Bi-Eliptic-Transfer... again

[Tokay Gris]

So the way I understand it, a bielptic transfer is more efficient when trying to reach a target that is in another plane (i.e. Moho, Eeloo and such.

But how do you PLAN one?

The last mission to Moho almost failed because of lack of delta-V. Normal ejection burn, HUGH mid-transfer burn and impossible injection burn at Moho. Just not enough delta-V left over. (So the "orbit Moho" mission turned into a "low flyby at Moho, then return to Kerbin" mission)

If I can minimize that mid-transfer burn by doing a bieliptic... I could have made it. The orbit that is.

And the actual landing mission wouldn't either need the tanker currently on route to Moho or would be able to make the way back to Kerbin with the delta-V left.

IF I could plan a bieliptic and IF such a bieliptic on such mission reduces the delta-v needs sufficiently.

Two "IFS". Big IFS.

[capi3101]

I've got a bi-elliptic in progress at the moment - its target is not Moho, but rather a heliosynchronous orbit.

The big trick of course is to increase the apoapsis of the craft to twelve times the distance that you want for your final periapsis - for Moho, that's about 63,157,659,648 m or higher (roughly out to the orbit of Jool). The plane change burn occurs at the ascending/descending node as normal.

Getting the final intersect with Moho - that's the part I have no experience with. I've tried standard Hohmann transfers for Moho - the result of which was pretty much the same as it is for all who attempt that route (vis-a-vis my salad bar).

I will say that a bi-elliptic transfer is totally unnecessary for Eeloo - a Hohmann transfer takes long enough as it is. Got a craft en route to there too and I know it'll get there in another year or so. Again, make a plane change at the ascending/descending node.

[Traches]

A different way to get to moho is to eliminate the plane change altogether--

Instead of going for a half-orbit transfer, ignore the transfer window and just aim for either the ascending or descending node between kerbin and Moho. Once you get to periapse, play with your orbital period to get an intercept in the next couple of orbits (preferably by decreasing it rather than increasing it.)

[allmhuran]

A bi-elliptic may make getting your periapsis to the desired altitude cheaper, but if you want to actually orbit your destination it will make the orbit insertion burn much more expensive, because your velocity relative to the target will be much much higher than it wouild be for a Hohmann transfer. (Not an issue if you can aerobrake). The best way to get to Moho is, in every case I can think of, a Hohmann transfer down to Eve and then a slingshot to drop your periaspsis to Moho.

Edited December 1, 2013 by allmhuran

[Kasuha]

For most purposes in KSP, bi-elliptic transfer is impractical. Delta-v savings are not substantial in the first place, and there are few occasions in the game when the maneuver can be used to save anything.

You only start saving dv if the target orbit is at least 12 times the size of your current orbit, either smaller or larger. Moho is not that case and so isn't Jool/Eeloo. Maybe going from Eeloo to Moho would be that case but it would mean you need to spend substantial time in time warp.

You might use bi-elliptic transfer if you're in low Jool orbit and go visit Bop or Pol. But if you go visit Bop or Pol, you don't need to park in low Jool orbit - you'll save way more dv if you park right at the target orbit upon arrival.

A bi-elliptic may make getting your periapsis to the desired altitude cheaper, but if you want to actually orbit your destination it will make the orbit insertion burn much more expensive, because your velocity relative to the target will be much much higher than it wouild be for a Hohmann transfer. (Not an issue if you can aerobrake). The best way to get to Moho is, in every case I can think of, a Hohmann transfer down to Eve and then a slingshot to drop your periaspsis to Moho.

The idea behind bi-elliptic transfer is that sum dv of the three maneuvers is smaller than sum dv of two Hohmann transfer maneuvers. If doing bi-elliptic transfer to higher orbit, you'll spend less dv on orbit insertion than you would if you came directly from lower orbit.

[Kasuha]

But how do you PLAN one?

That's not hard. But as I wrote before, for Kerbin to Moho transfer, you won't save anything. The difference in orbits is not big enough. You'll just spend a lot of time on it.

[allmhuran]

The idea behind bi-elliptic transfer is that sum dv of the three maneuvers is smaller than sum dv of two Hohmann transfer maneuvers. If doing bi-elliptic transfer to higher orbit, you'll spend less dv on orbit insertion than you would if you came directly from lower orbit.

No, the total delta-V may be lower, but the orbit insertion part of the burn is definitely higher. Think about it: Your tangential velocity relative to the sun at periapsis will be way, way higher than Moho's (enough to get you back up to your apoapsis). For a Hohmann transfer it will only be "somewhat" higher than Moho's (enough to get you back up to Kerbin).

This may be an issue if your TWR is low (eg, using nuclear engines) since you will zip by the planet extremely quickly and you may not have time to slow down.

Edited December 1, 2013 by allmhuran

[Kasuha]

No, the total delta-V may be lower, but the orbit insertion part of the burn is definitely higher.

That depends on whether you're going from higher orbit to lower (then yes) or lower to higher (then no). Bi-elliptic transfer can be used for both.

[allmhuran]

We're going to Moho here, so there's no question as to whether we're going from lower to higher or higher to lower. But I expect the insertion burn is larger in both cases anyway (depending on the ratio of the major axes, but if the ratio is low then there's no point doing a bi-elliptic in the first place).