Getting into Lunar Equatorial Orbit

[Ker Ball One]

I am trying to complete RP-1 program contracts for "targeted lunar landings". I see several historic Apollo landing sites fairly close to the Moons equator. So I want to establish a lunar orbit with near 0 degree inclination.
I can use MechJeb and the Lunar Transfer Planner mods to minimize delta-v during TLI... but I don't know how to adjust my maneuver node to minimize my destination orbit's inclination. I don't even know if it is possible. (also, MJ doesn't show destination orbit inclination, only Pe, so I am eyeballing it).

I know I can wait until reaching the Moon's SOI, and then get the inclination and LAN info. At that point, I can somewhat adjust inclination with a burn just inside the SOI. But I cannot get close to 0 degrees unless I burn at the closest Ascending Node or Descending Node, at which point, I am close enough that it will cost a lot of fuel.

I would think that the Moon's equatorial/rotational plane would intersect the Earth twice per month, and that would be my window for a Hohmann transfer, but can't seem to see that happening. To clarify before any confusion, I am not asking about the Moon's 'orbital plane', which intersects my launch site's plane twice a day. That'll get me a lunar encounter just fine, but puts me into a higher inclination orbit about the moon (usually 28 degrees or so).

I could be thinking about this all wrong, so please point me in the right direction.

My questions:

Is there a way to minimize the delta-v needed for that late plane change to 0 inclination, with better timing or adjustments during the TLI burn back at LEO? Or perhaps a mid-course correction burn (half way) still days away from Moon SOI?

Is it a mistake to even try for a lunar equatorial orbit for this mission? Can this only be done with an expensive burn close to the Moon? Is a MechJeb Hohmann transfer simply the wrong starting point, and I need to instead make several manual burns? Or is there a calculator that exists to do this?

How did the real Apollo missions seem to arrive at the Moon? I've read "Apollo by the Numbers", which gives lots of details, but still can't find out the inclination in their lunar orbits (just Ap/Pe). All I can tell is, most Apollo CSM crafts did a single plane change while in a circular orbit after they decouple the LM, so as to make rendezvous easier. That may imply were not equatorial. But I still don't understand what inclination they arrived into orbit with.

[farmerben]

Inclination changes cost much less fuel if you are at apogee and have a highly elliptical orbit.  It might be possible to set your perigee as low as possible for the intercept.  Take the most elliptical orbit you can.  And do your inclination changes where your speed is minimum.

[Ker Ball One]

10 minutes ago, farmerben said:

Inclination changes cost much less fuel if you are at apogee and have a highly elliptical orbit.  It might be possible to set your perigee as low as possible for the intercept.  Take the most elliptical orbit you can.  And do your inclination changes where your speed is minimum.

That may save some fuel, but at the expense of a lot of time.  Which I don't have in a RP-1 crewed mission.

From what I understand, that method succeeds best when apolune is near the ascending or descending node.  But you still have to be in an orbit already. 

I'm looking for a point in the journey that is between the Earth and the Moon, but not yet within the SOI of the moon.  There should be a point that is on the equatorial plane of the Moon, and thus equivalent to the ascending or descending node.  But since patched conics don't display that kind of orbital information until you are inside the SOI, I'm finding it difficult.

Edited July 18 by Ker Ball One

[darthgently]

51 minutes ago, farmerben said:

Inclination changes cost much less fuel if you are at apogee and have a highly elliptical orbit.  It might be possible to set your perigee as low as possible for the intercept.  Take the most elliptical orbit you can.  And do your inclination changes where your speed is minimum.

This is the way

Especially if the final orbit is to be polar

[Ker Ball One]

41 minutes ago, darthgently said:

This is the way

Especially if the final orbit is to be polar

It's not the way Apollo was done.  As I am playing Realism, time is a factor and cannot afford long elliptical orbital maneuvers.

The final orbit is to be equatorial.

