using "cheat" to set orbit?

[strider3]

I'm not quite sure why using the "Alt+F12" menu to set a craft on a certain orbit height around a selected celestial body has to be so...difficult? Why can't I just select the body and orbit height, for God's sake?? But, since it apparently has to be "rocket science" (on a cheat menu???), please explain to me how one uses the cheat menu to set a craft on an orbit of a certain altitude around any given celestial body...before I throw a large hammer through my monitor.

[Foxster]

It's something that annoys the hell out of me too. I'm guessing the number is the distance from the centre of the planet rather than altitude above the surface that we more commonly use. 

[4x4cheesecake]

The cheat menu tells you exactly which values it needs:

These values are necessary to define any orbit. If you want just a simple circular, equatorial orbit, you need to edit the Semi-Majo Axis (SMA) and leave the other values as they are.
In this case, the SMA would be the same as the radius of the orbit with the center right at center of the planet. To define the orbital altitude, you need to add the desired orbital altitude to the equatorial planetary radius. The equatorial radius of Kerbin is 600.000m so the default values displayed in the cheat menu will put your vessel into an orbit with an altitude of 86.750m.

Well, usually you don't know the equatorial radius of any celestial body, so you can a) look it up in the wiki or b) use a SMA which allows you calculate it easily (1.000.000m for example). In this example, your vessel would end in an orbital altitude of 400.000m and 1.000.000 - 400.000 = 600.000. Now you know that you have to add 600.000 to your desired orbital altitude and put the number as SMA into the cheat menu  

Using the SMA is necessary, even in the cheat menu, since the orbital altitude of a vessel in an eccentric orbit will change over time but the SMA will always be the same.

[Superfluous J]

Alt-F12

Set the orbit to a number.

Note your altitude.

Subtract number you set from your altitude. This is the radius of the planet.

Alt-F12

Set the orbit to the altitude you want minus the radius of the planet.

[ElWanderer]

Orbits are stored internally in that format (e.g. that's what you see inside the save file), which is why I guess the cheat menu gets you to enter those values rather than Ap/Pe and calculate the SMA/ECC for you.

https://en.m.wikipedia.org/wiki/Orbital_elements

(see the section on Keplerian orbital elements)

[strider3]

OK...thanks gang. Sorry I was so...frustrated...in my original post. I do apologize.

[goldbird09]

What do all of the other things do? (MNA,OBT,LAN, etc,,)

[AHHans]

5 hours ago, goldbird09 said:

What do all of the other things do? (MNA,OBT,LAN, etc,,)

Have a look at Orbital elements on Wikipedia.

MNA: Mean anomaly (in radians)

LAN: Longitude of Ascending Node (in degrees)

Arg PE: Argument of Periapsis (in degrees)

OBT: ??? What do we need a 7th parameter for?

 

 

[Zhetaan]

3 hours ago, AHHans said:

OBT: ??? What do we need a 7th parameter for?

Six parameters define the orbit in three-dimensional space.  You need a seventh to define which celestial object you're orbiting, but that's handled here, obviously.  Technically, you need an eighth to define the time element:  mean anomaly defines one's place on the orbit as a radial angle from the periapsis, and mean anomaly from epoch (also sometimes called time of periapsis passage) defines position with respect to some time, but without a reference time to respect, it doesn't mean much.  That time parameter is usually called epoch; I have no idea what 'OBT' describes.

Granted, these extra two are not, strictly speaking, orbital parameters.  They are properly considered part of the reference frame; an orbit can be just as well defined without knowing what you're orbiting or when (incidentally, orbits don't care about the surface elevation of the primary, either), but the practical value of knowing these things should, I hope, be readily apparent.

Edited January 6, 2020 by Zhetaan

[AHHans]

1 hour ago, Zhetaan said:

You need a seventh to define which celestial object you're orbiting

Yes, that's what the selection at the top is for. The one where you can click through all the CBs.

1 hour ago, Zhetaan said:

That time parameter is usually called epoch;

Well, I would assume that KSP works with a fixed epoch. I don't see any advantage for them to allow for a variable epoch (well, except maybe making some hard-core astronomers happy), but it would make the program more complicated.

Hmmm....

I just has a look into a savefile, there an orbit is defined with the following values: ["savefile-name": "What I think it is"] SMA: semi major axis, ECC: eccentricity, INC: inclination, LPE: longitude of aeriapsis (= argument of periapsis), LAN: longitude of ascending node, MNA: mean anomaly, EPH: ??? epoch?, REF: reference == number of celestialbBody

[...]

O.K. "EPH" seems to be the time (in seconds) when the craft was active (i.e. for which the orbital parameters were calculated).

Still no clue what the "OBT" in the cheat menu does. I didn't see how (if at all) it affects the values in the savefile.

 

[Zhetaan]

1 hour ago, AHHans said:

Well, I would assume that KSP works with a fixed epoch.

It doesn't, as you discovered.  I believe that the reason for it has to do with the on-rails versus off-rails way that KSP calculates orbital information.  When the craft is active, the orbital parameters are dynamic, but once you switch focus to something else, those parameters are treated as constant initial values in the orbital equation.  Knowing that the craft's mean anomaly was a given value at the specific time you last controlled it is exactly as useful for calculation purposes as knowing what the mean anomaly was at some arbitrary zero time.

That being said, I don't know that for certain, so I may be far and away off the mark.  It doesn't tell any more about what OBT is, either.

[AHHans]

1 minute ago, Zhetaan said:

It doesn't, as you discovered.

Gnnnaaahh! Sorry, my bad.

What I meant with "fixed epoch" was that I believe that KSP uses a fixed reference system, i.e. the reference direction for the longitude of ascending node. In real life the astronomical reference systems wobble around over time, so they are defined at an epoch and you can have fun times (or a computer and a preferably a program that someone else wrote ) converting from one to another.

But the reference time for when this set of orbital parameters was valid (in particular: at which time the craft was at the given mean anomaly) is something else. And I didn't see that it was that what you meant. In part because when you set an orbit with the cheat menu then I expect that the reference time taken is that current game time.

12 minutes ago, Zhetaan said:

That being said, I don't know that for certain, so I may be far and away off the mark. 

No, I think you are completely correct.

[jamescraftian99]

Can anybody tell me the numbers I need to enter in to get a low orbit around duna?

[AHHans]

5 hours ago, jamescraftian99 said:

Can anybody tell me the numbers I need to enter in to get a low orbit around duna?

To just enter into a low, equatorial orbit around any planet or moon use the default values except for "Semi-Major Axis" in which you can put a zero. Then when you click "Set Orbit" the first time you get an error message that your orbit was too low and that the value has been adjusted. Clicking on "Set Orbit" a second time will then get you into the lowest orbit that the cheat menu will let you.

Another issue is: this thread is already pretty old, it would have been better to start a new thread with your question.
[Edit:] I just saw that you did start another thread, so disregard that part. And welcome to the Forums!

Edited June 10, 2020 by AHHans