lowest dV way to get to circular equatorial orbit from highly eliptical - non 0 APe

[KerikBalm]

So I want to give Kerbin some new moons, I had my eye on a rendevous with an E-class, but by chance a C-class self captured (I think it was by interaction with minmus, but I didn't see what happened).

Its in a relatively stable orbit, it doesn't cross Mun's SOI at all, and it only crosses Minmus' orbit at the descending node. Its Apopapsis is roughly 50% farther out than the orbit of Minmus.

Its perapsis is pretty close to Kerbin's so it should be easy to aerobrake it to a stable orbit.

Of course, out at apopasis is the easist place to make a plane change... but its argument of perapsis of about 15 degrees (eyeballing it here) means i can't change it to an equatorial plane witha burn at apoapsis... so now ... I'm not sure what to do with it...

I'm guessing the most efficient way to change APe is with burns at the ascending/descending nodes?

I'm thinking what I should do is at Apoapsis, change the plane so it intersects Mun's orbit, and try to get mun to slingshot it into an equatorial+ aerobraking trajectory.... thoughts?

Any of you have experience in efficiently getting your captured roids into circular equatorial orbits?

[Geschosskopf]

If you want to end up in a circular equatorial orbit, the APe doesn't matter, does it? Sure, you're never quite circular but if you're close enough, then essentially everywhere on the orbit is both your Ap and your Pe so APe becomes meaningless. So I'd think you don't have to worry about your current APe at all. I'd just do any required plane change to get equatorial while the orbit is still highly eccentric, then go straight into circularizing.

Of course, orbital mechanics ain't my forte.

[Kasuha]

First, burn at periapsis to lower its apoapsis somewhat. Not much, but make sure it won't easily exceed SOI boundary. It shouldn't be more than single units of m/s.

Second, burn at apoapsis prograde and make the orbit circular. That's going to cost you some 100 m/s.

Third, perform plane change at inclination point. You can lower the periapsis down to Kerbin with the same maneuver. That'll cost 100-200 m/s depending how is your orbit oriented.

Aerobrake to make the orbit circular.

[KerikBalm]

If you want to end up in a circular equatorial orbit, the APe doesn't matter, does it? Sure, you're never quite circular but if you're close enough, then essentially everywhere on the orbit is both your Ap and your Pe so APe becomes meaningless.

Once it is circular, it is meaningless, but its not circular yet, and its not meaningless yet.

I'd just do any required plane change to get equatorial while the orbit is still highly eccentric, then go straight into circularizing.

Of course, orbital mechanics ain't my forte.

I can't do the required plane change at apoapsis... because apoapsis is well above the equatorial plane

Its only a C-class though, so I think I'll do as Kashua said and not bother with gravity assists... and I'll try to capture that E-class I'm looking at cleaner than this self captured C-class has ended up.

[Geschosskopf]

Once it is circular, it is meaningless, but its not circular yet, and its not meaningless yet.

I can't do the required plane change at apoapsis... because apoapsis is well above the equatorial plane

Looking at your pic, you're just going from essentiall a polar orbit to an equatorial orbit. So yeah, your APe and LAN are essentially meaningless already. Just do a plane change, nothing special. Don't worry about AN and DN as they're essentially the same energy. Compare normal +/- burns as may be at Pe and Ap. Pe gives you Oberth but Ap gives you leverage. Whichever one cranks your orbital plane down for the least dV, go with it. Then circularize.

[Red Iron Crown]

Oberth doesn't help you with normal +/- burns, higher speed actually hurts for plane changes. Kasuha's plan to circularize at Ap, plane change, lower periapsis into atmo, aerobrake to lower Ap and then circularize is the minimal dV strategy.

[Geschosskopf]

Oberth doesn't help you with normal +/- burns, higher speed actually hurts for plane changes. Kasuha's plan to circularize at Ap, plane change, lower periapsis into atmo, aerobrake to lower Ap and then circularize is the minimal dV strategy.

