Question about orbit MFD

[Pishky]

Hi there folks.
Please don't laugh at what is probably a really noob question but I am really puzzled by the following two buttons on the orbit MFD

FRM
PRJ

this gives rise to four possible combinations in relation to ship to the equator and ecliptic.

Could somebody please point me in the right direction?
Regards

 

[tblaxland]

From the manual:

FRM:

The orbital elements can be displayed with respect to one of two frames of reference: ecliptic or equatorial. The plane of the ecliptic is defined by the Earth’s orbital plane, and is useful for interplanetary flights, because most planets orbit close to the ecliptic. The equatorial plane is defined by the equator of the current reference object, and is useful for low orbital and surface-to-orbit operations. Use [Shift][F] to switch between the two frames of reference. The current mode is displayed in the top line of the display (Frm).

PRJ:

The plane into which the graphical orbit displays are projected can be selected via [Shift][P]. The current projection plane is indicated in the top right corner of the instrument (Prj). ECL or EQU project into the plane of the ecliptic or equator, respectively. SHP projects into the vessel’s current orbital plane, and TGT projects into the target’s current orbital plane, if a target is specified.

EDIT: Welcome to the forum, by the way.

 

[Pishky]

First of all many thanks for your reply. I have read this part in the manual and for some strange reason it makes no sense to me.
For example suppose I am in a low planet orbit and have the following settings
Frm Ecl Prj Ecl
should this not always give a perfect circle as I am projecting from the ecliptic plane to the ecliptic plane?

 

[Xantcha]

PRJ is only for projection of picture in MFD. z.b. Equatorial means you are "looking" at planet from North pole and equ plane lies flat below. Same logic for ECL and SHP(the plane of your orbit). If you will switch PRJ only the shape of orbit will change but orbital params will remain unchanged.

Frame sets plane for orbital parameters, not for the projection.
If it is set to Equatorial then Inc is equatorial inclination and Ascending/descending nodes are where your orbit intersects equatorial plane. If it's Ecliptic then inclination is for eclipitic plane, same for nodes. You can try switching FRM back and forth and see that only nodes on picture and only reference plane dependant values (everything below T) are changing.

 

[tblaxland]

The only time the OrbitMFD will display a circle is when your orbit is circular (Ecc=0.000) and PRJ is set to SHP. Any other setup will result in an ellipse.

Grpahics would probably explain this better, but I'll try with words.

Let's assume you are in a circular orbit around Earth, ie, your altitude is constant and your inclination is 45.0deg. What will this look like in OrbitMFD:

FRM/PRJ

EQU/SHP - Circle, Inc=45.0
EQU/EQU - Ellipse, INC=45.0
ECL/SHP - Circle, Inc=21.55 to 68.45 (depending on LAN)
ECL/ECL - Ellipse, Inc=21.55 to 68.45 (depending on LAN)

The reason why projections not equal to SHP show an ellipse is that you are not looking straight down on your orbit. Pretend that the rim of your coffee cup is your circular orbit. Tilt your cup over, the rim now looks like an ellipse. That is what OrbitMFD does when selecting ECL or EQU projection planes. Does that help?

 

[James.Denholm]

Pretend that the rim of your coffee cup is your circular orbit. Tilt your cup over, the rim now looks like an ellipse.

Hint: Make sure the cup is empty first.

 

[Pishky]

Yes that makes a lot more sense. Many thanks

 

[tblaxland]

Hint: Make sure the cup is empty first.

:rofl:Good point. I guess I didn't run my usual JSA (job safety analysis) on this task