Why can't I get perfectly circular orbits with manual editing?

[Frostiken]

Eccentricity of 0. Inclination of 0. Yet my apoapsis and periapsis are always slightly off when I take control of the craft. It\'s perfectly circular in the tracking station, but when I control it, it gives me an eccentricity of 0.00001 (or thereabouts) and moves my periapsis and apoapsis around correspondingly. It\'s not much, but enough to make me ask:

What gives?

[Causeless]

Eccentricity of 0. Inclination of 0. Yet my apoapsis and periapsis are always slightly off when I take control of the craft. It\'s perfectly circular in the tracking station, but when I control it, it gives me an eccentricity of 0.00001 (or thereabouts) and moves my periapsis and apoapsis around correspondingly. It\'s not much, but enough to make me ask:

What gives?

Yeah, I\'ve noticed this too. It\'s a little annoying, but nothing too major. I first assumed it was floating point errors but the offset seemed a little too much for that.

[togfox]

it gives me an eccentricity of 0.00001

We\'re talking five decimal places here. That\'s good enough for me.

[softweir]

That\'s Floating Point rounding errors for you. What starts off as perfect becomes imperfect in an instance. Unfortunately, there\'s nothing can be done about that.

Given that NASA considered '+/- 5 miles is circular enough for the shuttle', I think we do pretty well!

[The_Duck]

In addition to floating point errors, when vessels are off rails the physics is being propagated forward in small, discrete timesteps. This is inherently an approximation, and introduces small errors. Avoiding these errors (which build up over time) is the reason for the 'rails' system.

[samstarman5]

Consider that this plays into the part on all the little gravitational forces that normally pull on a satellite orbiting the Earth, as well as the part that gravity on Earth isn\'t a constant over its entire surface. In KSP the rules on gravity are a bit different(Okay, maybe a bit more than a bit), so the errors that come up are a good substitution on how your craft can be affected.

But don\'t let a little deviation discourage you. It doesn\'t stop Jeb, and he is the most deviant of them all.

...what?

[Frostiken]

That\'s Floating Point rounding errors for you. What starts off as perfect becomes imperfect in an instance. Unfortunately, there\'s nothing can be done about that.

Given that NASA considered '+/- 5 miles is circular enough for the shuttle', I think we do pretty well!

Well what made me wonder about the \'floating point errors\' is that the KSP orbital code is accurate to 12 decimal places. Given that, the five decimal places seems an odd place for the errors to propagate.

[PrISM]

It\'s perfectly circular in the tracking station, but when I control it, it gives me an eccentricity of 0.00001 (or thereabouts) and moves my periapsis and apoapsis around correspondingly.

Jeeze guys...it\'s not like we\'re trying to launch the Space Shuttle here.

[Caesar15]

Jeeze guys...it\'s not like we\'re trying to launch the Space Shuttle here.

That\'s Floating Point rounding errors for you. What starts off as perfect becomes imperfect in an instance. Unfortunately, there\'s nothing can be done about that.

Given that NASA considered '+/- 5 miles is circular enough for the shuttle', I think we do pretty well!

If they were.

[Alchemist]

A funny thing about this: I deployed a PowerSat in orbit with less than 100 m difference, switched to it, opened the map and saw apoapsis and periapsis moving along the orbit (and almost changing places) just because of attitude control torque. So, there really is some imprecision even in rotation affecting the orbit.