Can someone tell me why the LV-N is prefered for orbital engines?

[RoboRay]

Well, generally but not always true.

One example: Harnessing Oberth Effect.

If the TWR is too low, the ship will have a harder time taking advantage of Oberth Effect. The amount of Oberth assistance missed out on will, of course, depend on the specifics of the ship and burn. However, it can be significant.

Not really. It just depends on how many periapsis kicks you're willing to make. And, by extension, how patient you are. And, how well you planned your transfer window.

[Master Tao]

I've had ships where the TWR was so low that had 6-7 minutes of burn left even when my AP was out around Minmus. At that point, you're losing a fair amount of the Oberth Effect because the fastest part of your orbit carries you 1/3 of the way around Kerbin in that time. At that point, it's a choice between major correction burns or a short burn at PE and less Oberth Effect in solar orbit.

[Laie]

This is probably true. But then again, for a lot of the designs that people make in which they use several LVNs, the incentive may be to use only one or two, which is more efficient than spamming them all over the back of your ship anyway.

This is a part of the career mode that I'm really looking forward to.... some trade-offs. Will the LVN save me money over a conventional rocket and a little more fuel? Each situation may be a little different. Hopefully the prices will be well balanced.

Not too well balanced, I hope. IIRC, the original Nerva would have increased the payload fraction by so much that it's use even as an upper stage of Saturn Rockets was the obvious way to go. And, of course, the *key* to Mars.

If LV-Ns become so expensive that one seriously considers anything else for interplanetary missions, something has gone horribly, terribly wrong. OK, make that "interplanetary missions of more than a few tons". Not that I don't support the use of re-usable tugs and all that, and interplanetary should be expensive. But LV-Ns should by all means be the way to go, even for a one-way trip to Duna.

[Idrryl]

One other problem with the LV-N (aside from it's mass and low thrust) is it's size. It's fairly big to put it in a stack, I usually use two and put them radially. That can of course create problems if you have FAR, but all in all: it's the best engine for interplanetary missions.

[lammatt]

Because It's the most efficient.

Period.

[technion]

Not really. It just depends on how many periapsis kicks you're willing to make. And, by extension, how patient you are. And, how well you planned your transfer window.

The problem I run into is that the first of those points makes the second more difficult.

From a 75km, round orbit, missing a window by one orbit is of negligible consequence. Now imagine I've done a few kicks, and the ap is out past the mun. Making another orbit now takes a long time, and can have a huge impact on the transfer accuracy.

[Jouni]

One other problem with the LV-N (aside from it's mass and low thrust) is it's size. It's fairly big to put it in a stack, I usually use two and put them radially. That can of course create problems if you have FAR, but all in all: it's the best engine for interplanetary missions.

FAR is quite forgiving with rockets, as long as you put nose cones everywhere. When I started using FAR, I noticed that every reasonable-looking rocket I had built in 0.23.5 (and a few unreasonable ones as well) worked even better with it.

[Kryxal]

I don't like to put my apoapsis to the Mun's orbit till I'm ready to BREAK orbit ... there's little enough point, 800 m/s delta-v keeps you below the Mun (a few hours to an orbit), 950 gets you near Minimus (several days to return), and 1000 is getting you out of Kerbin SoI. Once you're in these ranges you're not adding much energy but really changing your time (and needing to watch for possibly getting caught by the Mun).