[Ker Ball One]

Apollo 12 did a 107m/s plane change after getting into a ~110km near circular lunar orbit and decoupling the LM, then a 116m/s plane change just before TEI.  Apollo 14 and on, were single plane change burns of similar delta-v.  I cannot see what the actual inclinations were, but that's what I got, and that's what should be possible in RO.  The other burns like TLI, circularizing, landing, ascent, TEI, etc... all match real world.  It seems like they all arrived into the lunar orbit at a the correct inclinations.  Especially Apollo 11 which did not seem to perform any plane changes at all.

Edited July 18 by Ker Ball One

[Ker Ball One]

The only thing I can think of... is the Mid-course Correction that took place about 1/3 on the way to the moon after TLI.  They didn't burn the SPS for this correction right after the S-IVB TLI burn and seperation.  Why?  Is it because they weren't yet intersecting the plane of the desired insertion inclination at the moon?  I would think that plane intersection would occur twice per month.  I just have no clue how to calculate that.

[magnemoe]

Did you launch from KSC or some higher attitude ones? For KSC I would wait until inside Mun SOI as its couple of degree. 
You use mechjeb so assume KSP 1 and you can preview burn costs inside other SOI. 
For larger changes the mid course correction is probably cheaper if you don't want to wait with an high Ap after particular circulation, you can also do plane change inside Mun SOI coming in. It all depend on cost and your budget. 

[Ker Ball One]

1 hour ago, magnemoe said:

Did you launch from KSC or some higher attitude ones? For KSC I would wait until inside Mun SOI as its couple of degree. 
You use mechjeb so assume KSP 1 and you can preview burn costs inside other SOI. 
For larger changes the mid course correction is probably cheaper if you don't want to wait with an high Ap after particular circulation, you can also do plane change inside Mun SOI coming in. It all depend on cost and your budget. 

Yes, from Canaveral.  I am not seeing a couple of degrees difference when I arrive at the moon (not Mun, if it were Mun there'd be no tilt, no problem).  I see about 28 degrees.

I know about a plane change inside the SOI.  A node just inside the SOI can't get me to equatorial, probably because I am not actually on the equator yet.  Only if I set the node on either AN or DN, which are on the equator.  But the cost is very high.

I think the fact that I'm not seeing a few degrees difference, means I am not launching at the right time of the month. 

Edited July 18 by Ker Ball One

[darthgently]

1 hour ago, Ker Ball One said:

It's not the way Apollo was done.  As I am playing Realism, time is a factor and cannot afford long elliptical orbital maneuvers.

The final orbit is to be equatorial.

Ah, severe time budget.  Yes, that nixes the high AP angle

My bad, I didn't read thread

[magnemoe]

7 hours ago, Ker Ball One said:

Yes, from Canaveral.  I am not seeing a couple of degrees difference when I arrive at the moon.  I see about 28 degrees.

I know about a plane change inside the SOI.  A node just inside the SOI can't get me to equatorial, probably because I am not actually on the equator yet.  Only if I set the node on either AN or DN, which are on the equator.  But the cost is very high.

I think the fact that I'm not seeing a few degrees difference, means I am not launching at the right time of the month. 

So not at equator, Then I try an plane change 2/3 -3/4 the distance to Mun, You can add an circulation node inside Mun SOI, 
Apollo also used patched conic for planning so KSP physic  

[mikegarrison]

Looking at the book Apollo By The Numbers (can be found online), I don't see any evidence that there was a specific plane change burn for Apollo 11. For Apollo 12 there was a 350 m/s lunar plane change burn, but interestingly it happened when the LM was on the surface. So it seems to have been for the purposes of setting up the ascent rendezvous rather than reaching a desired landing site. I didn't check any of the other missions.

[Ker Ball One]

2 hours ago, mikegarrison said:

Looking at the book Apollo By The Numbers (can be found online), I don't see any evidence that there was a specific plane change burn for Apollo 11. For Apollo 12 there was a 350 m/s lunar plane change burn, but interestingly it happened when the LM was on the surface. So it seems to have been for the purposes of setting up the ascent rendezvous rather than reaching a desired landing site. I didn't check any of the other missions.

