Can't get Duna encounter

[hhatch]

I cannot get an encounter with Duna after days of trying. I've tried everything without success. This photo shows the closest I've been so far. I've tried making all kinds of adjustments but nothing works. I am lost.

[Cpt Kerbalkrunch]

You're right on it, so try not to be too frustrated. It takes a while to learn the little fine-tune adjustments that need to be made. For now (until you get the hang of it) just make your burn as-is. Then, set up a another maneuver node afterward and make the small adjustments you need to get your encounter. This will be cheapest while still at Kerbin, but you can do this from solar orbit as well. It looks a lot better than my first Duna encounter. 

[Foxster]

Just a thought...You do have the same inclination, right?

[Gordon Fecyk]

Looks like you're pretty damned close.

If you don't mind flying out of Kerbin's sphere of influence before placing another manoeuvre node, try putting that right where you will have Sun Periapsis. It looks like Sun PE is really close to the descending node as well, so you can do orbital plane adjustments at the same time and save fuel.

Unless you're playing in Hard mode or something, you can always save the game before flying out there.

You can also add one or more nodes ahead of your current node to project course corrections. Try placing a second node out at Sun PE once you have the first node as close as you can get it.

Edited January 10, 2018 by Gordon Fecyk
grammar and ninja'd by Cap'n

[Kryxal]

From the looks of that, adding a bit of delta-v to your maneuver node or making the burn a tiny bit earlier (and therefore angled more solar-radial-out) will likely get you a better encounter.  Something like PreciseNode would be perfect for making these changes.

[Rocket In My Pocket]

Fine tuning a maneuver node with the stock system can be tricky.

I really like using:

Makes it so much easier to make the maneuver node do what you want it do to. It's just more pleasant from a quality of life standpoint and/or usability as well.

Edited January 10, 2018 by Rocket In My Pocket

[hhatch]

33 minutes ago, Foxster said:

Just a thought...You do have the same inclination, right?

This picture was prior to the burn. I was going to adjust inclination with another burn if needed. Should I adjust inclination before the burn ?

[Rocket In My Pocket]

1 minute ago, hhatch said:

This picture was prior to the burn. I was going to adjust inclination with another burn if needed. Should I adjust inclination before the burn ?

Afaik it's cheaper to change inclination "higher up" or "further" from the body you are orbiting.

So it's probably best to do it after?

[Cpt Kerbalkrunch]

40 minutes ago, hhatch said:

This picture was prior to the burn. I was going to adjust inclination with another burn if needed. Should I adjust inclination before the burn ?

It'll be a bit easier afterward, because you'll be able to see exactly what you need to fine-tune your encounter. But as always, the further away, the cheaper.

Also, you're hitting Duna asst exactly at the node. You shouldn't really need to adjust inclination for the encounter itself. The prograde/retrograde and radial in/put dials should do the trick. You can then make another maneuver node after you have an encounter (while focused in on Duna), to fine-tune your encounter even further. So that you can make your most efficient capture possible, and leave yourself in a good orbit.

And a quick question; are you intending to aerocapture?

Edited January 10, 2018 by Cpt Kerbalkrunch

[Zhetaan]

52 minutes ago, hhatch said:

This picture was prior to the burn. I was going to adjust inclination with another burn if needed. Should I adjust inclination before the burn ?

The best place to adjust inclination changes depending on when you set up your transfer window.  This is because the cheapest transfer window from Kerbin to Duna depends on the phase angle (this is the angle made between planets and the sun if you're looking at the face of Kerbin's orbital plane), but phase angle doesn't care much whether the orbital planes match.  The standard approach is to burn for transfer first, then correct for inclination somewhere on the way.  This is because, when the planes of two orbits intersect,  the line of intersection defines two nodes on those orbits (shown by the right-pointing green marker next to the dotted line--it's close to the Pe marker), and since a Hohmann transfer is one-half of an orbit, you are guaranteed to pass a node somewhere on the trip.  Save your inclination burn for then and you'll get the cheapest one possible.  Duna is nearly the same inclination as Kerbin; whatever the correction burn costs, it will be low.

Unless your node and transfer window occur in the same place and you can do both in one burn, it rarely makes sense to adjust inclination before you burn for transfer (Moho is the notable exception; it's extremely inclined and there are a couple of other factors that help).  This is because correcting for inclination requires you to do that when Kerbin is at a node; you have to then make a second burn for a transfer window.  While a Hohmann transfer is guaranteed to pass a node where you can correct your inclination within one-half of an orbit, a correction for inclination does not guarantee a similarly timely transfer window.  It might be years away, so it is simpler in terms of mission planning to do the transfer when it is available and correct inclination while on the way.

In summary, your current approach is correct.

To get an encounter, you may be falling victim to the coarseness of the manoeuvre controls.  At the start of the transfer, you are as far away from Duna as you can be, so any tiny change will result in massive changes in your arrival.  Vessels that leave minutes apart from Kerbin can arrive a week or more apart at Duna.  The solution to this is to wait until you're about halfway to Duna to fix your approach and correct the encounter.  You spend slightly more in fuel but you have much finer control because 1 m/s there doesn't push your trajectory from one side of the sphere of influence to the other.

