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# Rule of thumb interplanetary transfers?

## Question

So I've got reasonably good at shuttling between Kerbin and MÃƒÂ¼n/Minmus, without doing any calculations and instead just pointing the nose and throttling up (using the wait-until-the-destination-comes-over-the-horizon technique). It's not very efficient but it works, and my current lander has absurd amounts of fuel so I don't really care about efficiency right now.

However, interplanetary transfers are a lot harder. I've read up on it and how to do the standard Hoffman minimum-energy transfer orbit, and, basically, it involves doing the maths first. There's nothing wrong with that, but I feel that given the general seat-of-the-pants KSP style, there ought to be a more Jeb-like way.

In particular, a fast high-energy transfer should be relatively easy: get into Kerbol orbit, raise/lower apoapsis until your orbit crosses your destination, tweak until you enter the destination's SOI, wait until you get there and burn like hell to circularise. It would use a huge amount of fuel but it should be doable.

Unfortunately I have yet to make this actually work. None of the planets are in the same plane, they tend to move a lot over time so trying to find where a planet will be in the future is hard, and the SOIs are so small on an interplanetary scale that they're really easy to miss.

Has anyone had any luck with this approach, and if so, any good tips? (Any suggestions involving protractors or trigonometry will be staked out on the launch site the next time there's an engine test.)

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I Duna has the same inclination as Kerbin, so this approach might work there. One way to do it is to get into a low kerbin parking orbit, quicksave, then burn into a transfer orbit and work roughly how much you missed by (either eyeball it or use the mission clock). Then reload and time accelerate till you get into the rough launch window.

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To get to Duna I got into a circular orbit around Kerbin & then did a long burn to escape Kerbins gravity, then once in orbit around the sun I did another long burn on the periapsis to send the apoapsis out to meet the path of Duna. As I was on an eccentric orbit inside Duna it was just a case of waiting for an encounter to come round. It only took 5 years!

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That's Hohmann, sir.

As far as Jebbing to other planets goes, I haven't heard of anyone actually getting anywhere without at least being in proper phase angle. If you don't at least have the right phase angle, you're probably never going to get anywhere outside of Kerbin's SoI.

And yeah, Duna is on a flat plane so that's the easiest to get to with a mathless, imprecise approach. Otherwise, good luck.

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Well the phase angle changes depending on what transfer you want. The phase angle for a high speed transfer (i.e. the transfer obit extends beyond the orbit of the target planet) is different from a Hohmann transfer. What hjalfi is proposing is to keep the phase angle constant while changing the transfer orbit - untill he finds the orbit that matches the angle.

There are some phase angles that have no orbit that works for them though.

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I am too lazy and impatient to do the math. I match orbits at a slightly different altitude, then fast-forward until I get an intercept. Sloppy, but it works.

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Seat of the pants, no trig, no protractors, ok, got it. Let's do some eyeballing, guestimating, and saving/reloading. For the purposes of seat of the pants guessing, lets take out the changing ejection angles.

Lets make a couple of easy to follow rules.

The blue line is actually Kerbin's orbital path. Kerbin's prograde orbit is to the top in this photo. Let's call that North, Kerbin's retrograde orbit would be down, or south. If you wanna go to a higher orbit around the sun, burn when you are to the SE, if you want to go to a lower orbit, burn when you are in the NW where the craft is now.

Let's try to go to Duna. Burn when you're in the SE until your orbit matches Duna's.

Now all we need to know, is how far does Duna travel in the time it takes us to reach our apoapsis? Notice in the last photo I centered the camera on the sun and positioned Duna so it was to the east. Now we fast forward to apoapsis and see how far Duna went.

I have not moved the camera, so we can easily see how far Duna traveled in the 72 days it took to get to apoapsis.

