inserting at a good inclination

[Steampunked]

Hey folks,

I have just started doing interplanetary missions. So far Duna is the only planet that Ive attempted to visit. However, every time I set up my maneuver nodes and tweak them until I get an SOI encounter, I find that once I am in Duna SOI and approaching PeA, once I complete my burn to drop in to orbit around the planet I find that 2 things have happened:

(1) I am on a very highly inclined orbit

(2) My orbital speed relative to Duna is very, very low. Like 120m/s and my altitude is ~20,000 km.

What am I doing wrong here? How can I insert in to Duna SOI such that I will not end up with a highly inclined orbit?

Thanks!

[Kryxal]

Probably the easiest thing to do is to make a correction burn, not too long after your transfer burn ... set it maybe 10 minutes ahead and see what happens on the other end (conic patch mode 0 helps a lot with this, so you can just tab over to the planet and see what you will end up with).

The second possibility is a burn at the AN/DN to put you on a good trajectory to intercept Duna, and you can also get the encounter closer at this point.

[cantab]

Make a correction on entering Duna's SOI, using a combination of normal and radial components to get your periapsis at the right latitude and altitude.

Duna is less massive than Kerbin, your orbital speeds will be lower. If you're in a high orbit you really won't be moving very quickly. 120 m/s at 20,000 km sounds reasonable, considering Ike does about 300 m/s at 3,200 km.

[KSK]

If I remember rightly, you can click on the target planet now and select 'view from here' or some such (it's been a while since I've gone interplanetary), which does the same as Conics mode 0. Steampunked - this lets you see your current trajectory relative to Duna, which is very helpful for doing correction burns and lining up your insertion. Getting Duna and your maneuver node both visible (and useable!) on the screen at the same time can be a bit fiddly though.

[Kasuha]

After interplanetary transfer, you're coming to the planet from certain direction, but the exact trajectory along which you enter its SOI can be tuned during the transfer and I highly recommend doing so. There are two good spots to issue such corrections, one is right after you leave your starting SOI and enter interplanetary space, the other is halfway during the transfer. If you use transfer with inclination change, perform this tuning as soon as possible after the change.

The tuning is usually best done using fine maneuvers or even RCS. Focus on your target planet, orient your ship some controlled way (e.g. prograde with north up), use short RCS translation burns (HNIJKL) and watch what is your trajectory doing near the target planet. What you need is periapsis as close to the planet as possible and in its equatorial plane. You also may want to make sure you'll be flying prograde around the planet.

Make sure you cross your target SOI using as low time warp as possible (preferably 5x) and if necessary issue another correction right after you enter the SOI. Then brake at the periapsis you have already positioned above equator but don't circularize - stop braking when you're in highly elliptic orbit (set up some reasonably high apoapsis so it doesn't take eternity to get around). If you did it right, the apoapsis should be in equatorial plane as well. Then coast to the apoapsis and perform inclination change at that point. Coast to periapsis again and now you can circularize already in equatorial orbit.

Another option is of course to put up with inclined orbits. They are not a problem in most cases.

[Geschosskopf]

Hey folks,

I have just started doing interplanetary missions. So far Duna is the only planet that Ive attempted to visit. However, every time I set up my maneuver nodes and tweak them until I get an SOI encounter, I find that once I am in Duna SOI and approaching PeA, once I complete my burn to drop in to orbit around the planet I find that 2 things have happened:

(1) I am on a very highly inclined orbit

(2) My orbital speed relative to Duna is very, very low. Like 120m/s and my altitude is ~20,000 km.

What am I doing wrong here? How can I insert in to Duna SOI such that I will not end up with a highly inclined orbit?

Thanks!

