Adjusting Longitude of Ascending Node or Argument of Periapsis

[aoeud]

So in threads about getting satellites into orbits, it's often said to just "match them visually" but I can get all the characteristics of the blue circle correct: periapsis, apoapsis, AN, PN and inclination, but I have no idea what longitude of ascending node or argument of periapsis even are.

How can I adjust this part of my orbits?

[Streetwind]

These are more or less just there for the flavor. I personally have no idea how to deal with them either, and I complete satellite contracts just fine - even my first try ever worked flawlessly.

I think these numbers describe the detailed shape of the orbit, by the way. I mean, just try launching a spacecraft into a 90 degree polar orbit. Then go back to the space center, launch a second vessel also into a 90 degree polar orbit, with the exact same apoapsis and periapsis. You will notice that even though you launched both craft exactly the same way, they ended up in completely different orbits. That is because KSC rotated a ways around the planet during the time it took you to bring the first craft into orbit. I am guessing that this would be the longitude of ascending node - in other words, the point where the inclined orbit intersects the equator (ascending/descending node) is at a different point (a different longitude coordinate). Similarly, you can't just tell someone to launch into a 90km periapsis, 110km apoapsis orbit... after all, where exactly should that periapsis be? I am guessing that the argument of periapsis is an angle with respect to some sort of standardized reference point that describes where exactly the periapsis of the orbit sits (and thus at the same time also defines the apoapsis, which is always on the opposite side).

I tried reading up on this stuff on wikipedia months ago and drew a complete blank, because it's full of math and special chararcters and devoid of examples. So I can only try and make up stuff like the above for myself and hope it's correct... at least to me, it makes sense

For ingame purposes, you can fly completely by eye. The destination orbit the game draws for you already has all the information you need - it shows whether it's prograde or retrograde, how it is inclined, where the ascending and descending nodes are, and where the apoapsis and periapsis nodes are. All you need to do is match the orbit, and the contract will complete. Now, as to how to pull that off, you're going to have to draw on your experience on how the six velocity vectors affect your orbit. Prograde pushes the opposite side away from you, retrograde pulls it towards you (limited by the focus point of the orbit, i.e. the object you are orbiting). Radial-in twists the entire orbit to the left around your craft, limited by the focus point; radial-out does the same but to the right, again limited by the focus point. Normal tilts your orbit around an axis between your current position and the focus point, increasing the inclination angle; antinormal does the same, just decreasing the inclination angle.

It bears mentioning that since you are moving, your 'current position' constantly changes, so changing your inclination correctly can be a bit tricky - for any given inclination change that you need, there are only two points in the orbit where a normal/antinormal burn will tilt it exactly the way you want it. At all other points, you are just going to make it weird. Most commonly, these points will be located on the equatorial plane, because most commonly you are trying to change inclination with respect to the equator. Because they are so common and important, these points have special names: the ascending node (going upwards from south to north) and the descending node (going downwards from north to south). Because Kerbin conveniently has the Mun orbiting in a perfectly equatorial orbit, you can always display your current Kerbin-equatorial AN/DN for any spacecraft by setting the Mun as a target. For other bodies, you'll have to eyeball it or install a mod. For the satellite contract missions, the destination orbit helpfully shows AN/DN at all times.

Generally, I do the following:

- Launch a craft into low Kerbin orbit

- Burn prograde so that I raise my apoapsis 95% of the way towards the destination orbit's AN or DN (which one doesn't matter, either will do)

- Burn normal/antinormal at apoapsis to match inclination, then burn prograde there to raise my periapsis just short of the other side of the destination orbit

- Create a maneuver node

- Use the radial-in and radial-out markers while dragging and dropping the node around the orbit to figure out how to twist apoapsis and periapsis into the correct spots

- Execute the maneuver node

- Perform final apoapsis/periapsis adjustments and inclination tweaks as necessary until the orbit matching part of the contract requirements shows green

- Turn off SAS, take my hands off the controls for 10 seconds and collect reward!

Edited January 5, 2015 by Streetwind

[Spheniscine]

I am guessing that the argument of periapsis is an angle with respect to some sort of standardized reference point that describes where exactly the periapsis of the orbit sits (and thus at the same time also defines the apoapsis, which is always on the opposite side).