Ok, so Duna traveled more than 1/4 of her orbit but not quite half her orbit. She's making it easy for us, let's say she went 1/3 of her orbital distance, since we are guessing, no protractors, remember.. So now all we need to do is reload, time warp until duna is 1/3 of her orbital path behind where our Apoapsis had been. Our Apoapsis had been directly opposite where Kerbin was, so that's easy too. Then we burn at the outward orbital burn location we picked (That Southeast corner, heh, corner. Ziff is so silly.)

Ok, so I burned until my apoapsis was at the location it was before and didn't get an intercept. Well, we were only guessing, remember? Let's burn a little more and see what happens.

Is it efficient? Hell no. Does it work? Yes, depending on how good you are at guestimating. It probably won't work for Moho, with its really eccentric orbit. But seriously, it took me less time to play around, take the screenshots, and get that intercept (5 minutes) than it did to write this post.

Edit: For planets on a different plane, wait until you are at the correct location to do an orbital inclination change. Once that is done you can burn a little bit to see if you get an intercept. I've managed to do this with Eve quite a few times.

Edited by Ziff
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Hey guys,

So I've been metaphorically banging my head against my desk trying to get an encounter with Eve. The orbital inclination is just killing me...and I feel a little defeated because of that. I was able to do it no problem with Minimus. I just can't seem to get the orbital inclination right with Eve. So I suspended my operations aimed at getting to Eve and shot for Duna. Got there no problem. I've heard it said that orbital inclination changes take less fuel as the orbit gets larger, that is there is a negative correlation between fuel usage for inclination change and parking orbit height. Is that true?

Also what I've been doing to correct my inclination is I wait until I'm halfway between the periapsis and apoapsis to make the inclination change.

Does anyone have any suggestions regarding this? Is there something I'm missing? Or rather am I just making it harder than it has to be?

Thanks,

- Eckert

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I'll admit I had been trying for Eve, not Duna. I will give Duna a try. Thanks very much. (A really nice-to-have feature on the map is to show projections forwards in time. It already does it for intercepts *anyway*, it'd just be nice to have it more flexible...)

For planets in a different orbital plane, I wonder whether boosting into roughly the same orbit as the target planet, doing a plane change manoeuvre, and *then* attempting a rendezvous would be at all feasible? I suspect that would raise the fuel consumption to silly levels as you're effectively doing two complete orbital transfers instead of one, but it's worth a shot.

@Ernst: what you're talking about is a bielliptic transfer orbit, IIRC. If you want to get somewhere with very little fuel you raise your apoapsis hugely, do a tiny correction at apoapsis, and circularise again when you've got where you want to go. Wikipedia has an incomprehensible maths-filled page with a good picture:

http://en.wikipedia.org/wiki/Bi-elliptic_transfer

It's cheaper than a Hohmann but takes forever.

I think changing inclinations is supposed to be done at the point when your ship's crossing the line when your orbital plane and the target plane meet. In other words, rotate the map view until you see the two orbits as lines, not ovals; and it's when your ship reaches the point where the two lines cross. Actually picking the optimum angle to do the burn is beyond my expertise but for minor changes a vector due north or south seems to be good enough, but you may need to correct your trajectory afterwards.

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Bi-elliptic is only cheaper than the Hohmann for certain circumstances. For instance, if your aim is to dive into the sun, a bi-elliptic sundive can be a heck of a lot cheaper than a hohmann sundive. If your intention is to reach any of the planets from Kerbin, Hohmann transfer is less expensive for all of them.

For plane changes, while it's always less expensive to do a plane change when you're moving slower, the method of pushing the apoapsis out to a far distance, doing the plane change there, then returning to periapsis and recirularizing tends to only be worth it for extreme plane changes, on the order of 60 degrees or more.

Edited by maltesh
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Hey guys,

So I've been metaphorically banging my head against my desk trying to get an encounter with Eve. The orbital inclination is just killing me...and I feel a little defeated because of that. I was able to do it no problem with Minimus. I just can't seem to get the orbital inclination right with Eve. So I suspended my operations aimed at getting to Eve and shot for Duna. Got there no problem. I've heard it said that orbital inclination changes take less fuel as the orbit gets larger, that is there is a negative correlation between fuel usage for inclination change and parking orbit height. Is that true?

