How to plot a course for Duna ?

[Khazar]

Hi,

I can't plot a course for Duna.

I have used calculator to get proper angles and values for duna transfer. It looks like that :

I have got those values :

Phase anfle 117.10'

Ejection angle 18 to prograde

Ejection Dv 3261 ms

As I understand it Phase angle is angle between Kerbin and duna, and when I looka at the ingame map it loks like this angle is 117'

Ejection angle is angle between Kerbin prograde and point where I'm supposed to perform ejection burn.

Calculator says it's supposed to be 18', so if we assume that kerbin prograde would be 12 o clock, then I should burn at around 13, right ?

So I place maneuver node at 13 o clock and ad 3261 dv prograde burn, and that should do the trick. However no matter how much dv I put, I won't get encounter. I tried to put maneuver node in different places, I tried to add a lot more prograde dv (10k and more), tried to tweak normal/abnormal. No matter what I do, I won't get encounter. All I have is Kerbin escape marker, but no Duna encounter. What am I doing wrong ?

Edited April 22, 2015 by Khazar

[HoloYolo]

You have the worst phase angle ever. Duna should be at around 44 ish degrees for the phase angle. Also, plot a maneuver a few minutes after the sun goes behind Kerbin, like 3 to 7 minutes after.

[Starhawk]

In the initial image, you have moved the trajectory away from the optimal. The image shows required delta-v is over 7000 m/s!

Click the mouse in the centre of the dark blue area inside the Delta-V Transfer Plot. The numbers will change and you will have a transfer with reasonable values.

Hope this helps.

Happy landings!

p.s. When you first click the 'Plot it!' button, the crosshairs should be over the optimal transfer to begin with.

Edited April 20, 2015 by Starhawk

[Superfluous J]

You should be going for the dark blue part of the plot, if you want an efficient transfer. This is so inefficient I find it hard to think in these terms

18 degrees to prograde means that if Kerbin is at 3:00 in your diagram (you have it at like 4:00. I like to rotate the map so Kerbin is at 3:00 for some reason) then your ship will be 18 degrees counter-clockwise from noon, or at about 12:40 or so. Then you burn prograde (relative to your orbit around Kerbin) 3261 m/s and you should have a terribly inefficient transfer orbit to Duna.

Edited April 20, 2015 by 5thHorseman

[Khazar]

But are those calculations correct ? It's just small sattelite that I wanted to launch as fast as possible so I wasn't interested in efficiency that much. I have over 8k of dv and I would reather spent 3-4k for transfer than wait couple dozens of days. But I just cant have encounter no matter how much dv I would invest.

I have marked the kerbin prograde with black line labeled "a", and my craft's position with black lane labeled "b". You have said counterclockwise so I guess it's supposed to be the other way around. That would probbably be the case.

Edited April 20, 2015 by Khazar

[HoloYolo]

In the initial image, you have moved the trajectory away from the optimal. The image shows required delta-v is over 7000 m/s!

Click the mouse in the centre of the dark blue area inside the Delta-V Transfer Plot. The numbers will change and you will have a transfer with reasonable values.

Hope this helps.

Happy landings!

p.s. When you first click the 'Plot it!' button, the crosshairs should be over the optimal transfer to begin with.

He needs a better transfer window.

[Superfluous J]

I reread your post and yeah, you're about right for what you're thinking. Note, though, that these (did I mention terribly) inefficient transfers have extremely tiny windows of opportunity and nailing Duna from Kerbin in one burn will be tough. If you target Duna you should be able to fiddle to at least get the encounter markers close to each other. Get them as close as you can, do the burn, and then do a mid-course correction to actually nail Duna.

[HoloYolo]

I reread your post and yeah, you're about right for what you're thinking. Note, though, that these (did I mention terribly) inefficient transfers have extremely tiny windows of opportunity and nailing Duna from Kerbin in one burn will be tough. If you target Duna you should be able to fiddle to at least get the encounter markers close to each other. Get them as close as you can, do the burn, and then do a mid-course correction to actually nail Duna.

Why doesn't he burn into Kerbol orbit, then plot a maneuver to encounter Duna. Easier and more fuel efficient.

[Starhawk]

Ah, gotcha!

It's doable, but it takes a lot of tweaking. When I participated in the fastest small step challenge, I used that same planning tool and it worked. I did find that I had to tweak the trajectory quite a bit by fiddling around with the maneuver node. I seem to recall that I lost the encounter after SOI transition and had to do another maneuver to reestablish it.

Good luck, and happy landings!

[maltesh]

Why doesn't he burn into Kerbol orbit, then plot a maneuver to encounter Duna. Easier and more fuel efficient.

Easier than what he's doing? yes. More efficient than the plotted burn? Assuming he's trying for about the same flight time and the same departure date? No, it isn't.

[HoloYolo]

Easier than what he's doing? yes. More efficient than the plotted burn? Assuming he's trying for about the same flight time and the same departure date? No, it isn't.

I'm pretty sure his rocket (seeing as he's most likely a noob and has mastered asparagus staging) is dwarfing the Burj Khalifa right now.

