How to repair a bad inclination with minimal fuel waste?

[panzer1b]

Well as teh title says, ive recently started doing alot of interplanetary stuff, and i often find myself thrown into a terribly inclination (more then 20 degrees) often when i dont set up 100% during my initial burns (hard to do without mods ect). Is there any way outside of the horribly inefficient method of burning at purple triangle and then citrcularizing to fix a botched up inclination. Also, im fairly new to thge whole interplanetary stuff, so any tips regarding the best way to get to a planet ect is appreciated. Also, when is it bets to correct for bad alignment, way before reaching planet, ect?

Edited November 18, 2014 by panzer1b

[Kerbart]

The further out the easier it is, best to fix it when you just enter the SOI of the planet/moon.

Also, raising the apoapsis (in such a way that it coincides with the AN/DN) and performing the inclination change there really helps. Inclination change is all about changing the direction of your velocity vector, and the lower your speed, the less energy it takes.

[mdkendall]

Not sure if you are trying to fix a Kerbin orbit prior to doing a transfer burn, or trying to fix a botched transfer orbit.

If the former, combining the inclination change with the transfer burn will be a lot more efficient than doing them separately.

If the latter, you must burn normal (purple) when you are at the ascending or descending node (i.e. the points where your current orbit intersects the plane of the desired one, which likely is the plane of the orbit of the target body).

In general, changing inclination is cheapest when you are moving slowest, i.e. apoapsis. But the orbits available by burning at this point may or may not include the one you want (clearly both the before- and after-burn orbits must intersect the point at which you do the burn).

A third possibility is that you are trying to fix a final orbit around the destination body after a sub-optimal capture. In this case the cheapest thing is to not get yourself in that situation. At some point well before you reach the target, when you are still moving slowly through interplanetary space, set up a maneuver node and experiment with radial and/or normal burns while watching your predicted orbit after the encounter with the target body. If it rises or falls out of the plane you are encountering the target non-equatorially. Tweak the burn to get the exit vector to stay in the plane. This should only take a very small amount of delta-V.

Edited November 18, 2014 by mdkendall
typo

[Starman4308]

For plane changes relative to a planet, you're going to want to set up a maneuver node and drag out one of the purple triangles to fix it. If the inclination change is sufficiently large, it can save you fuel to raise apoapsis to almost escape velocity, perform your plane change at apoapsis, and then shrink the apoapsis back down, because inclination changes require delta-V proportional to velocity. If it is a target orbiting the same body, you will set up your plane change at the ascending/descending node. The same logic for plane change efficiency applies: if it is a sufficiently large angle, it saves fuel to burn prograde at the lower-altitude node (thus creating a very high apoapsis at the other node), and match planes near apoapsis.

For planetary encounters, one simple way to do it is simply to burn out to an encounter, target the planet, and set up a plane change at whichever ascending/descending node pops up. Optimal transfers can be planned here, or using MechJeb functions.

[Stratzenblitz75]

Could you show us a picture of your orbit? Without a picture, I can't really see how you should go about fixing your inclination efficiently, as there are differing solutions based on the situation.

As for setting up your interplanetary burns, I recommend you use this launch window planner. Just give it the planet of Orgin, the destination planet and the earliest departure date and it'll tell you how much Delta V you need for the transfer, and when you should depart.

[panzer1b]

Thanks for the quick replies, so let me get this straight, if im off by ALOT, eg more then 15 degrees, extend my orbit, make it have a higher apoasis that happens to be near AN/DN, and burn towards the purple things when im at the AP. For small correction (10 degrees ect), dont bother and just burn how i would normally.

Could you show us a picture of your orbit? Without a picture, I can't really see how you should go about fixing your inclination efficiently, as there are differing solutions based on the situation.

As for setting up your interplanetary burns, I recommend you use this launch window planner. Just give it the planet of Orgin, the destination planet and the earliest departure date and it'll tell you how much Delta V you need for the transfer, and when you should depart.

I dont have any pictures as i have nothing in a bad orbit as of now, its just theoretical, say i arrive at duna (or whatever plane), and instead of being nicely lined up i get into a terrible orbit.

One more question thats kindof related, when im entering a planet, how do i determine what direction i will orbit in before its too late, say i want a normally counterclockwise orbit liek teh entire solar system has, how do i guarantee it is a counterclockwise (or the other way depending on what i want, and ofc if i mess up and end up on the wrong orbit as soon as encounter starts, is there any way to fix that without essentially landing on planet and then taking off in correct direction?

[SRV Ron]

Before even entering the SOI, set up a maneuver node and correct as much of the error as possible. It will take very little fuel at that distance to do so. Additional correction can be made when you enter the SOI of the planet. Major corrections can be done using slingshot maneuvers around a moon of the planetary system. The node for a final correction in orbit should be set where your orbit crosses the equator of that planet or moon.

Example of a bad Duna encounter corrected using Ike for the slingshot.

Edited November 18, 2014 by SRV Ron

[Wanderfound]

Well, the simplest answer is: why do you care what inclination you're on when you're orbiting Duna? A high inclination orbit is no more difficult to land from than an equatorial orbit.

If it's about returning from interplanetary, well, again: you don't have to travel in the equatorial plane all the time (it can be more efficient to do so, but it isn't compulsory).

