The time given in launch window planner is completely orientational - definitely not to be used as the precise departure moment. You need to fine-tune the time for the exact departure angle and inclination.
The window planner has no clue about your initial/current argument of periapsis, therefore it can't suggest the precise moment to start the burn - that moment completely depends on your current position in your current sol's orbit.
Imagine you launch two probes into the same orbit, exactly half an orbit apart. When one flies over the dusk terminator, the other crosses the dawn line. Since the orbits and the destination are identical, the window planner will give both exactly the same departure date and delta-V components. But at the moment of the optimal burn they will be on two opposite sides of the planet, flying in opposite directions - how would you expect both, performing exactly the same burn relative to their current speeds, to end in the same place if they depart in two opposite directions?
What you do instead, you set the burn values, set the time to have roughly the right departure moment, and then adjust the precise burn moment to have the desired ejection angle and ejection inclination values. The departure time will be off relative to the "optimal" but that's a relatively small error which you should fix by a mid-course correction.
(if you desire to avoid mid-course correction, can perform a very precise, high-thrust burn, and want to arrive precisely at the target, you may do the above, note down the time when the burn should occur, convert the seconds back to date, find the new point on the graph, note down the new departure angles, fine tune the time and speed for the new values... repeat as necessary iteratively increasing precision.)
I want this answer to be relevant to all players so it is bloated with images and basic stuff. Answer to your real question should be at end.
TLDR: Aerocapture is more efficient most of the time. Especially if you land anyway.
You can use aerobraking to capture in planet orbit, which will save up to 700m/s on Duna.
It really, really, depends on your craft. Heatshield is a tonne or two, and it does not care much about size of your craft. On small craft it might weight more than its equal in fuel+engine. On bigger ones it will be hardly noticeable while required fuel keep its proportions to cargo.
If your craft is solid enough to survive in atmosphere, or simply will be landing anyway, aerobraking is perfect choice.
Example for Yes: This horrible lander I quickly glued together will benefit greatly from aerobraking. It have lots of fuel, but aerobraking will increase it by ~18%.
Hint: you can decrease amount of Ablator in heat shield to greatly reduce its mass (1 unit of Ablator is 1kg). Even empty heatshield will work great due to its high heat tolerance. It will still protect fragile parts from wind and heat (until it explodes). In case you are curious: at crazy speeds heatshield can overheat if ablator won't cool it down quick enough.
Of course there are notable exceptions. Fragile, tiny, Ion probes can pack 10k Delta V and still be below 10 tonnes. Big nuclear interplanetary vessel can pack 10k dV as well and still not collapse into black hole. Anything with ISRU can just mine more fuel when needed. In such cases aerobraking will needlessly complicate thing and introduce new dangers for no good reason.
Example for No: This probe can go anywhere without any aerobraking, so why bother?
Aerobraking is free Delta V. But how free it really is?
It takes roughly 700ms to circularize in low Duna orbit from fuel efficient transfer. How much is 700ms? Time to do Science! (If you want to do Math, then you need fuel mass, ISP and DeltaV formula)
Lets take 100 tonnes as cargo, to make math simple:
Ion is ~4 tonnes, ~90k credits, ~18k Xenon and will take many hours. It won't work because you need to burn as fast as you can to avoid wasting fuel. You will need more thrust (more mass in engines) and fuel (mass again...). And I haven't even added solar panels yet ;)
Lets go nuclear! 14k credits, 15 tonnes. 20 minutes. Still too long for burn in low PE. And I haven'y even added radiators yet ;)
Fuel efficient classic rocket isn't much better. 30 tonnes, 7k credits. 5 minutes. Still too long to burn efficiently.
And when you want to make that burn short enough, you end up with a joke. It would be better to circularize on Duna with same engine block used to circularize on Kerbin and transfer to Duna, otherwise such big engine will be dead mass.
And now aerobraking with tiny circularization (because you need to lift PE from atmosphere to stay in orbit). ~9 tonnes without min-maxing.
If we ignore issue of dead mass (engines, solars etc) propulsion to payload ratio will be roughly same for different cargo mass.
- Ion: 4%
- Aerobraking: 9%
- Nuclear: 15%
- Weak chemical rocket: 30%
- Strong chemical rocket: 40%
Of course in "real" world mass penalty will be higher (more engines, more structural parts and so on).
This means that aerobraking saves you somewhere between 5% to 50% of vessel mass. And the bigger your vessel, the higher penalty you will need to choose.
So how free it is? Ask Tyrannical Rocket Equation, or just look at images here. After all you need to get that thing into Kerbin orbit and then transfer it to Duna!
Note: You can split capture burn to increase fuel efficiency (ideal burn is instantaneous, fuel is wasted as time goes up). And aerobraking into multiple passes to increase survivability. Just make sure to burn/brake hard enough to stay in duna sphere of influence (SoI) which requires roughly 300m/s delta-V from low PE.
Example: Two burns, 300m/s to stay in SoI, then 350m/s to circularize. Every additional burn can be longer because with dropping speed at PE you will stay longer near it.
[Work in progress] How to include aerobraking in mission planning?
