Retrograde or Prograde Orbit?

[Damooer]

I enjoy KSP! But when making maneuvers between planetary bodies, how do you tell if your orbit will be retrograde or prograde around the target?

Edited June 12, 2013 by sjwt

[Uberick]

an East facing orbit, or counter-clockwise, is the normal orbit. To see, just look at your intercept with the planet and see if you are coming in on the planet such that it will establish an east facing orbit.

If you enter the planet's SOI, and have a west facing orbit, you can use a maneuver node, and use the inclination or the blue circles to move your orbit to the other side of the planet before you circularize. It should only cost about 100 d/v usually.

[Rickenbacker]

Since "prograde" just refers to the direction you're orbiting in, orbits are always prograde . Sorry, just had to.

[Poison Penguin]

Since "prograde" just refers to the direction you're orbiting in, orbits are always prograde . Sorry, just had to.

No prograde refers to the planets rotation.

[Fractal_UK]

I enjoy KSP! But when making maneuvers between planetary bodies, how do you tell if your orbit will be retrograde or prograde around the target?

What I tend to do is when making my mid-course correction, usually at the ascending/descending node I drag the radial (blue) markers around in addition to the normal (purple) markers around. I carefully watch the periapsis I will achieve around the destination planet to see which movements will lower my periapsis. I then lower my periapsis to a reasonable value (less than 1,000,000m often works well).

Once you've lowered your orbit this much you should be getting a big gravity assist from the destination body. If the orbit, after you emerge from the sphere of influence of the destination, is larger than your present orbit then you are coming in prograde. If it's smaller, you're coming in retrograde. Obviously, if you're coming in retrograde you will usually want to correct this (more little tweaks of the blue markers). I find it's easier to see this on the orbit screen than trying to examine which side of the planet I appear to be coming in from.

This also has the advantage of getting you into a good approach orbit for minimal delta-v cost.

[tntristan12]

You can plan for this when flying interplanetary missions. Currently all planets and moons (as far as I know of) rotate from West to East, or counter-clockwise if you're looking at the kerbol system from the top down. As such, any transfers you do to planets farther away from kerbol will end up with your orbital velocity on approach being less than the planet's. In other words, you will be coming from the prograde direction, so you will need to try to undershoot and enter the planet's SOI on the side closest to kerbol.

Now, when you are going to a planet that is closer to the sun, the opposite holds true. You will be moving faster than the planet at periapsis, so you will be overtaking it. Try to overshoot so that you enter the planet's SOI on the far side, away from kerbol. If you follow these steps you will get prograde orbits every time.

[Dunbaratu]

No prograde refers to the planets rotation.

No it refers to the direction of the object in question, whichever object that happens to be depending on context. It MIGHT refer to the prograde direction of a planet, but only if you explicitly said you were talking about the planet and not about the spacecraft.

If your spaceship is orbiting to the west instead of to the east, it would be correct to call west "prograde" in that case.

[Procyon]

I thought that Prograde refers to your ships direction of movement, while a prograde or retrograde orbit refers to the direction the planet rotates in respect to your ship. IE a prograde orbit is where the planet"s rotation in the same direction your ship is moving and a retrograde orbit is the planet rotating in the opposite direction of your ships direction of movement. but even if you are in a retrograde orbit your ships direction of movement is still referred to as prograde.

Or am I mistaken?

[Dunbaratu]

I enjoy KSP! But when making maneuvers between planetary bodies, how do you tell if your orbit will be retrograde or prograde around the target?

I assume you mean whether it will be rotating west-to-east or east-to-west?

It's a matter of which side of the target you happen to pass by on when it's gravity grabs you, obviously, but I assume you knew that and the real question is "How do I tell if I'm passing the target planet on the inside or on the outside"? (To get the same counterclockwise rotation as the rest of the solar system, you want to pass it on the outside. TO get an opposite rotation, pass it on the inside.)

And the answer to that is to watch which way the predicted flyby path bends. The prediction path you see graphed on the map presumes you don't do a de-orbit burn and that there's no atmospheric drag, so it shows you which way you'd be flung by the gravity assist as you pass the target by.

Assuming you're looking at the map view down from the north rather than up from the south:

- If the target's gravity assist causes your path to bend left, then the game is predicting you'll be passing on the outside of the target (You will end up counterclockwise and aligned with the rest of the solar system after you slow down to be captured in orbit.).

- If the target's gravity assist causes your path to bend right, then the game is predicting you'll be passing on the inside of the target (You will end up clockwise and counter to the rest of the solar system's rotation after you slow down to be captured in orbit.)

This direction of turn is the easiest clue to see when not zoomed in, and when not near enough to the target planet yet to visually tell. Knowing this ahead of time before you enter the planetary sphere of influence can save some fuel because the further back you make your burn to correct this, the less delta-V it takes to do it. (It's the difference between "deflect my direction by 5 degrees to east" versus "deflect my direction by 0.01 degrees to the east.") Of course this same effect can also make the burn harder to perform accurately, so be careful. ("Darn I wanted to burn for 1 second but I burned for 1.2 seconds! That puts me another 300,00 miles further out from the target!")