Yes, I aid previously, "Especially Apollo 11 which did not seem to perform any plane changes at all. " which is what I'm trying replicate.  They went to a landing site near the lunar equator.

I also mention Apollo 12's plane changes in my previous comment.  But converted their 350 ft/s into metric, about 107 m/s.

These numbers make me think that there was no plane change of significance during Apollo while in orbit of the Moon.... But since that maneuver is typical with stock Kerbal players, most of the advice I get is to just do it the Kerbal way instead of the Apollo way.

2 hours ago, magnemoe said:

So not at equator, Then I try an plane change 2/3 -3/4 the distance to Mun, You can add an circulation node inside Mun SOI, 
Apollo also used patched conic for planning so KSP physic  

I'm trying to model this in a way that is repeatable.  I'm trying a mid-course correction, as a second maneuver node that I will create before burning TLI.  I need to play with the timing of the TLI node, because that mid-course node cannot seem to get into an equatorial lunar orbit unless the TLI node is the correct time of the month.

It's just really hard to eyeball this, and I feel there should be a repeatable calculation that I can do to tell me not only the transfer window per day to get an encounter, but also a transfer window per month to get the proper destination inclination.

Edited July 19 by Ker Ball One

[Ker Ball One]

4 hours ago, magnemoe said:

So not at equator, Then I try an plane change 2/3 -3/4 the distance to Mun, You can add an circulation node inside Mun SOI, 
Apollo also used patched conic for planning so KSP physic  

Also, you keep saying Mun.

I'm talking about the Moon.  This is Real Solar System.  The distance are greater and orbits and rotation are all inclined.  Plus with RP-1, I don't have the extra fuel or time to do the stock KSP moar boosters, big plane changes that are common advice.

[mikegarrison]

You don't have to be in an equatorial orbit to land on the equator. Every orbit passes over the equator. I'm not sure why you seem to assume that Apollo was in equatorial orbit over the Moon, or why you think you have to be in your simulation.

Obviously it complicates the timing of the rendezvous after ascent if the orbital path of the CSM doesn't pass over the landing site every orbit, but Apollo had a lot of people doing a lot of math and a lot of planning, so they worked it out.

[Ker Ball One]

1 hour ago, mikegarrison said:

I'm not sure why you seem to assume that Apollo was in equatorial orbit over the Moon,

https://www.nasa.gov/wp-content/uploads/static/history/alsj/a11/A11_MissionOpReport.pdf
"Command/Service Module Plane Change - The CSM will perform a plane change of 0.18° approximately 2.25 revolutions after LM touchdown . This maneuver will permit a nominally coplanar rendezvous by the LM." -p43
"On future Apollo lunar missions, landing sites at higher latitudes will be achieved, with little or no plane change, by approaching the moon on a highly inclined trajectory." -p18

So the assumption is correct, Apollo 11's inclination was near equatorial.

1 hour ago, mikegarrison said:

it complicates the timing of the rendezvous after ascent if the orbital path of the CSM doesn't pass over the landing site every orbit,

This is the main reason.  Apollo 11's landing site, the Sea of Tranquility was on the equator, and the mission/contract requires a 2 - 3 day stay on the surface.  The lower the latitude, the more the site rotates away from the orbiter's (inclined or polar) plane.  This was dangerous because such a small window meant that if they missed it... they would be down there for another 12 days or so.  Not enough supplies like O2, food, water, scrubbers, etc.  For RP-1 with Kerbalism, the same concerns apply.  So equatorial landing sites prefer near equatorial orbits.  Later missions have sites at higher latitude, but those are not what I am doing now. 

The current challenge is to do it the Apollo way.  I just want to know how they did it.  And it seems like their mid-course correction about 1/3 from Earth to Moon, was the place they lined up to the lunar equatorial plane.  Last night I simulated a TLI that took place when the Moon was near its own AN/DN (map view, target Moon, yellow dashed lines), and the mid course (2nd maneuver node) seemed to fair much better with less than 10m/s of delta-v in the normal vector.  It seems with that better timing, I may be able to enter the Moon's SOI pretty close to equatorial.

