Why Circularize Orbits?

[Brainlord Mesomorph]

Do most of you guys circularize your orbits as a matter of standard procedure?

I don’t. I mean I can see why you would do that for certain purposes. Putting a satellite or a space station in a particular orbit. But as far as a tug and lander are concerned, or just trying to get two ships to rendezvous, elliptical orbits seem to provide a lot more opportunities.

If two orbits have a common perigee, and vastly different apogees, you’ll get rendezvous quicker. (Two ships in matching circular orbits will chase each other around forever.) Elliptical orbits have high-speed perigees, and a low speed apogees, which provide other opportunities for maneuvering.

In short, I see lots of reasons for elliptical orbits, but not very many reasons for circular orbits. Which is why I think I’m I missing something.

Is there some tactical advantage to circular orbits I don’t understand?

Edited August 18, 2015 by Brainlord Mesomorph

[Dafni]

As far as I can tell, circular orbits around planets are easier for getting encounters with the corresponding moons, because when you move the maneuver node around you don't change the apoapsis of your transfer orbit. But that's about it, I think.

[Yindy]

I like circular orbits more, because with elliptical orbits (for purpose of rendezvous etc.) you would have to match the angle of the two orbits (pe and ap on the same axis) or you have to make several adjustments to account for that.

[GoSlash27]

Well... in most cases, anything I put into orbit is going to dock with something else I put into orbit. Keeping everything in a circular orbit simplifies my rendezvous and gives me maximum flexibility for my launch windows.

Best,

-Slashy

[Red Iron Crown]

My standard orbit for assembly is at 100km and circular. When I launch something intended to go there, I put it in a 75km circular orbit for phasing. This makes getting the intercept relatively easy, I just set a node to raise Ap to 100km and move it around until the closest approach is sufficiently close. This is much simpler than trying to launch directly to rendezvous, and quicker than waiting for two orbits with a common periapsis to coincide.

It is probably not the most efficient method but it works reliably and simply.

[Brainlord Mesomorph]

My standard orbit for assembly is at 100km and circular. When I launch something intended to go there, I put it in a 75km circular orbit for phasing. This makes getting the intercept relatively easy, I just set a node to raise Ap to 100km and move it around until the closest approach is sufficiently close. This is much simpler than trying to launch directly to rendezvous, and quicker than waiting for two orbits with a common periapsis to coincide.

It is probably not the most efficient method but it works reliably and simply.

I'm really not talking about Kerbin. Launching from the ground, circular is pretty much a default. I'm talking about capture around a distant planet.

Once I have any safe orbit. I stop burning fuel. Until I make other much more precise plans. Maybe I need to circularize, or move the PE, maybe I don't.

I was wondering if there any reason other than "It might be convenient later" because (just as likely) it might not.

[Red Iron Crown]

Depends what you are trying to do, I suppose. If you are landing, you'll circularize on your way to deorbiting so it is convenient to circularize first to make choosing a landing site easier. If you are resource scanning then obviously the orbit must meet the requirements of the scanner. If you just want a satellite around that body then I'd agree circularizing is usually a waste (unless there are moons that need to be avoided).

One thing I have observed from the really efficient Jool 5 missions is that they almost never circularize their tugs or motherships; they leave them in highly eccentric orbits to make ejection cheap when they head to the next moon.

[FancyMouse]

If you decide to go out to another planet (return to Kerbin, for example), you'd better start with a circular orbit to give a more precise dV estimate, and less likely to waste fuel. With an elliptic orbit, if you happen to have to burn at apoapsis, you're wasting a lot of fuel, potentially ruining the whole trip.

For a station, also better circular to expect docking from something launched from that planet.

And no, elliptic orbit doesn't make rendezvous faster. You can pretty much achieve the same thing with the target ship staying at circular orbit with the same semi-major axis, while another ship using elliptic orbit to do rendezvous. I only have to elliptic orbit rendezvous towards an asteroid.

