AttitudeMFD R-Bar/Radial and Anti-Radial

[StargazerBranden]

The ones that read and reply to my multitude of questions on here (Lol) you all know that I have extensively been doing shuttle related ops and all of my questions are shuttle related. I have two questions this time regarding Attitude MFD. (I primarily use Velocity and Target Relative modes)

1.How do you go about setting up a radial or anti-radial attitude? Is it basically point the nose at Earth, have the cargo bay pointed North and undercarriage pointed South? (I'm guessing these are considered the geocentric radials. That is to and away from the planet)

And

2. What do maneuvers on the R-Bar do exactly?

All the documentation I've read basically talks about rendezvous ops on the R-Bar but don't go into much detail about why or what exactly it does. Nor have I found anything about what R-Bar maneuvers do in relation to orbital mechanics. All I know is basically V-Bar refers to the Velocity Vector and R-Bar is on the Radial Vector.

 

[Thorsten]

1.How do you go about setting up a radial or anti-radial attitude? Is it basically point the nose at Earth, have the cargo bay pointed North and undercarriage pointed South? (I'm guessing these are considered the geocentric radials. That is to and away from the planet)

To be 100% correct you point thrust axis towards the center of Earth.The roll attitude around that (i.e. where the payload bay points) doesn't matter. In reality, nose and thrust axis are some 13 deg different, remember?

2. What do maneuvers on the R-Bar do exactly?

In R-bar you atypically approach a target 'from below' and in v-bar 'from behind'.

Since below the target you're actually on an orbit with a different semimajor axis, your orbital period is smaller in - so stationkeeping (staying a constant distance to target) is costly as you have to constantly use thrusters to compensate for that.

And if you thrust towards target, since your orbital speed is faster, you'll have a tendency to get ahead.

In vbar you're in an orbit with the same semimajor axis, so you have the same orbital period, which is why stationkeeping is much cheaper. For 600 ft distance R-bar stationkeeping comes to 400 lb propellant per orbit, v-bar to 100 lb according to the Shuttle crew ops manual.

If you thrust towards target from a vbar stationkeeping position, you'll in essence do a prograde burn and develop a tendency to get 'above' your target.