Edited July 19 by Ker Ball One

[K^2]

On 7/18/2024 at 7:22 AM, Ker Ball One said:

Is there a way to minimize the delta-v needed for that late plane change to 0 inclination, with better timing or adjustments during the TLI burn back at LEO? Or perhaps a mid-course correction burn (half way) still days away from Moon SOI?

I'm going to assume Real Solar System but with stock KSP physics. The simplest dV-efficient plan is you burn to parking orbit that matches inclination of Lunar orbit. From there, you burn to LTO making any small corrections for inclination at that point. You aim to have the apogee of LTO to just clip the Lunar SoI at a point that crosses Lunar equatorial plane. As soon as you are in the SoI, burn to bring your periapsis down, at the same time aligning the orbit with equatorial plane. At desired periapsis, either establish parking orbit or burn for direct landing.

This can be optimized further. For starters, if the Moon axis is pointing towards or away from Earth at the time of capture, you don't need additional corrections - you'll be in equatorial orbit already. But you're still doing a 2-stage capture, which is not the most efficient. The most efficient capture is to bring your LTO apogee higher, so that your nearest approach to the Moon is at your desired periapsis. The problem is that this makes the rest of the math a lot harder. And it's only going to save you, maybe, a few percent on the dV. My sincere advice would be not to attempt this and to go for the two-step capture, with just barely scratching Lunar SoI with your LTO. That gives you very precise points where you can do the adjustments.

On that note, since inclination is all about angular momentum, you want to make changes as early as possible. If you realize that your ejection burn to LTO wasn't quite correct, fix it early. If you ended up hitting Lunar SoI at the wrong spot, circularize just inside SoI, then fix your inclination at the next ascending/descending node. You don't want to end up in low Lunar orbit and then start burning fuel to fix this. So long as you start fixing the problem while far enough from the Moon, you aren't going to lose that much fuel. Good planning is just shaving off these last few percent of the total requirements.

[Ker Ball One]

Thank you for a good explanation of something to try.  I was able to follow and can understand the concept.

2 hours ago, K^2 said:

You aim to have the apogee of LTO to just clip the Lunar SoI at a point that crosses Lunar equatorial plane.

This was a bit tricky, but I managed to get some visual aids with a couple of Waypoints created on the equator, to line it up with the start of the very short orbital line created by just clipping into the Lunar SOI.

2 hours ago, K^2 said:

As soon as you are in the SoI, burn to bring your periapsis down, at the same time aligning the orbit with equatorial plane.

730 m/s for this... and gets me about 1 degree inclined.
16 m/s to set inclination to zero at AN/DN.

1 hour ago, K^2 said:

At desired periapsis, either establish parking orbit

640 m/s for circularization (which is needed to do the contract).

This really made sense to try.  The only big show stopper is the total 7.5 day one way flight time is a bit too much.  It also makes sense, since we are basically pulling over at the SOI to make the changes at the minimum velocity.  I was hoping that paying the 730 to bring the perilune down would be offset by a much cheaper circularization burn, but it I didn't save that much.  Also the initial LTO cost of ~3200 m/s didn't change much by taking off some prograde to just clip into the Lunar SOI.

I am also testing lunar cycle timings.  I think I managed to get the plane change at AN/DN to less than 300 m/s on one test.  Combined with the direct circularization of ~800 m/s, this is still my best approach.  I last tried timing my Lunar SOI arrival when the lunar equatorial plane is parallel with the moon's orbital plane.
Do you have any thoughts on when in the lunar cycle I should launch from Kennedy and/or burn my TLI? 

[mikegarrison]

Apollo deliberately did not make a Hohmann transfer to the Moon. They burned until their apogee was well beyond the Moon's orbit, which (among other things) gave them a lot higher transfer speed than if they had made a minimum delta-V transfer.