I usually find that my bigger problem is that I didn't manage to keep the craft exactly in the level with the orbital rings, so I pass the target planet above or below it and end up in a polar orbit instead of an equatorial one. This too can be predicted and corrected before you get there by looking at whether your flyby prediction says you'll be flung north (up) out of the plane of the solar system (you are passing under the south of the target) or whether it says you'll be flung south (down) out of the plane of the solar system (you are passing over the north of the target).

[Ekku Zakku]

Well, it's just a matter of how easily you can visualize your orbit. However, it is a bit easier to figure out if you're using a different conic patch draw mode (this thread should help you choose a better one if you so desire). It can be changed in the settings.cfg file, "CONIC_PATCH_DRAW_MODE = ...", the default quite a while ago was mode 1, and I grew quite familiar with it, but a few updates ago the devs changed it to mode 3.

I don't think any of the other ones are that useful, but mode 1 is the easiest to visualize when you're trying to get a desired orbit around the target body, rather than the body you're currently orbiting. Mode 1 will be helpful for orbiting the correct way around a planet or moon, and Mode 3 is really helpful for free-return trajectories and gravity assists. Really though, with enough practice, any mode will be easy enough to visualize.

[sjwt]

Prograde is forwards, Retrograde is in Reverse. When referencing an Orbit as in the original post (how do you tell if your orbit will be retrograde) it refers in reference to the planets spin.

Pro tip. When transferring to an inner planet and your flight path references leaving Kerbins SOI in retrograde, set up your Orbit as normal in a prograde orbit, and just leave the planets SOI when you are above what would be Midday on the planet, this will give you a Retrograde orbit in reference to the planet. This saves a bucket laod of DV rather then trying to get into a Retrograde orbit to start with.

[maltesh]

In your settings.cfg file, change the line CONIC_PATCH_DRAW_MODE = 3 to CONIC_PATCH_DRAW_MODE = 0.

Mode 0 draws your trajectory through the SOI you're headed to in the frame of reference of the object you're headed to (So it looks like a normal hyperbolic trajectory instead of the mostly-useless curves of the default mode 3), and at the current position of the object (So you can easily center on it on the map screen for closer inspection.)

Mode 0 makes it very easy to see which direction your trajectory will be when you reach the object you're headed towards, as well as helping immensely with fine-tuning the periapsis of your arrival from weeks or months out with precisely-planned burns. Changing the default CONIC_PATCH_DRAW_MODE is the first thing I do on any new installation of KSP.

[Superfish57]

Prograde is where you are moving, not the planets rotation. Fly a plane and turn, you will see the prograde marker turn with the plane's nose.

[RoboRay]

No it refers to the direction of the object in question, whichever object that happens to be depending on context. It MIGHT refer to the prograde direction of a planet, but only if you explicitly said you were talking about the planet and not about the spacecraft.

If your spaceship is orbiting to the west instead of to the east, it would be correct to call west "prograde" in that case.

Actually, when discussing an orbital direction, it is standard practice to refer to an orbit opposing the rotational direction of the body as a retrograde orbit.

Prograde/retrograde have complex usage and are not limited to discussing the motion of a spacecraft. Referring to an orbit with an inclination of greater than 90 degrees (meaning that it's orbiting westward against the direction of the planet's spin) as "retrograde" is correct.

Edited June 7, 2013 by RoboRay
More accurate terminology

[rodion_herrera]

Referring to a westerly orbit around a body that rotates to the east as "retrograde" is correct.

Funny that this confusion arises among the young people today, when the fact is, people who are old enough to have experienced (or at least got stories from parents/grandparents) Apollo, are in fact aware of retrograde orbits, because that's how the CSM/LM orbits the moon when arriving there. The reason being that the LM should make an approach from east to west, so that when it encounters the terminator of the moon (which as of this time is in the "middle" of the moon, i.e. first quarter moon), the LM lands in such a way that the commander and LMP can see long shadows, which are crucial for distance and depth perception, important for manual control of the LM's descent. Thus, on T.V. newscasters interviewing Apollo astronauts and mission planners would often mention "retrograde orbit" and explain it to the public too. That is also how I got the idea, from watching archived footage of such interviews and TV broadcasts about Apollo.

[Horn Brain]

In your settings.cfg file, change the line CONIC_PATCH_DRAW_MODE = 3 to CONIC_PATCH_DRAW_MODE = 0.

Mode 0 draws your trajectory through the SOI you're headed to in the frame of reference of the object you're headed to (So it looks like a normal hyperbolic trajectory instead of the mostly-useless curves of the default mode 3), and at the current position of the object (So you can easily center on it on the map screen for closer inspection.)

Mode 0 makes it very easy to see which direction your trajectory will be when you reach the object you're headed towards, as well as helping immensely with fine-tuning the periapsis of your arrival from weeks or months out with precisely-planned burns. Changing the default CONIC_PATCH_DRAW_MODE is the first thing I do on any new installation of KSP.

