Eeloo Capture Burn Way Off dV Map At 2713m/s?

[NaniByte]

I am doing an Eeloo mission. I plan the whole thing using the most popular dV map. I plan the transfer burn, from LKO of 80km, so that my intercept happens on the plane nodes as well. I MJ it and fiddle around and manage to get a 10km encounter with a cost of 2127m/s.

This is all good. However I check to see what my capture burn will be. According to the dV map the burn should be around 1370. Instead for me it's 2713! How is this possible?

I will now have to go to Dres like a loser...

Edited May 4, 2022 by NaniByte

[TheFlyingKerman]

The orbit of Eeloo is elliptic and not co-planar with Kerbin, so the DV can differ a lot between each transfer window. 

The alexmoon launch window planner is more accurate

https://alexmoon.github.io/ksp/

[NaniByte]

7 hours ago, TheFlyingKerman said:

The orbit of Eeloo is elliptic and not co-planar with Kerbin, so the DV can differ a lot between each transfer window...

Like I said, my transfer burn meets Eeloo on the plane node is almost perfect. Just a little shy of the dV map value. What I'm having a problem with is my capture burn being too expensive for no reason I can tell.

[NaniByte]

I think I figured it out. It's because I'm not meeting Eeloo at my AP. So the craft has time to speed up. But I don't see know if it's possible. To meet Eeloo on a plane node and for that to be a transfer window AND for the encounter to happen on my AP.

[king of nowhere]

3 hours ago, NaniByte said:

Like I said, my transfer burn meets Eeloo on the plane node is almost perfect.

It is not perfect. if it was perfect, you'd have 1370 m/s intercept.

It's not a matter of ejection deltaV. Getting an ejection deltaV for the nominal amount does not mean it's a perfect ejection. there are many ways of making an ejection burn with 1370 m/s, and only one of them will result in the minimal injection.

2 hours ago, NaniByte said:

I think I figured it out. It's because I'm not meeting Eeloo at my AP. So the craft has time to speed up. But I don't see know if it's possible. To meet Eeloo on a plane node and for that to be a transfer window AND for the encounter to happen on my AP.

You have to burn normal, I think.

There are a lot of factors you don't know here.

You see, ejection deltaV brings you to touch eeloo's orbit. that's all it does, putting you in an intercept trajectory. for capture, you must match the orbit of your target closely. the more similar your trajectory and eeloo's trajectory when you meet, the lesser the cost of capture. This is why it is very important to meet the target body at your apoapsis, because then your velocity is parallel to the orbit, and so you have minimal difference with the target body.

https://forum.kerbalspaceprogram.com/index.php?/topic/207502-struggling-to-get-to-any-of-the-moons-of-jool/&do=findComment&comment=4110563

Here I explain the concept in more detail, the pictures show it very well, I'm talking of moons but it's the same concept.

For bodies in circular orbit it's not important, because if you make your ejection with the minimal deltaV then your apoapsis is on the target orbit, but for bodies in elliptic orbit and with inclination it's not guaranteed. the body in elliptical orbit is not moving exactly parallel to a circumference, so you don't want to meet it at exactly apoapsis, for example. and meeting at the orbital node is cheaper, but you still have to pay the difference in plane inclination; it's generally better to pay some of that during the ejection, but transfer with high inclinations are always very difficult.

Speaking of which, when you look the deltaV map and you read 1370 m/s deltaV, you ignore the other number, 1330, written diagonally over it. that 1330 is the deltaV for a plane change manuever, because the deltaV map assumes that you are going to launch purely prograde, then make a plane change at the orbital node. So you should pay 2700 m/s.

Which is a pretty inefficient way of making transfers, mind you. if you include the planar change in the ejection and injection burn, you can save deltaV, though it's harder to plan. Certainly, though, you have to pay something.

The alexmoon planner, the most reliable tool there, says 3550 as minimum deltaV needed to reach Eeloo.  Launching in 2:259. The transfer window in year 1, which you're probably using, that's 3750 m/s. So, your hopes of doing it with 2100+1400? nope, impossible. the deltaV map isn't all that accurate, actually, because of this assumption. when you reach the point where you understand how transfers work, you stop looking at it entirely. except the part to land from low orbit.

Of course, you can save even more deltaV by using gravity assists, but that's another matter entirely.

[NaniByte]

1 hour ago, king of nowhere said:

-snip-

I am in LKO with a craft that has 3461 dV. I want to get this craft to orbit Eeloo. Acroding to the dV map I should be able to do it with 950+1140 = 2090 dV. Right?

The only thing I see that can be wrong is the transfer window. More accurately it's a "bad" transfer window as someone said.

If I use the alexmoon calc it says that there is a window in y9d59 starting from my y7d41 and that the whole ordeal will cost 3657 dV. Still a far cry from the 2090 dV.

Even worst case plane change scenario is only 3840 dV. I'm confused.

[king of nowhere]

7 hours ago, NaniByte said:

I am in LKO with a craft that has 3461 dV. I want to get this craft to orbit Eeloo. Acroding to the dV map I should be able to do it with 950+1140 = 2090 dV. Right?

 

No. You are misreading the deltaV map.

You are in LKO. from there, according to the map, you need 930+1140=2070 to reach an Eeloo intercept, plus up to 1330 to equalize planes, plus 1370 for capture. total 4770.

But, as I said, the map is not reliable when significant inclination is involved. You can save some of the price of changing inclination. So the only thing you can tell from the map is that you'll need somewhere between 3500 and 4700 m/s.

The alexmoon tool, though, IS very reliable for those kind of transfers. If it says you need no less than 3657, then you need no less than that, period. That's the best possible trahjectory, if you're really good and really lucky you may save 20 m/s over that, at most.

By the way, if you tell the alexmoon tool to make a plane change, it will still not work as the map. the map assumes you reach the orbital node, then make a normal burn to zero inclination. the alexmoon will calculate the best, and that will often entail small plane changes that still leave a lot of inclination but do end up getting the lowest intercept cost for unfathomable reasons.

So, your craft with 3461 m/s cannot reach eeloo's orbit. Well, not unless you play smart.

First thing in your favor: orbit does not mean low circular orbit. both the deltaV map and the alexmoon tool assume that you end up in a circular orbit, which requires additional braking. You can brake enough to capture around eeloo, stay in an elliptic orbit, it will save some fuel. 200 to 300 m/s, not sure. anyway, you may just barely have enough to get captured into an elliptic orbit. I wouldn't count on it, though.

What you do have in your favor is that you can make gravity assists. Now, if you're in year 9, you're in trouble, because the only source of gravity assists in the outer system is jool, and the jool-eeloo transfer window happens between years 8 to 10. During that time, you could take a jool flyby (for 2000 m/s from LKO) and get a free gravity assist to an eeloo intercept, with low intercept speed. Now you can still take a jool flyby, but intercept speed on eeloo will be a lot higher. and jool-eeloo windows are every 15-20 years. You could take a trajectory spanning multiple orbits too. Anyway, that's very difficult stuff. if you aim for a straight transfer, then no, your ship does not have enough fuel.