Trying to use the "Interplanetary Calculator".

[strider3]

I am trying to use the Interplanetary Calculator to set up my burn from Kerbin orbit to Duna. For some reason I cannot seem to get an intercept even though I am at the coordinates required by the calculator. As you can see in the images, I am almost exactly at the required place and situation but I cannot get any closer than over 2 million kilometers?

 

Any ideas on why this is not getting me into a Duna intercept?

Thanks gang, as always.

Vic the Newbal

[Chiron0224]

I'm not sure.  I want to say that application is out of date but I could be wrong about that.  There is a transfer window planning mod though.  Let me find it and link it for you.

 

 

Edited September 27, 2016 by Chiron0224

[strider3]

That was my guess too...but I'm not sure. I will check out the Transfer Window Planner.

Thanks Chiron0224!!

[Andiron]

8 hours ago, Victor3 said:

That was my guess too...but I'm not sure. I will check out the Transfer Window Planner.

It's definitely what you need. In the map view, once you have found the perfect transfer burn, the mod have a button to plot the direction of that burn

[DD_bwest]

it can be pretty hard to get the timing of those burns right, since your ejection angle will vary depending on how close you are to the right phase angle

one thing that i do,  is play with the rad in and rad out.  itll change your ejection angle, which youll notice will move your closest approach closer or further, depending on if you arrive ahead or behind duna.   even if it takes alot, just finding the angle to leave kerbin at will help.   once youve done that, remember what it looked like and delete the node.   create a new node, with only prograde, and try to slide the timing of it so that your leaving at roughly the same angle as the node you deleted.

if you have trouble with that, you can also try to just budget an extra couple hundred m/s dv, and deal with a little bit of radial in your burn.  the distance in the picture to make up really wouldnt be that expensive.

 

edit: youll also notice, as per your picture, with that node you arrive behind duna, if that burn was done a few days later you would get a closer approach, thats another adjustment you could make.

Edited September 28, 2016 by DD_bwest

[Chiron0224]

6 hours ago, DD_bwest said:

it can be pretty hard to get the timing of those burns right, since your ejection angle will vary depending on how close you are to the right phase angle

one thing that i do,  is play with the rad in and rad out.  itll change your ejection angle, which youll notice will move your closest approach closer or further, depending on if you arrive ahead or behind duna.   even if it takes alot, just finding the angle to leave kerbin at will help.   once youve done that, remember what it looked like and delete the node.   create a new node, with only prograde, and try to slide the timing of it so that your leaving at roughly the same angle as the node you deleted.

if you have trouble with that, you can also try to just budget an extra couple hundred m/s dv, and deal with a little bit of radial in your burn.  the distance in the picture to make up really wouldnt be that expensive.

 

edit: youll also notice, as per your picture, with that node you arrive behind duna, if that burn was done a few days later you would get a closer approach, thats another adjustment you could make.

Agreed.  I just did my first successful Duna and back mission and I just planned a bit of wiggle room into my dV and I did the radial adjustment to get an encounter.  I also did another adjustment burn with a node at the ascending node so that I would wind up in an equatorial orbit of Duna when I arrived (thanks to Starhawk for the recommendation on that one).

[strider3]

HMmmm, I tried the Transfer Window Planner...with the same basic result. The numbers given by it changed a bit...but I still don't get "captured" by Duna? This is my first attempt at leaving the Kerbin/Minmus/Mun area. I guess I'll have to bring extra fuel and correct on the way? It's a tad bit disappointing that neither of these calculators are working for me...although I might not be doing something correctly. It seems pretty straightforward, however.

[Vanamonde]

There are a few things going on there, Victor3. Duna's orbit is not actually circular, so the precise transit will vary slightly depending on exactly when you are launching. The calculator doesn't take that into account. It's also really hard to place your outgoing maneuver marker on exactly the right spot, and to tug the little adjuster arms precisely. But if nothing you do is creating an intersect, then as others have said, you are probably missing the precise window by a few days in time. The good news is, none of these things will actually prevent you from getting to Duna. I plan on having to do a mid-course correction when I'm travelling between planets, if not to get an intercept, then at least to place my arrival exactly where I want it to be for aerobraking, or whatever I'm planning to do when I get there. There's no shame in it. Real spaceships do it, too.

