TransX Flytandem tutorial 1 vs. a newbie

[helgeras]

I am probably doing something stupid here, but I just cannot figure out the first of Flytandems tutorials. It is the Lunar return one (http://flytandem.com/orbiter/tutorials/lunar_return/index.htm).

The first problem I have is that I do not understand what it is I see on the right MFD when I add prograde velocity.
As I understand it, that view shows the moon orbit with earth in the center, and the green line that shows up is the projected orbit.
My problem is that when I set prograde velocity to 689 in the left window, then the green ellipsis in the right window does not reach the moons orbit. That cannot be true.
Also, I would assume it passes through the moons current position (where the blue radial crosses the blue moon orbit), but it doesn't.
The green curve starts to behave more like what I assume it would when prograde velocity is above 1000.

Flytandem then suggests to play around with the date and velocity until I get a maneuvre that looks like the picture in the tutorial, and I indeed get the picture with the values mentioned in the tutorial, but whats especially good with these values? Is it just to get a low "Focus PeD", which I assume is periapsis distance? (What does focus mean in this context?)

Can anyone explain what it is I see on the right display, and what it is I should be looking for when manipulating the velocity and date?

Regards,
Helge

 

[RisingFury]

Nothing is especially good about those values. Those values aren't constant - they change with time. One day they'll be lower, one day higher.

The numbers in the tutorial are just examples...

 

[C3PO]

Davs Helge

The MFD on the right shows your orbit after you've left the Moons sphere of influence (SOI). You have removed some of the energy so you're in a lower orbit then the Moon.

You can change the direction that you leave the Moon SOI by delaying the ejection burn. Try changing the the ejection date value using the finest adjustment. You will notice the orbit in the second stage (right MFD) becoming more eccentric. When Focus PeD is the same as Maj. Rad you are going to hit the Earth.

 

[helgeras]

Thanks for the replies.
So, that date and velocity was chosen to get a low "Focus PeD"?

"The MFD on the right shows your orbit after you've left the Moons sphere of influence (SOI)."

I still don't quite understand how I can get an orbit in the right display that does not go all the way back out to the moon?
I would have thought that energy preservation would result in a path that would go at least as far out as where it started?

Helge

 

[C3PO]

I would have thought that energy preservation would result in a path that would go at least as far out as where it started?Helge

It does! But you "start" the second stage orbit when you leave the Moon's SOI. If your ejection is pointing towards Earth, you will be a lot closer than the Moon by then. But you'll "inherit" enough velocity from the Moon to make your PeD way too high for an Earth transfer orbit.

What you want is to point your ejection opposite to the direction that the Moon is traveling, by doing the ejection burn 1/4 orbit later.

An accurate Moon --> Earth transfer is one of the harder things you can do in Orbiter, so don't worry if you can't visualize it right away.

 

[helgeras]

Ah, I think I understand it now.
When after the burn I start to fall towards earth, but slower and slower until I leave the moons SOI, then I speed up again. Thats why I dont go all the way back to the moon, but only to the SOI plus whatever velocity I had when I reached that spot on my way towards earth.

I will try this again before continuing to tutorial 2.

As General McArthur said... I will return

Helge

 

[C3PO]

Ah, I think I understand it now.
When after the burn I start to fall towards earth, but slower and slower until I leave the moons SOI, then I speed up again.

The Moon doesn't actually slow you down that much. The scale on TransX stage #1 is increasing, so it looks like you're almost not moving.

TransX doesn't calculate with more than one gravity source. Instead it breaks the trip into stages with one gravity source each. This means that during the transitions the displayed orbits aren't that accurate.

If you open the default OrbitMFD shortly before leaving the Moon's SOI, you can check how much each gravity force affects your craft. Change the reference body between Moon, Sun and Earth. At the bottom of the MFD there's a number that shows how much of the gravity comes from the reference body. In low orbits the number is 1.00 but as you leave the Moon's SOI no reference body shows up with more than 0.50.

The overall error of the whole plan is small, but the displayed orbits are not reliable during SOI transitions. You have to wait for the display to settle down.