Transfer burns and alarm timings

[Zo San]

I've been looking through a lot of guides and especially tutorial videos but I can't find anyone that discusses this in the detail I am looking for.

I set up alarms with Kerbal Alarm Clock (fantastic mod btw and I think the functionality should be built into vanilla, but that's another story for another day) so that I have a good idea of when a transfer window will be available. I assume that alarm timing tells you when you should be leaving (roughly).

The problem is then, that I don't know what the best technique for burning is. Let's use Moho as an example since it's a tricky one. My best understanding based on what I've read and seen is that you want to burn prograde from a Kerbin orbit a while after Kerbol has come over the horizon from the ship's point of view. With a bit of fine tuning this does seem to get the best solar periapsis. I've actually seen someone accomplish a Moho encounter by doing basically this and ONLY burning prograde, and I have no idea how he accomplished this black magic because I find at the very least you also need an ascending/descending node adjustment down the line.

But even when I am able to line up my trajectory with Moho's orbit after an adjustment, and even though I've left at the window that the alarm gave me, I find I'm not able to get an encounter around the far end of the orbit (which I assume is where you're supposed to be looking since it's the area that would use the least dv to get to). So instead I have to get there, burn retrograde to approximately line up with its orbit, and then shift inside or outside to eventually capture, but by that point I've wasted a lot of dv and I know I'm doing something wrong. It doesn't help that when you have to work with two maneuvers (a big burn from Kerbin to get the solar periapsis down, and then an adjustment burn for the ascending/descending angle much later) it becomes difficult to switch between the two to optimize since changing the prograde burn often shifts where the ascending/descending node will be.

TLDR: What's the best method for burning to an interior planet like Moho when the alarm says a transfer window is available? Where am I going wrong with my method of burning prograde from Kerbin to get the solar periapsis down, and then matching the orbital angle at an ascending/descending node such as to attempt an encounter at the far end of its orbit?

Thank you for reading!

[Guest]

I don't use KAC or any other tool for Moho. I simply eyeball it. See this post :

As far as how to perform the transfer burn, I would ask first, are you making maneuvers with the stock maneuver node tool to plan your burn first?

Edited July 28, 2016 by Otis

[DaMachinator]

59 minutes ago, Zo San said:

The problem is then, that I don't know what the best technique for burning is. Let's use Moho as an example since it's a tricky one. My best understanding based on what I've read and seen is that you want to burn prograde from a Kerbin orbit a while after Kerbol has come over the horizon from the ship's point of view. With a bit of fine tuning this does seem to get the best solar periapsis. I've actually seen someone accomplish a Moho encounter by doing basically this and ONLY burning prograde, and I have no idea how he accomplished this black magic because I find at the very least you also need an ascending/descending node adjustment down the line.

You can get to Moho without a normal/anti-normal adjustment by intercepting it at it's periapse relative AN/DN which should be at or close to Moho Ap/Pe.

Edited July 28, 2016 by DaMachinator

[bewing]

If you can't get an intercept during your midcourse correction, then the "window" that the clock gave you was a lie.

[Zo San]

2 hours ago, Otis said:

I don't use KAC or any other tool for Moho. I simply eyeball it. See this post :

As far as how to perform the transfer burn, I would ask first, are you making maneuvers with the stock maneuver node tool to plan your burn first?

Otis, thank you so much for that reference! Loaded to the brim with helpful advice and it already has me thinking very differently about how to approach this particular transfer as well as others in the future. Going at the apoapsis so that you can align the orbital planes at the same time is brilliant and will make it much less of a headache.

To answer your question: yes I usually start with stock maneuver node changes just to get in the ballpark, then I usually switch to the precise node mod to fine tune.

20 minutes ago, bewing said:

If you can't get an intercept during your midcourse correction, then the "window" that the clock gave you was a lie.

Haha I think you might be right. I might also just be doing it wrong. But I suspect it might be related to how fast Moho moves; most likely even small errors on my part will cause me to miss what would otherwise have been a decent window for any other planet.

Edited July 28, 2016 by Zo San

[Streetwind]

15 hours ago, bewing said:

If you can't get an intercept during your midcourse correction, then the "window" that the clock gave you was a lie.

It's not that the window was a lie, it's that the window is tiny. Due to the way Moho's orbit contrasts Kerbin's, the ideal time to leave requires precision down to the hour (or less), and then only for a very specific ejection angle. Even the precision required itself varies between different Moho windows, as there can be so many different possible constellations between the two orbits in terms of altitude, relative inclination and relative eccentricity.

Kerbal Alarm Clock doesn't give that kind of precision IIRC. It tells you roughly what day the window begins on. Additionally, it has two different modes: a simulated and a precalculated mode. The two modes generally disagree on the exact day - and thus, either one or even both of them will have an error regarding Moho's tiny window.

Still, if you leave on the correct day, you still should be able to get an intercept somehow. It may require a costly correction burn and a more expensive capture burn later-on, but it's not like the planet has suddenly teleported away. You just can't hit it with pure rograde/retrograde anymore. Use the radial directions in combination with prograde and retrograde to either slow or accelerate your travel time towards the destination. (Assuming, of course, you inclination is already corrected to the point where doing so doesn't cause you to miss the target in the normal/antinormal direction). It comes down to your understanding of orbital mechanics and your skill with the maneuver node editor.