If you do have a need to change inclinations (e.g. to rendezvous with a fuel depot), though: the higher and slower [1] you are when you make the change, the lower the delta-V required. Change the inclination on the way in to the encounter, not after you've circularised into a low orbit.

[1] These naturally go together. Although it's a bit counter-intuitive (due to the fact that you accelerate to raise an orbit), you're always going slower at apoapsis than at periapsis. Think of an orbit as like a playground swing; the apoapsis is the bit at the top of your swing, where you stop [2] and come back down, while the periapsis is at the bottom, where you're going really fast.

[2] Your horizontal speed doesn't actually stop and reverse, but your vertical speed does. And, while it's doing that, your horizontal speed reaches its orbital minimum [3].

[3] Nested footnotes! Yay!

[cantab]

Once you're out of Kerbin's SOI and on your transfer orbit, target the destination planet. You should get an AN/DN marker, put a manouvre node there. Alternatively just put a node about halfway round your transfer. Then focus the view on the destination planet, and adjust that manouvre node so you come in equatorially with a low periapsis.

If you need to avoid or get a moon encounter, trade off prograde/retrograde and radial burns, so you adjust your arrival time while keeping a low periapsis.

[Stratzenblitz75]

I dont have any pictures as i have nothing in a bad orbit as of now, its just theoretical, say i arrive at duna (or whatever plane), and instead of being nicely lined up i get into a terrible orbit.

Ok, this helps narrow it down. For Duna, or any other planet with large moons, you can use gravity assists to fix your inclination without spending much DeltaV. SRV Ron posted an excellent example of this.

Fixing a bad orbit around a planet that has no significant moons, such as Eve or Dres is a lot more difficult and can take a lot of Delta V depending on how bad the orbit is. The best way to get a good orbit around these planets is to get a good approach. Here's how I enter into an equatorial orbit around such planets:

Step 1: Adjust your periapsis so that it passes over or close to the equator of the target planet (since there are no stock tools that tell you where the equator is, just use your best judgment). Essentially, your periapsis is crossing the AN or DN of the planet's equatorial plane.

Step 2: Perform an insertion burn/aerobrake that is just enough to get captured by the planet. At this point, you will have an inclined orbit with both a low periapsis and a very high apoapsis that cross the AN or DN of the planet's equatorial plane.

Step 3: Fix your inclination at apoapsis. Since your apoapsis is so high, you will only be traveling 10-50 m/s, which means that inclination changes are very cheap. Your orbit should look something like this:

Step 4: Perform burn a periapsis to circularize orbit.

Step 5: Enjoy your equatorial orbit.

One more question thats kindof related, when im entering a planet, how do i determine what direction i will orbit in before its too late, say i want a normally counterclockwise orbit liek teh entire solar system has, how do i guarantee it is a counterclockwise (or the other way depending on what i want, and ofc if i mess up and end up on the wrong orbit as soon as encounter starts, is there any way to fix that without essentially landing on planet and then taking off in correct direction?

There's actually a simple trick to ensuring that you get a counterclockwise orbit. If you are transferring to an outward planet (Kerbin -> Duna, for example), adjust your periapsis so that it is on the day side of the planet. If you are transferring to an inward planet (Kerbin -> Eve, for example), adjust your periapsis so that it is on the night side of the planet.

Edited November 18, 2014 by Stratzenblitz75

[panzer1b]

Well, the simplest answer is: why do you care what inclination you're on when you're orbiting Duna? A high inclination orbit is no more difficult to land from than an equatorial orbit.

If it's about returning from interplanetary, well, again: you don't have to travel in the equatorial plane all the time (it can be more efficient to do so, but it isn't compulsory).

If you do have a need to change inclinations (e.g. to rendezvous with a fuel depot), though: the higher and slower [1] you are when you make the change, the lower the delta-V required. Change the inclination on the way in to the encounter, not after you've circularised into a low orbit.

[1] These naturally go together. Although it's a bit counter-intuitive (due to the fact that you accelerate to raise an orbit), you're always going slower at apoapsis than at periapsis. Think of an orbit as like a playground swing; the apoapsis is the bit at the top of your swing, where you stop [2] and come back down, while the periapsis is at the bottom, where you're going really fast.

[2] Your horizontal speed doesn't actually stop and reverse, but your vertical speed does. And, while it's doing that, your horizontal speed reaches its orbital minimum [3].

[3] Nested footnotes! Yay!

Its not that i CARE about what orbit im in, its just that i got my 1st ship on a 20 degree orbit (it was a fuel depo, well just a normal ship but it was half full of fuel so used as a fuel demo), and i got another ship on a 30 degree orbit the other way (so the angle between these orbits was 50). I got these 2 ships together, but it took like over 1000 bloody dV, and i couldve probably gotten back to kerbin with that much fuel wasted.

Also, it isnt as big a deal when landing, as i can more easily still land where i want to with a odd orbit (u dont need equatorial to land near a base), but still, thatnks for all the tips, really makes life easier

[LethalDose]

If it's about returning from interplanetary, well, again: you don't have to travel in the equatorial plane all the time (it can be more efficient to do so, but it isn't compulsory).

Well, to be fair, it could be a problem. If you were in a polar orbit around Duna, and the plane of that orbit was perpendicular to the planet's vector when your return window was open, there's no way you could eject retrograde (or prograded, for that matter) to Duna's orbit. It could be a major problem if you had a tight dV budget for the return trip.

This is a pretty edge case, though.