Minmus "equatorial" orbit?

[strider3]

Hopefully I am not, again, posting to the wrong forum?

I am orbiting Minmus with the beginnings of a new orbital refueling station. I am trying to determine when I am in an equatorial orbit of Minmus, and how to get there, if not. KER "Orbital" tells me I'm off (showing 6.1). When I try to change the orbital inclination I can never get to 0.0...I get to around 4.4 and then go back the other way. This may be because I am not burning at the correct place of the orbit...but since I no longer have the Ascending / Descending node indicators, I have no idea.

How does one go about getting into an equatorial orbit of a celestial body...once you are already orbiting it?

TIA, Vic the Newbal

[Superfluous J]

You got it right, you're burning in the wrong place. The galactic plane matches 0 degrees so put it as flat behind Minmus as you can and you should be able to estimate about where to burn.

[strider3]

20 minutes ago, 5thHorseman said:

You got it right, you're burning in the wrong place. The galactic plane matches 0 degrees so put it as flat behind Minmus as you can and you should be able to estimate about where to burn.

I'm dense, 5thH...could you elaborate?

[FullMetalMachinist]

Since you already have KER, this is pretty easy. 

You can customize the different info panels and add more things to them. So edit the "orbital" one to include the "time to AN/DN." Then when that time gets close to zero, you're pretty much right on the node. Then you make your normal/anti-normal burn to zero your inclination. 

[Kerbart]

Check the latitude, if you have access to it (KER can tell you). At 0° N or S, burn normal/antinormal until your inclination is 0°.

You could also burn when the distance to the ascending node or descending node is 0 but that measurement seems to be off in KER (either that or my KER install is borked somehow). At any rate, burning when crossing the equator always works.

[Superfluous J]

10 minutes ago, Victor3 said:

I'm dense, 5thH...could you elaborate?

It's hard without pictures and I'm on my phone so pictures aren't really an option. :/

EDIT: FMA has a much better and more accurate way. I've voted it up.

Edited September 10, 2016 by 5thHorseman

[Diche Bach]

4 minutes ago, FullMetalMachinist said:

Since you already have KER, this is pretty easy. 

You can customize the different info panels and add more things to them. So edit the "orbital" one to include the "time to AN/DN." Then when that time gets close to zero, you're pretty much right on the node. Then you make your normal/anti-normal burn to zero your inclination. 

I basically do this, but just use Kerbal Alarm Clock.

As far as it "getting to about 4.4 and then going the other direction" . . . someone with better comprehension of orbital mechanics should clarify, but I'll take my novice stab at explaining this.

AN and DN are just points in an orbit that define the hinge along which one orbit is inclined relative to another orbit (or in the case of "equator" an imaginary line).

It is my suspicion that, much like the apo and periapsis, the "most efficient" trajectory changes with respect to inclination can only be achieved right at these points. As one moves further from them, proportionally larger dV is required to lever the orbit one way or the other. Also, there is presumably a "mid point" (well two of them actually) that is in some since "midway" between the AN/DN and at which point it is effectively impossible to change inclination

So one passes lets say "AN" you start burning in the proper direction to regularize your inclination. The effect of this is to move the AN away from where you are in the orbit (don't ask me which direction), as you get farther way from that point, the inclination change "value" of each dV you are expending goes closer to zero. Then you cross the midway between AN/DN and you are now burning in the opposite direction relative to the change you were initially seeking to make to the relative inclination between your orbit and the target orbit.

In sum, it seems that inclination changes are best done:

A. With as high TWR as possible

B. With short burns that are "straddled" right at the AN/DN

The other thing is: as you get closer and closer to 0.0000 . . . relative inclination, the "window" of effectiveness seems to get smaller.

That is my social scientists amateurish take on this thing.

Edited September 10, 2016 by Diche Bach
answer OP's question

[swjr-swis]

1 hour ago, Victor3 said:

How does one go about getting into an equatorial orbit of a celestial body...once you are already orbiting it?

TL;DR: stock currently doesn't give us the information we need, for reasons unknown and unfathomable. Perhaps taniwha's work on orbitals for 1.2 will resolve this for us, but for now, hope that some mod gives you the indicators needed, or prepare for a lot of effort for an inaccurate approximation.

 

There is one thing we know: in KSP, there is no inclination of planets/moons themselves, only of their orbits. In other words, all planet/moon ecliptics are at 0.0 inclination relative to the solar system ecliptic. So you really just need to find the Asc/Des nodes of your current orbit.

Doesn't KER have an indicator of 'time to ASC/DES'? Go to the map view, focus on Minmus, check the time to ASC/DEC and then quickly hover over the orbit to place a maneuver node at a place ahead of you that is as close as you can get to that time. When you check again you will likely be a few secs off, but now that the node is placed, you can drag it along the orbit to a more exact location. A few secs off is acceptably close though. Now your node is at the right *time*, all you need now is to burn normal/antinormal as appropriate close around that node. You may need to do a few burns, but keep using this node trick node to time it.

