how on kerbin did you get that giant lander into orbit and all the way to eve? and you actually got it *back*? that is some advanced stuff, i'm hella impressed.
Much like a real mission to Mars - a kerbal transfer vehicle, a fuel cargo vehicle, and a lander cargo vehicle. First inserted the cargo fuel into high Eve orbit. Next inserted the crew into low eve orbit with enough fuel to go to high eve orbit. Finally, inserted the lander into low eve orbit, while using no fuel from the EDL stage and allowing Eve's atmosphere to kill my velocity and eventually a series of drogue chutes followed by main chutes to bring 'er down with a full wet mass (very difficult, need to pin your entry angle perfectly and do many passes through the atmosphere so it doesn't burn up). The lander's launch phase is planned so that it moves rather slowly through the lower atmosphere to avoid drag and then sheds mass quickly as it gains in altitude. Once it jumps out of the atmosphere it is fed by a highly mass efficient nuclear thermal rocket cruise stage to circularize the orbit. The launch timing should be synchronized such that rendezvous with crew costs minimal delta v on the crew end. They pick up the kerbal from the lander and then go up to high Eve orbit and RVD with fuel, just enough to get the command module into kerbal orbit on a trajectory scratching the Kerbin atmosphere.
Launch is a bit of a nightmare. All $500,000 payloads. Typically a core mounted stage that's lifted into kerbin exit by a 4x mammoth engine rig surrounding it.
my issue is always returning - i can get a single kerbal to duna and land, but if i use engines and not chutes to land (since i have only one kerbal i sorta need to use one of my pilots, and only engineers can repack chutes) i use up way too much fuel and don't have enough to escape/return. i can use a 3-man lander, but i can't seem to build them without them being enormously fuel-heavy and inefficient and thus gas guzzlers (which again doesn't leave enough to get back) or i make it too light and don't give myself enough dV in the first place.
I suggest actually doing a rocket equation to solve for staging of your lander's launch phases. If something is a "gas guzzler" it means the stage in front of it is too heavy. For Duna, you should be using a high thrust system. Maximum. Due to lack of real atmosphere, ballistics are best at getting out of a gravity well. Low acceleration is only useful under atmospheric conditions.
Also, consider cargo. Makes life easier if you can abandon stuff in other orbits.
i also really suck at mission planning. i know for efficiency i should be doing a rendezvous and LKO refueling etc but i hardly even know where to begin to get everything into LKO to do that. and i feel like i'm so bad at RV's that i would end up wasting a shit ton of fuel just getting the initial docking completed and sort of defeat the purpose. one thing i definitely need to start doing is launching during the most efficient window, but i also really don't like skipping months or years ahead if i can help it.
Yeah, KSP needs better launch planning. But. I would say just launch to escape and maneuver at a node. Pin your entry to make it such that you put a periapsis flyby in low orbit around the target's proper side (depending on which direction you approach from) - this should give a pretty nice gravity assist.
I'm afraid skipping months ahead is the best way to proceed.
i'm a pro at mun and minmus landings at this point but i feel like i'm kinda stuck when it comes to interplanetary missions.
Fear not, friend. I suggest you first practice RVD. It's not hard but at first it is daunting. Focus on the navball. When it changes from "Orbit" to "Target", that's when you need to start pointing at the target crosshairs and mess around with your relative velocity and when you're close enough, it's basically euclidien at that point.
Next, practice making surface to Kerbin orbit "landers." If it can make it off kerbin with fuel to spare, it can make it off Duna no problem. Remember, high thrust! You can always throttle down in space (where thrust is relatively unimportant).