Rendezvousing with Polar Munar Station

[Quadkiller]

I'm having trouble rendezvousing with a station that is in a low polar orbit of the Mun. I am able to get a Mun encounter and make a mid-course correction so that I end up in a polar orbit around the Mun. However, the polar orbit I end up in isn't aligned with the polar orbit of the station.

How can I time my launch so that I end up with the two orbits as closely aligned as possible?

[DD_bwest]

getting the timing right will be rather difficult,  it would be much easier to just adjust your orbit once you get to the mun.   Is there a reason your trying to do it without a correction in Mun's orbit?

[Tiafan]

Doing inclination or longitude changes close to a celestial body is very costly in terms of delta-v (requires much more fuel). Try to avoid them or make as little corrections as possible. Sometimes, you can't avoid them and planning fuel accordingly is crucial. When you intercept with the right inclination/longitude, it is then only about matching the AP and PE... much easier than matching everything all at once. 

Now your problem is more about timing than fuel, what I usually do is get the craft in orbit then prepare a transfer node. You can right-click the node to add orbital periods, effectively scheduling the burn for a later time.  Of course, you will have to keep adjusting the node along the way to intercept properly. However, this method is quite cumbersome in stock; a manoeuvre editing mod is very useful (mechjeb, precise node, manoeuvre editor, etc.), but it's possible.

Another method is to schedule the node, then purposefully skip the node and pass it by. Then create a new one and go around the orbit again; repeat until you are satisfied with the encounter angle. It is essentially the same method as above but without a lot of fiddling with the nodes; makes the process a bit easier to understand.

Ultimately, it is about experience; you will eventually be able to guestimate when to launch closer to a proper launch window and not waste too many hours/days in orbit (think electric charge!).

I hope this helps.

 

[bewing]

It's a hell of a lot easier if you use a vertical launch rather than a hohmann xfer. Because with a vertical launch you can control the exact height and timing of your Kerbin Ap when the Mun captures you. Your Munar orbit is basically determined at that moment. So basically the easy way to do it is to get to a particular altitude just in front of the Mun and then come to a stop. The Mun swallows you up, and because of your alititude you come down toward the Mun from a very particular angle.

It's also true that if you leave your Munar Pe very high, and just do your Munar capture burn at the An or Dn of your target orbit -- then you can combine the capture and plane change burns into a single very efficient burn.

Edited November 23, 2016 by bewing

[Orbin]

On 23.11.2016 at 2:06 AM, bewing said:

It's a hell of a lot easier if you use a vertical launch rather than a hohmann xfer.

This never ocurred to me but it sounds very intriguing. Did I get it right: with vertical launch you mean you just shoot straight up so that your Ap is just inside Mun's SOI and when you reach it the Mun will be there as well. Then, floating kinda stationary above the Mun, you burn in whichever direction you want your orbit to be?
Am I picturing it right? Does this even work?

[bewing]

41 minutes ago, Orbin said:

This never occurred to me but it sounds very intriguing. Did I get it right: with vertical launch you mean you just shoot straight up so that your Ap is just inside Mun's SOI and when you reach it the Mun will be there as well. Then, floating kinda stationary above the Mun, you burn in whichever direction you want your orbit to be?
Am I picturing it right? Does this even work?

Yes, it works. And you mostly have it right. If you put your Ap at 9Mm, then you end up in an extremely high prograde orbit around the Mun -- when it rushes up at 540 m/s and swallows you. So you aren't floating ... you have to burn retrograde to halt your 540 m/s hyperbolic trajectory. If your Ap is at 10.4Mm, then you end up in a lowish prograde orbit. If you put your Ap at 11.4Mm (and you time it so that the Mun grabs you as you come to a stop at your Ap), then you will be falling straight down toward the Mun's equator. If your Ap is higher than 12Mm, then you end up in a retrograde orbit. The other nice thing is that your ship spends days within 10km of the Ap, because your ship is very slowly coming to a stop (and then reversing direction) in very low gravity -- so you don't have to time things all that carefully. The Mun has lots and lots of time to come up and grab you. Just aim your launch 45 to 60 degrees ahead of the Mun (depending on how high your want your Ap to be). And you don't have to be particularly careful about that number -- judging by eye is more than accurate enough.