I will note that the closest approach markers don't align and that your transfer is more than a half-orbit; that means you're a little late in your transfer, so getting a direct encounter from Kerbin without a correction burn may not be possible.  You're close, naturally, but it will cost a bit more in fuel.  Also, since reverse time warp isn't a thing in KSP, you'll have to either make do or wait another two and a half years for another window.

You will definitely need to add a correction burn later in the trip.

I hope that helps.

[hhatch]

2 hours ago, Cpt Kerbalkrunch said:

It'll be a bit easier afterward, because you'll be able to see exactly what you need to fine-tune your encounter. But as always, the further away, the cheaper.

Also, you're hitting Duna asst exactly at the node. You shouldn't really need to adjust inclination for the encounter itself. The prograde/retrograde and radial in/put dials should do the trick. You can then make another maneuver node after you have an encounter (while focused in on Duna), to fine-tune your encounter even further. So that you can make your most efficient capture possible, and leave yourself in a good orbit.

And a quick question; are you intending to aerocapture?

Not sure on the aerocapture. I thought I would try it to save fuel. I need more data on the density and thickness of the atmosphere I suppose.

[Cpt Kerbalkrunch]

28 minutes ago, hhatch said:

Not sure on the aerocapture. I thought I would try it to save fuel. I need more data on the density and thickness of the atmosphere I suppose.

It basically has no density or thickness. The air is so thin, you can set your Pe to 20km and capture easily without a shield. You'll lose the ability to choose a landing site, though. If you flatten out your orbit so that it's equatorial, you'll at least land on the equator. As long as you're not planning to drill, it won't much matter where you come down. I usually like to bring a couple of drogue chutes along for the landing. They definitely help.

[Frosty Jack]

1 hour ago, hhatch said:

Not sure on the aerocapture. I thought I would try it to save fuel. I need more data on the density and thickness of the atmosphere I suppose.

I've just been sending surface modules to Duna today and one ship had no problem going down to 25km with 1x6 solar panels extended. Got a bit toasty, but they survived. Aero braking saved me 300m/s ish of a 400m/s DV to lower the orbit, so well worth doing without too much risk

[Spricigo]

50 minutes ago, Cpt Kerbalkrunch said:

It basically has no density or thickness. The air is so thin, you can set your Pe to 20km and capture easily without a shield..

The major problem I see its that Duna's terrain heigh varies  a lot more than Kerbin's. So the same Periapsis may result either in  a nice landing or a crash depending on if there is a valley or a hill when you get there.

Also, because of the thin atmosphere, you need to be a lot closer to the ground to it actually work efficiently in the first place, which further limite the 'security margin' you may have.

 

In any case is still quite doable, just save it so you can load and adjust if thinks don't go as expected in the first try.

 

 

[Cpt Kerbalkrunch]

26 minutes ago, Spricigo said:

In any case is still quite doable, just save it so you can load and adjust if thinks don't go as expected in the first try.

 

 

Absolutely. I neglected to mention (since it's such a habit), you should always visit everyone's good friend Alt-F5 before any such maneuver. (Or any maneuver at all, for that matter).

[Snark]

4 hours ago, hhatch said:

Not sure on the aerocapture. I thought I would try it to save fuel. I need more data on the density and thickness of the atmosphere I suppose.

You should absolutely aerocapture, if you can.  Duna is extremely friendly for it:

• Orbital radius relatively close to Kerbin's = your Duna-relative speed is reasonably low on arrival, assuming you had a good transfer window.
• Not a very big planet = low escape velocity = also helps keep your speed fairly low.
• Atmospheric heating generally isn't too bad.  Can probably get by even without a heat shield.  With a heat shield, it's so easy it's trivial.

As @Cpt Kerbalkrunch suggests, drogue chutes come in really handy for a Duna landing, since the air's so thin.  My typical Duna lander lands with a couple of drogue chutes, one regular chute, and a brief burst of thrust from the engines right at touchdown to cushion the landing.  (That's because the air's so thing that one measly parachute isn't even vaguely enough to slow enough for landing... and a smidgeon of braking fuel right at touchdown is generally more lightweight than carrying enough chutes to actually land on chutes alone.  But it's also totally possible to land with just chutes, if you spam them.  If you do plan on landing with chutes alone, though, it's worth trying to land at a low elevation-- the Duna atmosphere's density varies a lot between the highlands and lowlands.)

2 hours ago, Spricigo said:

The major problem I see its that Duna's terrain heigh varies  a lot more than Kerbin's. So the same Periapsis may result either in  a nice landing or a crash depending on if there is a valley or a hill when you get there.

My experience has been that this isn't an issue, at least for aerobraking.  Unless the ship has a super high ballistic coefficient, a Pe for braking to orbit will generally be higher than 11-12 km (more commonly up around 16-18 km), which is easily higher than the highest terrain on Duna.

Duna terrain does matter quite a bit for landing, if you're planning to do it with chutes alone.  But not for aerocapture.

(The variability of atmospheric density with terrain height is another reason why I like to use the landing strategy of "minimal parachutes, and a brief burst of engine thrust right at touchdown."  Because I can adjust the engine thrust as needed, regardless of whether I'm landing in highlands or lowlands.  A chutes-alone landing really needs to care about elevation of the landing site.)

8 hours ago, hhatch said:

I've tried making all kinds of adjustments but nothing works. I am lost.

Your screenshot shows you as being pretty close to an encounter.  Here's my suggestion for the easy way to fine-tune your trajectory, given the screenshot you've shown:

1. Keep the burn you've got right now, it looks pretty darn good.
2. Right-click on the "closest approach" marker, to "pin" it (so it will show you "how much distance at closest approach" even when you're not moused over it).
3. Drop a maneuver node right at the DN, as shown in your screenshot.
4. Now, carefully and gently pull on the maneuver node's and handles.  One of them will make the distance get bigger; one will make it get smaller.  Pick the right one and adjust it until you've made the distance as small as you can get.
5. Do the same thing with the and handles, again until you've made the distance as small as you can get.
6. Do the same thing with the and handles, again until you've made the distance as small as you can get.
7. Go back to step 4 again.  Repeat the cycle until the distance gets small enough that you've got an encounter.

It's quick, it's simple, and it won't be a very big burn.

Incidentally:  At some point during your "adjust / / " cycle, you'll get your approach close enough that you'll actually hit Duna's SoI and have an encounter.  You can continue to fine-tune there, if you want.  The thing to do at that point:  Click on Duna and choose "Focus View", and right-click on the Pe marker so it'll be "pinned", like you did before.  Now continue the same adjustment cycle, and you can get your Duna pass to go exactly where you want it to go, with your Pe at the preferred altitude.

[Vanamonde]

I suspect that you've plotted an optimal transfer path, but when you get there the planet's a little ahead of you because you are departing slightly too late. Remember, it's not just the path but the timing that are both required to get a hit. However, as others have pointed out, you are close enough that a mid-course correction will still get you to the planet. Fly the burn you have plotted, then once it's done, place a maneuver node at the half-way point and tweak it until you get an encounter. 

[Spricigo]

16 hours ago, Snark said:

My experience has been that this isn't an issue, at least for aerobraking.  Unless the ship has a super high ballistic coefficient, a Pe for braking to orbit will generally be higher than 11-12 km (more commonly up around 16-18 km), which is easily higher than the highest terrain on Duna.

Duna terrain does matter quite a bit for landing, if you're planning to do it with chutes alone.  But not for aerocapture.

(The variability of atmospheric density with terrain height is another reason why I like to use the landing strategy of "minimal parachutes, and a brief burst of engine thrust right at touchdown."  Because I can adjust the engine thrust as needed, regardless of whether I'm landing in highlands or lowlands.  A chutes-alone landing really needs to care about elevation of the landing site.)

Well, maybe my fault to assume aerocapture&landing. in any case bad choice of words,  major problem make it sound like a big deal when what I intended to say is that may not go exactly as planned. Lets say you still capture around Duna, but the orbital heigh is far off where you wanted it; or what was supposed to be a landing turn out to be an aerocapture in low orbit. Not a catastrophic failure but still a flaw in the mission. 

[Kryxal]

When you're looking to adjust a maneuver node while looking at a planet, it's probably easier to TAB through the various focus targets till you're looking where you want ... clicking stuff tends to close that node you're working with.

[peteletroll]

The easiest way to fix an encounter is to have some RCS on your ship.

[Rocket In My Pocket]

11 minutes ago, peteletroll said:

The easiest way to fix an encounter is to have some RCS on your ship.

Yes, RCS works very well for fine tuning maneuvers if you brought some!

As an alternative, you can also right click your engine and limit the thrust to 1%.

[Cpt Kerbalkrunch]

39 minutes ago, Rocket In My Pocket said:

Yes, RCS works very well for fine tuning maneuvers if you brought some!

As an alternative, you can also right click your engine and limit the thrust to 1%.

Absolutely. If you're not gonna do any docking, best to leave the monopropellent at home and save the weight. I'd love to see engine thrust limiting set to an action group. You just decide in the VAB what number you wanna set it to, then hit the appropriate button in flight. This would really help out when you have multiple engines. Trying to set them all to the same number can get annoying. Then remembering to put them back to 100% before your capture burn. 

[Geonovast]

5 minutes ago, Cpt Kerbalkrunch said:

Absolutely. If you're not gonna do any docking, best to leave the monopropellent at home and save the weight. I'd love to see engine thrust limiting set to an action group. You just decide in the VAB what number you wanna set it to, then hit the appropriate button in flight. This would really help out when you have multiple engines. Trying to set them all to the same number can get annoying. Then remembering to put them back to 100% before your capture burn. 

You could also slap on a few of the monodirectional RCS thrusters that don't use MP and only set them for prograde/retrograde use.

[peteletroll]

RCS is very useful because it allows to move in different directions without reorienting the ship.

Edited January 12, 2018 by peteletroll
Spelling

[hhatch]

After all this I don't have enough fuel. I need to go back and do some redesign of the ship then try again. I will try out some of these tips and hopefully will make it to Duna and back. Thanks everyone. This is a tough game.