Interplanetary burns...two or one?

[MajorThomas]

Can someone explain why one would want to first leave the SOI of the starting point (say, Kerbin) then perform a separate burn to get to the target (Duna, Moho, whatever)? I never thought anyone did this, but in at least one thread recently, and on a YouTube KSP video, I see that

1. Some people think it's common and/or recommended to do the two-step process. I don't yet see what is the advantage of doing this.

2. Some say it's better to do just one burn. My intuition also says to do just one burn. (I also learned about the "Oberth Effect" from this thread (thanks!) which may have been the source of this intuition. That said, I took a class in orbital mechanics 20 years ago. It's probably buried in here somewhere.) Apparently you can save a lot of fuel this way, so why would anyone do two major burns (one in the SOI and one out)?

Thanks-

[Tada]

Well, problem is, that you cannot do very precise burn from directly from Kerbin orbit, so light corection burns when in transition are way to go I think.

[Xaiier]

One burn is always more efficient due to the Oberth effect. One burn is also substantially harder to pull off without fancy calculations and autopilots. Two burns is much easier, it doesn't require precise launch and arrival windows, and it allows it to be taken more like a rendezvous than a transfer. Two burns does sacrifice the boost from the Oberth effect, which makes it more delta-v costly.

Other reasoning stems from the fact that the Oberth effect is difficult to understand. I still find it hard to fathom even though I understand why it works.

[MajorThomas]

Thanks folks.

Tada: I meant two major burns. Of course one must always do minor mid-course corrections

Xaiier: Yes, I don't have a feeling for the Oberth effect yet...haven't internalized it, can't explain it. That bugs me. I agree, "one-burn" interplanetary hops are hard to do well. There are webpages that give launch window times and angles that help A TON...I doubt I could do it without that help.

While it is fairly easy to setup a maneuver node in advance, knowing exactly when to start firing the engines and which way to point are both difficult. I find my burns take longer than the planning tools accommodate for, so I'm pointed more in an outward / away direction than I should. So frequently just after a major burn from Kerbin I must do a multiple 100 m/s delta V burn in an "inward" direction (toward the target planet). If I wait even longer, the corrective delta V gets much bigger.

So I guess the answer is, getting away from Kerbin makes the second burn easier, while escaping from Kerbin in the approximately correct direction is also easy. It just (in my experience) may cost you 500-1000 m/s delta v that way. That must be the price users are willing to pay for an easier sequence of burns.

Edited January 16, 2014 by MajorThomas