Can't figure out the Hohmann Transfer to intercept Duna

[Kookas]

I followed this tutorial to the literal letter:

http://forum.kerbalspaceprogram.com/threads/27236-Tutorial-Step-by-step-Interplanetary-Hohmann-transfer-guide-and-tips

The phase angle is 44.38, and my apoapsis touches Duna's orbit. I've ensured the escape trajectory is parallel to the planet's prograde. And yet, it's still not even close to an intercept. What's going on?

[Vanamonde]

A tiny imprecision in the transit burn can cause one to miss by quite a lot. Place a maneuver node on your Kerbin orbit and try both tugging on the adjusters, and moving the whole node around in your orbit. It may take some doing, but you should eventually get close enough to see the "closest approach" icons, and then you can see how you need to adjust from there. If it's still not coming together, fine-tune by plotting a mid-course correction.

[Raptor831]

You could get MechJeb and see how it plots the Hohmann transfers. It'll at least get you to see where the nodes need to be and the delta-V requirements. Using the nodes without MechJeb or at least Precise Node is a pain because those transfers need to be very precise.

And ditto on the mid-course correction. It's a bit less fiddly out there, and you can fine tune it much better.

[EtherDragon]

There is one REQUIRED mid-course correction that you may be missing: The plane-change at Ascending / Descending node.

Kerbin and Duna do not share the same orbital plane - so you need to complete a plane change at the nearest node to move you off of Kerbin's orbital plane, and onto Duna's orbital plane.

Typically, even a well planned mission will have the following maneuvers:

1. Ejection Burn - Burn to escape your source SOI and head in the general direction of your destination. Best if done at the optimal window (e.g. Kerbin -> Duna with a phase angle of 44.38 degrees).

2. MCC at SOI change - When you leave Kerbin SOI, a mid-coarse correction is a good idea to correct residual errors from the first burn. Once out of Kerbin's SOI - it gets much easier to be precise.

3. MCC at Ascending / Descending node - Used to change your orbital plane to match the target's. This is often the first time you will actually see encounter details for the target SOI encounter.

4. Encounter - As soon as you enter the target's SOI, a burn is planned to adjust your approach.

To answer your question, "What's going on?" I bet you have the X, Z, and T coordinates correct - but are missing the Y. Intercepting another planet requires correct positioning in four dimensions - X,Y,Z, and T(time). Too far to the Left or Right, and you miss. Too far Out or In, and you miss, Too far Up or Down (which is likely what is being missed in your case...) and you miss. Too Early or Late, and you miss.