 

Visually and in pure stock, you can approximate it by focusing on Kerbin and using Mun's orbit as the perfect 0.0 inclination guideline (or a bit easier but slightly more inaccurate: Duna's almost perfect 0.06 inclination, which you should be able to target from around Minmus and highlight yellow).

Rotate your view until Mun's orbit is as close to a flat line as you can get. Now, the farther away you are from ascending or descending from Kerbin's ecliptic, the more inaccurate, so you will want to rotate to where the Minmus orbit intersects right through the center of Kerbin and Mun's 'flat-lined' orbit. That point will be very close to the ASC or DES nodes through Kerbin's ecliptic. Time warp to get Minmus close to that location but still a bit ahead of it (you may need several burns if you are 6.1 degrees off, so allow some time before Minmus moves on too much from that point).

Now focus on Minmus, align your view so Kerbin is behind Minmus and at its 'center', with the flat Mun orbit slashing Minmus in half (Minmus isn't perfectly round, so again, some inaccuracy inevitable). Now time your burn appropriately normal/anti-normal at the time of passing through Mun's orbit line. Try to visually get your orbit as flat-lined and parallel (merged is best) with Mun's orbit as you can.

Edited September 10, 2016 by swjr-swis

[foamyesque]

AN/DN are where two orbital planes intersect. These points are unique for anything except exactly co-planar orbits. One of the basic principles of orbits is that you always return to your starting point (i.e. the last place you burned fuel). Therefore, if you do not burn at the node you will never be able to get a co-planar orbit. It isn't a question of efficiency; you could have infinite fuel and gravity would still say "no".

[Diche Bach]

2 minutes ago, foamyesque said:

AN/DN are where two orbital planes intersect. These points are unique for anything except exactly co-planar orbits. One of the basic principles of orbits is that you always return to your starting point (i.e. the last place you burned fuel). Therefore, if you do not burn at the node you will never be able to get a co-planar orbit. It isn't a question of efficiency; you could have infinite fuel and gravity would still say "no".

Interesting please elaborate if possible and reflect on what i said above. I'd like to understand this better.

[foamyesque]

1 minute ago, Diche Bach said:

Interesting please elaborate if possible and reflect on what i said above. I'd like to understand this better.

 

Basically, if you finish your burn at a point that isn't planar with your desired orbit, you will always return to it and therefore always be in the wrong plane.

[Diche Bach]

Ah, so the nodes (AN/DN) are where any impulse normal or anti-normal should _cease_?

If so, then I had it exactly in reverse.

I was under the impression that if:

you have a -5* at a descending node (and are trying to get relative inclination to 0.0000), then you should burn normal starting AT the descending node and can continue effectively to burn for some time after the node, and this will effectively change the relative inclination toward zero.

However at some point, between the DN and AN, continuing to burn normal stops working and you start moving the relative inclination back in the other direction (away from zero and in this case toward negative infinity . . . or whatever the maximum negative is . . .).

As a result of these effects, and depending on the size of the orbit (smaller orbits offering less of a "window" to burn around each node) as well as TWR, it may be that, one is only able to reduce the relative inclination by one or two degrees at a time, and that one must wait til a subseqeunt node to make additional adjustments.

[DD_bwest]

ive always use mj for informational readouts,  especially since i can place time to dn/an with target aswell as relative inclination right next to eachother.   what i always do to get right at 0.00,  is to burn just ahead of the actual  dn or an.   depending on the change needed and twr, when varies slightly.  but if you burn ahead of it,  it will push your an/dn  away from you, while also lowering the inclination. so burn a bit and wait to catch up to the node,  and repeat. you can then throttle down when you get to the end,   take care of that last little bit in the 5 seconds that surround the node itself.

[strider3]

Ladies and Gentlemen...I never fail to get more than enough info when I ask you guys a question! After years and years of dealing with other forums (Flightsim.com, subsim.com, etc.) it is amazingly fun to come hear...lost and perplexed...and walk away found and informed. THANK YOU!!

And, yes, KER gave me the info I needed to burn to an equatorial orbit...I just didn't know it was there. Like so many other things KSP, KER, KIS/KAS, I'm still discovering all the "tools" they contain.

Again...thank you so much, guys.

Vic the Newbal

[Starhawk]

On 2016-09-10 at 1:53 PM, Diche Bach said:

Ah, so the nodes (AN/DN) are where any impulse normal or anti-normal should _cease_?

If so, then I had it exactly in reverse.

I was under the impression that if:

you have a -5* at a descending node (and are trying to get relative inclination to 0.0000), then you should burn normal starting AT the descending node and can continue effectively to burn for some time after the node, and this will effectively change the relative inclination toward zero.

However at some point, between the DN and AN, continuing to burn normal stops working and you start moving the relative inclination back in the other direction (away from zero and in this case toward negative infinity . . . or whatever the maximum negative is . . .).

