Best way to MATCH a munar polar orbit

[tg626]

So I'm building a station in Munar orbit.

The wrinkle is that it's in a polar orbit (inc 90) and I am trying to puzzle out how to time my departure from LKO so that when I arrive I am close to being aligned with the orbital plane of my station.

Getting to the Mun - easy.

Getting into a Munar orbit with an inclination of 90 degrees - easy.

Not having to spend a bazillion m/s Dv to match planes with my station - that's the real trick.

I'll continue to experiment and puzzle over it, but in the mean time I know some of you are orbit wizards and might have a few tips...

TIA

BEST ANSWER HERE

Edited September 20, 2014 by tg626
Got a great answer!

[Claw]

Well, I'm sure you'll get replies like "Don't put your station in polar orbit." (My response is do it if you want. )

If you don't want to spend a lot of dV, what you've really done is limited yourself to two departure windows per Kerbin month. Since it doesn't take that long to get to Mun, you basically have to wait till the polar orbit is near tangent to the Mun's orbit (so that it's nearly a circle when you look at it from Kerbin). Personally I would still launch in an equatorial Kerbin orbit, then do a midcourse correction to a polar Munar orbit half way there. That should keep the overall dV low while still providing flexibility and the ability to recover from a mistake. Realize that at the two times during the Kerbin month when you can transfer, the polar orbit will be going opposite directions. So you'll have to pay attention to if your station is north-bound or south-bound on your approach trajectory when you set up the Munar encounter for the inbound ship.

Hopefully that makes sense without me making a picture.

Cheers,

~Claw

Edit: I should also recommend that you might want to set your station orbit so that the transfer windows happen during the full and new moon times (when the Mun is in-line with Kerbin and the Sun). This will also ensure your station has maximum sun exposure while you're trying to rendezvous and dock with it.

So in the image below, you want your station orbit matched up with the Munar terminus during the New Mun and Full Mun. Then it will receive sun during it's entire orbit while you rendezvous.

Edited September 18, 2014 by Claw

[Scarecrow88]

That doesn't sound right to me. With the station orbit matching the terminator at the full and new position, a ship approaching from Kerbin will be coming in at a tangent to the orbit, when he needs to come in parallel to it. If it was me, I would be aiming to arrive at the Mun when it was at the first or third quarter, again assuming the orbit of the target is in alignment with the terminator.

[Geschosskopf]

I put stations in Mun-polar orbits sometimes, and I favor a brute-force-and-ignorance method to rendezvous with them. This method is a lot easier than fretting over the math and timing, and the loss of efficiency isn't enough to worry about because Mun's right next door anyway. So what I do is this:

1. Launch the 2nd ship from Kerbin whenever, fly it to Mun whenever.

2. Do a mid-course burn to make the ship fly under Mun's south pole or over it's north pole, which will kick it into a high inclination when it gets there. Which pole to buzz is something I decide on the fly, whichever one will put me going the same way around Mun as the station.

3. Just barely capture at Mun so my Ap is just barely inside Mun's SOI.

4. Match planes with the station.

5. Adjust Ap and Pe as needed, burn to rendezvous with station.

Quick, simple, not requiring any forethought or planning or math, and not too much more expensive on fuel than doing it the complicated way.

[Wanderfound]

3. Just barely capture at Mun so my Ap is just barely inside Mun's SOI.

This is key.

Match inclination, then match altitude. Higher = cheaper for inclination shifts.

I've been playing around with Fine Print satellite delivery contracts, using a not-super-long-ranged spaceplane to do 'em. You can get three or four Munar satellites into sharply differing orbits on a single trip if you do things in the right order.

[tg626]

All interesting replies, thank you.

FWIW the station is indeed riding the terminator so it's always in daylight.

[Geschosskopf]

FWIW the station is indeed riding the terminator so it's always in daylight.

I don't think it will always be in daylight. Right now, the station's orbital plane might be perpendicular to the direction the sunlight comes from, but 1/4 of a year from now, Kerbin's orbit around Kerboal will put that orbital plane parallel with the sunlight.

[Aedile]

Claw, what do you mean by month? Why a month?

I thought Mun period was something like earth day and a half.

My strategy with new games is usually, send a lab+lander on polar orbit. Then while the lander is doing science (which can be quite a few landings), I refuel the base (later transfer to minmus).

