Hopefully simple question. Efficient inclincation adjustments

[goldenpsp]

So with 1.x and IRSU, I decided to setup a mining/refining operation on Minmus. It had some nice hotspots, lower gravity to get up and down from the surface etc. It all works fairly well, except I find the return trip annoying. Due to the inclined orbit of Minmus, my return orbit of Kerbin is usually (always) crazy inclined as well. I can get cirularized around Kerbin and then fix it, but that seems the most wasteful in terms of dV. I can fix a lot of it at Pe, but that generally requires a full orbit, which adds many days to the operation.

Thoughts?

[Richy teh space man]

Fix your inclination at apoapsis after leaving Minmus, it may take a bit longer before the right node is open, but it should be cheaper then doing it at periapsis.

[match]

Set up a way station to match Minmus orbit, but at 0* inclination- will want it to be in front or behind in the orbit to avoid Minmus' SOI at the AN & DN. It may add more annoyance with the extra hop, but if you intend to do many voyages to and from Minmus, it makes it more routine.

[Snark]

You're always going to have some sort of inclination issues with Minmus, but you have a few options available to you depending on where Minmus is when you launch.

A side note: If your goal is to end up in an equatorial orbit of Kerbin, a handy trick is to set your target to be the Mun, which orbits perfectly over Kerbin's equator. That gives you ascending/descending nodes, which you can then use for making course decisions as commented below. (Another alternative is to park a little satellite in an equatorial orbit and use that as your target).

Option 1: Be equatorial starting at Minmus launch.

How to do it: Wait until Minmus is at either the ascending or descending node of its orbit with respect to Kerbin's equator (i.e. where the plane of its orbit crosses the Mun's). Then launch directly into a Kerbin-equatorial orbit from Minmus.

-Advantage: Minimum dV requirements. Only have to make one burn. Time-efficient.

- Disadvantage: Have to wait until Minmus is at AN or DN, which could be days.

Option 2: Mid-course correction

How to do it: Just go ahead and launch from Minmus at any time. Your launch can include some normal/antinormal component at launch time to reduce your inclination relative to Kerbin's equator, but unless you happen to be at Minmus' ascending/descending nodes (see above), you won't be able to eliminate all of it. Set a maneuver node to make your target the Mun. At some point on your inbound journey to Kerbin, you'll pass through either ascending or descending node, hopefully when you're still very high above Kerbin. Set a maneuver node there to reduce your inclination to zero; as long as you're high up, this won't be a lot of dV.

- Advantage: Can launch whenever you want. Time-efficient.

- Disadvantage: More fiddly (have to do two burns). Not always super helpful for dV, depending where in its orbit Minmus is at launch time (e.g. if your AN/DN node on Kerbin approach is at low altitude, would be expensive to make that burn).

Option #3: this is option #2 + patience.

How to do it: Basically, do your burn, make your initial periapsis pass past Kerbin while still inclined, coast up to a high altitude and make an inclination-correcting burn at your ascending/descending node to set inclination to zero.

- Advantage: All of #2's advantages, minus the "can be expensive" disadvantage.

- Disadvantage: Needs patience, involves doing an extra orbital loop and therefore adds days to the trip.

In practice, I generally find that I go with #2, unless Minmus' placement at the time I happen to want to launch is especially lucky (in which case I go with #1) or especially unlucky (in which case I go with #3).

[cybersol]

If you don't want to wait until the Minmus orbit crosses Kerbin equatorial plane, then you are best off doing a mid-course plane change maneuver. The best place will be somewhere between just after you leave Minmus (slowest speed, so more plane change effect per dV) and mid-way to Kerbin (better angular leverage).

Edit: Ninja'd by Snark

Edited June 6, 2015 by cybersol

[Wanderfound]

Inclination changes increase in efficiency the slower you are moving; so, high orbit is better than low, and apoapsis is better than periapsis.

But you can do an inclination change anywhere in the orbit you want; you don't have to wait for apoapsis/periapsis (or the ascending/descending nodes, for that matter, although you need AN/DN if you want to perfectly match orbits).

The ÃŽâ€V cost of an inclination correction done immediately after departing the Minmus SoI should be trivial.

[goldenpsp]

Thanks everyone. I think the biggest tip is using Mun as the target. I still find it odd/annoying you can't choose kerbin as a target, and it is really hard otherwise to eyeball the inclination and AN/DN locations.

[Coga19000]

Actually, even if you don't start equatorial from Minmus surface, changing your inclination in Minmus orbit is still more fuel-efficient. For full-on conservation of fuel, raise your orbit so that the Apoapsis is both at the brink of escape velocity, on your Ascending/Descending node, and on the direction you need to get flinged towards Kerbin. Then, change your inclination then.

But whatever you do, if you can help it, NEVER change your inclination at Periapsis. In contrast to being more efficient when burning pro/retro when moving the fastest, moving the SLOWEST is actually the most efficient way to change inclination.

Edited June 6, 2015 by Coga19000

[cybersol]

But whatever you do, if you can help it, NEVER change your inclination at Apoapsis. In contrast to being more efficient when burning pro/retro when moving the fastest, moving the SLOWEST is actually the most efficient way to change inclination.

