Advice on Transfer Burns

[terrendos]

This is embarrassing to ask because I've been playing this game off and on since 2014 or so, and I've done more than my share of interplanetary trips in that time. But it's been a few years since I last tried, and I've hit a stumbling block on this probe I'm sending to Duna.

https://imgur.com/a/MfM2fdz

As you'll see in this photo, the planner says it's about time for the transfer window, and the phase angle looks at least approximately correct. But when I plan the burn, I keep getting this. Worse, no amount of fiddling I do with subsequent maneuver nodes, short of creating a brachistochrone trajectory, will get me an intercept. What am I doing wrong here?

[jimmymcgoochie]

It looks to me like you’re arriving at Duna’s orbit after Duna has already passed by, in which case the solution is to wait a bit longer before you do the transfer burn, allowing Kerbin to catch up so that your probe and Duna end up in the same place at the same time.

The general rule for outbound transfers (Kerbin to Duna, Mun to Minmus, Laythe to Tylo etc.) is: if you’re arriving too early (target behind you when you get there), you’re leaving too late but can delay your arrival with a longer trajectory (add more prograde to move your apoapsis higher) so the target catches up to you; if you’re arriving too late (target in front of you when you get there), you’re leaving too early so best to wait a while before doing the transfer burn.

That rule is reversed when heading inwards (e.g. Kerbin to Eve, Tylo to Laythe, Minmus to Mun etc.): in that case if you’re arriving early then you’re leaving early and need to wait for the target to catch up, but if you’re arriving late then you’re leaving late and lowering your periapsis will allow you to catch up.

[18Watt]

After briefly trying out the transfer planner I found it does not always give you an efficient transfer.  In particular, I found that my arrivals at the target CB ended up with a large velocity difference, yielding extremely costly capture burns.  I’ve stopped using that tool.

As @jimmymcgoochie noted, the screenshot indicates you should wait  X days before starting your burn.

I couldn’t tell from the photo if you’re burning directly from LKO or from a Solar orbit.  

• If you’re in solar orbit, drop a maneuver node ahead of you and slide it forward or backward until you find the right burn location.
• From LKO I usually time warp in small increments to find the best departure time.  When I start getting close I time warp in 10 day increments, then 5-day increments, to avoid overshooting.
• If you do overshoot (time warp a little too far, you can usually accommodate this by burning a little longer, allowing the target CB (Duna) to catch up.

[OHara]

The alarms in the screenshot were set from the built-in Alarm Clock, which for some reason sets an alarm 20--30 days before the ideal transfer (thread link).

The online tool at https://alexmoon.github.io/ksp/#/Kerbin/100/Duna/100/false/optimal/false/1/1 still works great. 
The first minimum-delta-V transfer is on day 231, so @terrendos has 40 days to wait, so probably easier to set a new node.

Leaving at the time in the image, it might cost another 200 m/s from low Kerbin orbit to get into a higher-energy transfer to catch up to Duna.  Doing that in a mid-course manuever would cost even more, as you say.

[Scarecrow71]

I will echo what has been said about TWP; it isn't very accurate at times, and you really have to watch what it tells you.  For the most part, creating your own nodes is going to be far more effective.

For myself, I couldn't live without MechJeb.  I use it to create nodes, and then before I execute them I can tweak them to get the biggest bang for my dV buck.  It still isn't always perfect, but it is better than having to guess!

[HebaruSan]

I'm quite biased, but Planning Node is how I think transfer planning should work (and yes, that means I think it should mostly replace my other mod Astrogator):

In the case in the OP, you would have been able to figure out the cause of and solution to the problem by dragging a planning node around until the close approach markers converged to an encounter, like you'd do for any other rendezvous. No more trying to guess the right burn based on a time, delta V, and an angle, or trusting heuristic-based tools that don't account for factors like eccentricity and then attempting to compensate by adding more burns because it happens to be easier than correcting the parameters that are actually off.

[AtomicTech]

6 hours ago, HebaruSan said:

and yes, that means I think it should mostly replace my other mod Astrogator

Why not have both?