The Launch Window Planner and MechJeb

[Palamedes]

Okay so the one thing KSP has taught me is that I'm nowhere near smart enough to be a rocket scientist.. I'd probably not make a good kerbal..

I'm trying to use the Launch Window Planner to get from Kerbin to Dres. ( http://alexmoon.github.io/ksp/ )

Now previously to get to Dres I have escaped Kerbin, circularized, match planes and then Hohmann transferred.. which always uses more ÃŽâ€v than what the launch window planner says it would use. And more importantly, my transfer times are long (and ugly to be quite frank..)

So I'd like to master this tool but damned if I can figure it out.

Mechjeb is the only mod I really use and I mostly use it for the orbit info, vessel info and ÃŽâ€v stats information panels -- very handy those.. Do I have enough to use the launch window planner?

Is there some magic I'm missing?

I have read over other tutorials but damned if I can get it to work for me.. I can't even get close.. I'm feeling very stupid..

Can someone please clue me in? Thanks.

[Tank Buddy]

I use this website to give me launch windows: http://ksp.olex.biz/

I then use those angles and a good ol' protractor to draw them out, then focus on Kerbol in map mode, and put it on the vertex, then wait for the planets to line up. You just have to make a maneuver node and one Hohmann transfer. For Dres, you can perform a mid course plane change, and if you can't see the encounter from your ejection burn, place a maneuver node on the ascending/descending node to show the predicted path, then fiddle with your ejection burn.

Here's a demonstration for Duna:

[Palamedes]

I can get to all the planets (though I haven't been to Eeloo yet) through the way I do it.. My question is more about understanding and using the other tool. Thanks though =)

[mediobogdum]

When you say the "other tool" are you meaning MJ or the launch window planner?

Assuming the latter:

You do have enough info to use the planner. It requires only origin and destination for inputs.

Once you have the resultant info create a manuever node faithful to those numbers. It seems from your post you are ok up till here.

If your manuever node isn't exactly like the planner (ejection angle, speed etc) then you will get different results. Is this where you're having problems?

Assuming yes:

If your results DV and time-wise are wildly different from the math than the issue is execution.

If you feel you are following the math exactly and are still not getting the correct trajectory after burn, then run two of them.

Let MJ plot and burn one, then plot and burn one yourself (the exact one MJ just did) and see what comes of it.

Something to keep in mind is the planner is a best case scenario. There may be many times when it's results are inconvenient, especially if LS is an issue.

In this case you can experiment with different departure times and angles, none of which are likely to be more efficient (dv) but may be much faster; i.e rather than ninety days to Duna for 2500k, seventy days for 3500k.

[Geschosskopf]

I can get to all the planets (though I haven't been to Eeloo yet) through the way I do it.. My question is more about understanding and using the other tool. Thanks though =)

If your problem is taking numbers from the launch window web site and applying them in the game, see Mr Shifty's EXCELLENT tutorial.

Note that this is using a ballistic trajectory, not a Hohman. That is, instead of doing a plane-change burn mid-course, instead you incorporate the plane change into your transfer burn leaving Kerbin.

Anyway, MJ has a Maneuver Node Editor that lets you type in exact values in all the different directions (prograde, retrograde, up, down, inwards, outwards). So, once you have a maneuver node for the burn, you can use this feature to type in the numbers you got from the website. But there's 1 problem: putting the node in the correct place (at the proper ejection angle in your orbit around Kerbin). There's nothing in the game or MJ that lets you do this manually, and eyeballing it isn't accurate enough for the website numbers to work. So, the whole process of using MJ to take advantage of the website's numbers goes like this:

1. Read the tutorial so you know what numbers on the website are important
   
2. Run the website for your date, target, and ballistic trajectory, and write down the numbers you need. You'll need a calculator that does trig functions.
   
3. Fast-forward time until you're less than 1 orbit before your burn time (15 minutes is usually good)
   
4. If you haven't stayed focused on this ship since you launched it, point prograde and give it 1 light tap on the Shift key to run the engines at their absolute minimum thrust. Let them run for 5 seconds and then stop them. If you don't do this, the burn time calculator won't know how long the transfer burn will last, so you won't know when to start it.
   
5. Go to the map view and set your destination as your target.
   
6. Open MJ's Maneuver Planet, cycle to "Transfer to Another Planet", and hit "Create Node". The reason for this step is to get the node created at the proper ejection angle on your Kerbin orbit. You're not going to use the burn values that this node starts with, but you ARE going to use this node exactly where it sits, so don't move it. See, MJ only knows Hohman, it don't know ballistic, but it does know ejection angles.
   
7. Open MJ's Maneuver Node Editor and type in the values you got from the website in their appropriate boxes, then hit the update button. This sets the pre-existing node to burn for the ballistic transfer you wanted or you wouldn't be going to all this trouble anyway.
   
8. Check the map to see that you're at least getting close to the target. Dres is tiny so even doing everything perfect, you'll probably still need to tweak your course a bit en route to get a good encounter. But you can't do that unless you're already in the ballpark when you leave.
   
9. Either do the burn yourself or use MJ's Maneuver Planner to "Execute Next Node".
   
10. Bon voyage!