It's more of that circular orbits don't give you more benefits than you know, but you underestimate a lot of matters that elliptic orbit complicates.

[Brainlord Mesomorph]

It's more of that circular orbits don't give you more benefits than you know, but you underestimate a lot of matters that elliptic orbit complicates.

Hmm. maybe I'm just used to them.

- - - Updated - - -

One thing I have observed from the really efficient Jool 5 missions is that they almost never circularize their tugs or motherships; they leave them in highly eccentric orbits to make ejection cheap when they head to the next moon.

THAT.

[Laie]

I'm really not talking about Kerbin. Launching from the ground, circular is pretty much a default. I'm talking about capture around a distant planet.

Oh, in that case...

One thing I have observed from the really efficient Jool 5 missions is that they almost never circularize their tugs or motherships; they leave them in highly eccentric orbits to make ejection cheap when they head to the next moon.

Exactly. I don't think I tried this before my own Jool-5, but that's how I've been doing it ever since. Not only does it save fuel: it's pretty much zero extra hassle compared to circularizing first, and provides more/better opportunities to fix your inclination if you mess up at launch time.

[Sharpy]

Encounter of two elliptical orbits (especially with much different argument of periapsis) is much harder than one elliptical and one circular. If something is meant to be encountered with, I set it on a circular orbit. If it's to encounter, I put it on elliptical.

For LKO, circular is the least delta-V and highest Oberth effect. For landers it's the lowest entry speed.

[GoSlash27]

I'm really not talking about Kerbin. Launching from the ground, circular is pretty much a default. I'm talking about capture around a distant planet.

Once I have any safe orbit. I stop burning fuel. Until I make other much more precise plans. Maybe I need to circularize, or move the PE, maybe I don't.

I was wondering if there any reason other than "It might be convenient later" because (just as likely) it might not.

Brainlord,

I don't put anything in an orbit of another body without a precise plan. If it's a mothership, it will have to be docked with, so same situation applies as with Kerbin. If it's a lander, I have a specific place I want to land it, and a circular orbit simplifies the problem. After I'm done, I'll want an efficient ejection angle. I can do that best from a circular orbit.

Best,

-Slashy

[Brainlord Mesomorph]

Encounter of two elliptical orbits (especially with much different argument of periapsis) is much harder than one elliptical and one circular. If something is meant to be encountered with, I set it on a circular orbit. If it's to encounter, I put it on elliptical.

For LKO, circular is the least delta-V and highest Oberth effect. For landers it's the lowest entry speed.

Don't I get the most Oberth benifit from coming down from a high AP to a low PE?

[Jouni]

It depends on the planet, whether I circularize or not. Around Duna, nobody cares, because the delta-v requirements are so low anyway. On a Tylo mission, I usually circularize the mothership, because it's cheaper and easier to have some extra delta-v in the mothership than in the lander. Around Moho, the situation is the opposite. I never circularize there, because the delta-v requirements for the transfers are quite close to what can be achieved with reusable transfer stages, while landing there is cheap.

[GoSlash27]

Don't I get the most Oberth benifit from coming down from a high AP to a low PE?

You do, but not if the Ap is in the wrong place WRT your desired ejection angle, which it almost certainly will be.

Best,

-Slashy

[Geschosskopf]

My primary uses for elliptical orbits are as follows:

Communications Relays:

At each planet where I have long-term stuff going on, I put up 2 commsats as the primary relays to/from that planet. I put these commsats into highly elliptical polar orbits, one going up and the other down, and timed so that when one is at Pe, the other is at Ap. Their Aps are way out there (I use 42Mm because 42) so at least 1 commsat will be able to see over/under any intervening moon or planet and their Pes are just outboard of the orbit of any station there. This all works great with AntennaRange so I plan to use a similar system when the stock relay thing comes out.

