Intercepting Inclined Planets

[Dizzle]

I was hoping someone could explain to me how to get an intercept with inclined planets. I've been to Moho and Dres once each by pure luck, but now I'm using maneuver nodes to set up an encounter with Moho and I just can't get close. My current method is using Kerbal Alarm Clock to tell me when the encounter is, but I don't think it takes inclination into account.

Any help is appreciated.

Edited February 1, 2014 by Dizzle

[Awaras]

I use the Kerbal Alarm Clock's 'Model' mode (as opposed to 'formula') for intercepts, since it is a lot more accurate for Moho in my experience.

First, I create a maneuver node that takes me from LKO to roughly the same altitude above kerbol as moho. Then I create a new maneuver node on the ascend/descent node of the projected path and reduce the inclination to 0/NaN. If I get an intercept, great. If I don't I fiddle with the first maneuver node until I do. Even then, I can never execute the maneuver node perfectly and need to make a correction burn later, but it is close enough...

[Bobe]

Does it make more sense to launch into a Kerbin orbit with an inclination that matches the plane of the planet you intend to transfer to, or to enter an equatorial orbit and change inclication en route at the ascending/descending node?

I imagine the former requires far less ÃŽâ€v, but will only work if you launch during the transfer window, rather than launching and then waiting for the window.

[relin]

Does it make more sense to launch into a Kerbin orbit with an inclination that matches the plane of the planet you intend to transfer to, or to enter an equatorial orbit and change inclication en route at the ascending/descending node?

I imagine the former requires far less ÃŽâ€v, but will only work if you launch during the transfer window, rather than launching and then waiting for the window.

Definitely the former: plane changes are very expensive. And since LKO has such a short period, you could easily get away with launching outside the transfer window and waiting for the alarm clock to go off. Even if you miss the node on your first orbit, you have ~30min to set up another one in pretty much the same spot and use it the second time.

[Geschosskopf]

Does it make more sense to launch into a Kerbin orbit with an inclination that matches the plane of the planet you intend to transfer to, or to enter an equatorial orbit and change inclication en route at the ascending/descending node?

This is easier said than done, however. Not only do you have to match the inclination itself, but also the longitude of the ascending node. (LAN) (NOTE: technically, it's the "right ascension of the ascending node" because this needs to be measured against the background stars, but I prefer to say LAN instead of RAAN). If you omit this latter part, getting into the same inclination could actually put you in for a bigger plane change later than if you'd launched into an equatorial orbit.

In case you don't grok LAN, it works like this. It's easier to visualize thinking about a long, skinny elliptical orbit but it works the same for circular orbits. Anyway, when viewed from above, this long, skinny orbit is pointing in some direction like the hand of a clock. Because it's inclined, it crosses the central body's equatorial plane at the ascending and descending nodes. The central body's equator is marked off in longitude, so the Longitude of the Ascending Node tells you where around the equator the ascending node is, which in turn tells you which direction the long, skinny orbit is pointing. If you don't know this direction, the orbit could be pointing in any direction, all its possible locations forming a funnel-shaped surface with the central body at the bottom. Every orbit that could be on this funnel has the same inclination but only those with the same LAN are in the same plane with each other.

So, if you want to really match planes with the target planet when you launch from the ground on Kerbin, you have to wait until Kerbin itself is at one of the 2 points along its orbit that are aligned with the line between the target's ascending and descending nodes. One these 2 days of the year, you can take off from KSC heading NE (if the target's ascending node is closer to Kerbin) or SE (if the target's descending node is closer to Kerbin) and end up in an orbit around Kerbin that's in the same plane as the target planet. Then you wait in this orbit until the "launch window" (actually, the transfer window) to the target comes along.

Note that physically launching from the ground into a plane-matched parking orbit and the "launch window" for transfer from that orbit to the target are entirely separate things, based on entirely different parameters. The first is based on Kerbin's position relative to the target's orbit as a whole. The 2nd is based on the relative positions of Kerbin and the target in their paths around Kerbol. These 2 circumstances rarely, if ever, happen on the same day. So the drill is, launch into the parking orbit first on the day Kerbin aligns with the target's nodes, then leave when the next transfer window comes along, whenever that is. If you don't follow this 2-step procedure, then you'll have to do some sort of plane-change burn en route or during the transfer burn, which sort of defeats the whole purpose of launching into a specific inclination to begin with.

