High Eccentricity for Refueling.

[957Chatterton]

I've rehashed my refueling station design countless times. This time, I'm done but want to know the ideal orbital altitude to refuel at and then set off for another planet. Specifically, I had my station at an 80Km periapsis and 80,000Km Apoapsis. The reason for that was that I wanted to use the Oberth effect while hardly using any dV to escape Kerbin. It's the best of both worlds.

Edited April 9, 2014 by 957Chatterton

[Random Tank]

I don't mean to be rude, but there is a thread on the exact same subject on the same page of the Questions and Tutorials page as this one... Please look there

[daniu]

Here's a graph Starstrider42 created for the optimal launch position for the different planets.

[957Chatterton]

I looked at it and they're not addressing my central question. I want to combine the benefits of low and of high orbits by having a high eccentricity. This would allow me to burn as a crazy speed while my apoapsis only has a few inched until I'm in Kerbol orbit.

Really, I'm most interested in having a flaw in that plan that I'm not yet aware of pointed out to me.

[daniu]

Oh, okay.

So you're planning on burning at periapsis to only have to add a few m/s to escape?

Sounds to me like a drawback may be you have to time your takeoffs and escape launches very carefully, to not waste fuel on the encounter maneuvers.

[mhoram]

By putting the refuling station into an 80x80000km orbit you gain some benefits, but

you lose the ability to to chose the phase angle for departure - or would need to make the ejectionburn not at the 80km periapsis of that orbit, but at a higher altitude where the oberth effect is not that effective.

And also the rendezvous with that refueling station will be harder to achieve than with a circular station.

Edited September 26, 2015 by mhoram
typo

[Streetwind]

By putting the refuling station into an 80x80000km orbit you gain some benefits, but

you lose the ability to to chose the phase for departure - or would need to make the ejectionburn not at the 80km periapsis of that orbit, but at a higher altitude where the orbeth effect is not that effective.

That's not as much of an issue as you think it is. Because you're falling in from such a high apoapsis, your speed near the planet will be far higher than if you were in a circular orbit. Thus, even if you're a little further away from the planet, you're still getting a lot of help from mister Oberth.

And also the rendezvous with that refueling station will be harder to achieve than with a circular station.

This is a valid concern, but ultimately a minor one.

Really, I'm most interested in having a flaw in that plan that I'm not yet aware of pointed out to me.

The major flaw is that Kerbin orbits its parent star.

Minimizing your ejection dV by having a high apoapsis on your refueling station requires that said apoapsis is pointing directly prograde (for going outwards) or retrograde (for going inwards) of Kerbin's orbit. So not only would you need two stations to cover these two cases in a perfectly static model environment, but you also get screwed over because the solar system isn't static. Kerbin follows its own circular orbit, but the orbit of your station doesn't turn along with it. The result is that the apopasis turns away from the planet's orbit vector in a hurry and spends most of its time not pointing anywhere useful.

Even if you said "okay, so I can get away with one station and I will have two launch windows per Kerbin year, one where the station points prograde and one where it points retrograde", it doesn't work - except by pure chance. You need your destination planet in the proper position relative to Kerbin too, after all, and because the orbital periods of the other planets are different from Kerbin's, they likely will not do you the favor of being right where you want them by the time your once-per-year window comes up.

EDIT: Just to clarify, you could still eject Kerbin in any direction you want from any position of the refueling station. It's just that the dV savings will not manifest. In fact, in 50% of all possible cases, your dV costs increase quite a bit.

Edited April 8, 2014 by Streetwind

[Kasuha]

I don't quite understand the original post and what the matter of the question is. Whatever the orbit of your refueling station is, you need to match orbit with it to refuel. Meaning you will enter an orbit with 80 km periapsis and 80 Mm apoapsis right before you dock for refueling. Ideal approach is to intersect its orbit at periapsis and brake/accelerate to match orbits and rendezvous from there because you get the most from Oberth effect at that place.

[Streetwind]

He was asking what the downside is to have a refueling station permanently in such an orbit for easier interplanetary transfers, because after docking and filling up, you only need a tiny burn a periapsis to reach escape velocity. If your station's apoapsis was out beyond Minmus and pointing perfectly prograde along Kerbin's orbital vector, your craft could reach Jool and Eloo with much smaller burns and fuel usage, leaving more for when you reach the target (or allowing smaller craft). Compare the old "drop to Kerbin from Minmus" maneuver, except without an orbital inclination to screw with you and without having to pay dV for exiting the SoI and dropping your periapsis close to Kerbin.

Another problem with such a station though: assume you fix the problem I mentioned earlier by sheer effort and launch for example 12 stations. One for each hour on the clock, so no matter where Kerbin is on its orbit and no matter in which direction you want to eject, you can always find a station to utilize that's matching your desired vector almost perfectly. But it still won't work, because Kerbin isn't alone in its SoI. There are two moons, both of which will be crossing the orbits of the refueling stations regularly, which will lead to encounters that alter the station's orbits. Best case scenario, they become less efficient for what you want to do and go out of sync with the others; worst case scenario, their periapsis gets dropped into the atmosphere or the station gets bowled out of the SoI entirely.

