How to stably get to Minmus

[numerobis]

Two or three times now I've been hit with the following scenario:

1. I burn at AN as Minmus rises, I set up a node 10 minutes later for fine-tuning, I set up a nice transfer to Minmus with 20km Pe at the destination. I save the game and go to bed.

2. Wake up, start up KSP, and my spacecraft now doesn't have an intercept with Minmus at all. I'm just barely out of the SOI when I cross the Minmus orbital path. It would take a rather considerable burn to get back into an intercept.

What's going on, and how do I avoid it?

Edited July 17, 2016 by numerobis

[ElWanderer]

What version are you running? I found in v1.0.x, the difference between the orbital calculation for a loaded vessel and that when on rails (the former is done by Unity and less precise) was big enough to cause major problems trying to get to Minmus. I'd get a perfect intercept that would vanish on hitting time warp, while back in real-time it'd constantly flicker between two different orbits, one good one bad. I was hoping this would be better in v1.1.x, but I've not had the chance to try it properly yet.

I think this is partly a case of floating point arithmetic being very susceptible to small errors, partly that Minmus's sphere of influence is tiny.

[Van Disaster]

There's an issue in various versions where it'll decide you don't have an intercept when you actually do, so you'll get a complete blue orbit line and then fall into the target SOI anyway. Not sure how you get round that, try putting a node down just behind you?

[bewing]

The code that says you have an intercept is much more trustworthy than the code that says you don't. Your speed when you are out at Minmus' orbit is so slow that it's very easy to make minor corrections. So go with your gut, and your first burn. If it said you have a 20km Pe, then you almost certainly do have a 20km Pe. Wait until you are out close to Minmus' orbit. Watch Minmus come closer and closer. Figure out how far 2 million meters is on your screen and measure with your fingers. If it looks like you are going to hit it, then you're good. Suddenly the game will realize that you have an intercept, and it will suddenly show it again.

[soulsource]

I'd like to second what bewing wrote, and back it up with a reference to a statement by the developers.
Currently the game can, under certain circumstances, miss intercepts. Once one gets close enough to the target body, the SOI change does take place properly though. For details see:

 

[numerobis]

I hit warp, and I missed Minmus completely. Then I reloaded the same save (from after the burn), and it did get the intercept -- plotted and in fact. That it's not consistent is really strange to me.

Oh well.

@ElWanderer: this is with 1.1.2. I guess I should update to 1.1.3.

[bewing]

Where did you warp to? If you are on a fast warp, you will often miss SOI changes (or atmospheric encounters). You have to stop before you get to your destination. Can't just expect the program to do it all correctly automatically! That's kinda the point of the lesson.

Edited July 17, 2016 by bewing

[numerobis]

@bewing: I warped to close to where I should have been intercepting Minmus, slowed down, tried some small maneuvers (via nodes), no dice. Then I reloaded (back to where I'd done my TMI) and I had an intercept after reloading, and the intercept was true, I landed and rescued Valentina as planned. Overall, I'm befuddled.

Whatever, it'll be a while again before I have time for my next session of rockets.

[Van Disaster]

You may have not been close enough still, or your intercept was past AP when you're falling back down the well. I've had the game decide not to display intercepts for Mun when it's blindingly obvious I'm on the right trajectory ( and I did in fact drop into it's SOI ), so it's not limited to small SOI.