Planning Launches - Longitude of Ascending Node?

[BentRim]

I am working on fulfilling Satellite contracts around Kerbin.  What I normally do is launch to the east and establish an equatorial orbit.  I then adjust orbit size and inclination to match the contract and then fine tune the Ap and Pe.  I find this very easy to do.  But I figure there must be a more fuel efficient way to do it. 

What I would like to do is plan my launch around Kerbin closer to the inclination of the target orbit for the satellite.  When I review the contract terms I see that the only data other than the orbit size is the Longitude of the Ascending node.  In this particular case that number is 339°

When I go to my map screen and then look at the target orbit for the contract I see that the An and Dn are at ±116°

Can someone offer advice on how to better plan my launch using this data so that I can be more efficient and not have to adjust my inclination as much.

[bewing]

First point. The An and Dn numbers you see in map view are relative to the orbit of the ship that you have focused. So if you change focus to 100 different ships, those node angles will have 100 different values, one for each ship.

I find that the easiest way is to understand that Mun is on an equatorial orbit. So you scroll out far enough to find the nodes where the Mun's orbit crosses the satellite orbit -- by looking at the Mun's orbit edge-on, and rotating so that the nodes point directly at you. Then you very carefully tilt the screen up 90 degrees, zoom in on Kerbin, and see where a flag placed at KSC is. Timewarp until KSC is at the top or bottom of Kerbin -- so KSC is right on the node. Then look at that satellite orbit again, to see if you need to launch a bit north or south. You can rotate the ship in the VAB so that (for example) the D key will make your ship tilt in about the right compass direction. Then do a standard gravity turn and go to orbit. Your inclination won't ever be perfect, but you can usually get it within 5 degrees, instead of the 20 degrees that you'd be off if you launched equatorial.

[HebaruSan]

You're not going to be able to use the LAN directly to do anything useful (unless you're writing a mod). It's just the angle between a (hidden) "reference direction" and your AN. You can eyeball the correct launch time in the map view by rotating the camera until the target orbit looks like a flat line instead of an ellipse and launching when your ship's icon is directly under it. Then fly northeast-ish or southeast-ish during launch to match inclinations.

Edited February 12, 2017 by HebaruSan

[BentRim]

10 minutes ago, bewing said:

First point. The An and Dn numbers you see in map view are relative to the orbit of the ship that you have focused. So if you change focus to 100 different ships, those node angles will have 100 different values, one for each ship.

 

I was not aware of that.  It makes sense now that I think about it and that explains why I see them change as I switch focus.

13 minutes ago, bewing said:

I find that the easiest way is to understand that Mun is on an equatorial orbit. So you scroll out far enough to find the nodes where the Mun's orbit crosses the satellite orbit -- by looking at the Mun's orbit edge-on, and rotating so that the nodes point directly at you. Then you very carefully tilt the screen up 90 degrees, zoom in on Kerbin, and see where a flag placed at KSC is. Timewarp until KSC is at the top or bottom of Kerbin -- so KSC is right on the node. Then look at that satellite orbit again, to see if you need to launch a bit north or south. You can rotate the ship in the VAB so that (for example) the D key will make your ship tilt in about the right compass direction. Then do a standard gravity turn and go to orbit. Your inclination won't ever be perfect, but you can usually get it within 5 degrees, instead of the 20 degrees that you'd be off if you launched equatorial.

I think I am envisioning what you are trying to describe.  I will give it a shot.

I was hoping to somehow use the numbers given in the orbit data to get it spot on.  

[BentRim]

1 hour ago, HebaruSan said:

You're not going to be able to use the LAN directly to do anything useful (unless you're writing a mod). It's just the angle between a (hidden) "reference direction" and your AN. You can eyeball the correct launch time in the map view by rotating the camera until the target orbit looks like a flat line instead of an ellipse and launching when your ship's icon is directly under it. Then fly northeast-ish or southeast-ish during launch to match inclinations.

I believe what you and @bewing are describing is essentially the same thing.  I was able to use this technique to get much closer to my intended target orbit.  

I have no background in programming at all and don't see my skillset ever progressing to the level that I would be trying to mod the game myself.  I do enjoy learning however and the fact that this "game" really challenges me to use my brain and learn rather than "shoot, reload, shoot more" has been what's kept me interested.  

My Kerbal journey is certainly a work in progress.  

Thanks to you both for your help.

[bewing]

1 hour ago, BentRim said:

use my brain and learn rather than "shoot, reload, shoot more"

What do you think the kerbals in the rockets are, if not ammo?

[PLAD]

The Longitude of Ascending Node is the longitude at which you cross the equator of the parent body going North. In this case the parent body is Kerbin, and you start at the equator, so if you launch from KSC heading North your LAN will be whatever the absolute longitude of the KSC was at the moment you launched. If we knew the absolute longitude of the KSC at Y1 D1 00:00:00 then we could compute it at any time thereafter and you could always figure out when the next time to launch into your desired LAN would be. I don't see that information in the wiki so I figured it out by launching a rocket with at 90 degree North azimuth as early as I could and reading the LAN of the resulting orbit using Mechjeb.
      I come up with a start longitude for the KSC of 16 degrees and about 10 minutes. I say about because it is too difficult to not swing back and forth a bit during launch, but I can repeat the 16 degrees 10' to about +/-20 minutes when getting into a polar orbit, your results may vary depending on the orbit inclination you are going for. (If you will get into an orbit with a very low inclination then tiny changes in your launch profile can make a big change in the LAN, +/- 7 degrees for a 1-degree-inclined orbit for me.) Also don't read the LAN out right after launch because it jerks around all over the place while your rocket is still almost exactly over the equator. Wait until you're in orbit.
    Note that if you are launching Southward then your LAN will be 196 degrees 10 minutes if you launch at Y1 D1 0:0:0, since your launch point will then be the descending node of the orbit and you have to go halfway around the planet before your orbit crosses the equator going North.
    Kerbin's sidereal rotation period is 21549.425 seconds.  From this we see that the KSC's longitude moves forward at a rate of about 1 degree a minute, and every 21549.425 seconds it will be back at 16d 10m longitude. I leave it to you to make a spreadsheet calculating the KSC's longitude at any given time. Careful if you are using an older version of KSP, Kerbin's sidereal rotational period has changed from earlier versions! The start rotation might have changed too but now you know how to measure it.
Inclination is a matter of launching and pointing in the right direction, remember that the KSC is moving at 175m/s due East when calculating what direction will get you into the right inclination.

[BentRim]

@PLADand everybody else.  You guys have convinced me that I'm not as smart as I think I am....

I have to disect each little bit of information and try to make sense of it.

I truly appreciate all of you taking the time to share your knowledge. It makes me realize that there is not just a simple answer and it also makes me think of how complex KSP really is and what it takes to design a program like this.