Can I put a station into a figure 8 loop-orbit around Mun and Kerbin?

[Valley]

Hello!

Was watching a video on mining the Moon and they were talking about ships going between the Earth and Moon in a figure 8 loop. I assume it would be using fuel to do so but I was wondering if, in the game, a station could be placed in some kind of orbit that would allow it to go back and forth between Kerbin and the Mun - allowing me to send landers, crew, and fuel back and forth without cost? I doubt it - but has anybody with better math skills and better skills at the game tried it? Thanks.

Edited January 27, 2015 by Valley
Question was answered

[Alshain]

It's called a Free Return Trajectory. You put your craft in just such an orbit that it encounters the muns gravity which then slingshots it back around to Kerbin. You do use fuel to adjust the orbit, but it's cheap because most the work is done by the mun. Yes you can do it in KSP, however the way the patch conics shows visually, you may not "see" the figure 8 even though it is happening. It makes a good cheap way to get early science in career from the muns orbit.

Also, This is how Apollo 13 got home safe.

EDIT: One other note, you can't do it continuously without spending fuel. You can get the free return once, but the mun will inevitably move out of your orbit or worse, move too close into your orbit.

Edited January 26, 2015 by Alshain

[Valley]

It's called a Free Return Trajectory. You put your craft in just such an orbit that it encounters the muns gravity which then slingshots it back around to Kerbin. You do use fuel to adjust the orbit, but it's cheap because most the work is done by the mun. Yes you can do it in KSP, however the way the patch conics shows visually, you may not "see" the figure 8 even though it is happening. It makes a good cheap way to get early science in career from the muns orbit.

Also, This is how Apollo 13 got home safe.

So if I put a station into that figure 8 loop orbit it would keep doing that orbit again and again?

[Alshain]

So if I put a station into that figure 8 loop orbit it would keep doing that orbit again and again?

Sorry, I edited too late. No you can't exactly do that as I mentioned above.

[NASAHireMe]

Yup, what Alshain said. If you're really, really good, you might be able to fine-tune your first free-return trajectory to get you into a second encounter. Actually, on second thought, it might not be possible. Returning from the Mun takes at least a day, by which point the Mun will have revolved quite a bit around Kerbin.

One extra free-return might be within the realm of possibility; two free-returns is almost impossible.

[Empiro]

You might be able to make repeated free-returns if you're exceptionally precise with your orbital period so that after your return, you'd get another free return encounter. You'd have to keep making corrections since even tiny deviations will quickly put you off course.

However, it still wouldn't allow you to ferry things back and forth for free. To dock with a craft doing this, you'd need to match velocities, which means that you're putting yourself in the same orbit. To drop things off at the Mun, you'd need to slow down into a Munar orbit.

[basic.syntax]

I think the overall problem with re-creating this in KSP, is that KSP does not simulate N-body physics. Each time you leave the "Sphere of Influence" (link takes you into waycomplex territory) of one body, your ship is handed off to the gravitational effects of the other. KSP does a good job of handing off your momentum from one transition to the next, but its not as smooth or realistic as it could be, if the simulation took each planetary body grav effect on your ship into account.

[Chewy]

So if I put a station into that figure 8 loop orbit it would keep doing that orbit again and again?

No... It is not a stable orbit in both RL and in KSP. When you do a figure 8 type slingshot, you will lose energy, so often you would be put on a trajectory that likely would re-enter Kerbin atmosphere. To have a continuous loop, each time around you would have to do one burn to keep your trajectory out of the atmosphere, and another burn to rotate you major axis enough for another Mun intercept.

[Alshain]

Actually, on second thought, it might not be possible. Returning from the Mun takes at least a day, by which point the Mun will have revolved quite a bit around Kerbin.

From the Mun? It should be a Kerbal day round trip.

[Valley]

Thanks folks. I figured you can't get free energy in this case but I was kind of wondering.

[Kerba Fett]

Even if you could get a stable orbit out of it, you'd still need to match orbit with the station on anything going to the Mun. That would take just as much dV as simply going directly to the Mun and would be much more difficult to rendezvous with due to the non circular orbit.