruise Alt (set with Mechjeb autopilot): 8,000m

Ground Distance Covered (from F3): 7,024,943m

Total Distance Covered (from F3): 7,033,795m

Fuel used: 639 units.

Also a polar circumnavigation is probably worth an experiment. I should get science points for this.

 

Results from Mach 2 trip:

Time (from T+ status and F3): 1h 55m 2s

Cruise Speed (set with Mechjeb autopilot): 615m/s

Cruise Alt (set with Mechjeb autopilot): 8,000m

Ground Distance Covered (from F3): 5,496,216m

Total Distance Covered (from F3): 5,497,833m

Fuel used: 474 units.

Edited July 10, 2018 by CrashyMcCrashFace
Added the 2nd flight

[Gargamel]

If this is accounting for the speed of rotation, it should also be accounting for solar orbital velocity too, unless of course the SOI physics is negating that.  So as you rotate around the planet, the planet is also moving 'laterally' in space, so your path of travel might look something like an E.      Or perhaps a C.