Flight Question Rinc?

[dykesc]

I have self flown the DG to Moon tutorial pretty much successfully, but I have a question regarding the RInc parameter in the Align Planes MFD.

I advanced time and waited for this parameter (Relative Inclination) to reach a minimum before I launched so that my earth orbit Align Planes burn would be minimal. Everything worked out fine with a before burn RInc of somewhere between 5 and 6 degrees.

What I can't visualize is what is causing this parameter to reduce as I advance time before launch. I know I'm advancing time until Canaveral is very close to the lunar orbit plane's intersection with earth but still can't visualize how RInc is falling. The moon's inclination is a constant just over 5 degrees. What exactly is RInc measuring? What angle? Regardless of time of day when I launch, if I hold a heading of 90 degrees to orbit insertion around earth, I am going to end up in the same orbit plane so I can't visualize how waiting to launch is minimizing my relative inclination to the moon's orbit.

A visual aid might help.

As always, thanks in advance for your help.

 

[boogabooga]

Regardless of time of day when I launch, if I hold a heading of 90 degrees to orbit insertion around earth, I am going to end up in the same orbit plane...

That is not a true statement.

First, you need to understand orbital elements:
[ame="http://en.wikipedia.org/wiki/Orbital_elements"]Orbital elements - Wikipedia, the free encyclopedia[/ame]

The heading you launch to sets the element known as inclination. The time you launch at determines another element called "longitude of ascending node." Actually, this is only true in an equatorial frame of reference. But at any rate, the time you launch very much determines your orbital plane as much as the heading.

Anyway, read the wikipedia article and see if that doesn't help.

Also, see this:
https://www.youtube.com/watch?v=bOxpvqrqLAo

 

[Urwumpe]

What exactly is RInc measuring? What angle?

RInc is relative inclination. It is not the difference in inclination (We call that [math]\Delta i[/math]), but the actual angle between the two orbit planes.

It is at least as big as the difference in inclination, and at least as big as the difference in LAN (Longitude of the ascending node at epoch). Inclination and LAN define the orientation of the orbit plane in space.

Relative inclination is the problem, that Russian engineers have to solve for Putins plan to recycle the Russian segment of the ISS and put it into an orbit with more than 61° inclination.

 

[dykesc]

Thanks to both of you!
Boogabooga that page on orbital elements is excellent. I now see how
launch time of day affects the ships orbital plane orientation. Obviously
I am new to orbital mechanics but I am beginning to understand the fundamentals.

Urwumpe many thanks for your explanation of RInc. I was indeed thinking
of it in terms of the difference in inclinations. Your explanation about the dependency
on the difference in LANs nailed it for me! I have searched the internet unsuccessfully for
a good definition of RInc but you came through!

Again thanks to both of you.

 

[Hlynkacg]

What I can't visualize is what is causing this parameter to reduce as I advance time before launch. I know I'm advancing time until Canaveral is very close to the lunar orbit plane's intersection with earth but still can't visualize how RInc is falling. The moon's inclination is a constant just over 5 degrees. What exactly is RInc measuring?

In the following screen shots show me getting ready to fly the stock Space Shuttle to the ISS (not the moon) but the basic principals are the same so hopefully this will help explain what is happening.

Open up your "Map" mfd and use the menu [MNU] button to select the "Orbit plane" display mode. Use the target [TGT] button to select your target and you should see something like this...

As you can see the ISS's orbital plane (the yellow line) is well to the east of me (the green +). However, as the earth spins it will carry me eastward, and approximately 2 hours later...

...the ISS's orbital plane is directly over my head, despite the fact that the ISS's orbital parameters have not changed.

Now to rendezvous with ISS all I need to do is ignite the engines and follow that little yellow line all the way to orbit.