Going to Eeloo; Launch Window Planner and Plane Changes

[davidpsummers]

I sent a ship to Eeloo using the great Launch Window Planner by Alex Moon. Eeloo is, of course, inclined and I chose to do a midcourse correction to account for that. It gives me a time to do the Plane Change. But the time is different than the ascending node. Wouldn't it be better to handle this at the node?

Edited June 18, 2014 by davidpsummers

[Tortoise]

Idk, but I recommend using kerbal alarm clock for all your planning needs. Everything is automatic.

[allmhuran]

Not always. Burning at the ascending/descending node is necessary if you want to match your orbital plane with the target, but it's not necessary if you just want to make sure you intercept it (and don't mind coming in on a more polar apprach).

Picture yourself in an orbit, and imagine a circular disk representing your orbital plane. When you perform a normal burn, the disk will rotate round an axis that passes through its center and through the point on the edge where you perform the burn. This means the part of the disk that moves out-of-plane the furthest will be the point 90 degrees around from where you perform the burn.

As such, the most efficient place to perform a normal burn in order to get an intercept is 90 (or equivalently, 270) degrees around from your planned intercept point, but with the proviso that if you do this you will approach the target from either above or below it rather than equatorially, and therefore if you circularize you will end up with an orbit shifted towards polar and away from equatorial.

[technion]

I'll say this - transfers to Eeloo, due to its elliptical orbit and inclination, often look about as inefficient as possible. I'm in the middle of one now, and the orbit goes out way, way past Eeloo and catches it on the way back, despite being setup with Precisenode to exactly match an Alexmoon recommendation.

Meanwhile, if the transfer burn takes any more than 2-3 minutes (and it will if you use LV-N), the slight misaim will be enough to cost an encounter, so I always recommend budging for a significant correcting burn.

[davidpsummers]

OK, I see the point. One advantage of the burn at the nodes is that the display will do a better job of showing the closest approach, to make it easier to find an intercept? How important is that?

[allmhuran]

The closest approach should always show if your orbital path takes you "close enough" to your target's orbital path. If you have an intercept with the target, the closest approach is usually going to be very close to either the ascending or descending node for fairly obvious reasons: If you are close enough to the target for an intercept, then your orbital line is probably crossing its orbital line at around the same point. The only time this won't be true is when you're already very close to the target's orbital plane around your whole orbit anyway (difference of 0.0 - 0.1 degrees).

An example of this can be seen in the video here from about 1:30

http://www.youtube.com/watch?v=mnPSNH8zlmw

[davidpsummers]

I was playing around with a mission to Jool. (Boy, is that a _lot_ easier to hit!) I guess the only advantage of burning at the nodes is that it is an easier (but not the only) way to come in to the orbital plane. (Since I'm hoping to do a bunch of flyby's of some of the satelites).

[allmhuran]

If you're just burning, then burning at the nodes is indeed necessary to match planes. But you can also use the gravity of your target(s) to assist. Jool makes things particularly easy, because in addition to using Jool itself you can use the moons... indeed, it's almost impossible to avoid getting pinballed if you are in an eccentric Jool orbit. Combined with a high altitude aerobrake on the first pass (leaving you with an extremely high Jool apoapsis) you can match planes with Laythe, Vall and Tylo for negligible delta-V, burning only to make the adjustments necessary for your various encounters.

[davidpsummers]

If you're just burning, then burning at the nodes is indeed necessary to match planes. But you can also use the gravity of your target(s) to assist. Jool makes things particularly easy, because in addition to using Jool itself you can use the moons... indeed, it's almost impossible to avoid getting pinballed if you are in an eccentric Jool orbit. Combined with a high altitude aerobrake on the first pass (leaving you with an extremely high Jool apoapsis) you can match planes with Laythe, Vall and Tylo for negligible delta-V, burning only to make the adjustments necessary for your various encounters.

Yeah, I was thinking about aerobraking, but for my first mission I don't know how high to pass the planet.

[StrandedonEarth]

Hard to aerobrake at Eeloo when there's no atmosphere. Jool, IIRC, was about 115km or so, but your best bet is to quicksave when you enter Jool's SOI (for the cheapest trajectory adjustments) and expect to quickload a lot until you get the post-aerobrake orbit you want.

[davidpsummers]

Maybe I could try "lithobraking" on Eeloo.

("Lithobraking" is a term for when you plough into the crust. It does quickly kill your velocity. Though I'm not sure Eeloo isn't made of ice instead of rock. :-)