Actually, the AoP is measured from the ascending node, along the orbital plane. The LAN is measured counterclockwise, along the equatorial plane, from a standardized reference point (on Earth, the position of the sun during the vernal equinox is taken to be that point).

I actually found this picture quite informative: except imagine the "Pe" flag right at the end of the purple line.

The only way you can actually see those values in-game is either by digging through your save file, or using mods like VOID. But you don't need to for satellite contracts and can just match it by eye.

Edited January 5, 2015 by Spheniscine

[DeMatt]

Imagine a top. Yes, the children's toy. Specifically, imagine a top like the one from Inception - with a thin but wide disc, and narrow handle. Imagine a dot of paint on one edge of the disc, so that when you spin the top, the dot goes around in a circle.

This is a very simplistic way of imagining a circular orbit, with the dot of paint playing the role of your satellite.

Hold the top straight up. This is a flat equatorial orbit.

Now tilt it on its side. This is an inclined orbit. Tilt it further, more inclination. Where the inclined orbit crosses the equatorial orbit (or more precisely, the equatorial plane), is where the ascending and descending nodes are. Which one's which, you ask? Well, where's the satellite crossing going "up" and where's it crossing "down"?

Now we get to the "longitude of ascending node". You know where the ascending node is on our top-model, yes? Where it is, along the equatorial orbit, is defined as the "longitude". For the obvious reason that "longitude" measures east-to-west angles and that is exactly what we're measuring here. It's just not referring to Greenwich or the Prime Meridian as the zero longitude.

The "argument of periapsis" is essentially the same as the "longitude of ascending node", except that instead of defining where the ascending node is, we're defining where the periapsis is.

If you've already visually matched the orbit pretty precisely, then you've also matched the "longitude of ascending node" (LAN) and "argument of periapsis" (LPE) "pretty precisely". A wildly incorrect LAN would certainly result in your orbit being improperly inclined relative to the target orbit, and a wildly incorrect LPE would have your periapsis being located far from the requested periapsis.

[Rayder]

To change your LAN, you need to wait until you are at the closest point to the pole in your orbit, so whether it's the most northern or southern point. You then burn normal or anti-normal to move the LAN around the planet. Only works if your orbit is already inclined however, and it isn't very efficient. Much like changing your inclination, but you're moving your orbit on a different axis. You will also need to make several burns if you need to make large changes in your LAN.

To match those orbits, I find it's easiest to line up the ascending/descending node line, with the KSC and launch straight into the inclined orbit. It's more fuel efficient that way.

Think of it this way. Launch into a polar orbit. Then wait for a bit, then launch another craft into a polar orbit. The vessels will be on different orbits, but they are both in polar orbits. The reason for this is that the Longitude (the lines that go North/South, from pole to pole) where the orbit crosses from the northern hemisphere to the southern hemisphere, is different. As Spheniscine mentioned, there is usually a determined point where the LAN is actually measured from and from a KSP standpoint, is completely arbitrary.

[RW-1]

Same here, and I did them early on without maneuver node capability, or SAS, just Mk1 eyeball, launch into appox inclination, and make aP, then circularize, make sure you are going in the direction they want the orbit BTW ...

Fine tune and keep the contract window open, while I have read some complaining about tolerances, I've found them to be fairly wide, on one I had a 0.9deg of offset to the target orbit plane and still got it, while looking at it readying a Anode burn to match it better ....

I also noted that if you have two sat contracts and launch your new probe, if you allow for extra fuel on your last stage or probe itself, you can hit both orbits with just the one and collect both contracts.

I'm doing a few more of these right now for the cash, as I need to pop over 610K to get past 100 science limitation and still have 100k to build the next generation of go fast toys.

Edited January 5, 2015 by RW-1

[Spheniscine]

Fine tune and keep the contract window open, while I have read some complaining about tolerances, I've found them to be fairly wide, on one I had a 0.9deg of offset to the target orbit plane and still got it, while looking at it readying a Anode burn to match it better ....

The tolerance varies based on star rating (you'll also note that the contract stipulation is worded differently; one star is "reasonable deviation", two stars is "marginal deviation", three stars is "minimal"). Personally I like to just get it almost perfect on the first go (matching Ap and Pe within three significant figures) as it is very annoying to adjust an orbit that is "almost" right.