But whatever you do, if you can help it, NEVER change your inclination at ApoapsisPeriapsis. FIFY.

[Coga19000]

But whatever you do, if you can help it, NEVER change your inclination at ApoapsisPeriapsis. FIFY.

Woops, you're right.Goddamn muscle memory Thanks!

[SanderB]

AN/DN can be adjusted by some off-prograde thrust from maneuvers made near the AN/DN and sometimes fully by losing merely 1-5% extra dV by going no more than 20° off prograde. Of course your maneuvers need to be near or on the nodes to be able to do this.

[mrklaw]

I'm confused - how can you adjust inclination at apoapsis if your position is above/below the kerbin Equatorial orbit? Surely you have to be at the AN/DN to do it?

Also so what direction would you burn at apoapsis to raise periapsis? Burning normal/anti-normal would keep periapsis where it is and raise the AN/DN wouldn't it?

[Coga19000]

I'm confused - how can you adjust inclination at apoapsis if your position is above/below the kerbin Equatorial orbit? Surely you have to be at the AN/DN to do it?

Also so what direction would you burn at apoapsis to raise periapsis? Burning normal/anti-normal would keep periapsis where it is and raise the AN/DN wouldn't it?

Meh, by Apoapsis, we mostly mean the highest of the two nodes. Though generally, burning prograde at the lower node to set Ap to the other one can actually save you Dv at the actual incl. burn.

And burning prograde at a point of your orbit will raise the opposite point, as a rule of thumb. So, Prograde at your Ap will raise your Pe, and vice versa (because Ap and Pe are always opposite point of an orbit, due to its elliptical nature.

[Laie]

I'm confused - how can you adjust inclination at apoapsis if your position is above/below the kerbin Equatorial orbit? Surely you have to be at the AN/DN to do it?

The veterans in this thread are two steps ahead of you: with some smart planning/maneuvering, you can make it so that AN/DN pretty much coincides with AP/PE.

Toy around a little with how you can leave Minmus. Not only consider the easiest/fastest/cheapest return but also see what would happen if you added as sizeable normal component to your ejection burn, and/or try to first fly out to an even higher apoapsis around Kerbin. Expending some 50m/s on top of the bare minimum when leaving Minmus can save you some 150m/s in course corrections on your way to Kerbin.

Myself, I prefer to launch with enough fuel if possible. If I have to do ISRU near Kerbin, I prefer the Mun. Plotting a return trip from Minmus is almost as much effort as a whole Munar round trip. If the (real life) time you put into it is of any value, the Mun wins by a large margin.

[OhioBob]

Toy around a little with how you can leave Minmus. Not only consider the easiest/fastest/cheapest return but also see what would happen if you added as sizeable normal component to your ejection burn ...

^This^ is an important point. If you depart Minmus along a trajectory that is parallel to the equatorial plane, your AN and DN will be very close to Kerbin. Instead, when you depart Minmus, you must incline your trajectory so that you cross Kerbin's equatorial plane when you are still far from Kerbin. If Minmus is below Kerbin's equatorial plane when you depart, you should leave Minmus orbit along an inclined trajectory that will take you northward. If Minmus is above Kerbin's equatorial plane, you should leave along an inclined trajectory that will take you southward. In this way you'll reach the ascending node (when traveling northward) or descending node (when traveling southward) relatively soon after leaving Minmus and while you are still a long distance from Kerbin. However, you don't want to incline your trajectory too much. Although you gain by performing the plane change farther from Kerbin, you lose by having to perform a larger plane change. Somewhere in the middle is an optimum balance, which is something you'll have to figure out.

[mrklaw]

Meh, by Apoapsis, we mostly mean the highest of the two nodes. Though generally, burning prograde at the lower node to set Ap to the other one can actually save you Dv at the actual incl. burn.

And burning prograde at a point of your orbit will raise the opposite point, as a rule of thumb. So, Prograde at your Ap will raise your Pe, and vice versa (because Ap and Pe are always opposite point of an orbit, due to its elliptical nature.

Sorry, my question was maybe worded badly. By 'raising periapsis' I meant relative to the equatorial plane.,mint raising it as in making it further out from the planet. My fault for using the wrong term.

I.e. If you're leaving minmus while its orbit is above kerbin's, then your apoapsis will likely also be above kerbin's orbit, and your periapsis will be below kerbin's orbit. that was the situation I was talking about with my confusion. I didn't think you could bring your periapsis point above the plane of kerbin's orbit by burning at an apoapsis which is also above kerbin's orbit.

Laie's post got what I meant and although I have no idea how to do the things suggested, I get the concept.

[Coga19000]

Sorry, my question was maybe worded badly. By 'raising periapsis' I meant relative to the equatorial plane.,mint raising it as in making it further out from the planet. My fault for using the wrong term.

I.e. If you're leaving minmus while its orbit is above kerbin's, then your apoapsis will likely also be above kerbin's orbit, and your periapsis will be below kerbin's orbit. that was the situation I was talking about with my confusion. I didn't think you could bring your periapsis point above the plane of kerbin's orbit by burning at an apoapsis which is also above kerbin's orbit.

Laie's post got what I meant and although I have no idea how to do the things suggested, I get the concept.

Woops, my bad '~' In this case, yeah, Laie pretty much got it spot on.