How to deal with extremely elliptic orbits and asteroids, in a efficient matter?

[panzer1b]

Well, as the title says, ive gotten quite far, ive even gotten to just about every planet except for very long range atmoless ones such as Eeloo, ect. Still making better and better dV SSTOs.

Anyways, one issue ive come up with recently, is dealing with eliptical orbits especially if you happen to be an elliptical orbit yourself, that is in a completely inconvenient direction (such as your long axis being 90ish degrees off). How do i EFFICIENTLY get onto an intercept and get to said place. Is it best to just mess with it and get the 1st encounter you can, or is it instead better to get into a close orbit somehow?

Another issue ive started actually working with is asteroids. They only give me one shot to rendezvous, and i seem to be eating well over 1000 dV minimum for even a lucky good intercept, sometimes im unable to get a good intercept and end up using 3K dV to get to the asteroid by inefficent path matching. Are there any tips you pros can give me on how to 1st of all approach an asteroid, and then how to get to this asteroid, intercept it, and do something with it without needing absurd levels of dV.

So well, i hope someone here has some tips, and preferably specifics on what to do in what situation. Im not gonna say im a pro or anything, but i do know how to intercept stuff, and i have managed to do all of the things im asking for help with, my main dilema is just that whatever i do i end up eating so much dV that i prolly could get to jool and back twice with the amount of fuel im wasting on just 1 lowly asteroid. Same with things like fixing eliptical orbits and rendezvousing with something else thats in a eliptical orbit that happens to be well off of me.

Edited February 8, 2015 by panzer1b

[cantab]

Regarding asteroids, I favour intercepting them in solar orbit. It takes a little more delta-V to get there, but that's delta-V without the asteroid in tow, and you can then adjust the asteroid's pass of Kerbin - most obviously, to aerocapture into Kerbin orbit - using only a little delta-V.

There's a tutorial on solar orbit rendezvous, but the "trick" is to head out of Kerbin's SOI in the direction the asteroid is set to come in.

[Empiro]

Under what circumstances are you trying to rendezvous with a target in an elliptical orbit? Are you trying to mount a rescue, or trying to dock with a space station around another planet etc.?

You can also intercept asteroids inside the Kerbin SOI. The trick is to launch into a circular orbit with the same inclination as the asteroid, with the AP/PE matching that of the asteroid's PE around Kerbin. When the asteroid enters Kerbin's SOI, you can fine-tune your orbital period by burning prograde at the point where your orbit touches the asteroid's PE so that when the asteroid reaches Kerbin PE, your ship is exactly there too, and you can perform a velocity matching burn to rendezvous.

[Superfluous J]

Intercept the asteroid at or before Kerbin Periapsis.

At Kerbin periapsis, burn retrograde until you have an apoapsis within Kerbin's SOI (i.e., you have an orbit).

Target Mun.

At apoapsis, set up a maneuver node that at least crosses Mun's orbit well, and at best gets a Mun encounter.

If you didn't get an encounter, go a few orbits ahead until you DO get an encounter.

Once you have a mun encounter, at your Kerbin apoapsis make a maneuver node and fiddle until Mun will circularize your orbit. Try to also get your periapsis where you want it, set up your ejection, whatever. If you can't, no bigs. You'll get another Mun encounter where you can do that.

In short, do everything you can at apoapsis, and "everything you can" should always involve using Mun to make the big changes.

But it'll still chew through fuel like it's water going over Niagara Falls.

[Frank327]

As someone else said, capturing asteroids outside kerbin SOI feels scary but is really effective. Indeed, escaping kerbin in the same direction that the asteroid is coming from will get you a decent intercept.

As for rendezvous at elliptical orbits, the first most important step is getting into the same plane as the target (get the AN/DN close to 0). From there, get your orbit to touch the target orbit at its periapsis. From there it's all about arriving at that point at the same time as the target. You can adjust the time it takes to reach the rendezvous point by burning prograde at the rendezvous point. Basically, make a manouver node at the point where the orbits touch and fiddle around with it until you've got a close encounter. If the target orbit is much bigger than your own, you might have to wait one or more orbits before burning.

As long as your approach orbit remains smaller than the target's orbit, it's 100% fuel efficient.

[Mythos]

As someone else said, capturing asteroids outside kerbin SOI feels scary but is really effective. Indeed, escaping kerbin in the same direction that the asteroid is coming from will get you a decent intercept.