[impyre]

If you're intent on doing it this way, my suggestion is this: Perform the planned burn at the correct time... but don't worry if you don't get an intercept. Only fiddle if you happen to notice an encounter "pop" up and then disappear... then you can just turn around 180 degrees and backtrack a bit at a time until you find it again. Adjust thrust limiters by right-clicking engine(s) to help reduce excess thrust for fine-tuning. If you cannot get an encounter during your ejection burn, that's fine. The reason we do ejection burns at low altitude is so that we get the most juice out of the fuel. That being said, burning 99% of it at low altitude, and 1% outside kerbin's SoI is almost the same thing (close enough that it won't make a big difference). So this means that you can perform course correction burns partway there (aim for about a third of the way up [distance between kerbin's and duna's orbits]... it's not exact, but very good). Once higher up, it may be easier to get an intercept... especially since you can once again play with maneuver nodes to get it precise. Setting duna as a target while setting nodes will also help you find an encounter, just keep an eye on the point of closest approach... you can mess with the node until you decrease the distance enough for an encounter. A final correction burn just before encountering duna can also help set up aerocapture... which can save a lot of fuel.

[Khazar]

​

The white line marks Kerbin prograde.

As I zoom in, the prograde line would be roughly where blue dot is :

So, would my ejection burn point would be somewhere near the red dot or green dot ?

Placing maneuvere node at red point with 3300 dv prograde gives me an orbit as seen at last picture.

I have Kerbin escape marker, but my new orbit doesn't change much no matter what how I tweak maneuver node. No closest approach point. No encounter.

As for dvs calculations I understand that ejection burn is burn I need to perform to be at proper course and insertion burn is burn I would have to du to stay in orbit after I reach capture node. Am I right ?

Setting maneuvere node at more vavourable phase is just cheapper or easier as well ?

[Superfluous J]

That last picture, where your orbit is exactly in line with Kerbin's orbit, is impossible with a 3300m/s prograde burn. Are you sure you're burning 3300 m/s, and not burning until your SPEED is 3300m/s?

You should have a resultant speed of about 5700m/s, assuming you're orbiting at 2400m/s when you burn 3300.

And yes, you should burn at about the placement of the red dot.

[Khazar]

I'm sure. the orbit seen at last picture is what I'd get after burning 3300 dv prograde at point marked with red dot at 3rd picture.

I'm starting to assume there might be something wrong with my game. I have installed some mods adding other planets, maybe they screw things up. I tried to use plugin that does same calculations in game and it showed some insane ammounts like 170.000 dv and higher. That would explain why maneveurs like 10k make very little difference.

I suspect that planet factory mod might be responsible...

[Khazar]

Thanks for help. It showed up that had installed some mods that screwed up Kerbon system and I couldn't get anywhere except for the moons.

Now I have removed every mod that had anything to do with planets/systems and everything is ok now. I had no trouble with finding course for Duna encounter.

I have problem however with ploting a course for Eve. It has smaller orbit than Kerbin, so my parking orbit around Kerbin should be retrograde. Then I have ejection angle at 160 to retrograde. Where would be that point ? Assuming that Kerbin prograde is at 12 o clock and retrograde at 6, should that point be around 5 o clock or 7 o clock ? Or maybe at 11 or 1 o clock ? I don't know how to count that angle.

[Superfluous J]

I have problem however with ploting a course for Eve. It has smaller orbit than Kerbin, so my parking orbit around Kerbin should be retrograde.

You don't need to orbit retrograde around Kerbin to reach Eve. Just burn on the day side in a similar way to how you burned on the night side to get to Duna.

[Khazar]

Ok, thanks for the info. Is there any reason to use retrograde parking orbit then ?

[TheXRuler]

Only if you are orbiting a moon and you wish to use the orbital velocity upon leaving it's SOI to change your orbit relative to the parent body, however that can also be achieved by leaving at a different time (when you are on the other side of the moon you will be going in the other direction. Also, it costs a few hundred dV more to get into a retrograde orbit if launching from the surface.

Then only difference (although I don't know if this has any impact) would be that you would be either slightly further up or down in the parent body's gravity well.

[Superfluous J]

Ok, thanks for the info. Is there any reason to use retrograde parking orbit then ?

Nope! Everything that can be done orbiting backwards can be done going forwards for cheaper (to launch) and more convenient (for that nearly guaranteed unexpectedly needed rendezvous you'll be doing for some random purpose).

[Khazar]

Ok, thanks again.

[jarmund]

Ok, thanks for the info. Is there any reason to use retrograde parking orbit then ?

The only times I've been in a retrograde orbit (-180 degrees inclination) is when I've ended up that way after arriving from a different body. I've never launched to one, and I can't see a need to.

The important part is that you always have to know prograde/retrograde in relation to what. If you're in a "normal" 0 inclination equatorial orbit around kerbin, burning prograde on the dayside is in effect burning retrograde relative to kerbol. So to travel inwards in the kerbol system, you should burn prograde between kerbin sunrise and mid-day, depending on your TWR. To travel outwards, you should burn prograde somewhere between sunset and midnight.

Edited April 22, 2015 by jarmund

[LethalDose]

Why doesn't he burn into Kerbol orbit, then plot a maneuver to encounter Duna. Easier and more fuel efficient.

You need to check your math. Burning an escape from LKO into interplanetary space, followed by a separate transfer burn from interplanetary space is never more efficient than burning a transfer directly from LKO.

[jarmund]

You need to check your math. Burning an escape from LKO into interplanetary space, followed by a separate transfer burn from interplanetary space is never more efficient than burning a transfer directly from LKO.

Especially if going for destinations with inclined orbits, such as moho.

Escape burn + plane change is a huge waste of fuel, rather than having the escape burn at the right angle.