Optimal approach query

[technion]

Hey guys,

I have a rocket that is in a "just captured" orbit at Jool. That is, highly elliptical.

I've found that, for just 120dv, I can make a manoeuvre node that provides an intercept with Tylo, which happens to be the first stop. However, that intercept involves an orbit that goes out nearly double the length of Tylo's orbit and comes screaming back at a weird angle, guaranteeing a very high cost capture.

Alternatively, I can burn fuel lowering my ap to something closer to Tylo's, and then plan a much less expensive capture. The question is, which is these will be more efficient?

I realise the default answer would be "aerobrake at Jool" but I'm playing with DRE, and through trial I've found this thing explodes if I try to shave off more than 100m/s each lap. In other words, that would be a very slow and painful aerobrake.

[Kasuha]

If you get the Tylo intercept, you're coming to it from certain direction - but you can still pass on its prograde side, retrograde side, above it, or below it. Selecting which way you'll come to Tylo is matter of a few m/s in corrections when done at the right time and place.

If one pass sends you in much higher orbit, then passing it on the opposite side is going to send you in much lower orbit - that's what gravity slingshots do. But ultimately, the speed at which you'll be passing Tylo is the same. So if you're going to stop there, then just make sure you're passing it on the right side to be able to enter your Tylo parking orbit easily, then brake at periapsis. The lower your periapsis will be, the less dv you'll need to close your orbit within Tylo's SOI (but the more dv you'll need to make it circular).

[KerbMav]

Shouldnt it be easier/take less dV to establish orbit around a moon when moving prograde relative to its movement?

I mean, you are orbiting the planet with x m/s when entering the moon's SOI that is rotating around its planet with y m/s - if you are coming in prograde, your orbital velocity relative to the moon will be x-y m/s - retrograde it will be x+y m/s, so you would have to burn longer to establish an orbit, no?

[Kasuha]

Shouldnt it be easier/take less dV to establish orbit around a moon when moving prograde relative to its movement?

I mean, you are orbiting the planet with x m/s when entering the moon's SOI that is rotating around its planet with y m/s - if you are coming in prograde, your orbital velocity relative to the moon will be x-y m/s - retrograde it will be x+y m/s, so you would have to burn longer to establish an orbit, no?

Not when you're coming from outside. Orbital speed is the same in both directions. It only plays role when landing/taking off. And since Tylo's rotational speed is about 17 m/s, it's almost irrelevant which direction you go even if you land.

[RenevB]

Not when you're coming from outside. Orbital speed is the same in both directions. It only plays role when landing/taking off. And since Tylo's rotational speed is about 17 m/s, it's almost irrelevant which direction you go even if you land.

I'm prettry sure he meant prograde with respect to Jool. In that case he's quite right. If you're orbiting in the opposite direction as the moon in question, your required capture burn will be incredibly higher, but this orbit is something you could and should have corrected just after or even way before entering the planet's sphere of influence.

Now back to the OP:

I was going to argue it wouldn't matter much, but then I had a thought about the Oberth effect. Since Jool is so massive, it's probably most efficient to do most of your burning low in the gravity well, at periapsis, but I doubt it will make a huge difference and I'm still not entirely sure I'm right with this.

[Taki117]

...

Now back to the OP:

I was going to argue it wouldn't matter much, but then I had a thought about the Oberth effect. Since Jool is so massive, it's probably most efficient to do most of your burning low in the gravity well, at periapsis, but I doubt it will make a huge difference and I'm still not entirely sure I'm right with this.

This is true if you are trying to change your apoapsis. If you are trying to change orbital inclination or direction you need to perform this maneuver as far from the planet as possible to save the most dV.

[Jimbimbibble]

So, to summarize what has been said, see if a gravity assist can help you. If it can't, match planes with the target as far from Jool as you can. Then lower your Jool Pe to just above the atmosphere and burn there to lower your Ap for encounter.