[WuphonsReach]

I also noted that if you have two sat contracts and launch your new probe, if you allow for extra fuel on your last stage or probe itself, you can hit both orbits with just the one and collect both contracts.

Yes, the way to profit greatly is to design something with 2500-3500 DV left over after you achieve orbit around Kerbin. Queue up four orbital contracts (my usual limit), launch the new probe, and do all four contracts with a single 15-20k launch.

[Rayder]

Yes, the way to profit greatly is to design something with 2500-3500 DV left over after you achieve orbit around Kerbin. Queue up four orbital contracts (my usual limit), launch the new probe, and do all four contracts with a single 15-20k launch.

This is how I broke the game. I set a strategy that converted funds to science and I literally earned enough science to unlock the whole tech tree after about 10-12 satellites. The problem is you can't get past 160 science in the tech tree without a building upgrade, which in hard mode is about 1.1 million.

[WuphonsReach]

Funds -> Science is definitely broken. Rep (25%) -> Funds is my preferred strategy.

Kerbal Construction Time (KCT) does give you things to do with all that extra science (like upgrading your SPH/VAB to build rockets faster)...

[AdamJon]

Hi i have a solution to your problem... First thing is that if you have a contract set to get a certain orbit around a planet or moon you have got to get EVERTHING within at least 1500m apoapsis or periapsis. Once you have completed ALL of these perimeters, you should have completed the contract, BUT if this contract is still being uncooperative but you are sure you have got everything complete you need to check if your ascending node and/or descending node are the same. If not, you are orbiting the planet or moon in the wrong direction, to fix this you will either need to completely flip your orbit around the planet/moon by setting a maneuver between the apoapsis and the periapsis, then flipping the inclination of your orbit or you can do a full retrograde burn until your orbit has flipped in rotation. those are two solutions to this thread hope it helped you

[Kirogn 1]

   It has to do with orbital direction.  My mission said i needed a longitude of AN of 256.5 but i had 70.4.  after experimenting i found out that if i aim retrograde and full throttle it'll reverse.  just fire retrograde but don't follow the node.  it will seem like you're going for a landing but as long as you keep firing where retrograde was, it'll turn into prograde and back to orbit. after this you should change your angle (if polar then 90 etc) by using normal and antinormal and lastly adjust apoapsis and periapsis.  If any more help is needed just ask

[Voodoo8648]

Can somebody help me please? I need to launch into a specific orbit around Kerbin but the visual target orbit is not displaying in the map screen. I checked another body where there is a contract available for a specific mission and the visual target orbit is correctly displaying for that contract... it's just not displaying for my current contract for some reason. 

So I need to launch into this orbit but I am not sure when to launch into the plane of the orbit. I need to know what to look. Do I need to look at my Longitude of Ascending Node? I'm running Kerbal Engineer.

The Longtitude of Ascending Node for the target orbit is 136.9 degrees.

thanks for any help

Edited December 30, 2016 by Voodoo8648

[bewing]

3 hours ago, Voodoo8648 said:

Can somebody help me please? I need to launch into a specific orbit around Kerbin but the visual target orbit is not displaying in the map screen. I checked another body where there is a contract available for a specific mission and the visual target orbit is correctly displaying for that contract... it's just not displaying for my current contract for some reason. 

What version of KSP are you running? Are you using mods? It's rather impossible to get a satellite orbit correct unless you have a visual target orbit. You may have to just launch something and then set the contract as being "complete" by hand. It is supposed to always show a target orbit in the current version. In a previous version, there were occasions when perfect equatorial target orbits were not shown, but those were still easy. Have you tried closing KSP and reopening it, to see if the orbit shows up in the tracking station?

 

[Voodoo8648]

2 minutes ago, bewing said:

What version of KSP are you running? Are you using mods? It's rather impossible to get a satellite orbit correct unless you have a visual target orbit. You may have to just launch something and then set the contract as being "complete" by hand. It is supposed to always show a target orbit in the current version. In a previous version, there were occasions when perfect equatorial target orbits were not shown, but those were still easy. Have you tried closing KSP and reopening it, to see if the orbit shows up in the tracking station?

 

I created a separate question regarding this issue. Please see my response here: