How to transfer from Mun to Minmus ?

[Tatonf]

Hello !

I'm trying to do a transfer from Mun to Minmus.

I'm using Alexmoon's Launch Window Planner to find a window. Here's the window it gives me after clicking "Refine Transfer" :

Problem is, it doesn't work.

It doesn't even show the blue makers that appears when you cross your target's orbit. What should I do ?

 

 

[Superfluous J]

I don't know about the planner (I don't use it for moon-to-moon transfers, as the scale is so small it's easier to just go) but based solely on that 3rd pic, you're either going way too fast or ejecting at way the wrong time. Based on your Kerbin Pe (which looks good for an efficient transfer) I'd say the problem is your exit velocity. Bring it down until your Ap gets closer to Minmus' orbit.

Getting intersect markers is not a science, it's an art. A dark art that usually involves sacrificing chickens. Smaller worlds like Minmus and Gilly (And Pol and Bop) can be really finicky about it. I can't give you much advice there except "Keep fiddling with the maneuver node" :/

[Leeman]

Also check the angle you come in. Hard to tell from the pictures however the Mun is on an orbit with 0 inclination while the orbit of Minmus is a bit off, so you might need to burn normal / antinormal on the asc/desc node whatever the degree is

[Tatonf]

Yes Minmus is quite inclined (by 6° if I recall correctly), this is why I'm struggling with finding a good transfer window.

I'm also trying to be very efficient on this transfer (I'm quite short in delta-V), which is why I'm using Alexmoon's planner.

[Superfluous J]

Just eyeballing the picture, it looks like you're totally fine except you're escaping too fast (my first guess). If you just tone down the prograde on the maneuver node a little bit I bet you'll nail it. Super easy to do with PreciseNode installed. Just dial it down to 0.1 or so and click it down.

Regarding dV to the transfer, if time is of no importance you can get to Minmus with just about the minimum dV possible by just burning to escape Mun at the point where your resultant orbit will be crossing Minmus' orbit right at the An/Dn markers (which you are just about doing) and also nail your orbit's Ap to touch Minmus' orbit right at that point (which slowing down here should do). When those are touching, the game should most assuredly give you close approach markers. One will be right there showing your ship's position at that time, and the other will be somewhere else on Minmus' orbit (in this case I bet very nearby, but in general - with no pre-planning or timing - it could be anywhere).

The trick is, after you get this near encounter, to place a maneuver node just AFTER your ship's location marker (up just past Ap) and set it to burn PROGRADE a little, which will cause Minmus' location marker to move around its orbit until you get an encounter. You need to burn that fuel anyway to stop at Minmus, and Oberth gains at Minmus aren't THAT great, that doing this small burn really doesn't waste enough fuel to worry about. And, as a bonus, it always works.

I should do a video on this some day.

[Superfluous J]

And one final thing (until I think of another final thing), I was once where you are now, and eventually decided that the transfer planners are great for telling you WHEN to transfer, but when it comes to telling you HOW you're better off just fiddling with maneuver nodes until you nail it. That's the engineer in me beating the mathematician. I don't care what the equations say SHOULD work. I care what actually DOES work.

Edited April 15, 2016 by 5thHorseman

[Alshain]

Just for future information and planning, if you can then it is easier to go from Minmus to the Mun.  The Mun moves considerably faster so it is much more likely to hit it at some point if you get your inclination right, even if you have to go around Kerbin to do it.  This can help if you are planning a mission to go to both, go to Minmus first.  There will be many times that you wish the Mun would just get out of your way.

The launch Window planner gets you in the ballpark but it can't know exactly where your craft will be in it's orbit around the Mun, so it is nearly always not going to give you the single perfect solution for any transfer.  Once you are in the ballpark you can usually play with it until you find a capture.  I would try moving the maneuver along your orbit, rather than altering the burn itself.  Sometimes burning a little sooner or later will find it.

Edited April 15, 2016 by Alshain

[Tatonf]

Hi ! Sorry for not posting sooner.

I appreciated your help, but as I said I'm trying to be very efficient on delta-V.

On 15/04/2016 at 3:02 AM, 5thHorseman said:

Just eyeballing the picture, it looks like you're totally fine except you're escaping too fast (my first guess). If you just tone down the prograde on the maneuver node a little bit I bet you'll nail it. Super easy to do with PreciseNode installed. Just dial it down to 0.1 or so and click it down.

Regarding dV to the transfer, if time is of no importance you can get to Minmus with just about the minimum dV possible by just burning to escape Mun at the point where your resultant orbit will be crossing Minmus' orbit right at the An/Dn markers (which you are just about doing) and also nail your orbit's Ap to touch Minmus' orbit right at that point (which slowing down here should do). When those are touching, the game should most assuredly give you close approach markers. One will be right there showing your ship's position at that time, and the other will be somewhere else on Minmus' orbit (in this case I bet very nearby, but in general - with no pre-planning or timing - it could be anywhere).

The trick is, after you get this near encounter, to place a maneuver node just AFTER your ship's location marker (up just past Ap) and set it to burn PROGRADE a little, which will cause Minmus' location marker to move around its orbit until you get an encounter. You need to burn that fuel anyway to stop at Minmus, and Oberth gains at Minmus aren't THAT great, that doing this small burn really doesn't waste enough fuel to worry about. And, as a bonus, it always works.

I should do a video on this some day.

Your technique unfortunately didn't work for me, as the orbit of my ship is going beyond Minmus's orbit before I can get an encounter.

I've found a much better (= more efficient, not easier) method : I directly launch my ship into an inclined orbit around Mun (about 60°), and when I perform the transfer my inclination relative to Minmus is like 0.1°, which make it super easier to have an encounter. I've been able to transfer and capture my ship into a 10x10 km orbit with 285 m/s. I think you're closer to 400 m/s with your method.

On 15/04/2016 at 3:35 AM, Alshain said:

Just for future information and planning, if you can then it is easier to go from Minmus to the Mun.  The Mun moves considerably faster so it is much more likely to hit it at some point if you get your inclination right, even if you have to go around Kerbin to do it.  This can help if you are planning a mission to go to both, go to Minmus first.  There will be many times that you wish the Mun would just get out of your way.

The launch Window planner gets you in the ballpark but it can't know exactly where your craft will be in it's orbit around the Mun, so it is nearly always not going to give you the single perfect solution for any transfer.  Once you are in the ballpark you can usually play with it until you find a capture.  I would try moving the maneuver along your orbit, rather than altering the burn itself.  Sometimes burning a little sooner or later will find it.

The main problem with doing Minmus and then Mun is that it is less efficient if you have to go back to Kerbin after (which I do). Going from low Mun orbit to a kerbin periapsis of ~30 km cost around 275 m/s ; while the same thing from Minmus is more like 170 m/s. And you can even reduce the cost to less than 100 m/s by performing a Mun gravity assist.

As for the Launch window planner, I think it knows where your ship is, because it tells you both your departure time and your angle of ejection. Since you provided the altitude of your orbit (assumed to be equatorial, prograde and circular), the node can only be at a very specific point in space. Well, I think.

I think the planner struggle because the window form a Mun to Minmus transfer is super small. I've found the correct window to be at Year 1 Day 22 Hour 4. The planner told me it was almost 2 hours later, and it is too late to get your encounter by then.

Edited April 17, 2016 by Tatonf

[Alshain]

34 minutes ago, Tatonf said:

The main problem with doing Minmus and then Mun is that it is less efficient if you have to go back to Kerbin after (which I do). Going from low Mun orbit to a kerbin periapsis of ~30 km cost around 275 m/s ; while the same thing from Minmus is more like 170 m/s. And you can even reduce the cost to less than 100 m/s by performing a Mun gravity assist.

Why on Kerbin would you do that?  You can get a Mun encounter coming directly from Minmus rather easily.

34 minutes ago, Tatonf said:

As for the Launch window planner, I think it knows where your ship is, because it tells you both your departure time and your angle of ejection. Since you provided the altitude of your orbit (assumed to be equatorial, prograde and circular), the node can only be at a very specific point in space. Well, I think.

Yes it gives you a departure time, but it doesn't know that your craft will be at that specific point at that departure time.  It could be on the other side of the Mun, so you have to wait till your craft moves around to the other side of the Mun to actually burn, of course at that point for something as quick moving as the Mun, it could completely mess up your trajectory just the time it takes to get half way through the orbit. 

Most people use it for interplanetary transfers, which don't change all that quickly like a moon will.  Even then, it can often be a little off and you have to search around a bit for the encounter.  It just isn't perfect.  Moon transfers are so incredibly easy to do, there is simply no reason to use that tool at all.  I fear this may be a case where you jumped to the mod instead of learning how to do it properly first, something I strongly advise against.  You really need to learn to manipulate maneuvers manually before relying on mods, and that is something you learn when going to the moons of Kerbin.

Edited April 18, 2016 by Alshain

[Tatonf]

2 minutes ago, Alshain said:

Why on Kerbin would you do that?  You can get a Mun encounter coming directly from Minmus rather easily.

Because of efficiency, as I said. It's cheaper to get back to Kerbin from Minmus than it is from Mun. And yes, I can get a gravity assist quite easily, which is cool.

6 minutes ago, Alshain said:

Yes it gives you a departure time, but it doesn't know that your craft will be at that specific point at that departure time.  It could be on the other side of the Mun, so you have to wait till your craft moves around to the other side of the Mun to actually burn, of course at that point for something as quick moving as the Mun, it could completely mess up your trajectory just the time it takes to get half way through the orbit.  Most people use it for interplanetary transfers, which don't change all that quickly like a moon will.  Even then, it can often be a little off and you have to search around a bit for the encounter.  It just isn't perfect.

Hmmm ok, that's good to know.

[Alshain]

4 minutes ago, Tatonf said:

Because of efficiency, as I said. It's cheaper to get back to Kerbin from Minmus than it is from Mun. And yes, I can get a gravity assist quite easily, which is cool.

No, getting back from the Mun costs less than getting back from Minmus, though not a significant amount in any case, not enough to make you want to do it the hard way.

[Tatonf]

17 minutes ago, Alshain said:

No, getting back from the Mun costs less than getting back from Minmus, though not a significant amount in any case, not enough to make you want to do it the hard way.

Sorry but I don't think so. I know how to make an efficient retro burn, and going back from Minmus is costing around 100 m/s less than from Mun (and this is not accounting for the possibility of a Mun gravity assist). I think it's for the same reason that bi-elliptical transfers are more efficient than Hohmann transfers in certain conditions.

If you can make a return from Mun that cost less than a return from Minmus, please send pictures and explain how you do it.

Edited April 18, 2016 by Tatonf

[Alshain]

Well that isn't the most efficient, but it really doesn't matter, 100 m/s is nothing.  It's definitely not worth making it hard on yourself, you shouldn't be packing fuel quite that tight in the first place.

[Nich]

For a challenge 100 DV can be the difference between 1st and 5th place.  Although I have to agree minimus first is cheaper based off my gut feeling. You have to account for the entire trip and burning to minimus first takes more advantage of  oberith saving more DV then the transfer back cost from the mun.  I also have no idea what cost more transfer from mun to minimus or minimus to mun.  It may also depend on the mission landing vs orbit vs flyby. Without building each mission and totaling all the burns I just can't say.

[Plusck]

7 hours ago, Alshain said:

Well that isn't the most efficient, but it really doesn't matter, 100 m/s is nothing.  It's definitely not worth making it hard on yourself, you shouldn't be packing fuel quite that tight in the first place.

I think it is. I don't see how you could get back from either low Munar or low Minmus orbit for less than what is shown in @Tatonf's screenies, and his/her logic seems impeccable. Particularly tightfisted, one may add, but still impeccable.

[Plusck]

9 hours ago, Tatonf said:

[...]

I've found a much better (= more efficient, not easier) method : I directly launch my ship into an inclined orbit around Mun (about 60°), and when I perform the transfer my inclination relative to Minmus is like 0.1°, which make it super easier to have an encounter. I've been able to transfer and capture my ship into a 10x10 km orbit with 285 m/s. I think you're closer to 400 m/s with your method.

[...]

I've found exactly this technique works wonders for Moho. The correct launch angle from Kerbin is a lot less than this (and it has to be the opposite angle to Moho's inclination, since this'll reverse as you leave Kerbin's SOI), but if you get it right it can save hundreds to escape at AN/DN and match Moho's inclination.

[Superfluous J]

If you're going for a challenge and totally minimizing dV expenditure, then you shouldn't be going from Minmus to Kerbin. You should be going from Minmus to Mun for a gravity assist to slow you down to encounter Kerbin. Likewise, you shouldn't burn up to Minmus from Kerbin but use a Mun assist to get there.

It'll only save a few dozen m/s of dV but if you care about 100...

[Nich]

5 hours ago, 5thHorseman said:

If you're going for a challenge and totally minimizing dV expenditure, then you shouldn't be going from Minmus to Kerbin. You should be going from Minmus to Mun for a gravity assist to slow you down to encounter Kerbin. Likewise, you shouldn't burn up to Minmus from Kerbin but use a Mun assist to get there.

It'll only save a few dozen m/s of dV but if you care about 100...

Actually your right slingshot off the mun saves a good chunk of DV getting to minimus and capturing as you are less esentric.

[Tatonf]

On 18/04/2016 at 7:32 AM, Nich said:

For a challenge 100 DV can be the difference between 1st and 5th place.  Although I have to agree minimus first is cheaper based off my gut feeling. You have to account for the entire trip and burning to minimus first takes more advantage of  oberith saving more DV then the transfer back cost from the mun.  I also have no idea what cost more transfer from mun to minimus or minimus to mun.  It may also depend on the mission landing vs orbit vs flyby. Without building each mission and totaling all the burns I just can't say.

Well, let's try to do it. I have some empiric results. We'll ignore landings and ascendings, because the values are the same, no matter which route is taken.

1a) Mun then Minmus

• Kerbin to Mun : 860 m/s
• Mun insertion burn : 275 m/s
• Mun to Minmus : 210 m/s
• Minmus insertion burn : 90 m/s
• Minmus to Kerbin : 165 m/s
• TOTAL : 1600 m/s

2a) Minmus then Mun

• Kerbin to Minmus : 930 m/s
• Minmus insertion burn : 155 m/s
• Minmus to Mun : 85 m/s
• Mun insertion burn : 205 m/s
• Mun to Kerbin : 275 m/s
• TOTAL : 1650 m/s

So the Mun then Minmus route still wins with a small advantage. But also note that I did not take into account the low Kerbin orbit, which is more expensive for the "Minmus then Mun" route, since you have to launch in an inclined orbit. This loss is however accounted in the Mun to Minmus burn, but it's almost negligible due to the Mun's rotation speed being low (~10 m/s).

On 18/04/2016 at 5:19 PM, 5thHorseman said:

If you're going for a challenge and totally minimizing dV expenditure, then you shouldn't be going from Minmus to Kerbin. You should be going from Minmus to Mun for a gravity assist to slow you down to encounter Kerbin. Likewise, you shouldn't burn up to Minmus from Kerbin but use a Mun assist to get there.

It'll only save a few dozen m/s of dV but if you care about 100...

You can't benefit from both gravity assist, because you have to stop at Mun at some point. But you can still get one, and I mentioned it earlier. Problem is, I'm really bad at gravity assist, despite having read a lot of topics about this.

While getting a Mun gravity assist back to Kerbin is quite easy (it cost about 90 m/s), getting a Mun gravity assist that kicks you to a 10 km Minmus encounter AND correct your inclination (from 0° to 6°) is not something I'm able to do. If someone can do it, please send pictures and parameters.

Despite that, I can easily simulate it by placing a ship at Minmus's inclination, periapsis at Mun's orbit and apoapsis at Minmus's orbit. This would result in a Minmus capture of about 80 m/s.

So let's rebuild our two roads :

1b) Mun then Minmus

• Kerbin to Mun : 860 m/s
• Mun insertion burn : 275 m/s
• Mun to Minmus : 210 m/s
• Minmus insertion burn : 90 m/s
• Minmus to Mun gravity assist : 90 m/s
• TOTAL : 1525 m/s

2b) Minmus then Mun

• Kerbin to Mun gravity assist : 860 m/s
• Minmus insertion burn : 90 m/s
• Minmus to Mun : 85 m/s
• Mun insertion burn : 205 m/s
• Mun to Kerbin : 275 m/s
• TOTAL : 1515 m/s

So, in this particular case, the Minmus then Mun route wins, by almost nothing.

However, please note that all those results are empiric results. It means that I had lot of fun with Hyperedit, created a lot of maneuver node and look at them. I would say that each result have an uncertainty of 5 m/s, and therefore each total have an uncertainty of 25 m/s.

So, while in the no gravity assist case, the Mun then Minmus road wins ; if you start playing with gravity assist, you can't really say which route is better. (but maybe Slashy could come with his awesome math skills and clear this out for us ).

On 18/04/2016 at 11:24 AM, Plusck said:

I've found exactly this technique works wonders for Moho. The correct launch angle from Kerbin is a lot less than this (and it has to be the opposite angle to Moho's inclination, since this'll reverse as you leave Kerbin's SOI), but if you get it right it can save hundreds to escape at AN/DN and match Moho's inclination.

Oh yeah, Moho is on my to-do list, but I intend to try the route of PLAD who made an awesome double Eve gravity assist. You still have to launch in an inclined orbit though.

Edited April 22, 2016 by Tatonf

[Plusck]

7 hours ago, Tatonf said:

[...]

While getting a Mun gravity assist back to Kerbin is quite easy (it cost about 90 m/s), getting a Mun gravity assist that kicks you to a 10 km Minmus encounter AND correct your inclination (from 0° to 6°) is not something I'm able to do. If someone can do it, please send pictures and parameters.

[...]

Oh yeah, Moho is on my to-do list, but I intend to try the route of PLAD who made an awesome double Eve gravity assist. You still have to launch in an inclined orbit though.

Just by taking a look at the porkchops on the alexmoon planner for Mun to Minmus transfers, with plane change, it seems that the Mun and Minmus are correctly aligned every 7.3 days or so, and require a minimal plane change every 3 or 4 windows (i.e. every 22 or 29 days). If you compare the same plots with "ballistic" trajectories, those same best transfers are extremely fine (i.e. you either nail it, or it's impossible).

Top graph is Mun to Minmus with plane change (lowest dv requirements corresponding to very minor change in inclination) and bottom graph is ballistic. Lowest dv corresponds to about 240 m/s, including insertion burn (100km orbit to 100km orbit).

Checking various plots with no insertion burn (just to get the ejection burn figures) gives 211 m/s at a 20km Mun orbit to 146 m/s at a 200 km orbit.

So yeah, it's no great suprise that you're having difficulty getting a gravity assist via the Mun because the parameters need to be so precise.

In addition (assuming that we're both talking about a 20km orbit at the Mun) that (approx) 210 m/s ejection burn corresponds to what would be a 275 m/s circularisation burn if you were coming from Kerbin, so you will necessarily have to make a 65 m/s retrograde burn at Mun Pe during the gravity assist if you pass it at an altitude of 20km. So without even opening the game, it's clear that you will make zero (or maybe 5 m/s) savings with that gravity assist if you pass the Mun that close.

Since there is a possible alignment at year 1 day 1, according to the graph, I'll try a fresh sandbox game to see what might work...

[edit: tried in KSP 1.1 on year 1 day 1 - got a gravity assist from Kerbin 75km equatorial orbit for 853.1 m/s which got me a Minmus encounter directly. However, it wasn't perfect and came in too high at Minmus. Doing the plane correction at the Mun was expensive - about 60m/s. Doing the plane correction halfway to Minmus would probably be much lower, but I couldn't continue due to 1.1's horrible horrible UI blurriness doing my eyes in...]