[Ker Ball One]

I did it!

With careful timing of the launch when the moon was in the correct phase (precise day of the month), I was able to enter the Lunar SOI on the equatorial plane.

The visualization was done by cheating a satellite into a near SOI altitude (at 60Mm), equatorial orbit. This gave me a nice white line orbit that I can look at on its edge.

The real missing piece that nobody was touching on, was the lunar phase. I was right in thinking that there would be a monthly window when this was possible.

With that reference orbit, I could go set a regular direct Translunar Injection manuever node and see how far that path was from the equatorial orbit at SOI. And using time warp, could determine precisely when that TLI burn would be needed. I used MJ to quickly remove and add a hohmann transfer node between time warp intervals to hone in on the day and hour I needed.

1. 3150 initial TLI

2. 10 m/s correction in 4hrs to SOI to eq plane and pe of 110km (10 degree inc at start of SOI)

3. 13.5 m/s plane change to 0 degrees inc at nearest AN/DN (2h into SOI)

4. 788 m/s circularization burn at 112km

   
   There is no practical way to get the point of entry into the Lunar SOI, onto the plane of the equator... other than waiting.
   I have tried reducing Prograde so that the apogee just barely reaches the Lunar SOI, and although that does get that point on the equatorial plane, the perilune is so far away that I need a huge additional burn (750m/s) to bring it back down.

   So by waiting a few days for that white reference orbit plane and the purple maneuver path to align, I wind up spending less than 25 for all course corrections and plane changes to get my final goal, lunar equatorial orbit.
   

   This question is solved. However, it's still eyeballing the measurement with a cheated reference orbit, time-warping to get a date time, and then reverting. There honestly should be a chart or calculator, or some way to do this the NASA way.   At this point I'd settle for a mod that could show equatorial planes.  EDIT:  Found one https://github.com/StrikeForceZero/KSP-OrbitPOInts

Edited July 22 by Ker Ball One

[darthgently]

1 hour ago, Ker Ball One said:

I did it!

With careful timing of the launch when the moon was in the correct phase (precise day of the month), I was able to enter the Lunar SOI on the equatorial plane.

The visualization was done by cheating a satellite into a near SOI altitude (at 60Mm), equatorial orbit. This gave me a nice white line orbit that I can look at on its edge.

The real missing piece that nobody was touching on, was the lunar phase. I was right in thinking that there would be a monthly window when this was possible.

With that reference orbit, I could go set a regular direct Translunar Injection manuever node and see how far that path was from the equatorial orbit at SOI. And using time warp, could determine precisely when that TLI burn would be needed. I used MJ to quickly remove and add a hohmann transfer node between time warp intervals to hone in on the day and hour I needed.

1. 3150 initial TLI

2. 10 m/s correction in 4hrs to SOI to eq plane and pe of 110km (10 degree inc at start of SOI)

3. 13.5 m/s plane change to 0 degrees inc at nearest AN/DN (2h into SOI)

4. 788 m/s circularization burn at 112km

   
   There is no practical way to get the point of entry into the Lunar SOI, onto the plane of the equator... other than waiting.
   I have tried reducing Prograde so that the apogee just barely reaches the Lunar SOI, and although that does get that point on the equatorial plane, the perilune is so far away that I need a huge additional burn (750m/s) to bring it back down.

   So by waiting a few days for that white reference orbit plane and the purple maneuver path to align, I wind up spending less than 25 for all course corrections and plane changes to get my final goal, lunar equatorial orbit.
   

   This question is solved. However, it's still eyeballing the measurement with a cheated reference orbit, time-warping to get a date time, and then reverting. There honestly should be a chart or calculator, or some way to do this the NASA way.   At this point I'd settle for a mod that could show equatorial planes.

You might want to look at kOS, the Kerbal Operating System mod.  If doesn't specifically do what you did, but is rather a scripting environment that makes that sort of thing easier. 