Arriving at Duna in at least a slightly inclined orbit relative to Duna's equator is almost inevitable because Duna's orbit is inclined relative to Kerbin's. Thus, unless it works out that your ship arrives just when Duna's sitting on the ascending or descending node relative to Kerbin's orbit, your ship will necessarily be moving somewhat upwards or downwards relative to Duna upon arrival. Furthermore, the axis of every single planet points straight up (IOW, all their equators are parallel) so Duna's orbit is inclined to its own equator. That means that if you match planes with Duna's orbit while en route, you will arrive at Duna with the same inclination as the planet's orbit. So, your pretty much stuck with at least some amount of inclination relativel to Duna' equator upon arrival and if you don't want to end up in an inclined orbit, all you can do is fix it once you get there.

However, it's a lot easier to fix if your Duna Pe is close to where you want it before you get there. IOW, you have a "good" encounter set up beforehand. Duna's SOI is pretty big for its size so there's plenty of room for "bad" encounters (retrograde, highly inclined, Pe outside Ike's orbit, etc.). "Good" encounters (prograde, Pe at the desired altitude, little inclination) are hard to just get from the get-go back at Kerbin. Usually you have to deliberately make them over the course of the trip by doing 1 or more minor correction burns along the way. Often folks do 1 correction about a week outside of Kerbin's SOI, often another about the midpoint of the trip. If done correctly, these will get you a "good" encounter if you don't have one to start with, and then you just need a final fine-tuning once in Duna's SOI.

To make these correction burns, it really helps to be in conic draw mode 0. This will show your Duna Pe right there next to where Duna currently is on the map. So you create a node somewhere along your ship's path, double click on Duna to focus in on it, then adjust the node to move your Duna Pe to where you want it, then do the burn. Because future paths tend to wobble around somewhat, and crossing SOI boundaries at >1 time warp will make them jump, don't expect perfection here. Accept something that close enough with the knowledge that it will probably not quite be that way when you actually get there, but shouldn't be too far off. Then, once you're inside Duna's SOI you can do a another tweak of your Pe.

All the above is concerned mostly with setting your Pe. Your Pe determines whether you're going prograde or retrograde, your closest approach altitude to Duna, and the general nature of your inclination (equatorial vs. polar). If you want to get into an exactly equatorial orbit, you normally can't do that until after you've captured. So capture into a pretty elliptical orbit, change your inclination at the node furthest from Duna, then bring your Ap down. If you want a polar orbit, it's the same: capture into a big ellipse that's close to the inclination you want, then tweak inclination.

NOTE: If you plan on using aerocapture, you'll want your Pe usually somewhere between 11-13km, depending on 1) how fast your ship's going when it enters Duna's SOI, and 2) the height of the Ap you want afterwards. The map view doesn't display the effects of aerobraking on your future trajectory so this is all trial and error, where the difference of a few hundred meters Pe altitude can have a large effect. I recommend setting your aerobraking Pe about even with Ike's orbit and quicksaving there. Then, if you don't get what you want afterwards, you can revert back, tweak the Pe up or down a little as desired, and try again.

[Mister Spock]

I always end up with crazy orbits at Duna too, so I'm glad the original poster asked.

Duna's SOI is pretty big for its size so there's plenty of room for "bad" encounters (retrograde, highly inclined, Pe outside Ike's orbit, etc.). "Good" encounters (prograde, Pe at the desired altitude, little inclination)

By prograde, you mean that you arrive a little behind Duna, so that you're already heading counter-clockwise around Duna? And retrograde means you arrive a little ahead of Duna, so that you're turning clockwise around Duna? And arriving retrograde means you have a costlier orbital-insertion burn?

Also, how important is it to aerobrake there? In the past, I've done what you said -- saved and reloaded, trial and error. In my current game, I'm playing with no reloads, so if I'm going to aerobrake, I'll probably send a couple unmanned probes ahead of a manned mission. (I treat manned missions with kid gloves.) But is it that costly to simply insert into a high orbit? More generally, is it more efficient to plan for a periapsis very close to the target planet, or very far?

[Tex_NL]

Arriving at Duna in at least a slightly inclined orbit relative to Duna's equator is almost inevitable because Duna's orbit is inclined relative to Kerbin's. ...