You and Kasuha know way more about this than I do so I totally defer to you all when it comes to doing things right. But if somebody needs any help with orbital mechanics as misunderstood and misapplied by a hunter-gatherer, and don't mind getting sidetracked into the reltaive lithotechnic merits of blade-and-core vs. bifacial flintknapping, I think I can claim membership amongst the top 10,000 or so of available pundits .

[KerikBalm]

Looking at your pic, you're just going from essentiall a polar orbit to an equatorial orbit. So yeah, your APe and LAN are essentially meaningless already.

I really have no idea what sort of reasoning leads you to the statement that APe and LAN are meaningless already.

Btw, its not a polar orbit, you can't see well from that angle, because the orbit is "coming out of the screen".

That angle really shows the deviation from the equatorial plane, as I said, its really only about 15 degrees.

this is another view

Just do a plane change, nothing special. Don't worry about AN and DN as they're essentially the same energy. Compare normal +/- burns as may be at Pe and Ap.

Assuming it was a polar orbit, with the Apoapsis above th north pole, doing a plane change there will always still result in a polar orbit... you can change if it intersects the equatorial plane between kerbin and the sun/with kerbin between it and the sun, to crossing in front/ behind kerbin's orbit (ie kerbin prograde/redtograde directions)... like a spinning coin standing on its edge, the plane of the coin is rotating, but the coin is still standing up and not laying flat on the table.

Pe gives you Oberth but Ap gives you leverage. Whichever one cranks your orbital plane down for the least dV, go with it. Then circularize.

Oberth describes the ost efficient way to change your ships energy. Going from one plane do another plane does not increase your orbital energy.

This is why plane changes are most efficient at apoapsis -> oberth doesn't apply. Suppose you are in a perfectly polar orbit, and want to go perfectly equatorial. Suppose at the ascending/decending node, you're going 2,300 m/s North/south, and 0 m/s East/west, you need to kill 2,300 m/s of north/south velocity, and add 2,300 m/s of east/west velocity. Pythagoras tells us a burn of 3252 m/s is required to do that.

In contrast, a 900 m/s burn puts us out to the edge of the SOI, where the orbital velocity is about 250 m/s, we then do the plane change for sqrt(250^2+250^2)... 353 m/s. If it were a world where we can't aerorake our apoapsis back down, then another 900 m/s would be needed, for a total of 2153.. over 1000 m/s less (but since we can aerobrake, we save nearly 2,000 m/s doing a plane change like this)

In this scenario, we chose our APe by doing the burn to increase our apoapsis.

but that applies to highly inclined orbits... for an inclination of say... 5 degrees, you don't want to spend 900 m/s to extend your apoapsis.

But if I were in an eccentric orbit with an APe of 90, then doing plane changes there would be like spinning a quarter standing on edge when I want to to lie flat.

I'm basically wondering what the most efficient way is to set APe to 0

[Kasuha]

I'm basically wondering what the most efficient way is to set APe to 0

I thought you want to put the asteroid on equatorial orbit. Zero APe does not mean zero inclination.

In general, finding most efficient way is an optimization problem. You set up equations for your orbital parameters after certain number of burns, set the parameter you want to achieve to the desired value, and derive by dv to find local minima. Resulting burns are non-obvious.

[John FX]

Sometimes it uses less Dv to go from an equatorial LKO to a very high Ap, change inclination then drop Ap again. You should try to adjust your inclination as far from kerbin as you can and at the slowest speed you can, this will use the least Dv. You may find that making your orbit a bit fatter will use less Dv than adjusting inclination in your current orbit.

Then drop your Pe just into the atmosphere and use aerobrakes to get your desired Ap then circularise.

[KerikBalm]

I thought you want to put the asteroid on equatorial orbit. Zero APe does not mean zero inclination.

Correct.

But I'm already aware of the most efficient way to change inclination to an equatoraial orbit once at zero APe.

Of course, if getting to zero APe takes away all my eccentricity... well, then that makes the plane change harder, and its an optimization problem as you say.

