General Question Prograde / Retrograde

[DH106]

Does Prograde/Retrograde selection always point you forwards/backwards to your absolute direction of travel? Sometimes when I set the HUD reference to some other body than it's default reference and then select say, Prograde, the spacecraft doesn't get aligned to the +ve velocity vector symbol. Perhaps I'm mis-understanding the way it works?

 

[dbeachy1]

to Orbiter-Forum!

Your "direction" of travel, and therefore prograde and retrograde, is always relative to something, whether it's the sun, a planet, a moon, or whatever else you're orbiting. So whether you're moving prograde or retrograde all depends on what you set your HUD reference to, because those terms inherently refer to a body of reference. In other words, "What are you orbiting?" Does that make sense?

 

[boogabooga]

It points you Prograde/Retrograde relative to the body whose gravity has the strongest influence on you at the moment.

If you do an Earth-Moon transfer, there is an annoying phase where your Prograde/Retrograde is relative to the Sun, until you get close enough to the moon.

Edit: If this bothers you, Pursuit MFD allows you to control attitude relative to an arbitrary body.

 

[ADSWNJ]

Another interesting thought experiment is if you consider going from Earth to Venus. The Trans Venusian Injection burn needs to be on the sunny side of Earth, reaching Earth's escape velocity whilst reducing your Solar velocity enough to target inwards towards Venus. In other words, a burn can be Prograde to one reference body, whilst simultaneously Retrodgrade to another reference body.

 

[boogabooga]

Another interesting thought experiment is if you consider going from Earth to Venus. The Trans Venusian Injection burn needs to be on the sunny side of Earth, reaching Earth's escape velocity whilst reducing your Solar velocity enough to target inwards towards Venus. In other words, a burn can be Prograde to one reference body, whilst simultaneously Retrodgrade to another reference body.

Returns to Earth from Lunar orbit are like that, too.

 

[Notebook]

Another interesting thought experiment is if you consider going from Earth to Venus. The Trans Venusian Injection burn needs to be on the sunny side of Earth, reaching Earth's escape velocity whilst reducing your Solar velocity enough to target inwards towards Venus. In other words, a burn can be Prograde to one reference body, whilst simultaneously Retrodgrade to another reference body.

Wouldn't it be interesting for someone to list all solar-system source/destinations, this applies to.

Not me obviously...

N.

 

[Thorsten]

Wouldn't it be interesting for someone to list all solar-system source/destinations, this applies to.

It also depends on what direction you orbit Earth initially - you could have been on a retrograde orbit to begin with, which wouldn't really change the direction of your velocity around the Sun so much.

I think the real take-away from all of this is that there's no absolute direction of travel.

 

[Notebook]

How about wrt the Galactic centre. Or are our Solar System velocities (assuming not leaving)relevant wrt our velocity around the Galactic centre?

 

[GLS]

Another interesting thought experiment is if you consider going from Earth to Venus. The Trans Venusian Injection burn needs to be on the sunny side of Earth, reaching Earth's escape velocity whilst reducing your Solar velocity enough to target inwards towards Venus. In other words, a burn can be Prograde to one reference body, whilst simultaneously Retrodgrade to another reference body.

Not exacty in the "sunny side" but more as the spacecraft enters the "sunny side". One has to take into account that Earth will still exist after the burn and it will curve your trajectory, so a burn at "orbital noon" will not only decrease the orbital speed around the sun, but will also send you away from the Sun (for a while).

 

[ADSWNJ]

Not exacty in the "sunny side" but more as the spacecraft enters the "sunny side". One has to take into account that Earth will still exist after the burn and it will curve your trajectory, so a burn at "orbital noon" will not only decrease the orbital speed around the sun, but will also send you away from the Sun (for a while).

LOL - I was trying to keep it a bit ELI5. I think it helps to think about being on the sunny side for this burn, as it gives you a good mental picture where the burn should be. (Assuming normal eastward launch from Earth into orbit, e.g. out of Cape Canaveral.)

 

[DH106]

Thanks everyone for their replies - very enlightening!

 

[statickid]

I originally posted this on some other thread but it seems relevant here