Also what I've been doing to correct my inclination is I wait until I'm halfway between the periapsis and apoapsis to make the inclination change.

Does anyone have any suggestions regarding this? Is there something I'm missing? Or rather am I just making it harder than it has to be?

Thanks,

- Eckert

No, you don't do a plane change when you are halfway through the orbit. You want to do it where the two planes cross. For all of you who don't mind using Mods, this is the best one to use, and the only one you need, for interplanetary travel. The Rendezvous Calculator Shows you exactly when your launch windows are, and exactly when to start your ejection burn based on your current T:W ratio. (Click the little Adjust icon to do this.) Ok, that done, here's how you transfer to Eve.

First, use the calculator (or whatever method you use) and get your transfer burn lined up like this.

Once you have that lined up, click on the Sun to set it as the viewpoint. Then, and this is the tricky part, you want to flatten out your craft orbit and rotate the camera until the orbit of Eve flattens out as well. I typically zoom the camera in just enough to remove Duna's orbital path from the screen. Eventually I am sure we will have a way to turn on/off other orbital paths, but for now, it just looks messy. Anyway, this is what it will look like.

You can see how nice the orbits look flattened out and lined up. Now timewarp until your craft just reaches where those two orbits cross. In this case, it happens to be dead center on the sun as well. This is where your perform your plane change burn.

Burn at either at the N location on the gimbal, or 180, or you can use MechJeb's Normal and Anti-Normal buttons. In this case, it's Anti-Normal or 180. Now burn until the planes line up.

And bang, we didn't even finish the plane change, instead we had an intercept because of the super awesome job the Planetary Calculator does at timing everything for us.

Now check this out, if you look from the top-down view you can see where we are when we performed the plane change. We are well past half-way to the (original) periapsis when we burned.

Edited by Ziff
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Doing the inclination burn halfway to the target is the most efficient way, but not the easiest to eyeball. If you match planes at one of the two spots where your ship's orbit crosses that of the objective, you can watch from the side and burn until they coincide fairly precisely. And yes, it can be done after matching orbital height, if you don't mind the inefficiency.

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No, you don't do a plane change when you are halfway through the orbit. You want to do it where the two planes cross. For all of you who don't mind using Mods, this is the best one to use, and the only one you need, for interplanetary travel. The Rendezvous Calculator Shows you exactly when your launch windows are, and exactly when to start your ejection burn based on your current T:W ratio. (Click the little Adjust icon to do this.) Ok, that done, here's how you transfer to Eve.

First, use the calculator (or whatever method you use) and get your transfer burn lined up like this.

Once you have that lined up, click on the Sun to set it as the viewpoint. Then, and this is the tricky part, you want to flatten out your craft orbit and rotate the camera until the orbit of Eve flattens out as well. I typically zoom the camera in just enough to remove Duna's orbital path from the screen. Eventually I am sure we will have a way to turn on/off other orbital paths, but for now, it just looks messy. Anyway, this is what it will look like.

You can see how nice the orbits look flattened out and lined up. Now timewarp until your craft just reaches where those two orbits cross. In this case, it happens to be dead center on the sun as well. This is where your perform your plane change burn.

Burn at either at the N location on the gimbal, or 180, or you can use MechJeb's Normal and Anti-Normal buttons. In this case, it's Anti-Normal or 180. Now burn until the planes line up.

And bang, we didn't even finish the plane change, instead we had an intercept because of the super awesome job the Planetary Calculator does at timing everything for us.

Now check this out, if you look from the top-down view you can see where we are when we performed the plane change. We are well past half-way to the (original) periapsis when we burned.

That is an excellent explanation Ziff. Thank you very much. Possibly the most well articulated explanation of orbital mechanics I've seen yet.

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