[Kasuha]

Of course, if getting to zero APe takes away all my eccentricity... well, then that makes the plane change harder, and its an optimization problem as you say.

Eccentricity is not a concern here. Plane change is the easier the higher the AN/DN you use is so you need to make one burn to raise the AN/DN to a suitable altitude, and another burn to fix your inclination and bring periapsis down again.

Care to share orbital parameters of your asteroid (copy/paste from persistence/quicksave)? Maybe I could implant them to some ship and check what can be done about it.

[Geschosskopf]

I really have no idea what sort of reasoning leads you to the statement that APe and LAN are meaningless already.

Because LAN and APe are totally arbitrary values having nothing to do with the amount of energy needed to move from Oribt A to Orbit B. LAN and APe (which is derived from LAN) simply say which direction the "slope" of your inclined orbit is pointing relative to a fixed celestial coordinate system, measured from that system's prime celestial meridian. The direction of the celestial prime meridian is totally arbitrary, so it can point in any direction. So picture your orbit sitting there and the direction of the celestial prime meridian spinning around through the whole compass. As it moves, your orbit's LAN and APe will change with the difference in the reference direction they are measured from, but the orbit's Ap, Pe, and inclination won't change at all. Which means the dV required to change your inclination and eccentricity won't change regardless of LAN and APe.

Seriously, if LAN and APe were important for gameplay purposes, people would be talking about them all the time here in the forum. But it's quite possible to play KSP for years and never hear these terms mentioned, let alone understand what they mean. And even if you do know what they mean, and have a mod that will drag these vaules up from under the hood and display them for you, they still do you zero practical benefit because you have no idea at all in which direction lies the celestial prime meridian. In this world, the celestial coordinate system is based on a "fixed" landmark in the sky, but to what point on the skybox would you draw Kerbin's celestial prime meridian?

Bottom line is, don't worry about your LAN or APe. Their values are arbitrary so make no difference whatsoever to KSP gameplay and because the map view doesn't draw the celestial coordinate system over the skybox texture, we can't use these values anyway. Instead, we look at and change the shape of the orbits themselves as drawn in 3D space.

Btw, its not a polar orbit, you can't see well from that angle, because the orbit is "coming out of the screen".

Yeah, I see that now.

[Mr Shifty]

Seriously, if LAN and APe were important for gameplay purposes, people would be talking about them all the time here in the forum. But it's quite possible to play KSP for years and never hear these terms mentioned, let alone understand what they mean.

They're important for launching directly to a particular inclined orbit to, for instance, [thread=88371]rendezvous with a satellite or station in a Molniya orbit[/thread] or with a captured asteroid.

[Red Iron Crown]

They're also important for some of the contracts in Fine Print, which give specific parameters for some orbital insertions.

[Mr Shifty]

They're also important for some of the contracts in Fine Print, which give specific parameters for some orbital insertions.

Right, and they're very important for all real-life launches because: 1) The Earth has an axial tilt and 2) Most launch sites are not on Earth's equator. (Kourou is probably the most equatorial major spaceport at 5°N.)

[Geschosskopf]

They're important for launching directly to a particular inclined orbit to, for instance, [thread=88371]rendezvous with a satellite or station in a Molniya orbit[/thread] or with a captured asteroid.

But you don't need to know the values for LAN and APe to do this in KSP. That's my point.

LAN and APe, and the totally arbitrary celestial coordinate system they depend on, exist solely to help visualize the orientation of an orbit. If you have KSP drawing a 3D picture of the orbit already, then you don't need to know LAN and APe. Especially because KSP doesn't show you the celestial coordinate system anyway. All you have to do in KSP is place nodes and pull them in various directions, and you get immediate visual representation of what your orbit will look like once you do that burn, without having to do any calculations or maintain a mental image of the situation, or even knowing anything of the math going on under the hood or the coordinate system upon which it's based.

EDIT: Even with Fine Print, that still draws your target orbit so just as with rendezvousing with a ship, you can still use a totally eyeball method to get into the desired orbit, without knowing the numeric values of any of the orbital elements.