[strider3]

I did notice, just now, that the planner mentions an "insertion DeltaV" and "Insertion inclination"...which seems to indicate that I'm expected to do a second burn to insert into Duna orbit?

Edited September 28, 2016 by strider3

[Superfluous J]

I used to try to nail these things exactly the way the calculator said and experienced the same feelings you are having right now. Then I realized that the important thing is NOT nailing the exact burn, but nailing the ENCOUNTER. If the calculator's wrong (and people above have given good reasons for it being wrong) then don't follow it. You've got a very close encounter there. Tweak your Kerbin ejection burn prograde, retrograde, radial, anti-radial, and time until your approach is as close as you can get it.

If you don't mind a mod suggestion, I strongly recommend PreciseNode or PreciseManeuver, both of which do the same thing in slightly different ways: They let you tweak your burn to the centimeter per second, and your burn time to the fraction of a second. At the very least, though, you should use the in-game +/- orbit buttons (right click the maneuver node to switch modes) to see if going later will get you even closer. Frequently, it will.

[DD_bwest]

18 minutes ago, Victor3 said:

HMmmm, I tried the Transfer Window Planner...with the same basic result. The numbers given by it changed a bit...but I still don't get "captured" by Duna? This is my first attempt at leaving the Kerbin/Minmus/Mun area. I guess I'll have to bring extra fuel and correct on the way? It's a tad bit disappointing that neither of these calculators are working for me...although I might not be doing something correctly. It seems pretty straightforward, however.

most likely your ejection angle is off, changing when you burn around kerbin will change that.  try sliding your node forward and back ever so slightly and youll see how it changes when you zoom bacl out ti look at the whole system. and youll need your closest approach to be less then 50,000 km for the encounter

you can also just correct it by adding radial in or out to your burn.next time you try making a node, pull on the blues and see which one brings it closer and which one further.  you can adjust to get a capture and burn,  or look at it and use the angle as a guide for a less expensive burn.

and always plan an extra burn to correct your approach.  many things can move it around, especially going to warp. and burning so you  complete your node perfectly is very hard.  if you plan an adjustment about half way there you can assure that your encounter will be closer to it.

 

maybe ill try to post pictures when i get home later to explain better

[Vanamonde]

If the destination does not have an atmosphere, you will have to make a braking burn to "insert" yourself into orbit. If it does have an atmosphere, you can "aerobrake" to shed some or all of the speed required to end up in orbit. The degree of braking, though is dependent on the thickness of the atmosphere and how low you pass through it. What is a good periapsis for aerobraking? That can be tricky. There's a mod which helps, I think, but I usually trial-and-error until I get it right. (I don't know what the values are currently since we have some new heat shield parts and drag has been adjusted recently.) At any rate, aerobraking may reduce your apoapsis to within the target's SOI, but the periapsis will remain within the atmosphere, causing more braking every pass, unless you do another burn to raise it and make your orbit stable. 

[Superfluous J]

9 minutes ago, Victor3 said:

I did notice, just now, that the planner mentions an "insertion DeltaV" and "Insertion inclination"...which seems to indicate that I'm expected to do a second burn to insert into Duna orbit?

Yes this means you're expected to do a mid-course correction, which you should likely just ignore and do yourself at your An or Dn around Sun, on the way to Duna. Right there, you want to do a normal burn to get exactly in Duna's plane, and then prograde/retrograde/radial to get right in. And don't sweat the fuel it'll take. Usually it's just drops, if you got a halfway decent ejection burn.

[DD_bwest]

37 minutes ago, Chiron0224 said:

Agreed.  I just did my first successful Duna and back mission and I just planned a bit of wiggle room into my dV and I did the radial adjustment to get an encounter.  I also did another adjustment burn with a node at the ascending node so that I would wind up in an equatorial orbit of Duna when I arrived (thanks to Starhawk for the recommendation on that one).

there are alot of little tricks youll pick up over time.  i always make an adjustment part way to fix my trajectory when i get to the encounter.

if the burn in kerbol orbit is expensive, which it can often be, dont circulize on first approach. just get yourself captured and plan a node at the far edge of the SOI where itll be cheaper.   this also works good for matching an inclination of a moon or ship to rendezvous

[strider3]

My Son just came by and did almost everything you guys mentioned. The first words were "forget the calculator as the final word" and proceeded to move my wonderfully placed node until I got a capture. It ain't pretty and he told me to do a correction burn on the way as it is MUCH easier to grab and move the node while away from other bodies. So, as always, it comes back to the "pilot" to get what they want done.