Parking Oribts for Interplanetary Ships

I launch my ships days/weeks before the actual transfer window. The ships start out in the lowest possible circular orbit but once there, I use MJ to plot the burn. This shows me where the node will be along my path days/weeks in advance. So then I burn like 500-800m/s at the node on the day of launch, putting the ship into an elliptical parking orbit with its Pe (close enough to) where the ejection burn will be when the transfer window comes along. Then I remove the node and do a new one when its actually time to leave.

[Sharpy]

Don't I get the most Oberth benifit from coming down from a high AP to a low PE?

If you arrive from the surface, you get exactly as much Oberth effect benefit from coming down from high AP, as it had cost you to set up that high AP in the first place.

That is providing you set it up with the right argument of periapsis. If your departure/teansfer burn is anywhere but the periapsis, that's a pure loss; both less Oberth effect and fuel wasted to bring that periapsis high,

Edited August 18, 2015 by Sharpy

[Brainlord Mesomorph]

Brainlord,

I don't put anything in an orbit of another body without a precise plan. If it's a mothership, it will have to be docked with, so same situation applies as with Kerbin. If it's a lander, I have a specific place I want to land it, and a circular orbit simplifies the problem. After I'm done, I'll want an efficient ejection angle. I can do that best from a circular orbit.

Best,

-Slashy

Wow. I don't plan that far ahead. I tried when I first started, it never worked.

These days, I make sure my ships are capable of a "variety of missions" and I empower my commanders to make decisions once they get there.(LOL) Not like NASA. (every last detail pre-decided) more like Sir Francis Drake. I send out ships and they explore.

Where do we land? The most interesting place under whatever orbit we managed to grab. Where do we go next? Whatever is orbiting into a convenient position.

- - - Updated - - -

You do, but not if the Ap is in the wrong place WRT your desired ejection angle, which it almost certainly will be.

Best,

-Slashy

AH but you forget that I know how to place my PE exactly at my ejection angle because I line it up with the Sun!

[Sharpy]

AH but you forget that I know how to place my PE exactly at my ejection angle because I line it up with the Sun!

For interplanetary burns? Not very useful.

The only actual benefit is if you use a low TWR propulsion like nukes and your burn would start early enough that it would drive you into the atmosphere. In that case distributing ejection between 2-3 apoapsis burns may be preferable.

[Brainlord Mesomorph]

For interplanetary burns? Not very useful.

The only actual benefit is if you use a low TWR propulsion like nukes and your burn would start early enough that it would drive you into the atmosphere. In that case distributing ejection between 2-3 apoapsis burns may be preferable.

I do. My TWRs are usually around 0.2. Its very useful, and explained thoroughly in the tutorial in my sig.

[GDJ]

Makes docking with space stations a heck of alot easier. Docking is tricky enough, and having an elliptical orbit makes it that much harder.

[rofltehcat]

Circular orbits for:

1. Stuff you want to dock with (just don't place it in a too low orbit)

2. Remotetech: Equatorial communication satellites (short range/local)

3. Preparation for landing (circularizing into a low orbit reduces overall velocity and makes hitting targets easier)

4. ScanSat scanning satellites (polar)

Non-Circular orbits:

1. Remotetech: Long range relays satellites in a polar orbit with high Ap and low Pe --> long range connection can only break off 1-2 times a year, often for less than 1 hour.

2. When I just don't care what orbit something is in.

3. For inclination changes (circularize afterwards if you want circular orbit)

Edited August 18, 2015 by rofltehcat

[Sharpy]

Actually, an extended orbit makes hitting targets on the surface easier.

keyword hitting.

[Brainlord Mesomorph]

actually, an extended orbit makes hitting targets on the surface easier.

Keyword hitting.

lol

[LordCorwin]

To me, low altitude circular orbits are a good idea if you are going to land using aerodrag and you're not sure that any part will explode. The low altitude circular orbit is the one with least energy, so less chances of getting burnt.