I've used kOS to take the cross product of Kerbin's rotational angular velocity vector and Minmus orbital plane normal to find where the planes intersect.  Then launch into Minmus' orbital plane as KSC reaches the upcoming intersection point.  So now the craft is in a low orbit coplanar with Minmus' orbit.  Then one would transfer at one of the intersection nodes again to get that equatorial periapsis.  Mid course corrections and tunings may be needed. 

kOS' vecdraw command allows drawing colored lines in the map view so makes visualizing one's calcs fairly easy.

kOS isn't totally perfect experience, but still very much  worth it if you like the kinds of problems you just solved, imo

[Ker Ball One]

54 minutes ago, darthgently said:

You might want to look at kOS, the Kerbal Operating System mod.  If doesn't specifically do what you did, but is rather a scripting environment that makes that sort of thing easier. 

I've used kOS to take the cross product of Kerbin's rotational angular velocity vector and Minmus orbital plane normal to find where the planes intersect.  Then launch into Minmus' orbital plane as KSC reaches the upcoming intersection point.  So now the craft is in a low orbit coplanar with Minmus' orbit.  Then one would transfer at one of the intersection nodes again to get that equatorial periapsis.  Mid course corrections and tunings may be needed. 

kOS' vecdraw command allows drawing colored lines in the map view so makes visualizing one's calcs fairly easy.

kOS isn't totally perfect experience, but still very much  worth it if you like the kinds of problems you just solved, imo

Wow, thanks for the tip. 

I have a feeling that using KOS will be a deep dive.  I do some light programming in Python and JavaScript already and I'm learning c# for modding.  I have a feeling that once I get into KOS, it's going to be a major part of playing KSP. 

Do you know if a repository of KOS scripts that maybe someone already has something similar?

Edited July 20 by Ker Ball One

[K^2]

4 hours ago, mikegarrison said:

Apollo deliberately did not make a Hohmann transfer to the Moon. They burned until their apogee was well beyond the Moon's orbit, which (among other things) gave them a lot higher transfer speed than if they had made a minimum delta-V transfer.

That was part of a plan to make it a "free return" mission, in case they had to abort the landing, wasn't it? Allowing them to circumnavigate the moon and get a safe return trajectory with maneuvering thrusters alone if there was engine trouble.

[Ker Ball One]

1 hour ago, K^2 said:

That was part of a plan to make it a "free return" mission, in case they had to abort the landing, wasn't it? Allowing them to circumnavigate the moon and get a safe return trajectory with maneuvering thrusters alone if there was engine trouble.

Learning a lot with this challenge, about the free return and the hybrid trajectories used.  With Apollo 12 and on, they would start with free return, then after testing the SPS to ensure it would fire, their mid course correction would change it to the hybrid to allow for more landing sites at higher latitudes.  And they still could return with the much weaker LM descent engine.

https://www.nasa.gov/history/afj/launchwindow/lw1.html

Edited July 20 by Ker Ball One

[darthgently]

8 hours ago, Ker Ball One said:

Learning a lot with this challenge, about the free return and the hybrid trajectories used.  With Apollo 12 and on, they would start with free return, then after testing the SPS to ensure it would fire, their mid course correction would change it to the hybrid to allow for more landing sites at higher latitudes.  And they still could return with the much weaker LM descent engine.

https://www.nasa.gov/history/afj/launchwindow/lw1.html

kOS can be quirky as all programming languages that start with the premise of being "a programming language for non-programmers".   I've never seen this approach succeed really because, well, it's still programming at the end of the day.   Like building a car for non-drivers.  But not a robotaxi.  You still end up needing to learn how to drive, but eventually the "non-programmers" aspect makes deeper programming harder.

Still, I've been playing with the visual play-school Lego block scripting in Juno New Origins which embraces the "non-programmer" concept even more ludicrously deeply and will never seriously complain about kOS ever again.  Ever.

You might also look into the kRPC mod.  It has a bit more lag, or hysteresis in craft control than kOS but you can use Python and other languages

Edited July 20 by darthgently