I should also note that I didn't install the LWP mod, as yet (trying to keep Mods to a minimum) but went to the web app.

5thHorseman: I never mind a good Mod suggestion and, after dealing with the imprecise maneuver node controls, I will definitely look into the 2 mods you mentioned.

As always, thank you all!! I'll let you know how this first trip to Duna goes.

[DD_bwest]

gj. im sure youll get the hang of it in time.  i still remember when i had mj do everything for me lol

[Plusck]

A bit late to the party, but there is one very simple thing to bear in mind when trying to get an intercept:

The most efficient transfer in the "ideal" transfer window is always going to be directly opposite the sun from where you are now. Looking at the pic in your OP, you're obviously meeting Duna much earlier than "opposite", which means that your ejection burn has to be excessive and/or not quite at the right angle.

 

Since you're heading away from the sun, you'll be travelling slower than the target when you get to its orbit. Since your screenshot shows that Duna is ahead of you at "closest approach", you can only meet it at that point by going faster, which means a bigger ejection burn.

However, your angular velocity while down by Kerbin is faster than Duna's. Therefore, counter-intuitively, you need to spend more time travelling near Kerbin's orbit before heading out to Duna. The simple solution is to eject at a slight angle towards the sun (i.e. you move the burn node a couple of degrees further along your orbit). You'll gain on Duna while heading towards a solar Pe which is slightly lower than Kerbin, then swing up to a solar Ap which is almost directly opposite your position now as Duna catches up with you again.

 

I'm pretty sure that the failure to get an encounter was due to two things. First and foremost what @Vanamonde said: Duna's orbit is not perfectly circular, and the interplanetary calculator doesn't take this into account at all (otherwise you'd have to specify a date), while Alex Moon's porkchop planner does. From the screenshot, it looks like Duna is about to reach its solar Pe, so it's going faster than average, which tallies with the fact that your calculated burn ends up with you lagging behind it.
Secondly - there is a normal component to Alex Moon's projected burn: the effect of the normal component will be to increase your total ejection burn without causing a correlative increase in your ejection velocity. Therefore you eject on a more curved trajectory, meaning that your ejection vector will be angled more towards the sun than if you just burned prograde for the same total dv, meaning that you end up spending more time zipping along ahead of Kerbin and, therefore, catching up (angularly) on Duna... if you see what I mean.

[strider3]

Made it to Duna! Now it looks like it will be a long time before I can get back? The angles are all wrong according to the calculator and I'm going to have to do a lot of fooling around to find a burn that will get me close to Kerbin...or wait the phase angle out.

5thHoreman...I never seem to have any luck searching for mods at Curseforge? Neither PreciseNode or Precisemaneuver shows up when searching...I'll try Google. 

[Zhetaan]

@Victor3:

Try the forum search tool, instead:

Precise Node

Precise Maneuver

There are quite a few mods that are not posted to Curseforge; many post to SpaceDock or Github exclusively.  The forum is your best place to look for a mod, either because it's here or because someone who uses it is here.

Also, and this is even later to the party, but your son is right:  calculators and planners are great, but at the end of the day, you're the one who has to fly the ship. 

[strider3]

Thanks Zhetaan...that did the trick.

One more question about "calculators". Can I ignore their results completely and just try and burn back to Kerbin? If I wait for the phase angle to be close...I'll be orbiting Duna forever LOL. I'm going to just try it as soon as I get Precise Maneuver or Precise Node working. I just can't deal with the squirrely stock maneuver node anymore.

[Kelderek]

4 hours ago, Victor3 said:

Made it to Duna! Now it looks like it will be a long time before I can get back? The angles are all wrong according to the calculator and I'm going to have to do a lot of fooling around to find a burn that will get me close to Kerbin...or wait the phase angle out.

...

