When and how to go for bi-elliptical transfers?

[Andersenman]

Hello,

When do you decide for bi-elliptical transfers, and where do you place your new Ap and Pe points to get around? I am looking for simple decision helpers, rules-of-thumb and the like.

Many thanks!

A.

[Kasuha]

Wikipedia is your friend

Apparently bi-elliptic transfer is good idea when the radius of your target orbit is approximately 12 times bigger/smaller than radius of your initial orbit (assuming circular orbits). For initial orbit at 100 km above Kerbin (radius = 430 km) it would mean transfer to at least 4830 km orbit.

And the saved delta v does not even seem worth it.

[Artorp]

It's mostly useful for a transfer to low Kerbol orbit, there are few cases where the ratio of semi-major axes are high enough for the savings to matter.

Another example would be when you desperately need to change your inclination from say a prograde to a retrograde orbit, but I'm not even sure if raising your apoapsis to revert the orbital velocity can be called a bi-elliptic transfer.

As a rule, when it is worth it to do a bi-elliptic orbit you want your apoapsis as high as possible, the higher it is the more ∆V is saved. So if you're doing a transfer around kerbin you want your apoapsis to be at the very edge of Kerbin's Sphere-of-Influence.

Edited November 4, 2013 by Artorp

[capi3101]

I've heard of folks doing it for transfers to Moho and for getting into heliosynchronous orbit around Kerbol; in both cases, the recommended apoapsis is somewhere in the neighborhood of the orbit of Jool.

[SpaceSphereOfDeath]

I've heard of folks doing it for transfers to Moho and for getting into heliosynchronous orbit around Kerbol; in both cases, the recommended apoapsis is somewhere in the neighborhood of the orbit of Jool.

Why not go to Jool then?

Bi-elliptic orbits are good IRL, but in game they just seem a bit, excessive.

[Nao]

I use Bi-elliptic orbits for:

Getting faster planet (and other ships) intercepts at smaller fuel cost than forcing one burn transfer.

Making big inclination changes (above 38,94 deg inclination change its more efficient to make bi-elliptic 3 burn plane change) (KSP Orbit Mechanic has a chart of terminal radius ratios for different angles)

Also while not pure bi-elliptic there are some transfers that use more than one burn to get some dV for free from other bodies like:

Going to Dres and Eeloo using Jool aerobrake can save some dV sometimes

Going to Moho or low kerbol orbit through gravity slingshot off Jool (never tried it thou)

[capi3101]

Why not go to Jool then?

Bi-elliptic orbits are good IRL, but in game they just seem a bit, excessive.

Well, the whole point of doing a bi-elliptic transfer is to save delta-V. Case in point: you do a Hohmann transfer to get to Moho, you can expect to spend somewhere in the 7,500 to 10,000 m/s of delta-V range getting there and back again. If you do a bi-elliptic transfer instead, you can pull it off for around 4,000 (i.e. what the delta-V map will tell you). It just takes a lot longer to do, obviously (it takes somewhere around 45-50 days to get to Moho with a Hohmann transfer, about 220 days just to get out to Jool's orbit).

Nao mentions that you can get a gravity slingshot off of Jool, so perhaps going there on your way towards Kerbol/Moho isn't that bad of a notion...