Flight Question Free return using LT MFD

[supersonic]

Does anyone know how to create a free return trajectory in Lunar Transfer MFD? I can't seem to figure one out.

 

[Tommy]

If I can, I'll look into that tonight. Otherwise, IMFD can do this, and is one of the "new" sections of the IMFD Full Manual version 2 that will be released soon - hopefully in the next couple weeks.


About the only way to set up a free return in LTMFD is to have a time of flight of about 75 hours (a couple or three hours either way is OK) and have a Pe Heading of around 270 degrees. The TLI scenario included with LTMFD is more or less a free-return, but the Earth return PeA is high, around 10M. This can be fixed on the way back to Earth with a burn of about 10 seconds. A lower Earth return PeA can be sometimes be arranged by changing the time of flight, but there is no way to predict the return PeA until after the TLI burn. Also, unless the target base is fairly close to the Moon's equator, a low return PeA may not be possible. If you target Brighton Beach, for instance, expect a return PeA of over 9M at the lowest.

In short, if you are going to be inserting (or Landing) at the Moon, LTMFD will get you there very accurately, and very easily. IMFD will get you there just as accurately, but will require some "fiddling around" with the offset values. If you are going to simulate some type of failure (ie, Apollo 13) and will be coming right back to Earth, IMFD can predict your return PeA before the TLI burn. It's not perfectly accurate, but a PeA of 1M or 2M can be lowered with a few seconds of linear RCS at the Moon Pe (use fore - aft) or when re-entering the Earth's SOI (use inward - outward).

The new version of IMFD Full Manual will have a much more detailed section on Target Offsetting, and includes instructions for setting up and flying a full (round trip) lunar free return. Hopefully it will be out in a few weeks (we aren't quite done yet, and want to avoid the incompleteness and errors that came from trying to meet a deadline on the first version).

 

[george7378]

I'd be interested in trying this too - I made a flight to the Moon anticipating free return, but LTFMD kept giving me errors when I tried to set it up, so I went into lunar orbit, circumnavigated the Moon once, and ejected back to Earth one orbit later. It stretched my fuel reserves though!

 

[Southwell]

I used LTMFD to perform a TLI burn only needed one mid course correction and then i displayed the map in IMFD with the cnt as r-moon target moon and it showed somewhat of a free return trajectory. I dont know if it was because i inserted into orbit but i guess if thats what you get a TEI MCC burn would get you right on track

 

[george7378]

I tried it, and it does work if you fly to the Moon with a heading of 270. I haven't tried certain flight times yet, but for the three flights I've done so far, it works well, with a 60-second correction burn at the Moon's periapsis.

 

[jarmonik]

You can use the Earth Return view mode of the TLI program in LTMFD 1.4 to setup a free-return trajectory.