But also note that the rotational axes all planets in KSP point straight N-S, even those in inclined orbits. So, suppose you follow the above procedure and make your trip to the target all within its plane, so you don't need any plane-changing burns along the way. When you get to the target, you'll be in an inclined orbit relative to the target's equator. So, if you're interested in achieving an equatorial or polar orbit at the target, you'll have to do a plane-change anyway upon arrival. But this is only 1 plane change, compared to 2 if you launched equatorial at Kerbin and made a plane change somewhere en route, and it's usually less expensive than an en route change.

[Dave Kerbin]

There is already a lot of good advice here so I won't repeat it. But I do want to suggest that if you are learning how to reach an inclined planet that you practice on Dres rather then Moho. If Eve is the most advanced planet to return to orbit from its surface, Moho is the most advanced to enter orbit from Kerbin. Dres will let you learn about inclined transfers without subjecting you to massive delta-v costs if you don't pull off a perfect approach.

[alexmun]

The only way to match inclinations upon ejection is to leave Kerbin's sphere of influence at one of the nodes (either ascending or descending). Your ejection trajectory will actually need to have a greater inclination than your target orbit's inclination because when you leave Kerbin's sphere of influence your net solar orbital velocity is the sum of Kerbin's orbital velocity (which has no inclination) and your velocity relative to Kerbin. But the fact that you have to leave the sphere of influence at the node makes matching the inclination directly from Kerbin orbit usually infeasible because it is rare for a transfer window to line up with a node.

The good news is you don't actually have to match inclinations at all in order to transfer. All you need to do is get a node (either ascending or descending) of your transfer orbit to intersect your target's orbit at the right time. This method can save a ton of delta-v relative to trying to exactly match the target's inclination, but it can be more difficult to find an intercept.

[Tank Buddy]

I use http://ksp.olex.biz/ to give me the launch windows, then I use a good ol' protractor to draw the angle on a sheet of lined paper (thin). Kerbol is at the vertex, and I set the focus in the map view to Kerbol, and place the vertex of the angle on Kerbol. I time warp to line up the planets to their respective positions.

For Moho you have 2 options:

A. Mid course plane change- If you used my method to to launch windows, you should just launch a normal Hohmann transfer, with your Pe on Moho's orbit. Then just perform the inclination change at the ascending/descending node. To be safe you could use maneuver nodes to plan your burns. You can place a second one on the Ascending/Descending node.

B. Launch so Kerbin is at Moho's Ascending/Descending node- Your solar Pe will be at one of the nodes, and so will the intercept (hopefully). You won't have to do an inclination change, which is helpful for Moho, because, you know...

Also, Duna and Eve are much easier targets for beginners going interplanetary, + they have an atmosphere.

[fireflower]

Basic interplanetary travel. This part you should know:

1) From LKO, create a maneuver node of @ 1 km/s ÃŽâ€v, at the planet's terminator.

2) Burn, warp to SOI transition, and target your destination planet.

3) From heliocentric orbit, create a maneuver node equal to the ÃŽâ€v for a Hohmann trajectory to your target.

4) Move that maneuver node around until it vaguely intersects with your target at some point, and burn.

This is the part you don't seem to know:

5) Find the next AN/DN with your target along your Hohmann trajectory and burn normal/antinormal until inclination is 0.0° exactly.

The rest follows for completeness:

6) Warp to SOI and burn radial in/radial out until your periapsis is above the surface.

7) If target has an atmosphere, consult data on aerocapture. If not, set up a retrograde burn at periapsis for orbital insertion.

Edited February 2, 2014 by fireflower
clarity

[ComradeGoat]

I've started doing Moho by a slightly longer route, but one which avoids the need for a plane change, and doesn't involve trying to match orbital inclinations at launch. On two days a year, as said above, Kerbin crosses the plane of Moho's orbit. Launch on one of these days and burn for escape velocity with an ejection angle for Moho. Lower your perihelion until it touches Moho's orbit, then wait.

Once you reach perihelion, select Moho as your target and burn retrograde to lower your aphelion until you get an encounter.

With this encounter, you'll find yourself in an inclined Moho orbit, but fix that by stopping your capture burn the moment you get an orbit. As long as your periapsis isn't too near the poles, you should be able to fix your inclination near apoapsis. If not, consider this a good time to go explore the Moholes!