plane changes and orbit reversals - most efficient way?

[KerikBalm]

I'm unsure of what the most efficient way to do plane changes and orbit reversals (ie change a retrograde orbit to a prograde).

Initially I'd just think "oberth effect" and do everything at perapsis... but... then I think in a very eccentric orbit, suppose at perapsis my velocity is 2,000 m/s and at apoapsis, it is 200, then to reverse the direction, perapsis burns would require 4,000 dV, but apoapsis burns would require only 400.

Plane changes would work similarly I would think, so then I start to wonder if the most efficient way to make these changes from a low orbit is to burn prograde (not retrograde) at perapsis to put the apoapsis way out there, then change the plane/direction at apoapsis, and then a retrograde burn at perapsis to circularize again.

but then I wonder if the dV needed to make the orbit highly eccentric, and then circular again would result in more total dV use.

Later today I may have time to experiment, but if I could get an answer now, it would save me some time

Edited April 26, 2014 by KerikBalm

[FREEFALL1984]

Unlike burning retrograde or prograde, changing your orbital inclination is best performed at the slowest orbital speed. imagine you're travelling at 2200m/s in order to reverse your orbit (ignoring DV expended in recovering your orbit) you need to expend 4400m/s of DV, now if you spend a few hundred m/s burning prograde and raising your orbit, you might then only be travelling at 400m/s at apoapsis, then in order to reverse that orbit you only need 800m/s, So assume a 300m/s burn to climb, an 800m/s burn to reverse the orbit and another 300m/s burn to circularize. and you have a total burn of 1400m/s in my experience I find that anything less than 30 degrees it simply isnt worth changing the orbital altitude. anything more than 30 degrees and you should really start raising apo. That being said there is rarely an instance where you dont have the option to change inclination before to circularize or climb into orbit.

So as a rule of thumb, when entering a new system, its always best to perform your inclination changes as soon as (or even before) you enter the SOI, and if I'm making a targeted landing at laythe or duna I'll do a light aerobrake maneuver until my apo just drops into SOI and then once at apo i'll perform my plane changes

Edited April 26, 2014 by FREEFALL1984

[O-Doc]

I know that orbit reversals are most efficient at the very edge of the sphere of influence. I'd say the same is true for plane changes given that they would operate on the Oberth effect as well. Which, basically states that, your orbital velocity is greatest the closer you are to that body. Ergo, your speed is slowest the further away you are from it meaning, the smaller the burn required to change your orbital trajectory.

[BudgetHedgehog]

This will answer all your questions.

[KerikBalm]

This will answer all your questions.

Thanks, its as I suspected. I guess it makes sense, pretty much any plane change can be done for about 2x the dV need to go from low orbit to "intercept" on the dV map (once to go out, miniscule dV to change inclination, then you pay the "going out" dV again to recircularize)

I know that orbit reversals are most efficient at the very edge of the sphere of influence. I'd say the same is true for plane changes

Indeed, a complete orbit reversal is nothing other than a 180 degree plan change. Although, for small inclination changes, its surely not worth going out to the edge of the SOI.

when entering a new system, its always best to perform your inclination changes as soon as (or even before) you enter the SOI,

I'm well aware, but right now I'm doing a new career mode, as soon as I got the docking ports and some science instruments + the lab, I set up stations/fuel depots around mimus and the mun->initially in equatorial orbits. Then I used a small lander to just go harvest science. As the lander was designed as minimal as possible for deorbit, science, back to orbit, dock and clean instruments, it lacks the dV to visit the poles. The stations/fuel Depots have a LV-N, so they are mobile under their own power and actually have quite a bit of dV when full (I guess you could call them ships, but they weren't really intened as such) now I want to move my stations to a polar orbit so I can get the last of the biomes. I want to do it as efficiently as possible, because I plan to again put them into equatorial orbits afterward.

I guess I could have just started the mission in a polar orbit, or built a bigger lander with more dV(but I'm not sure if that would be better, as the lander would then use much more fuel per landing and ascent)

[Red Iron Crown]

Oberth effect only applies to when you wish to change your specific orbital energy (i.e. raise or lower periapsis or apoapsis). For inclination changes you are not changing your orbit's specific energy, just the direction. It is best to do so when moving as slowly as possible, in fact it usually works out to be most efficient to raise your apoapsis as high as possible, burn the inclination change there, then recircularize at periapsis.