Spacing long burns over multiple orbits

[Tyko]

I was having a heck of a time with a 20 minute burn for a transfer orbit from Kerbin to Jool. My maneuver plot told me it would be ~2400m/s DV, but the actual burns ended up being over 3500. After some experimenting I figured out that most of my 20 minute burn was happening too far from PE to be efficient and I was wasting a lot of DV at the ends of the burn.

My solution was to do 2 shorter 4 minute burns on successive orbits and then do a final burn on the 3rd orbit. This method got my total DV very close to the 2400 I'd initially plotted.

Have others tried this?

[Aegolius13]

Yep, it's a pretty popular technique for long burns.   Sometimes known as "Periapsis kick." 

[pincushionman]

Yes, and it's almost essential for ion use.

[FullMetalMachinist]

Perhaps this will help? 

 

[Red Iron Crown]

59 minutes ago, FullMetalMachinist said:

Perhaps this will help? 

One hopes so.

[Tyko]

Thanks @Red Iron Crown  ..good stuff. Since I only did three burns I just recomputed my course after the first 2 4-minute burns.  I was still able to get within a few percentage points of my original calculated DV budget. I imagine I can get even closer with this.

Do you just hit the "advance to next orbit" on the maneuver node after each pass?

Edited August 15, 2016 by tjt

[Red Iron Crown]

2 minutes ago, tjt said:

Thanks @Red Iron Crown  ..good stuff. Since I only did three burns I just recomputed my course after the first 2 4-minute burns.  I was still able to get within a few percentage points of my original calculated DV budget.

Yeah, I got reasonably good at doing them by hand before actually setting out to do the math. As long as your last interim orbit is well under the Mun the orbital periods are short enough to not make much difference.

[OhioBob]

When I perform a two-burn strategy, I like to make the first burn just big enough to place the spacecraft in an orbit with a period of exactly 6 hours.  This should take about 785 m/s and place the spacecraft is an orbit with an apoapsis about halfway to Mun.  This orbit brings the spacecraft back to the exact same spot the next day to complete the burn.  This makes it easy to plan things so that the final ejection burn occurs at the right time and at the right location.  I simply perform the first burn at the exact spot and time of day that the final burn should occur, I just do it one day earlier.  Fortunately I've never had a burn long enough that it couldn't be completed in two parts.

Note that in 6 hours Kerbin revolves around the Sun about 0.85 degree. Therefore, it is necessary to perform the first burn at an ejection angle that is 0.85 degree less than the computed transfer ejection angle.  This assures that when the spacecraft returns to the same spot 6 hours later, the ejection angle will be exactly correct.

Edited August 17, 2016 by OhioBob