Yeah, inclined at a whole whopping 0.06 degrees. Any inclination that close to zero would be considered perfectly equatorial.

[comatosed]

I achieve this by setting up a correction burn at the ascending or descending node relative to the target planets orbit. This is usually a very low Delta V manouver.

Something that I rarley see people doing (so it might be a mod like precise node that allows it, but not sure) is once you have an SOI change focus view on the planet in question and you should be able to see the projected trajectory next to the planet. This is really handy because you can zoom right in and tweak it to your exact needs. You'll also need to position the camera so the manouver node is visable in the background.

[cantab]

A retrograde orbit isn't any harder to get into, but it makes landing on the primary (eg Duna) and orbiting any moons need more delta-V.

To make sure you don't enter such an orbit, set a capture burn at your periapsis then target a moon. If the AN/DN are near 0 that's prograde, if they're near 180 that's retrograde.

[Scarecrow88]

I find it easiest to tweak my arrival inclination immediately after entering the SOI of the body I am travelling too.

[Tex_NL]

I find it easiest to tweak my arrival inclination immediately after entering the SOI of the body I am travelling too.

That is pretty much the only way to correct your orbit.

Burn radial to bring your Pe up/down to the desired altitude. Then burn to the normal to place your Pe at the equator. And as a third and final step plan a manoeuvre at Pe to circularize and correct inclination simultaneously.

[Geschosskopf]

By prograde, you mean that you arrive a little behind Duna, so that you're already heading counter-clockwise around Duna? And retrograde means you arrive a little ahead of Duna, so that you're turning clockwise around Duna? And arriving retrograde means you have a costlier orbital-insertion burn?

Correct. In general, you usually want to capture into a prograde orbit as it makes life easier in many ways. But OTOH, the purpose of the mission is just to land on Duna and go home, or put a mapping satellite into polar orbit, it doesn't matter much if at all.

Also, how important is it to aerobrake there? In the past, I've done what you said -- saved and reloaded, trial and error. In my current game, I'm playing with no reloads, so if I'm going to aerobrake, I'll probably send a couple unmanned probes ahead of a manned mission. (I treat manned missions with kid gloves.) But is it that costly to simply insert into a high orbit? More generally, is it more efficient to plan for a periapsis very close to the target planet, or very far?

If you brought enough fuel to capture (and return home) without benefit of aerobraking, then aerobraking isn't important at all. But most folks rely on it so they don't have to lug so much fuel around, in which case it makes the difference between success and failure.

Aerobraking without trial and error is IMHO foolhardy because in real life nobody would plan such a maneuver without beaucoup calculations and simulations beforehand. Beacuse the game provides exactly zero opportunity for this and AFAIK nobody knows enough about how the wonky atmospheric things work to calculate the effects on paper, it's just silly to risk a big, complicated, time-consuming mission on just guessing what aerobraking Pe to use and be stuck with it. But if that's your thing, go with it. Don't say I didn't warn you, though .

There is no way to test this out beforehand with probes, either. This is because no 2 ships arrive at Duna with the exact same velocity. I've sent scads of mutli-ship flotillas to Duna (up to 14 ships at once), all doing their departure burns back at Kerbin within the course of the same 1-2 hours. And the ships of these flotillas arrive spread out over several days to several weeks depending on how accurately I do the burns because they're all going different speeds. Each one requires aerobraking at a different altitude to end up where it needs to be. There is no way to determine this beforehand.

[Geschosskopf]

I find it easiest to tweak my arrival inclination immediately after entering the SOI of the body I am travelling too.

Unless you matched planes with Duna en route, or arrived just when it was sitting on an ascending or descending node with Kerbin, you can't get a zero inclination until after you capture. This is because when you enter the SOI on your initial hyperbolic trajectory, there is by necessity only 1 ascending or descending node on your path, and it will be in the vicinity of your Pe. The best you can do prior to capture is to put both ends of your trajectory on the same side of Duna's equator instead of 1 above and 1 below, but there will still be an angle between the plane of your trajectory and Duna's equatorial plane.