Maneuver marker on Navball confusion

[Biggen]

I'm been planning a mission to Moho (because I like the pain) and I've read just about every post out there about getting there and what method to use.  I am doing a test flight to there by launching a few relay sats into Moho orbit first before I begin sacrificing my poor Kerbals...

Anyway, using @maccollo method described in this thread, I'm having an issue.  I'm in LKO (~100KM) and when I setup my maneuver node it gives me like a 26 minute burn time (single nuke pushing approx 49 tons of mass) to Moho.  This is a prograde burn of 2000 and a antinormal burn of 1170.  I'm trying to combine the ejection burn with the inclination change.

Using @Snark's BetterBurnTime mode, it gives me the count down, and tells me to go in approx half the time it takes to arrive at the maneuver node (pretty much like expected).  I make sure the maneuver marker is locked and give it full throttle when the countdown is done.  However, after I start the burn, the dV countdown next to the Navball increases instead of decreases and I eventually end up going back into Kerbin's atmosphere and blowing up.  Looking at the maneuver marker on the navball when I'm supposed to start the burn it appears to be taking me close to radial in when I'm supposed to begin my burn.

What am I missing here?  Is the burn time just too long (longer than my orbit around kerbin) and that's causing the issue?

Edited February 6, 2017 by Biggen

[renhanxue]

44 minutes ago, Biggen said:

What am I missing here?  Is the burn time just too long (longer than my orbit around kerbin) and that's causing the issue?

Yep, that's it. I don't remember the exact numbers off the top of my head but the orbital period for a 100 km Kerbin orbit is something like 30-35 minutes, so by halving a 26 minute burn you'll end up starting on the wrong side of the planet. You need to split your burn into many smaller ones - probably around 3-5 minutes each. The longer they are the more dV you lose to not burning in the right direction at the right time, but on the other hand the longer you take to complete the burn the further off the correct ejection angle you'll be. It's a tradeoff.

Edited February 6, 2017 by renhanxue

[Biggen]

Ahh I see!  Ok, I figured that was the problem.  It becomes hard to time long burns like this when close to a body as you end up running into the body (like I did) if you just follow the maneuver marker.

I guess the solution is multiple burns or bigger/more engines to overcome the TWR issue.

[Nicias]

Or transfer out to a larger orbit. You loose some Oberth then though.

[FullMetalMachinist]

@Biggen what you're looking for is a method called 'periapsis kicking'. As was suggested earlier, you split your single long burn into many smaller ones. The trick then becomes the timing of all the small burns so that your final one has the correct ejection angle and you do it at the correct time for your transfer window. Luckily, someone has already done the work for you. 

 

[Biggen]

9 minutes ago, FullMetalMachinist said:

@Biggen what you're looking for is a method called 'periapsis kicking'. As was suggested earlier, you split your single long burn into many smaller ones. The trick then becomes the timing of all the small burns so that your final one has the correct ejection angle and you do it at the correct time for your transfer window. Luckily, someone has already done the work for you. 

 

Ahh Great!  I'll read up on that now!

I finally got to Moho, btw.  I ended up manually piloting out of Kerbin SOI and ended up pointing my craft between the prograde and maneuver marker.  This allowed me to miss Kerbin but keep some of my anti-normal burn going as well.  After I got to the correct side of kerbin for the maneuver node, I locked the maneuver marker back in and was on my way.  I got within .4 degrees off the inclination (from 7 degrees at the start) of Moho after the burn was completed by doing this so I'm quite proud of myself!

Edited February 6, 2017 by Biggen