Getting to Duna

[Plusck]

3 hours ago, MrOsterman said:

... so for using the gravity of the planet to Oberath my way out to Duna would require, if I'm guessing right, a push off of Minmus when it's roughly between Duna and Kerbin, then creating an really big elipse around Kerbin to set it up so that a Prograde burn at PE pushes the AP of the orbit out of the Kerbin SOI, and into an intercept with Duna.

...maybe it's time to up my game, take the Athena back out with a new crew and repeat my last trip but do it directly from the Mun (where my current group of tourists want to visit first.)

Again, yes, quite right. Push off Minmus into highly eliptical orbit around Kerbin, then wait for Kerbin Pe and a relatively small burn to push Ap out of the SOI and all the way to Duna (should be about 250 m/s or thereabouts).

However, from Minmus you need to leave Minmus much later than that - Minmus has to be further round so that it's on the other side of Kerbin's orbital trajectory. This is because when you do your burn at Pe, you turn an eliptical trajectory into a hyperbolic one that is much closer to a straight line tangential from Pe. So if you leave when Minmus is between Kerbin and Duna, your Pe will be slightly to the sunny side of Kerbin and your escape trajectory will be almost at right angles away from Kerbin's orbit around the sun - certainly not the efficient way to get to Duna unless you want to get there quick and have masses of fuel to burn both on the way out and then to capture.

The final disadvantage with going from Minmus is planning. I just tried it and it took a 160m/s burn from relatively low Minmus orbit to an eliptical Kerbin orbit, then about a 300 m/s burn at Kerbin Pe to get to Eve (Minmus was in entirely the wrong place to test for Duna, and by the time I'd finished trying to place those nodes I was seriously hacked off and not ready to warp forward for more of the same). It is a pain. Once you've plotted your escape from Minmus, it is extremely difficult to place a node at Kerbin Pe (too many ships in orbit getting in the way) and as soon as you try to adjust it, it jumps all over the place.

So basically, if you refuel at Minmus and then drop back to Kerbin for an escape burn, you would essentially have to plan your escape burn after getting back into a stable Kerbin orbit. Let's hope that the KSP 1.1 fixes some of the jitteriness in the patched conics whatsitscalled.

Going from the Mun, however, is much easier. I did some tests there too, and can confirm that you're unlikely to gain anything going back to Kerbin orbit (for a burn to Duna, in any event). Starting in a 40km-ish Munar orbit, I was getting a Duna close encounter with 430 m/s - escaping from the Mun directly prograde along its orbit. Getting back to Kerbin, however, required a significant burn to bring Pe down, again a problem with plotting angles, and (since I couldn't get a Duna encounter with that angle) almost the same combined total of around 400 m/s just to escape Kerbin's SOI.

The "planning" advantage with going from Mun orbit is also clear - you just need to wait until the Mun is almost eclipsed by Kerbin, so that escaping prograde sends you out of Kerbin system prograde. It is certainly no harder (or at least, only marginally harder) than plotting an escape burn from LKO.

 

Finally, I'd just add that plotting an escape burn from LKO isn't as hard as it might seem to be at first.

As soon as you are on an escape trajectory, the new orbit around the sun is visible on the map. If you have checked a launch window planner such as https://alexmoon.github.io/ksp/ then you roughly know how much dv you need. So even without zooming out at all, you just need to adjust prograde to roughly that figure (say, 1050 m/s for Duna), then move the maneuvre node around until you can see that your resulting orbit passes through where you are now, with Pe relatively close to where you are now.

That will guarantee that your ejection angle is almost directly prograde (which is what you want in 99% of cases) and it's just a matter of setting Duna as target and adjusting prograde/retrograde (which is easy to do zoomed out, or zoomed in and looking at Duna's orbit behind the sun).

Edited January 20, 2016 by Plusck

[davidy12]

You can get there at almost any time, but it's EXTREMELY DIFFICULT AND INEFFICIENT to try and do it. I'd use MechJeb.

[Giacomo]

Thanks for all your help! I'm trading efficency for convinience; I'm try to get their in as little time as possible. I'll try out what some of you suggested.