[KerikBalm]

Eccentricity is not a concern here. Plane change is the easier the higher the AN/DN you use is so you need to make one burn to raise the AN/DN to a suitable altitude, and another burn to fix your inclination and bring periapsis down again.

Care to share orbital parameters of your asteroid (copy/paste from persistence/quicksave)? Maybe I could implant them to some ship and check what can be done about it.

I was speaking generically, like if in the process of changing my APe, I lower my eccentricity, then the plane change will be more expensive.

I think I'll first do a normal plane change at apoapsis, and then tweak it to see if its worth raising my perapsis for the 2nd plane change at the AN/DN, knowing that I'll have to bring it down again to aerobrake.

This is only a C-class, so it should be about 100 tons, I should have no problem getting 100 tons of fuel up to orbit with LV-Ns, which tells me that I'll have a wet:dry ratio of about 2:1, so the rocket equation tells me I should be able to get about 5,000 m/s of dV with the roid... so I'll be able to do it on my own, I'm more of asking for the techniques/methodology, so that when I go after E-class roids, I don't run out of fuel, or need a ship so big that my computer crashes. (with 100 tons of fuel, and a 1000 ton roid, I'll get a dV of less than 800 m/s)

Geshosskopf - until the introduction of asteroids, there was little reason to use the terms, as except Eeloo, most bodies were in pretty circular orbits.

And most ships travel in equatorial or near equatorial orbits, were APe is nearly zero.

These roids though... get captured into some weird orbits, and as a result, these terms are now useful.

[Kasuha]

I think I'll first do a normal plane change at apoapsis, and then tweak it to see if its worth raising my perapsis for the 2nd plane change at the AN/DN, knowing that I'll have to bring it down again to aerobrake.

That will IMO come out even worse than if you coasted to the inclination point further from Kerbin and fixed the inclination there. By burning normal at apoapsis you're not changing inclination of your major axis, and you're moving the more distant inclination point towards the planet where it'll take more dv to align it.

I suggest you at least to "simulate" those scenarios in map view using maneuvers. You can see dv of (second, third, ...) maneuver if you collapse it (to the "star" shape) and hover you mouse above it. That will tell you how much dv will your maneuvers use in total without actually making them. Then you can try a different scenario and compare the dv. I really believe raising the periapsis, and fixing the inclination from high inclination point together with lowering the periapsis again will be the more economical approach.

Edited September 13, 2014 by Kasuha

[MarvinKitFox]

Just accept the fuel surcharge for the asteroid's mass.

Just use kasuha's plan as from the first page of this thread.

You need at absolute most about 500m/s, and that assumes that your rock needs to be completely u-turned around in addition to getting its inclination fixed. Orbital speed out near the edge of Kerbin SOI is *very* low.

With nukes, your fuel burn need only be 1/16th the mass of the rock, to achieve that.

For a c-class, one full orange tank, squeezed out through a nuke, should provide ample impulse for your needs.

[Geschosskopf]

These roids though... get captured into some weird orbits, and as a result, these terms are now useful.

I intercept and redirect roids all the time without the slightest reference to any of their LAN and APe values.

In KSP, the coordinate system doesn't matter. You never see it displayed and nothing in the stock game displays values in terms of the coordinate system. The reason the coordinate system doesn't matter is because KSP navigation is done entirely with graphical cues relating where your orbit is now to where you want it to be. So when it comes to rendezvousing with anything, or changing your trajectory, you're comparing 2 orbits directly to each other: the one you're in now and the one you want to end up in. In effect, this is making your present orbit the basis of an arbitrary coordinate system used only for that 1 burn. Next burn, you'll have a different arbitrary coordinate system because your trajectory will be different then. So because in KSP the coordinate system doesn't matter in the slightest, orbital elements defined solely in terms of the coordinate system (such as LAN and APe) are utterly and completely meaningless in KSP. Which is why KSP doesn't display these values; if you want to confuse yourself with them, you need a mod .