I also skipped from capturing asteroids from inside kerbin SOI to outside. But I'm still not shure where to do the plane change. Matching plane inside SOI (like in escaping same direction as asteroid would come in) feels somewhat less efficient, but I have not really compared. Does anyone know better?

[Superfluous J]

I also skipped from capturing asteroids from inside kerbin SOI to outside. But I'm still not shure where to do the plane change. Matching plane inside SOI (like in escaping same direction as asteroid would come in) feels somewhat less efficient, but I have not really compared. Does anyone know better?

To get your asteroid elliptical, you MUST do a plane change at the AN/DN. Period. You can fiddle around to make that take less dV, and the best way to do THAT is to use Mun to alter your orbit in any major way. Note: some alterations are so large in dV that you simply can NOT use Mun to do them in one try, but you CAN use one Mun encounter to do half of it, then in the next Mun encounter do the other half.

It's a bit of an advanced maneuver but if you fiddle around with maneuver nodes enough you'll see how it works. And as a bonus you can launch a small ship into a very elliptical orbit and practice long before you capture your asteroid.

[panzer1b]

Thanks for the tips.

One more question, if im trying to get to say gilly (or any body/ship in a extremely elliptical orbit), should i start out from as close to circular as i can? Most of my craft maximize what they can do via aerobrake, so ofc i aerobrake at eve. Should the ending orbit be more or less circular before i try to match my PE with the PE of the gilly orbit?

[Xavven]

Thanks for the tips.

One more question, if im trying to get to say gilly (or any body/ship in a extremely elliptical orbit), should i start out from as close to circular as i can? Most of my craft maximize what they can do via aerobrake, so ofc i aerobrake at eve. Should the ending orbit be more or less circular before i try to match my PE with the PE of the gilly orbit?

Not necessarily. It's most efficient to get plane changes done when your orbital velocity is the lowest. As you may know, your orbital velocity is lowest at the apoapsis of a highly eccentric (elliptical) orbit. It's also pretty low at high orbits (high altitude, very big circle or ellipse).

Exactly how you get to Gilly will depend on its orientation when you arrive, so there's no 1 prescriptive method for getting there cheaply. There are some basic principles that can help you, though:

Firstly, fine tune your approach while in your transfer orbit from Kerbin to Eve. There are several good places to do this: Right after you leave Kerbin's SOI; at an ascending or descending node (burn normal/antinormal to match planes with Eve); and 90 degrees from your encounter (burn radial in/out to get your periapsis close to Eve, and fine tune your inclination by burning normal/antinormal). Try to get your periapsis close to Eve (target 100km - 200km) and make sure your hyperbolic orbit goes COUNTER-CLOCKWISE around Eve (West to East)!

Secondly, do a final fine-tune right when you enter Eve's SOI as there will have been some floating point errors at the SOI change that throw your approach off. The goal will be to get your periapsis close to Eve in order to minimize the delta-v you expend in orbital insertion (you're using the Oberth effect). Some people like to aerobrake at this stage as well, but I usually put my periapsis about 30km above the edge of the atmosphere (in Eve's case, the atmosphere abruptly ends at 90km, so I target 120km). Also, are you going counter-clockwise?

Next, perform your orbital insertion burn (burn retrograde at periapsis) and stop with a highly eccentric orbit. I like to have my apoapsis altitude to be much higher than Gilly's highest altitude. Now target Gilly and check where your nodes are.

Scenario A: If you're lucky, you'll have a node close to the apoapsis of your highly eccentric orbit. If that's the case, you want to perform your plane change at the node closest to the apoapsis. Your next move is to burn prograde at apoapsis until your periapsis is at or above Gilly's orbit.

Scenario B: If you're unlucky, your nodes are going to both be near your periapsis. When that happens to me, I wait until I'm at my apoapsis and burn prograde until I have a very high altitude, almost circular orbit. That gets those nodes out to a high altitude where your orbital velocity is low. Perform your plane change at whichever node is closer to your apoapsis for maximum efficiency, but if your orbit is pretty circular at this point it won't matter a whole lot which one you chose anyway.

Whether you had scenario A or B, your last step is to play around with maneuver nodes until you get an intercept. But the hard part is over. At this point, your delta-v expenditure is going to be pretty minimal no matter what strategy you choose. The big parts are your Eve orbital insertion and the plane change.

[Kryxal]

Side note: if you can combine two burns, so much the better, like the final plane-change for Gilly and placing your periapsis near where it needs to be.