In any case, never needed to wait more than 2 days. (same as return to Kirbin from polar orbit)

I usually launch eq kirbin.

Once I reach Mun's SOI, I make a course correction to fix inclination and bring pe down. (usually involves radial+normal). I try to bring Pe closer to the pole.

Slow down at Pe, to orbit. Not circular orbit, leave Ap high. If it's too near equator, I make my burn a bit earlier.

If Pe is around the pole, Ap, and one of the AN/DN (to base) will be around Ap. Fix relative inclination to base there. You shouldn't need more than 120-180dV for 90% change.

Rendezvous as usually.

So to answer the question when to leave - I don't know the exact answer, as it depends on where you arrive (e.g near or far side of the Mun). I'm sure there is way to know, but I never bothered, as dV for inclination change is not that big.

[Claw]

Geschosskopf's suggestion also seems reasonable to me. I don't know if it would be minimum dV, but it shouldn't be too expensive either. Maybe the key point for you (if you didn't already know) is to adjust the inclination when you are at AP, and to keep that AP as high as is practical when capturing around the Mun.

That doesn't sound right to me. With the station orbit matching the terminator at the full and new position, a ship approaching from Kerbin will be coming in at a tangent to the orbit, when he needs to come in parallel to it. If it was me, I would be aiming to arrive at the Mun when it was at the first or third quarter, again assuming the orbit of the target is in alignment with the terminator.

You don't fire straight toward the Mun like a bullet. Orbits swing around the sides. So while most of your time appears to be spent pointing straight at the Mun, you need to do the midcourse correction so that it swings over the poles while approaching Mun from behind or ahead (whichever you prefer).

By the way, tangent is what you want because tangent applies to curves. You want to come in tangent to the orbit where you're intersecting it. Parallel is for lines. If you prefer, one could imagine coming in parallel if viewed from above, but it should be noted that the approaching craft's trajectory isn't going to be a straight line when it's reaching the Mun's SOI.

Really it comes down to controlling exactly where you hit the SOI transition if you want to minimize dV consumption. You can open up the transfer window more as you get better at adjusting the midcourse correction to put you on the right trajectory to match up with the station. However, I think worst case is if the station orbit is "pointed at" Kerbin when you arrive. You'll have more maneuvers to accomplish, and need more dV to spend.

Claw, what do you mean by month? Why a month?

I thought Mun period was something like earth day and a half.

I mean a Kerbin month, not an Earth month. 1 Month = 1 orbit by Mun around Kerbin. I suppose it's not a standard ingame measurement, so sorry for the confusion.

Cheers,

~Claw

Edited September 19, 2014 by Claw

[Wanderfound]

I mean a Kerbin month, not an Earth month. 1 Month = 1 orbit by Mun around Kerbin. I suppose it's not a standard ingame measurement, so sorry for the confusion.

A munth?

[Geschosskopf]

Geschosskopf's suggestion also seems reasonable to me. I don't know if it would be minimum dV, but it shouldn't be too expensive either. Maybe the key point for you (if you didn't already know) is to adjust the inclination when you are at AP, and to keep that AP as high as is practical when capturing around the Mun.

The place to burn to match planes with the station's orbit is your ascending or descending node with its orbit. This might not necessarily be at your Ap but hopefully it'll be in the general vicinity of it .

[Claw]

The place to burn to match planes with the station's orbit is your ascending or descending node with its orbit. This might not necessarily be at your Ap but hopefully it'll be in the general vicinity of it .

Oh yeah. I guess I was thinking of shifting into any polar orbit, instead of matching the station. But then I suppose you're not minimizing dV usage.

A munth?

Yes, a munth. I like that.

Cheers,

~Claw

Edited September 19, 2014 by Claw

[Claw]

Here you go, example of what I'm talking about. Using the transfer windows I talked about don't cost much (if any) additional dV as compared to any other Munar polar orbit insertion. You can see the transfer timing in the first two pictures. I'm sure you could probably shave off a few dV from this method, but to me it seems to be the lowest dV while being straight forward and maintaining flexibility.

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[tg626]

Although my preliminary attempts eventually tried tilting my inclination at Kerbin to get above or below the Mun, the preceding answer is really the best I think!

Thanks!