Edited April 8, 2014 by Streetwind

[Kasuha]

If your station's apoapsis was out beyond Minmus and pointing perfectly prograde along Kerbin's orbital vector, your craft could reach Jool and Eloo with much smaller burns and fuel usage

Not really, if you burn at periapsis your trajectory opens. Ejections towards even nearest planets are hyperbolic meaning you are not going in the direction of your former apoapsis. Also the direction is different depending on how much dv you need to apply.

But it still won't work, because Kerbin isn't alone in its SoI. There are two moons, both of which will be crossing the orbits of the refueling stations regularly, which will lead to encounters that alter the station's orbits.

If you make the orbit just slightly inclined you can be able to avoid Mun completely without real effect on ejection (you don't have to burn exactly prograde to eject). And there is no point to draw the apoapsis all the way to Minmus, there's almost no dv gain from that.

In general it is not a bad plan, I have very good experience with such elliptic parking orbits. Restriction to one launch window per refueling station per orbit is not very comfortable though and matching it with incoming ships might be a problem even with twelve such stations.

[Streetwind]

Not really, if you burn at periapsis your trajectory opens. Ejections towards even nearest planets are hyperbolic meaning you are not going in the direction of your former apoapsis. Also the direction is different depending on how much dv you need to apply.

Has anyone told you before that you have a rather obsessive tendency to latch onto and argue minor semantics?

[Superfluous J]

The biggest problem with the 12-refueling-stations-in-highly-elliptical-orbits plan is you'll spend far more time setting it up and refueling them than you spend doing anything else.

[Aedile]

He was asking what the downside is to have a refueling station permanently in such an orbit for easier interplanetary transfers, because after docking and filling up, you only need a tiny burn a periapsis to reach escape velocity. If your station's apoapsis was out beyond Minmus and pointing perfectly prograde along Kerbin's orbital vector, your craft could reach Jool and Eloo with much smaller burns and fuel usage, leaving more for when you reach the target (or allowing smaller craft). Compare the old "drop to Kerbin from Minmus" maneuver, except without an orbital inclination to screw with you and without having to pay dV for exiting the SoI and dropping your periapsis close to Kerbin.

Another problem with such a station though: assume you fix the problem I mentioned earlier by sheer effort and launch for example 12 stations. One for each hour on the clock, so no matter where Kerbin is on its orbit and no matter in which direction you want to eject, you can always find a station to utilize that's matching your desired vector almost perfectly. But it still won't work, because Kerbin isn't alone in its SoI. There are two moons, both of which will be crossing the orbits of the refueling stations regularly, which will lead to encounters that alter the station's orbits. Best case scenario, they become less efficient for what you want to do and go out of sync with the others; worst case scenario, their periapsis gets dropped into the atmosphere or the station gets bowled out of the SoI entirely.

The problem with eccentric orbit

I hope you realize that dropping the periapsis at the exact position and time around Kirbin and burning there is the whole point of it. In fact high Mun orbit works better, as you would miss launch window with less, due to lower time to travel, and lower Mun orbital period. Install a mod such a precise node, add a number of small satellites so you could test encounters from different start orbits.

Then even 6 stations won't work - why - because orbital period is like what 5 days? You need to be at periapsis at the exact point and time.

Launch ship few days early, using nodes, get it in correct elliptical orbit, so that the next perhaps is he phase you'll need to burn for ejection. Send a fuel ship, rendezvous and refuel. Easily accomplished if you start with identical circular orbits, with fuel ship trailing you few minutes. Not sure why you need refueling station on a wrong orbit as a middleman. Or better yet build a mother ship/fuel ship which can burn together.

In ksp refueling and bases make most sense for landers/rovers, as well as refueling over low orbit of the body producing

[Streetwind]

Yeah, you want your return to periapsis to fall into the transfer window. Which isn't that big of a deal, since such a window usually lasts a few days. But it adds yet another point on the growing list of reasons why this isn't as good an idea as it might seem at first glance

The "12 orbiting refueling stations" example isn't there to say "it might work if you did this", it is there to say "you can bend over backwards with ridiculous effort and it still won't work right".

[Kasuha]

Has anyone told you before that you have a rather obsessive tendency to latch onto and argue minor semantics?

I'm sorry if you feel that my fixing minor point in your post is an attempt to invalidate it, I did not mean it that way. I do not disagree with anything in your post except what I quoted and commented on.

I hope you realize that dropping the periapsis at the exact position and time around Kirbin and burning there is the whole point of it.

I don't see that as major problem. Exact position yes, but you don't have to hit exactly the periapsis, the further from it you get the less effective but it still can be pretty effective. And exact time is not really necessary, you can match time in one orbit for insubstantial dv.

Matching the inclination - or more generally the major axis direction - is the tricky part. But even that can be done.

[John FX]

Has anyone told you before that you have a rather obsessive tendency to latch onto and argue minor semantics?

I`ve noticed a few people say that...

In the spirit of fairness though the thread on fuel flow rules is awesome and deserving of respect.

Edited April 9, 2014 by John FX

[Streetwind]

(I included that smiley in my post to indicate that I did not feel insulted or anything of the sort, btw.)

[stupid_chris]

Alright guys, back on topic please. It's not a question of who's right and who's wrong, just stick to the question on the table and things will be fine

[TheD007]

It is a good idea for the outer planets as they have wide launch windows and can save much delta-v. One suggestion would be to have the refueling station have engines and be on a high circular orbit, initially, to make rendezvous simpler and avoid gravity slingshots out of the system. After refueling the docked ship, have the station use its fuel to drop the periapsis to a LKO at the right phase angle. Then undock the ship and have it do the escape burn. Afterward have the station return to a circular orbit if it has enough fuel to continue.

[Claw]

By putting the refuling station into an 80x80000km orbit you gain some benefits, but

you lose the ability to to chose the phase angle for departure - or would need to make the ejectionburn not at the 80km periapsis of that orbit, but at a higher altitude where the orbeth effect is not that effective.

And also the rendezvous with that refueling station will be harder to achieve than with a circular station.

That's not as much of an issue as you think it is. Because you're falling in from such a high apoapsis, your speed near the planet will be far higher than if you were in a circular orbit. Thus, even if you're a little further away from the planet, you're still getting a lot of help from mister Oberth.

I think the wrong part of mhoram's point was latched onto. You WILL miss out on Oberth by not burning at PE, especially from such an elliptical orbit, but I don't want to turn this thread into an Oberth thread because those go for pages and pages.

Anyway, with an 80 km x 80,000 km orbit...

- The orbital period is roughly 10 Earth days. Sure, you can lower this down by burning retrograde at PE to adjust the orbital period for the next pass, but then you're basically reducing the benefit of making such a huge orbit to begin with. And you'll need to put your craft into orbit well in advance to join up, refuel, and readjust the orbital period for a timing pass. So you'd typically require two orbits.

- Probably the bigger problem is, as stated above, more often than not your elliptical orbit won't be pointing in a convenient ejection angle. That's all fuel that you were trying to save.

Personally I think it's easier to orbit around the Mun, or along the Mun's orbit. You're looking at a difference of about 350dV, including burning away from the Mun. (That's all assuming you don't have to make any corrections on the elliptical orbit.), However, the ejection angle problem just about goes away, and with the roughly 1.5 Earth day orbital period, you get window visits more often without having to play with orbital periods as much.

Edited April 8, 2014 by Claw

[Rus-Evo]

In the time it would take you to set that up you could easily rendezvous and dock an extra fuel tank on to the front or rear of your ship and explore half the solar system (if all you care about is assisting your exploring).

But if this is simply an exercise in its own right could you just stick a circular orbit a little beyond the Mun and sacrifice Oberth for flexibility?

Otherwise sounds like spending 30,000DV to save 30DV

[Kasuha]

Okay I'd like to once more weigh in favor of eccentric orbits as I really have very good experience with them.

The matter is what are your playing priorities. In case you want to save in-game time for any reason (be it roleplaying, life support limitations or just your personal preference), then they're not very good and you should stick to equatorial orbit at suitable height.

If you're trying to save some dv, are willing to "buy dv for dt", and have sufficient experience to handle them efficiently, they're definitely very good. You will probably need multiple such refueling stations to accomodate ships coming from all directions but that's all there is about it and refueling missions to your stations will not be any more frequent than if you had just one station in circular orbit. Actually they will be less frequent because your ships will use less fuel.

[Superfluous J]

In the time it would take you to set that up you could easily rendezvous and dock an extra fuel tank on to the front or rear of your ship and explore half the solar system (if all you care about is assisting your exploring).

But if this is simply an exercise in its own right could you just stick a circular orbit a little beyond the Mun and sacrifice Oberth for flexibility?

Otherwise sounds like spending 30,000DV to save 30DV

Better to put the orbit past Minmus and then you don't have to sacrifice either Oberth OR flexibility. For a couple hundred dV you can get a nice tight periapsis back at Kerbin and you'll easily gain that back from Oberth. You can even use your "fuel station" to push your ship into that elliptical orbit, undock, and then burn your station back into regular orbit for the next ship.

Works better if you're mining Kethane on Minmus, of course, so you don't have to carry all that fuel up there.

EDIT:

Okay I'd like to once more weigh in favor of eccentric orbits as I really have very good experience with them.

I agree. Nothing feels better than setting up an elliptical orbit several weeks before your window, so that you can burn at Periapsis into a perfect transfer to another planet. And nothing is more annoying to find out that the orbit before that Periapsis, a Mun encounter messes everything up.

I almost refuse to go to other planets any other way.

Edited April 8, 2014 by 5thHorseman

[957Chatterton]

I, as the OP, will go ahead and say I've received sufficient reason to back off of high eccentricity. I'll simply use my standard 250Km circular orbit for interplanetary travel. I guess my focus would be perfecting my rocket design. Hell, I'm launching with at least 10k dV, lol!

[FenrirWolf]

The biggest problem would be that depending on your chosen orbit, you would only be set up for transfers to outer planets or inner planets. Though I guess that's less of an issue of you don't plan to go to Eve or Moho