It works tho same way as any ordinary burn.  You want to burn so that you hit the AN/DN exactly at the halfway point of the burn.  This is the same as burning at any maneuver node where you want to start your engines half the 'burn time' before the node so half of the burn is before the node and half after.

On 2016-09-10 at 1:53 PM, Diche Bach said:

As a result of these effects, and depending on the size of the orbit (smaller orbits offering less of a "window" to burn around each node) as well as TWR, it may be that, one is only able to reduce the relative inclination by one or two degrees at a time, and that one must wait til a subseqeunt node to make additional adjustments.

Yes indeed.  If your TWR is too low or your orbital velocity is very high then you may need to make several burns to get the inclination where you want.  ALL of these burns will occur right at (very near to) the AN or the DN.

I once captured a class E asteroid and it took many, many, orbits before I could get it's inclination to where I wanted, changing it by a tiny amount every orbit.  I just kept it pointed the same way and waited for it to come around to the same node over and over.  With a more nimble craft you can change the orientation every half orbit and burn at both the DN and at the AN every orbit.

Happy landings!

[Diche Bach]

Well . . . I'm not so sure it works "the same way as an ordinary burn exactly" though do correct me if I'm not quite grokking it yet (still!).

1. Prograde burn: effect? ALWAYS pushes the point on the opposite side of the orbit farther away from the center of the gravity well being orbited.

Will it ever have the "reverse" effect (bringing the point on the opposite side of the orbit closer . . .): As far as I know, NEVER.

2. Retrograde burn: same as prograde, but reversed.

 . . . skipping Radial / Anti-radial  . . .

In contrast.

3. Normal burn: effect? depends on where you are relative to the AN/DN. If you are  near enough to AN, the effect will be to increase the inclination. However, if you get too close to the DN, suddenly you will progressively decrease the inclination. No?

I'm a bit embarrassed to admit that this still puzzles me but it does, and my observations may be completely erroneous.

[Plusck]

42 minutes ago, Starhawk said:

It works tho same way as any ordinary burn.  You want to burn so that you hit the AN/DN exactly at the halfway point of the burn.  This is the same as burning at any maneuver node where you want to start your engines half the 'burn time' before the node so half of the burn is before the node and half after.

Well... I humbly suggest that this explanation will leave some confusion.

You want to burn so that if you had not changed course you would have hit the AN/DN exactly at the halfway point of the burn...

In fact, by starting the burn you never actually hit that point, and assuming constant acceleration you will in fact end the burn exactly on AN/DN.

 

34 minutes ago, Diche Bach said:

Well . . . I'm not so sure it works "the same way as an ordinary burn exactly" though do correct me if I'm not quite grokking it yet (still!).

1. Prograde burn: effect? ALWAYS pushes the point on the opposite side of the orbit farther away from the center of the gravity well being orbited.

Will it ever have the "reverse" effect (bringing the point on the opposite side of the orbit closer . . .): As far as I know, NEVER.

2. Retrograde burn: same as prograde, but reversed.

 . . . skipping Radial / Anti-radial  . . .

In contrast.

3. Normal burn: effect? depends on where you are relative to the AN/DN. If you are  near enough to AN, the effect will be to increase the inclination. However, if you get too close to the DN, suddenly you will progressively decrease the inclination. No?

I'm a bit embarrassed to admit that this still puzzles me but it does, and my observations may be completely erroneous.

I think the easiest way to consider it (though this is entirely personal...) is actually to consider that the most significant effect is always one quarter of an orbit ahead and behind you. The effect on your orbit now, and on the opposite side of the body you're orbiting, is only incidental to that "most significant effect".

So burning prograde/retrograde affects your angle of flight compared to the horizontal one quarter orbit ahead/behind. The "incidental" effect is to change the height of the orbit half an orbit later.

Burning radial in/out will affect the height of the orbit one quarter orbit ahead/behind. The "incidental" effect is to change the angle with respect to the horizontal, now and one half orbit later.

Burning normal/antinormal will affect the latitude one quarter orbit ahead/behind. The "incidental" effect is to change the angle with respect to the vertical, now and one half orbit later.

 

As for the normal burn's effect on AN/DN, I consider it more a question of "pushing and pulling". Burning "up" will push DN away from you and pull AN towards you, and vice versa. Whatever is pushed away (while it is less than one quarter of an orbit away) is decreased, whatever is pulled is increased. Once it is more than a quarter orbit (90°) away, the effect is the opposite...
And this ties in with making rendezvous using target mode on the navball: you push retrograde (within 90°) to decrease relative velocity (and by pushing it the right way, decreasing intercept distance), you pull prograde (again within 90°) to increase relative velocity.

Edited September 15, 2016 by Plusck

[Diche Bach]

That is a good description, though I'm too brain dead to fully grasp it. Will come back to this tomorrow!

[strider3]

I've always just split the burn time with good results? If a 10 second burn (at full throttle) is required, I will split that (5 sec.) and add some seconds to account for my reduction of throttle towards the end of the burn. In this situation I would start my burn 7-8 seconds before the node...but that's just me ;-P