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michal.don

Free return-ish trajectory to Mars and Venus (RSS/RO)

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Hi guys,

Since I have managed several succesful lunar crewed mission in my current save, I think it is the time to move to exploration of other planets. The game offers a Mars crewed flyby and Venus crewed flyby, and a generous amount of funds for completing it. For the first mission, I'd like to do just the flyby, and do it on the cheap side, so to utilise the gravity assist to bring me as close to Earth intercept as possible. Do you have experience with similar missions? Is there a way to plan it, or should I just go for Mars and try to set the flyby on mid-course correction so it takes me close to Earth later on?

I'm quite nervous to take such a gamble, considering I haven't lost a single Kerbal in space so far.

Thanks,

Michal.don

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I also suggest FFRSS  (I wrote it). In one of the guides I made a panorama of Earth-Mars-Earth free returns, it was in the pre-RSS v12.0 version of FFRSS so it will be a bit different now if you are using the latest RSS (which is what the latest version of FF is for). Here it is:

I also did a piloted mission to Mars using a flyby of Venus to get there, the path I chose could have aborted the Mars landing and done a free return to Earth, so you can flyby Venus and Mars and get back to Earth with only a few small mid-course corrections. It's here. Watch out though, the arrival speed at Earth can be pretty high if you're not careful.

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Setting up the flyby won't be hard, and if it's not a close flyby you should return to where you left Kerbin without much of a correction burn, the problem will be getting there at the right time.  If you can set up a transfer orbit that's close to 3/2 of Kerbin's orbital period, it would probably do the trick in a reasonable timeframe, more or less.

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2 minutes ago, Kryxal said:

Setting up the flyby won't be hard, and if it's not a close flyby you should return to where you left Kerbin without much of a correction burn

Well, that's the first problem - it should be a close flyby, for scientific reasons. And the contract says so, so the influence on the resulting orbit around Sun will be significant.

3 minutes ago, Kryxal said:

probably do the trick in a reasonable timeframe, more or less.

And that is the second problem. Since every kilogram counts in RO shipbuilding, I just can't afford tu pack life support supplies for several years of drifting around the Sun. It has to work the first orbit, or the crew are dead.

Michal.don

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Assuming Hohmann transfer, reaching high orbit of destination planet, doesn't want to burn much, and want to return when you come back to 1AU the first time, then I don't even think such free return trajectory exists. Unlike Moon situation, for other planets, if you want some sort of free return trajectory, then you'll need the period of the transfer orbit to be very close to an integer multiple of Earth's orbital period. This doesn't happen for both Venus and Mars. In general, if you go inwards, then when you finish a period of your transfer orbit, Earth is definitely not catching up. And for Mars, it was ~1yr5months, not whole years.

So you have to give up at least one of the assumptions. You could do a non-Hohmann transfer, where you could let the transfer orbit to intersect Earth's on two different points, and leave at one, returning to another. But you'll need careful calculations and it might take much more fuel (because the wasted radial component must have been huge)

Or you could reach a low orbit, take advantage of gravity assist somehow. Note that you might still spend more fuel because the transfer orbit that can take advantage of gravity assist to achieve free return might be quite different from traditional Hohmann transfer, and that means much more fuel. I'm not saying it doesn't exist, but this requires numeric calculation and there's a decent probability that a cheap solution doesn't exist.

Or if you don't need to return home the first time it comes back - then theorerically any orbit is free return because eventually they will meet. It might takes thousands of years, though.

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This tool can help you:

You can use it to find Earth-Venus-Earth (or Earth-Mars-Earth) trajectories using theroretically no more than that of the initial ejection burn (keep a small margin for correction burns).

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9 hours ago, FancyMouse said:

So you have to give up at least one of the assumptions. You could do a non-Hohmann transfer, where you could let the transfer orbit to intersect Earth's on two different points, and leave at one, returning to another. But you'll need careful calculations and it might take much more fuel (because the wasted radial component must have been huge)

Yes, I'm aware of these limitations, I meant "free-return-iish" as a trajectory where I won't have to do any major burns adter my ejection burn from LEO. Using the gravity assist and plotting a low flyby seems like a reasonable option to me, I'm just not confident that I can set up the flyby at ejection to bring me back home - heck, even setting up the ejection to intercept your target is not very easy in RSS.

8 hours ago, Gaarst said:

This tool can help you

This looks exactly like the thing I need, I'll take a look at it and I'll see what I can do.

Thank you, 

Michal.don

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20 hours ago, PLAD said:

I also did a piloted mission to Mars using a flyby of Venus to get there, the path I chose could have aborted the Mars landing and done a free return to Earth, so you can flyby Venus and Mars and get back to Earth with only a few small mid-course corrections. It's here. Watch out though, the arrival speed at Earth can be pretty high if you're not careful.

Thank you very much, this should be very helpful, as soon as I wrap my head around how exactly it works :)

Michal.don

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Posted (edited)

Are you aware of NASA's trajectory planner? It has some limitations when applied to KSP and in particular you cannot get trajectories for any arbitrary time (and they probably wouldn't be accurate anyway due to drift), but is awesome to get a general idea of the requirements for a Mars/Venus flyby.

Edited by blakemw

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