Why a Circular Orbit?

[iPat]

I suspect there may be a brilliant answer to this, rendering it perhaps a dumb question, but having put my first Kerbal space station into orbit, I'm wondering why make it's orbit circular, as opposed to the elliptical one it's in now?

[Red Iron Crown]

Welcome aboard!

The biggest benefit is it makes it much easier for other vessels to rendezvous with it.

[Dangerous_Beans]

what Red Iron Crown said. circular orbits also look neater.

for the space station contracts there is no need (especially if you are just going to de-orbit it and recycle it). elliptical orbits can also be useful where you want to use the Orbeth effect but can't do the burn in one go, or possibly if you want low and high orbit gravity scan data for every biome (do a 70k pe, 270k ap polar orbit, it should fly over every biome low and high eventually, provided the ap is over a pole).

[Snark]

Aside from making it easier to rendezvous other vessels with it, it also makes for somewhat easier planning if you're planning to then send it elsewhere (e.g. to the Mun, or another planet, etc.). If I want my apoapsis to be here, then if I'm in a circular orbit, I just do a prograde burn 180 degrees opposite. Whereas if I were in an elliptical orbit, it would take more fiddling to figure out how to burn.

Many transfer-planning tools (such as http://ksp.olex.biz, my favorite) assume that you're in a circular orbit to start with.

[paul23]

Aside from making it easier to rendezvous other vessels with it, it also makes for somewhat easier planning if you're planning to then send it elsewhere (e.g. to the Mun, or another planet, etc.). If I want my apoapsis to be here, then if I'm in a circular orbit, I just do a prograde burn 180 degrees opposite. Whereas if I were in an elliptical orbit, it would take more fiddling to figure out how to burn.

Many transfer-planning tools (such as http://ksp.olex.biz, my favorite) assume that you're in a circular orbit to start with.

Actually, advanced planning requires starting orbits to be circular. It's otherwise often simply unsolvable, and if the eccentricity is large even numerical approaches may not converge.

[LostOblivion]

For the ISS, I believe it's a combination of ease of access and not having to pass through the Van Allen belts of radiation.

[mabarry3]

Question about getting into a circular orbit: it seems that when I try to burn into a circular orbit, when my periapsis is within a few km of my apoapsis, the apoapsis starts increasing to the point I can get a circular orbit down to 1-2 km difference but I can't go less than that (as some contracts require). I try to burn pro/retrograde exactly at apoapsis/periapsis to make a circular orbit, but is there something else I need to do instead? Or is it just as simple as I'm not burning exactly on top of my apoapsis/periapsis which causes eccentricity?

[paul23]

Well don't strive for perfectism.

And to get as close as you can don't try to reach it at once. burn at apoapsis/periapsis untill i'ts roughly 10km accurate (or when you see the apo/peri quickly moving away). then keep doing very small (less than 1/2 a second) burns near the apo/peri until you have the accuracy you wish to get. The "best" I got was a 180m difference, but a 2km difference is already low enough that it doesn't matter if you wish to do other manoevres afterwards.

[Snark]

Question about getting into a circular orbit: it seems that when I try to burn into a circular orbit, when my periapsis is within a few km of my apoapsis, the apoapsis starts increasing to the point I can get a circular orbit down to 1-2 km difference but I can't go less than that (as some contracts require). I try to burn pro/retrograde exactly at apoapsis/periapsis to make a circular orbit, but is there something else I need to do instead? Or is it just as simple as I'm not burning exactly on top of my apoapsis/periapsis which causes eccentricity?

You can also burn radial in/out. Here are the "four points of the compass", so to speak:

• "I'm at Ap." Burn prograde until Pe rises up to equal. (The symptom of this is that the Ap/Pe markers will start to rotate around the orbit. When they're 90 degrees off from your ship, that's a good place to stop.)
  
• "I'm at Pe." The same, except burn retrograde instead of prograde, to lower Ap.
  
• "Ap is about 90 degrees ahead of me, and Pe is 90 degrees behind me." Burn radial-inwards. This will cause Pe to rotate towards you from behind. Stop when Pe arrives where you are.
  
• "Ap is about 90 degrees behind me, and Pe is 90 degrees ahead of me." Burn radial-outwards. This will cause Ap to rotate towards you from behind. Stop when Ap arrives where you are.
  

Each one of those will nudge your orbit closer to the perfectly circular. How close is "close enough" depends on your circumstance.

In my experience, contracts that require matching a particular orbit aren't super-finicky-- I'm surprised that an orbit-around-Kerbin contract would fail to complete because of a measly 1km difference between Ap and Pe. Are you sure there's not some other problem going on?

[mabarry3]

In my experience, contracts that require matching a particular orbit aren't super-finicky-- I'm surprised that an orbit-around-Kerbin contract would fail to complete because of a measly 1km difference between Ap and Pe. Are you sure there's not some other problem going on?

I suppose then it was more or less my inexperience with those kinds of contracts. I didn't typically take the ones that required a perfectly circular orbit because I was unsure what "marginal deviation" actually meant, since there are sometimes where you really do want a truly circular orbit.. I suppose if they're not that finicky then there's no real reason I should care beyond that.

[paul23]

Well I've had marginal "complete" already when the periapsis was within a few meters (probably I couldn't see the difference in orbital map), but the apoapsis was like 4% closer to kerbin than required. So actually quite a big difference (delta v of 10m/s required), around 20-30km altitude