direct transfers to moons of other planets?

[TrooperCooper]

So I am wondering if there is any more or less feasible way to do direct transfers to the moon of another planet. I am certainly no expert on interplanetary maneuvers, but I can hold my own using the launch window planer, precision nodes etc. and get to my target without burning more than a few extra tanker loads of fuel.

But I am struggeling on how to get from... say Kerbin orbit to a Gilly orbit. Well, unless I get a lucky shot on my approach, I can go to an Eve orbit and then do another regular transfer to its moon. But somehow I think thats not how NASA would do it.

I know Gilly is small and its tiny SOI makes it more difficult to move in on it. But I'm wondering about the principle...

Using just a swing arround the planet without an actual stable orbit of course is an option, too. But then the question arises of when its best to approach and from where to minimize dV costs... and thus the launch and initial transfer burn moment becomes a critical factor as I see it.

The launch window planer from Alex Moon doesnt give me an option to choose moons as targets. So how are other people solving this problem. And is there a feasible way to do this without spending an afternoon with the calculator (nope, dont have a degree in astronomy )?

Sorry if this was answered elsewhere already. I checked several guides and tutorials, but they seem to usally cover regular planet-to-planet-transfers or just going from a planet to its own moon...

Edited June 20, 2014 by TrooperCooper

[Epthelyn]

Perform a burn to encounter the planet with the desired moon as normal, and then fine tune your approach as necessary to encounter that moon. The further away you do so, the less dV will be required, as is the case with all maneuvers of a similar nature.

You may have to mess around with nodes a bit to get exactly where you want to go (and don't warp over SoI boundaries, the orbits will change and break a carefully planned encounter) but that's about as close as you can get to what you're asking for.

Basically you want to pass the planet in such a way that you just so happen to pass within the SoI of its moon. After that it's just a case of slowing down when you arrive.

[Jimbimbibble]

If you want to minimize required dV you also need to aerobrake as you're coming in. The ideal transfer will put you on a course so that your orbital plane is more or less lined up with gilly after the aerobrake and your Ap is close to Gilly's orbit. Then continue to adjust and raise your Pe until you get the encounter you want. I find it helpful to get the trajectory reasonable half way to the planet and fine-tune immediately after entering the SoI.

[Mr Shifty]

But I am struggeling on how to get from... say Kerbin orbit to a Gilly orbit. Well, unless I get a lucky shot on my approach, I can go to an Eve orbit and then do another regular transfer to its moon. But somehow I think thats not how NASA would do it.

NASA has never transferred directly to any moon (besides our own, of course.) In fact, no moon in the solar system besides Luna has ever been orbited by a human-made object. The Huygens probe was released during one of Cassini's fly-bys of Titan.

[Kasuha]

Direct transfers to moons are possible but not always optimal. For instance if you're going to Gilly, it costs less dv to set up your transfer to go right above Eve's atmosphere and aerobrake or even perform powered brake there than to brake at Gilly's orbital altitude.

There are just two moons where I would consider direct transfers - Tylo and Laythe. And for Tylo I'd do it only if I can meet it on favourable part of its orbit, i.e. when it is moving along my own ship's velocity vector. Laythe is fine from any angle if you aerobrake.

[cantab]

The launch window planer from Alex Moon doesnt give me an option to choose moons as targets.

That's because the ejection burn will be virtually identical.

Anyway, what you'll probably need to do is set up a mid-course correction, and play around with that to see if you can get an encounter with the moon in question. I suggest trying with something like Ike or Tylo before you attempt to hit Gilly, mind you.

In terms of delta-V, it's only really worth going direct to the moon if it's big; helpfully, it's the big moons that are easy to hit the SOIs of. For a small moon, you're better off setting up a low equatorial periapsis over the parent planet, then making a capture burn or aerocapture there to put your apoapsis level with the target moon's orbit. Then make a phasing burn at apoapsis to get an encounter in a few orbits.

[Kasuha]

That's because the ejection burn will be virtually identical.

I believe it has rather something to do either with complexity of the calculation or practical usability of the result.

Some time ago I was sending three ships to Jool and since I sent them independently and did the burn manually, even though I sent them the same day they arrived to Jool almost two (Earth) weeks apart. If I was trying to land them on Tylo, initially minute differences in ejection speed and direction would translate to meeting Tylo at completely different points in its orbit. Even Pol can do complete orbit within that time.

[TrooperCooper]

All righty. Thanks for the input, guys. Much appreciated!

[SRV Ron]

Try to encounter the moon retrograde so that encounter slows you down therefore requiring less fuel for a capture.

[cantab]

That works if you're seeking to use a gravity assist from the moon to get captured into orbit round the planet, but not if you're trying to go direct to the moon.

[Tortoise]

Use KAC and fine tune your node.

[SRV Ron]

That works if you're seeking to use a gravity assist from the moon to get captured into orbit round the planet, but not if you're trying to go direct to the moon.

Going to a direct landing can use more fuel then shedding some of the approach speed by a retrograde gravity assist prior to landing. Still, you are better off going into low orbit first so that you can pick out your landing spot.

The most efficient landings are those where you can use an atmosphere to aerobrake to a landing, or to slow down enough to reach the intended target moon. Those maneuvers are difficult to set up but not impossible.

Rare is the ability to set up a gravity assist capture. I almost got one on Laythe.

Edited June 21, 2014 by SRV Ron

[metaphor]

Getting to Gilly:

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The most important part is that in order to get a cheap inclination change, you want your incoming planet periapsis to be on the same orbital plane as the target moon.

[MarvinKitFox]

My advice is basically same as metaphor above, just less picturesque.

When heading to a planet's moon, it is almost always less delta-v, and always easier, to dive past the planet first.

points to consider:

1) You get a healthy boost to efficiency from Oberth effect, effectively usign the planet's gravity to 'boost' your engines.

2) You get a virtually free plane-change maneuver. In the case of eve-gilly, this is significant. By spending as few as 1.5m/s course adjustment shortly after leaving kerbin, you can tweak your encounter with eve to bend your eve-orbit into exactly gilly's plane.

3) if you are gutsy, you can save 600-ish d-v by aerobraking on arrival at eve. More, if you are on an accelerated transfer and arrive with 1000's of excess V.

4) This also eliminated the very hairy problem of timing your arrival to coincide with the right position of the moon!

Edited June 23, 2014 by MarvinKitFox