Here in the real, we're in the habit of using a celestrial coordinate system that's been in place for thousands of years. So instead of comparing 1 orbit directly to another, we compare them both to the coordinate system and thus indirectly to each other. Doing things this way, LAN and APe have meaning because the coordinate system never changes.

[KerikBalm]

Kashua's pretty much the only one in this thread that seems to get the point...

LAN I pretty much don't care about, but APe is absolutely relevant if you want to go to an equatorial orbit... surely Geschosskopf (projectile-head? shot-head?) you pay attention to the equatorial plane, no?

Its not that "hard", what I'm talking about is an optimization problem. If I don't care about optimization, then its just burn normal/anti-normal at the AN/DN nodes, circularize at perapsis/apoapsis.

You don't even hear much about the most efficient way to change inclination on these forums. Guides to getting to minmus will tell you to get in orbit, match planes, and then go... fine, but not efficient. Better guides will tell you to launch into the correct inclination, or time your burn so you meet Minmus at the AN/DN.

Not much discussed (although it is mentioned on this thread), is how to most efficiently change planes by sending your apoapsis way out and doing it there - implicit that this is done at an AN/DN node.

Now I'm faced with a scenario where my Apopasis is already way out there, but its not at the AN/DN node. This scenario would never reasonably happen in the first place for a ship under player control, and cellestial bodies can't have their orbits altered, so its been quite irrelevant until asteroids are introduced.

Marvin:

"you need at absolute most about 500m/s, and that assumes that your rock needs to be completely u-turned around in addition to getting its inclination fixed. Orbital speed out near the edge of Kerbin SOI is *very* low."

That assumes you can fix the inclination at the edge of the SOI... in its current orbit, I can't. If your Apopasis is directly above a pole of Kerbin at the edge of the SOI, you can't put yourself in an equatorial orbit with a plane change at apoapsis. If the apoapsis is on the equatorial plane, then its easy to turn a polar orbit into an equatorial orbit.

Its not that bad in my case, but I'm just trying to figure out the principles in general for dealing with these orbital changes in the most efficient manner.

Kashua, I need to rotate the plane of my orbit about 2 axis, if I burn at Ap to raise my Pe for a more efficient change along one axis at An/Dn, that means the other axis I need to rotate will take more dV because I'm now moving faster at Ap.

My plan is to rotate the plane along the first axis at Ap, this is the plane that is most out of what (since APe is not all that high), then raise my Pe from there, to fix the 2nd axis at AN/DN... although I'm still hoping to be able to use Mun to save a lot of fuel.

I was also considering lowering the PE with aerobraking, so that I can raise the Ap again, but also move it closer to the equatorial plane - for example, if my PE is below the 15 degrees below equatorial plane, wait until I pass PE and the equatorial plane, and then burn prograde 15 degrees above the equatorial plane.

I worry about the cost of raising my PE for changes at the AN/DN, if I then want to lower my PE again.

So far I havent rendezvoused with it yet, as I said, its my career save, and I've got other things happening before that burn is scheduled (I need to get Kerbal Alarm clock). I've got a manned ship nearing the edge of kerbin's SOI that I will need to send back to Kerbin, I've got a transfer window from Mun to Minmus for my science lab+fuel depot, a manned craft returning from minmus(maybe I should burn some more to speed up its return, I don't have DRE installed, so ) - and orbital assembly of some other ships for interplanetary missions (if I don't do the rendevous now, it will be many many orbits before they are close again).

Of course, now that its easy to have multiple saves, maybe I should just time warp ahead and skip all that stuff.

Or maybe I should just make a sandbox save, and put a craft into an orbit with an APe of 45, and a PE right on the edge of the SOI, and then try different approaches to getting back to equatorial orbit, and see which one has the most fuel remaining.

[MarvinKitFox]

... if I burn at Ap to raise my Pe for a more efficient change along one axis at An/Dn, that means the other axis I need to rotate will take more dV because I'm now moving faster at Ap....