That's the real question you asked. Mission planning is rather complex. And it really, really, depends on your mission. I will do aerobraking where it makes some sense, and skip it where it makes none, hopefully displaying both pros and cons in simple form. I will also throw in bunch of crafts unrelated to aerocapture simply because I am having too much fun to limit myself, it will take two or three days to finish :)
By the time this expedition ends we should have examples of quite a lot of practical Duna approaches. I hope your project will benefit form it.
I will be doing it in many launches, building a fleet, to avoid game slow down and ugly rockets. All launches and maneuvers will be efficiently done by MechJeb, because after few hundred times such thing gets boring and I have Netflix on second screen ;) Transfer to Duna will be lowest Delta-V Hohmann style. I believe your mission planner will suggest to do it similarly.
All crafts should be easy to use, with huge error margin, and not limited to Duna, as I would like all players to be able to use them. It will be done in Sandbox so no tech level constrains will be applied.
Two LittleBrother's for Ike and Duna polar orbits. Hopefully I will do some cool gravity assists with them. I used small Cubic Strut to build quad-Dawn module. As it is Ion craft, I used spare fuel to get to 25km orbit, reducing escape burn from ~1100m/s to ~600m/s.
GentleImpactor's will land on various spots on Duna, I think four would be good number. Two will go for poles after bouncing around Ike, remaining two will land directly on Duna. Nothing special, just bunch of science parts. They will perform aerocapture and then land.
Crew transfers will be done with two SpaceBoat's. It should be able to land on Ik and, I hope, reach orbit again as well. They are also equipped with emergency planetary landing system.
And Kerberus itself! It will carry all crew, and serve as fuel storage. Almost a thousand tons of explodium! You should be able to notice two SpaceBoat's docked at its flanks. At its top you can see return module. It flew better than expected. I plan to capture it with nuclear burn in high orbit, so it will be close to both Ike and Duna (Boats have short range).
Of course proper mission requires laboratory to maximize Science. Sadly some parasite drone attached itself to last empty side of lab ;)
I am still assembling the fleet, there is need for miner/refinery with support crafts and crewed Ike/Duna landers. I would also like to drop some rovers and deliver a plane or spaceplane, maybe ion, to fly around Duna. If anyone have an idea what also to send (stock only), feel free to comment, expedition will take a lot of time so I will start it this friday.
Best Answer
Third method is better
Oberth effect + Aerobraking + Gravity assist.
Aerobraking happens at low PE so it will be coupled with Oberth effect. Aerobraking alone will save you enough Delta-V to encounter moon later on, with some planing you can even encounter it for free. Quick saving and Trajectories mod are advised. Of course it will require craft capable of aerobraking, inflatable heat shield is good starting point.
You might want to have high thrust-to-weight ratio, long burn will waste propellant. But try not to hurt your total Delta-V. Before you go for low PE on body without atmosphere, check its highest point. You do not want to encounter mountain at 5000 m/s.
So in perfect encounter you:
Capture burn from interplanetary transfer involves braking, a lot of it. Delta-V requirements tend to be in thousands. Savings in capture burn tend to be higher than moon encounter burns. Do not forget mid course corrections, long before planetary rendezvous you can make high changes in planetary orbit for very low delta-V cost, for example changing orbit from prograde to retrograde will be in tens of m/s instead of thousands.
Jool is an exception, as always.
Laythe and Tylo have 80% of Kerbin mass, which gives them notable gravity assist and Oberth effect. Those moons can capture you with nothing more than gravity assist and some luck in alignment. Orbit changes on Jool itself will cost a lot, due to its massive size, so use moons as much as you can.
Example of aggressive Ike/Duna transfer
I found some time to play so here is my take at red planet.
I went with lowest Delta-V Hohmann-style transfer so Duna PE at encounter was rather low ~1500 m/s.
With some orbit tweaking, and bucket of luck, I got direct encounter at Ike with low Duna PE. Quite good starting point for tests.
Have in mind that:
Plan your mission accordingly if you have return in plans. If you want to visit both Duna and Ike, aerocapture on Duna would be cheapest in Delta-V overall. Especially when you consider that Duna escape craft can easily be capable of Ike landing.
(I simply can not give precise numbers because it heavily depends on your orbit and planetary alignment)
I used Duna PE to tweak Ike PE for ~100m/s. Ike low orbit needed ~900m/s to circularize.
Now that's some orbital billiard. ~700 m/s to capture in Duna low orbit, coupled with cheap orbital plane change by bouncing on Ike (300m/s, same change with engines alone would cost above 800m/s).
Ike gravity is too low for meaningful gravity assist, but here is one anyway:
Note: I had horrible position for Ike gravity assist, simply shooting through its sphere of influence, if ship trajectory would be tangent to Ike orbit it would be much more effective.
Afterword
In the end, it will mostly depend on your mission profile, especially your plans for return. You have lot of options, with plenty of possible combinations. Remember, even if you will use most effective transfers, you will waste fuel if orbits are not in line with your exploration plan. Plan in advance. And use Precise Maneuver Node Editor (MechJeb comes with one, but there are more to pick from).
If you want some fun ideas for Duna/Ike check out Duna Ore Bust web comic.
See you at Barsoom!