 

[Superfluous J]

All you need to do is time your burn so you arrive at Mun when your station is orbiting parallel to Mun's orbit around Kerbin. This means you can do such a transfer twice a month.

I'm on my phone now so can't really explain it better or use pictures.

[monstah]

I have a different suggestion:

• do what you usually do, and have it arrive at any polar orbit; make sure you match your Pe with the station's orbit at the pole.
• When you get there, capture it in a highly elliptical orbit (i.e., Ap almost out of Mün's SOI). If both orbits are polar and the Pe is at the pole, then the AN and DN should coincide with Ap and Pe.
• Keep lowering your Ap until you get an encounter (the higher it is, the longer it'll take to complete this orbit, but the less DV you'll spend to correct it).
• Now, match planes at Ap. You'l be going a lot slower there than closer to Mün, and you already have an encounter at Pe.
• Finally, match orbits at Pe

20 minutes ago, 5thHorseman said:

All you need to do is time your burn so you arrive at Mun when your station is orbiting parallel to Mun's orbit around Kerbin. This means you can do such a transfer twice a month.

I'm on my phone now so can't really explain it better or use pictures.

This works too, but don't like the windows (it's just over there, for crying out loud!).

Here's how I eyeball it:

• Look at the orientation of your polar orbit from "above" (the North pole);
• Imagine a line perpendicular to that one;
• Now shift this perpendicular line, so it passes through Kerbin;
• This line will intersect Mün's orbit in two points.
• You'll want to arrive at Mün when it is in one of those two points (hence, twice a month window). Just time your launch so your Ap is around there.

[Quadkiller]

Thanks for the help everyone. I'll give both suggestions a try. It turns out I was completely wrong in terms of timing the launch. I'll give it a try and hopefully it will work better now.

I'll also try out a vertical launch but I'm sure that'll involve me flying by hopelessly far away from the Mun until I get the hang of it :).

I just started playing KSP and it's amazing. It's awesome to have a community of people who can help out with getting started.

Also by the way, I couldn't figure out how to comment on other people's answers. Can someone help me out?

[Superfluous J]

1 hour ago, Quadkiller said:

Also by the way, I couldn't figure out how to comment on other people's answers. Can someone help me out?

There are 4 main ways to do it.

1. Click "quote" under a post, and it'll quote that post and you can answer it. You can also edit within what you're posting, to pare it down like I did above.
2. Click the "+" next to the word "quote", this will add the quote to a queue and you can then, when you're ready to post, click "Quote 2 posts" or whatever it says in the bottom right of your browser and it'll insert them all at once.
3. Use your mouse to highlight some text, and the forum should bring up a "quote this" button that will immediately put just what you highlighted into a quote. I actually did this to quote you above. Sometimes it's wonky and you have to try twice, and on mobile it's almost impossible.
4. And finally, if you type a @ followed by someone's name, a drop down will come up and you can select the person's name. You MUST actually select the name, and not just type it. This way you can reference someone without directly quoting them. Example: @Quadkiller

[Quadkiller]

2 hours ago, 5thHorseman said:

There are 4 main ways to do it.

1. Click "quote" under a post, and it'll quote that post and you can answer it. You can also edit within what you're posting, to pare it down like I did above.
2. Click the "+" next to the word "quote", this will add the quote to a queue and you can then, when you're ready to post, click "Quote 2 posts" or whatever it says in the bottom right of your browser and it'll insert them all at once.
3. Use your mouse to highlight some text, and the forum should bring up a "quote this" button that will immediately put just what you highlighted into a quote. I actually did this to quote you above. Sometimes it's wonky and you have to try twice, and on mobile it's almost impossible.
4. And finally, if you type a @ followed by someone's name, a drop down will come up and you can select the person's name. You MUST actually select the name, and not just type it. This way you can reference someone without directly quoting them. Example: @Quadkiller

Thanks!