I've been trying to figure out some stuff with the KSP Interstellar, and my biggest problem is that I don't know how to enter orbit of another planet. When I leave warp, I'm basically left with a trajectory that is either sub orbital or hyperbolic (i.e. an escape trajectory), or worse than both of those, hyperbolic with a periapsis below the surface of the planet. How can I tell what my orbit will look like when I drop out of warp? I'd really like to be able to drop out of warp directly into a stable orbit.
If you're talking about the alcubierre warp drive, the idea with that is that is creates a bubble around you for the duration of the warp and after you come out of the warp you have the exactly same velocity vector as before entering warp. The warp drive just adds a temporary speed increase and removes it after you stop it.
You can not. Or: you can plan that using the map - you keep your original vector. But
I'd really like to be able to drop out of warp directly into a stable orbit.
Will NEVER work because you will never have 2 stable orbits with exact the same vector. Ok, like nearly never. You can minimize the needed dV but not get rid of it.
And that is good - and planned - otherwise the warp drive would be totally overpowered. Basically: learn to live with it. Get out of warp far enough that a good hard burn can be used to circularize. Remember, KSP Interstellar also has a lot of interesting other technology - and you need antimatter anyway for a good usable warp drive, so you can use those reactors for their high performance engines.
In my limited testing of the warp drive so far, I could immediately reenable the drive after turning it off. So, turn it off for a second, look at your orbit, and reenable it if you're not happy with it.
In order to drop directly into a stable orbit, you will need to make an intermediate jump or two. The big idea is to use the gravity well of the sun to change your current vector to the one you need at your destination.
With some planning you can avoid big differnces in dV. But im not patient enough for this.
I usually jump directly to the target. If im too fast i place my vector in a way, that i travel directly away from the planet. Once im a bit away i warp again straight towards the planet, getting as close as possible (dangerous). After that your velocity gets drained pretty fast by the gravity while flying straight away. Repeat it a few times until you get an apoapsis. Then warp to the side of the planet and you get a somewhat stable orbit. Rest is for the conventional engines.
This works great for Jool or Eve and even Kerbin, but planets with a low gravity can be a pain, because you need a lot of jumps that way.
You can use a maneuver called gravity breaking to slow your craft down. You just warp when you're about to leave the objects S.O.I. to your original periapse, letting the gravity of the planet slow you down after a series of successive jumps. You can accomplish this with a fission generator, since you will have ample amounts of time to recharge the warp drive between jumps. This kind of maneuver works best with high gravity bodies like Jool, so its good for quick antimatter runs. You still will need some propellant to adjust your orbit, but you can avoid that with a quantum vacuum plasma thruster.