Intercept planet after X orbits?

[aluc24]

Hello,

I'm using KSP 1.04 with Mechjeb 2, KSP Alarm Clock and several other mods. I'm now into advanced interplanetary trajectory planning. Could someone please tell me how can I set up a maneuver so that I see target intercept predictions after more than 1 orbits?

What I mean is that I burn now, but intercept target planet after, let's say, 3 or 4 orbits. Right-clicking maneuver node and clicking "add" button only delays the maneuver itself, not the predictions. KSP Alarm Clock can check target distance up to 20 orbits, but it doesn't work with maneuver nodes and planets (only vessels).

The point of doing this is that it might save a lot of fuel in some cases (like returning from Moho).

So, how do I do it?

[Retread]

I can't help you with a mod that would show this, but I'm wondering why you would want to do it this way instead of just waiting for a transfer window.

[RA3236]

I can't help you with a mod that would show this, but I'm wondering why you would want to do it this way instead of just waiting for a transfer window.

The point of doing this is that it might save a lot of fuel in some cases (like returning from Moho)

Its to save fuel and while it takes longer, it could also be safer in fuel costs.

[nuclearping]

MechJeb has an "Advanced planet transfer" planner. If you select "Lowest dV" it will setup a node in the future to transfer with lowest fuel consumption. Additionally you can select the transfer time manually clicking the image map. Basically the bluer the area, the lower dV you need. Using MechJeb's "Maneuver Node Editor" you can manually tweak the transfer maneuver node.

[Retread]

Its to save fuel and while it takes longer, it could also be safer in fuel costs.

Maybe I didn't ask the question right, then. As far as I know, there are certain alignments between two planets that will allow for minimim dV transfers between them, usually referred to as launch or transfer windows. Are you saying that by allowing several orbits to elapse between escape from one planet and insertion to another, there's additional dV savings to be had?

[aluc24]

MechJeb has an "Advanced planet transfer" planner. If you select "Lowest dV" it will setup a node in the future to transfer with lowest fuel consumption. Additionally you can select the transfer time manually clicking the image map. Basically the bluer the area, the lower dV you need. Using MechJeb's "Maneuver Node Editor" you can manually tweak the transfer maneuver node.

I know, but I don't think it accounts for multiple orbits.

Maybe I didn't ask the question right, then. As far as I know, there are certain alignments between two planets that will allow for minimim dV transfers between them, usually referred to as launch or transfer windows. Are you saying that by allowing several orbits to elapse between escape from one planet and insertion to another, there's additional dV savings to be had?

In certain cases, yes. My example. I have a ship in Moho orbit that I'd like to return to Kerbin. Time frame isn't of importance. Now, to minimize dV, I have to burn at Moho periapsis, and raise apoapsis just to touch Kerbin orbit. However, the Kerbin will not be there at the right time. So, I coast to apoapsis, and then adjust periapsis just slightly so that I intercept Kerbin again at apoapsis after some orbits (usually 3-4). This can be much more fuel efficient than usual Hohman transfer.

But for it to work, I need to see predictions for future orbits, not just this one.

[Snark]

There's an easy trick to do this. Works in stock, doesn't need any mods. It relies on the fact that the "closest approach" markers shown in map view are always the closest approach after the last maneuver node.

So what you do is this:

1. Place a maneuver node at the point where you want to do your burn.

2. Go ahead and give that maneuver node some dV, something that is at least in the general ballpark. Doesn't have to be accurate.

3. When you do that, it will show you a dotted-line ellipse which is your new trajectory after doing the maneuver.

4. Drop another maneuver node somewhere on that dotted oval, but don't give it any dV at all. Just leave it at zero. This is a "dummy" node.

5. You now see the closest approach you get on your first orbit after the dummy node.

6. Click the "+" button on the dummy node to move it forward a few orbits. Each time you do this, you'll see the closest-approach markers jump around, because each time it's showing you the closest approach one orbit farther into the future.

7. When you get a closest approach that you like (i.e. one that's at least in the reasonable ballpark), go back to your "real" maneuver node (i.e. the first one, the one that actually has dV on it) and start adjusting it slowly. You'll note that it will have a greatly-magnified effect on closest approach, since you'll be seeing "if I make this adjustment now, here's where the closest approach will be N orbits from now."

8. So all you have to do is slide it around a bit until you get your intercept.

Ta dah! Works like a charm.

Edited October 26, 2015 by Snark

[magnemoe]

One way to do it in mechjeb is to first add an 0 m/s node, then add an mechjeb burn like circulate then your orbit already is circular but add say 200d for 200 days after last node.

Now use the advanced transfer.

Note that 200 days is over 2000 orbits in low kerbin orbit so it will probably build up some errors making it smart to create an new one some weeks before you go.

[aluc24]

There's an easy trick to do this. Works in stock, doesn't need any mods. It relies on the fact that the "closest approach" markers shown in map view are always the closest approach after the last maneuver node.

So what you do is this:

1. Place a maneuver node at the point where you want to do your burn.

2. Go ahead and give that maneuver node some dV, something that is at least in the general ballpark. Doesn't have to be accurate.

3. When you do that, it will show you a dotted-line ellipse which is your new trajectory after doing the maneuver.

4. Drop another maneuver node somewhere on that dotted oval, but don't give it any dV at all. Just leave it at zero. This is a "dummy" node.

5. You now see the closest approach you get on your first orbit after the dummy node.

6. Click the "+" button on the dummy node to move it forward a few orbits. Each time you do this, you'll see the closest-approach markers jump around, because each time it's showing you the closest approach one orbit farther into the future.

7. When you get a closest approach that you like (i.e. one that's at least in the reasonable ballpark), go back to your "real" maneuver node (i.e. the first one, the one that actually has dV on it) and start adjusting it slowly. You'll note that it will have a greatly-magnified effect on closest approach, since you'll be seeing "if I make this adjustment now, here's where the closest approach will be N orbits from now."

8. So all you have to do is slide it around a bit until you get your intercept.

Ta dah! Works like a charm.

Wow. This is really smart. Worked like a charm. Thank you!!!

[nuclearping]

I know, but I don't think it accounts for multiple orbits.

It does. It will setup a node in the future, no matter if the maneuver is 1h or 1d or 1y away.