When doing it this way (which is the very best way, btw), just hit tab until your camera is focused on your target body and look at which end of the hyperbolic encounter orbit has the "Jool/Duna/Eve/Whatever escape" icon. Your craft will come from the other end and travel to this end as it passes through, so that will show you the direction of your orbit. Prograde orbits are counterclockwise when viewed from above. Use radial burns to switch which side of the body you're arriving on!

Good luck!

[rodion_herrera]

In Orbiter's TransX, setting Conic Patch DM to 0 is like having the "Encounter" view, which is "zoomed into" the target planet.

[Ekku Zakku]

Referring to a westerly orbit around a body that rotates to the east as "retrograde" is correct.

Sorry, just wanted to point out that "westerly" means from the west, not to the west. An "Easterly orbit" would indeed be one that is orbiting from east to west, or retrograde as you are mentioning. "Northerly, Easterly, etc." are really only terms used for weather, as it indicates what direction the weather is coming from; it doesn't have much use otherwise and just causes confusion.

[RoboRay]

Sorry, just wanted to point out that "westerly" means from the west, not to the west.

Your correction of my correction is correct.

[Damooer]

Steven Mading, I just want to say that I found your answer the most satisfactory. I now know how to tell what my orbit will be just from the navigation ui! Thank you.

[Dunbaratu]

Actually, when discussing an orbital direction, it is standard practice to refer to an orbit opposing the rotational direction of the body as a retrograde orbit.

Prograde/retrograde have complex usage and are not limited to discussing the motion of a spacecraft. Referring to an orbit with an inclination of greater than 90 degrees (meaning that it's orbiting westward against the direction of the planet's spin) as "retrograde" is correct.

Thanks for the terminology info, but in my defense the only thing I did wrong was presume that people wouldn't have deliberately developed a terminology with such a dangerous ambiguity in it. But apparently they did. That is absolutely terrible terminology. If you radio to the spacecraft to "make a retrograde burn now", it's horrible that the term "retrograde" in use there has two equally valid but utterly opposite interpretations if you happen to be orbiting east-to-west.

[RoboRay]

Thanks for the terminology info, but in my defense the only thing I did wrong was presume that people wouldn't have deliberately developed a terminology with such a dangerous ambiguity in it. But apparently they did. That is absolutely terrible terminology. If you radio to the spacecraft to "make a retrograde burn now", it's horrible that the term "retrograde" in use there has two equally valid but utterly opposite interpretations if you happen to be orbiting east-to-west.

Well, no, not at all. A "retrograde burn" would be describing your craft's orientation in respect to your orbit, so it's not ambiguous. You would always burn against your orbital motion.

Anyway, "retrograde" isn't a technical term that was invented for spaceflight or astronomy... it has broader applications.

Edited June 11, 2013 by RoboRay

[Dunbaratu]

Well, no, not at all. A "retrograde burn" would be describing your craft's orientation in respect to your orbit, so it's not ambiguous. You would always burn against your orbital motion.

And you don't think it's ambiguous that "retrograde direction" has two different meanings here? Yes, it's ambiguous.

[Frederf]

Prograde is an adjective. Etymology for its antonym is here: http://www.etymonline.com/index.php?term=retrograde&allowed_in_frame=0

Anyone who says that the meaning of "go forward" is specific to a certain context is too isolated within a convention. There are plenty of these conventions where longer phrases are abbreviated to shorter ones where the explicit meaning of the wording gives way to implicit meaning. The implied meaning is totally dependent that all members share the same convention of abbreviation. When differing or non-participating groups interact you get confusions over assumed meanings of words such as this one.

Hopefully any critical situation would sort out the exact meanings of words or not rely on ambiguous phrasing. Very costly calamities have resulted from ignoring such advice historically.

[Colonel_Panic]

wow lot of misinformation in here.

quite simply, prograde and retrograde are velocity vectors toward and away from the orbiting body's direction of travel relative to the body of reference. In layman's terms, they are the directions in which acceleration will speed you up or slow you down without changing your relative direction of travel.

This means that every object (except Kerbol itself) has a prograde and retrograde vector relative to either Kerbol, or one of the planets, moons or other objects in the kerbol system. The prograde is the direction the orbiting body is moving toward, and the retrograde is the direction it is moving away from. If the orbiting body is a ship with a navball, then the prograde vector is marked by a yellow circle, retrograde is marked by a green Y, and the body of reference is listed at the top of the navball (and can be changed by clicking on it)

Other examples: Kerbin's prograde around Kerbol points along the orbital line in the direction that kerbin orbits (counter clockwise) so if you want to "burn toward kerbin prograde" you need to begin your burn in the middle of the dark side of kerbin (assuming a 0 inclination or 90 degree orbit to the East) which will serve to bring up your kerbol apoapsis upon kerbin escape, while burning toward the retrograde of kerbin, on the day side of kerbin, will lower your kerbol periapsis on escape.

For more information, read the wiki on the navball http://wiki.kerbalspaceprogram.com/wiki/Navball

Edited June 12, 2013 by Colonel_Panic