 

As for going to Moho via Eve, I did it with a trio of ships once, with a single Eve assist, but it ended up being of little benefit because I didn't pass Eve at the Eve/Moho AN/DN. It wasn't too far off, but enough to require an expensive correction, far too close to the sun to be economical.

Edited April 22, 2016 by Plusck

[Tatonf]

Well thank you you for taking your time testing this. I was doing it at a 10 km Mun orbit, but I guess the results would be very similar at 20 km.

If you can't get a perfect Mun gravity assist to Minmus then you'd better take the route with a Mun gravity assist to Kerbin !

2 hours ago, Plusck said:

[...]

but I couldn't continue due to 1.1's horrible horrible UI blurriness doing my eyes in...]

 

As for going to Moho via Eve, I did it with a trio of ships once, with a single Eve assist, but it ended up being of little benefit because I didn't pass Eve at the Eve/Moho AN/DN. It wasn't too far off, but enough to require an expensive correction, far too close to the sun to be economical.

For your blurriness problem, are you talking of the orbit lines ? You can change that in the parameters.

For Eve gravity assist I would recommand you the PLAD route, so you already have the parameters for doing it :

Someone even did 8 Moho gravity assist !

 

Finally, if you want to see the results of this topic, you can take a look at my video, featuring the Mun then Minmus route :

 

[Plusck]

50 minutes ago, Tatonf said:

Well thank you you for taking your time testing this. I was doing it at a 10 km Mun orbit, but I guess the results would be very similar at 20 km.

If you can't get a perfect Mun gravity assist to Minmus then you'd better take the route with a Mun gravity assist to Kerbin !

For your blurriness problem, are you talking of the orbit lines ? You can change that in the parameters.

[...]

 

The blurriness is the general 1.1 UI (particularly text blurriness) which kills my eyes. I was also having problems since I was on normal physics not in warp, and with very precise maneuvre nodes it is impossibe to do without being in warp and on rails because the constant recalculations of the orbit means the nodes keep changing.

However, I perservered and got a good hit. This is the savegame (from a fresh 1.1 save called "SandboxDefaultNormal"): minmus grav assist persistent.sfs

Roughly 76x77km equatorial orbit, 853.1 m/s burn to the Mun to get gravity assist to Minmus, followed by a 17 m/s correction (iirc) at Ap to get a 250km Minmus flyby. It was impossible to get a better approach because of the bug that causes the encounter to disappear if you try to make a correction burn too close (happens a lot for Minmus, Bop and Pol). When actually at Ap, you should be able to get the "real" orbit line down to a 10km flyby with just a couple m/s more.

So that gives 870 m/s to Minmus via gravity assist. 10m/s more than your option 2b) - which is neat because that gives exactly the same result for either way.

Edited April 22, 2016 by Plusck

[Tatonf]

12 hours ago, Plusck said:

It was impossible to get a better approach because of the bug that causes the encounter to disappear if you try to make a correction burn too close (happens a lot for Minmus, Bop and Pol). When actually at Ap, you should be able to get the "real" orbit line down to a 10km flyby with just a couple m/s more.

So that gives 870 m/s to Minmus via gravity assist. 10m/s more than your option 2b) - which is neat because that gives exactly the same result for either way.

Well, just make the node earlier. It won't disapear and it will actually be more efficient.

http://imgur.com/fvDgta5

You can see that I will crash on Minmus, but it's really a matter of 0.01 m/s for getting a 10 km periapsis. I actually got one, but it turned into a crash as soon as I returned to the Tracking Station. Related to that, you claimed to have a 250 km periapsis, but I got a 330 km periapsis with your save. I think there's a bug/feature (delete as appropriate) here.

I tried to actually make it but I didn't get the exact same Mun encounter, and the correction burn cost me 20 m/s instead.

You also have to account for the Minmus inserstion burn, because it will change depending of how similar your orbit is to the orbit of Minmus. I planned for 90 m/s in my 2b) route, but got 100 m/s instead.

But still, you proved that this route is indeed valid and possible, thanks for sharing your save file.

Edited April 22, 2016 by Tatonf