One more question about "calculators". Can I ignore their results completely and just try and burn back to Kerbin? If I wait for the phase angle to be close...I'll be orbiting Duna forever LOL.

If you fly to Duna from a proper launch window, by the time you get there the two planets have moved and you are no longer at the right phase angle to return.  Chances are that Kerbin is now ahead of Duna, but you need Kerbin to be behind Duna for a good window.  When moving to a higher orbit, like from Kerbin to Duna, the target needs to be ahead of you  -- the launch window planners and mods will show you this.  When moving to a lower orbit, like from Duna back to Kerbin, the target needs to be behind you when you start.  So generally you need to plan to linger on Duna to wait for a good return window -- I would recommend doing this before actually lifting off the surface as you can access full time warp while landed.

If you don't want to wait for a good return window then prepare to need a much higher DV amount for the burn to get an encounter.  I flew from Kerbin to Duna with only 9 days of travel time once for a challenge, but it was a crazy amount of DV for the burn and I was going too fast to do anything more than zoom past the planet or crash into it.  Nearly anything is possible, but most people prefer to design around efficient launch windows to reduce the DV requirements.

Your ejection burn will always be aimed with a prograde attitude with your pilot controls or very close to it.  The point along your current orbit for that burn will generally be on the night side for going to a higher orbit (such as Kerbin to Duna) -- or alternatively you could say that your ship is pointing in roughly the same direction that your current planet is moving around the sun (prograde for that planet).  If you want to go to a lower orbit, like from Duna back to Kerbin then you will burn from the daylight side -- which is the same as pointing your ship in the direction behind the planet's path, the planet's retrograde -- this will be prograde for your ship, but retrograde for the planet.  EDIT: assuming a standard west to east parking orbit.

Edited October 1, 2016 by Kelderek

[Plusck]

What @Kelderek said.

If you're really wanting to send your Kerbals on their way, and have the dv for it, you could send them back on a return that loops down low-ish over the sun - probably near Moho - before rising up to meet Kerbin again. Again, it's the high angular velocity low over the sun that you need to catch up on Kerbin.

Taking a look at Alex Moon's porkchop planner, for instance, you can leave Duna immediately after the first arrival window (year 2, day 69) but you'll spend 1800m/s retrograde instead of 640 to leave, and you'll arrive at Kerbin at high velocity - without aerobraking you'd need 3000m/s to return to LKO. Of course, if you're well shielded it's just a question of getting the reentry altitude right (I'd say 27-28 km Pe, but do alt-F5 when you arrive in Kerbin's SOI in case it's too hot to handle).  

edit: forgot to mention: if you're going to try the "looping-low" return tactic, do try to perfect the trajectory as soon as possible after leaving Duna. You'll be going fast later on, and course corrections will be much more expensive. All in all though, if you have about 2200-2500m/s dv in your tanks, starting in Duna orbit, you should be able to leave for home at just about any time.

Edited October 1, 2016 by Plusck

[strider3]

19 hours ago, Kelderek said:

  EDIT: assuming a standard west to east parking orbit.

Uh oh...I unfortunately ended up in an east to west orbit when I arrived at Duna. Now that I have a more precise add-on for tweaking nodes, that should be easier to avoid on future flights but...I think I placed my Kerbals in an awkward situation.

[Kelderek]

55 minutes ago, Victor3 said:

Uh oh...I unfortunately ended up in an east to west orbit when I arrived at Duna. Now that I have a more precise add-on for tweaking nodes, that should be easier to avoid on future flights but...I think I placed my Kerbals in an awkward situation.

Well if you are landing then this is no issue, just be sure to lift off toward the east after you are done on the surface and you will be going west to east for your departure orbit.

If your orbit ends up being retrograde or some other wonky orbit like a polar one then you just need to remember the following:

1.  You must ALWAYS burn prograde to leave your current body, just use your location on the orbit to control where you go.

2.  If you are trying to leave a planet to go to another planet in a lower orbit around the sun, then you want to choose a location on your parking orbit where pointing your ship prograde will also be be pointing as close to that planet's retrograde as possible.  It is the reverse if you want to go to a higher orbit, choose a spot on the parking orbit where you can burn prograde and also be going roughly in that planet's prograde direction.