IMFD Free Return Trajectory

[jambooger]

With the help from the manuel and the settings it suggested I was able to get a Free Trajectory Return Plan that had a little reverse loop at the moon. But when I try it at other planets to their moons I get a normal Eliptcal orbit showing. I would like to get the Figure Eight looking plan but I cant figure out what setting I need to change. Thoughts appreciated.

 

[Tommy]

The "figure 8" trajectory you get in an Earth-Luna flight is rare. It requires that the moon's size (relative to it's planet) be large enough, and the distance from the planet has to be right as well. You may be able to get a "figure 8" on a Saturn - Titan trip, but I'm not sure. You would need to start from a parking orbit at the right altitude (not sure what that is) or it wouldn't work.

In short, most moons don't have enough "pull" to do it. Your velocity at the moon Pe needs to be higher than the moons own velocity.

Also, Map needs to be configured properly. The CNT must be the periapsis (ie, CNT = p-moon, or p-titan) and INT must be enabled. Most times you will also need to set a time limit so the trajectory displayed doesn't get cut short by the period limit or hyper limit.

Free Returns don't need to have the "figure 8", the only requirement is that your trajectory will bring you back to your starting point if no insert burn is made at the target. It will look like an ovoid (egg shaped, sort of pointy at the high side) more often than it will a figure 8.

 

[jambooger]

Thanks Tommy for a great explanation as usual. I wont worry about the figure 8 anymore.

 

[Tommy]

No prob. I gave it some thought, and don't think Saturn - Titan will work. For one, Titans atmosphere would prevent you from having a low enough Titan Pe. Second, Titan is too close to Saturn, and has a very short period (about 10 days, IIRC) and a large (compared to the Moon) radius, which means it's travelling too fast.

Luna is unique in this solar system. It has a large mass (relative to Earth) so it has sufficient gravity. It orbits outside the Earth's SOI and has a long period - so has a low relative (to Earth) velocity. It's that combination that makes it so easy to do the figure 8. In fact, Luna is big and slow enough that we need some excess velocity - that's why free-returns take three days instead of the 5 days for a Hohman transfer. Shortening the time-of-flight is how we get IMFD to add the "excess" velocity. Without that excess, the lunar Pe would need to be higher than we want our lunar orbit, perhaps too high.

Also, assuming you want to land on the moon, a low insert altitude will be more efficient than a higher one (Oberth Effect).

There is good news, and bad news. The good news is that Target Offsetting is covered much better in IMFD Full manual version 2. The bad news is we aren't done with it yet, and it will be at least a couple more weeks until it's ready for release.

 

[jambooger]

There is good news, and bad news. The good news is that Target Offsetting is covered much better in IMFD Full manual version 2. The bad news is we aren't done with it yet, and it will be at least a couple more weeks until it's ready for release.

I am looking forward to its release.