Yes, you are now moving faster at Ap. by about 160m/s

You are also moving slower at the Ascending/Descending node. by about 2200m/s !!@!!!!!!

Your choice.

That assumes you can fix the inclination at the edge of the SOI... in its current orbit, I can't.

ONLY *you* are making that assumption.

The instructions are to:

1) at apogee, burn prograde until your orbit is circular

2) At ascending or descending node, burn to change planes

3) when convenient, burn retrograde to drop Perigee down to atmosphere.

Steps 2 and 3 can be more efficiently done as one burn.

There is room for further optimisation by not using a full circularization burn in step 1, etc.. but frankly, you are talking about single-digit deltav improvements.

Or maybe I should just make a sandbox save, and put a craft into an orbit with an APe of 45, and a PE right on the edge of the SOI, and then try different approaches to getting back to equatorial orbit, and see which one has the most fuel remaining.

And as a result you will find that Kasuha's advice will use very little fuel, compared to any of your schemes.

You only need, absolute maximum, 500m/s to go from your current orbit to an equatorial, atmo-grazing, zer0-inclination orbit using his method.

Much more likely, you will need about 310m/s.

Edited September 14, 2014 by MarvinKitFox

[Kasuha]

I need to rotate the plane of my orbit about 2 axis

Wrong. You only need to rotate the orbit around 1 axis. The axis going through your ascending and descending nodes.

Maybe you're just confused by knowledge of your orbital parameters and are trying to put them in order one by one. It's of course possible, but it's rarely optimal approach.

Edit:

I placed a ship at an orbit similar to what you describe and tried to play with it. Cheapest way to get the orbit to equatorial plane is of course Mun slingshot. That can be done in some 30 m/s, including navigating the ship to graze the atmosphere.

Avoiding Mun gravity assist and since it looks like one of your inclination points is already almost on Minmus level, probably simplest approach is to coast to that point, burn retrograde to stop all your orbital speed, then burn slightly east to raise your periapsis to just the right altitude in atmosphere suitable for aerobraking. It can be done in one burn but might be tricky to set up. And it should not need more than some 300 m/s.

Edited September 14, 2014 by Kasuha

[Geschosskopf]

LAN I pretty much don't care about, but APe is absolutely relevant if you want to go to an equatorial orbit... surely Geschosskopf (projectile-head? shot-head?) you pay attention to the equatorial plane, no?

OK, let me try a different approach....

I get the impression you're hung up on your APe because it happens that your roid's orbit has both its AN and DN close to your Pe, which is close to Kerbin, so doing plane change burns at either one would be expensive. But the thing is, this has nothing to do with your APe. It's instead a function of your Pe's altitude and your inclination.

APe is defined as the angle between LAN and Pe as measured in the plane of your orbit. Thus, it pays no attention to your inclination. And because APe just measures the angle between LAN and Pe, it doens't care about Pe altitude, either. APe's purpose is to help describe the direction in which your oribt's axes are pointing relative to a fixed, celestial coordinate system. That's all it's for.

The upshot is that orbits with the same orientation of their axes and the same LAN will all have the same APe regardless of the altitudes of their Aps and Pes. The value of APe, therefore, tells you nothing meaningful about your real problem. Your problem is caused by your Pe altitude and your inclination, which are 2 things APe doesn't even take into account.

This is why some folks have suggested you circularize at Ap. While this increases your major axis, it also increases your minor axis. Increasing the major axis moves the Pe further from your AN and DN. Increasing the minor axis moves the AN and DN further away from Kerbin. The combination of these effects makes it cheaper to do a plane change burn at AN or DN. During this process, your LAN won't change, and thus neither will your APE (unless you overshoot, which will change APe by 180^). Because of this, APe is irrelevant to this part of the process.

Now assume you've circularized (which probably isn't the best tactic--do what Kasuha says). Your APe is still the same as it was before but the plane change burn is now way cheaper because you've moved your AN, DN, and Pe to a better configuration. So now you do the plane change. Here, you still don't care about APe because now it's all about inclination.