CoL axis offset and what is CoP ?

[Jovzin]

Hello guys,

I want to know which axis on CoLoffset is up/down   left/right and forward/backward ? 

CoLOffset = -1.875, 0, 0

Example above this would move the CoL closer to CoM or more behind or ?   I need to know which axis I need to edit to move the CoL muych more closer to CoM with a spaceplane/shuttle.

Thank you

 

And what exatly is CoP ? ??

Thank you
  

[Rocket In My Pocket]

Wouldn't it be easier to just add some lifting surfaces and actually move the COL?

[bewing]

CoP is Center of Pressure, aka Center of Drag. It is the sum of all the drag vectors at any moment for your plane/rocket. If your CoP moves in front of your CoM, your rocket flips. But your CoP moves when your craft changes speed, so it can't be easily represented with a simple little dot in the editor. It's usually fairly close to your CoL.

Edited December 3, 2018 by bewing

[Jovzin]

Hello @bewing   Thank you for your reponse.

I watn to ask you one mor thing.

Do I need to have the same settings for CoL and CoP ?  For example I see in Cormorant Technology's Shuttle that in the wings, elevons, Tail, there is the same numbers for CoL offset and for CoP offset too.

For example from CADelta :

CoLOffset = -1.6, -0.4, 0
    CoPOffset = -1.6, -0.4, 0
    

Do I need to have same numbers too with my wings and elevons and tail control ? 

Thank you

 

[bewing]

No idea. I don't mess with config files.

[Snark]

On 12/3/2018 at 8:04 AM, bewing said:

It's usually fairly close to your CoL.

Well, that depends on the ship design.

The thing is, when KSP is calculating the displayed CoL location, it only includes parts that explicitly have ModuleLiftingSurface on them, i.e. mostly just wing parts, control surfaces, and the like.  It won't, for example, include other parts such as cylindrical fuselages and such.

This becomes an issue because the CoP contributions of those "non-lifting" parts can be huge, with the result that the CoP can be very far away from the CoL in flight, which results in horrible (and, to the player, confusing and unexpected) disparities between how the player thinks a craft will fly, and how it will actually fly.  For example, it can make a ship very aerodynamically unstable, when the user thinks it's stable.

The most common case I've seen for that sort of problem is when people are trying to design a space shuttle that looks like the US space shuttle.  That design has the wings and control surfaces way in the back of the vessel, but it's got a huge, bulky, body-lift-generating fuselage sticking waaaay out in front.  And the CoM is often way in the back, too, because often people build shuttles so that they've got a bunch of heavy engines on the back end, and the fuselage is mostly fuel tank and becomes very lightweight as the vessel empties.

This results in a craft whose CoL is displayed as being way in the back-- further back, in fact, than the CoM.  And since so many players get told the (incorrect and misleading) "rule" that "you're stable if CoL is behind CoM", then they become understandably baffled when they're supposedly stable craft is hopelessly unstable on reentry.  The confusion there happens because the CoP during a typical nose-up shuttle entry attitude becomes way far forward of the "CoL" (due to body lift from that big, lightweight fuselage).

So yes, CoP is often pretty close to CoL-- mainly for craft that have a lot of wing and not much fuselage, where the wings and control surfaces are by far the biggest contributors to CoP.  However, for craft that have higher ratios of "non-lifting" parts to "lifting" ones-- e.g. "big fuselage, small wings", like a shuttle-- then they can be very different, with significant consequences for gameplay.

[Snark]

On 12/4/2018 at 9:25 AM, Jovzin said:

CoLOffset = -1.6, -0.4, 0
    CoPOffset = -1.6, -0.4, 0

So you're talking part configuration here, which changes the conversation somewhat.  Here's what I believe to be the case regarding these parameters-- but mandatory disclaimer, this is not an area I'm expert in, so it's possible I could have some of the details wrong:

When any KSP part moves through atmosphere-- whether it's a "lift-generating" part (like a wing) or something else (like a fuel tank or whatever)-- it will experience aerodynamic forces due to dynamic pressure.  By default, if your part doesn't specify a CoPOffset, then the effective CoP will be determined by the physical shape of the part-- for example, for a cylindrical fuel tank, it will be in the geometric center of the part.  Take such a fuel tank and drop it from a high altitude, and it doesn't have any particular preferred orientation-- it'll rotate any which way.

On the other hand, if you apply a CoPOffset, that moves the position at which the dynamic-pressure vector is applied, which can cause the part to have a preferred orientation (since aero forces will make it want to orient with the CoP trailing behind the CoM.  For example, if you were to take the aforementioned cylindrical fuel tank and tweak its config to give it a CoPOffset in the +Y direction (i.e. moving the CoP towards the top end of the cylinder), then it will want to fall with the bottom end down.  If you turn it some other way and let go, it'll orient itself with the bottom end facing the airstream.

For example, take a look at the config for the Mk1-3 command pod.  It contains the following line:

CoPOffset = 0.0, 0.6, 0.0

What that's saying is that "the CoP of this part is offset 0.6 meters above where it would be if you just calculate it based on the shape of the part".  What that does is to give the part a "butt-first" aerodynamic bias-- that is, if you were to try reentry with just a Mk1-3 command pod, and turn off SAS and just let it tumble, it will automatically orient itself butt-first rather than pointy-end-first.  Which is, of course, exactly what you want in a command pod, because that's the end where the heat shield goes.    If you want to try a little experiment, try changing that value from "0.0, 0.6, 0.0" to "0.0, -0.6, 0" (i.e. with the CoP moved downward instead of upward) and then do a reentry.  I expect you'll see that it now wants to enter pointy-end first instead.

The above description, regarding CoPOffset, applies to all KSP parts, I believe.  However, some parts-- those with ModuleLiftingSurface-- also generate "lift" as an explicit force component separate from simple dynamic pressure.  (Here's where I'm a lot fuzzier on how it works exactly, so take what I say in this paragraph with a bigger grain of salt.)  When you apply CoLOffset, I believe what that does is to move the center of where lift gets applied (due to ModuleLiftingSurface).

Since most of the dynamic pressure of wing-style parts is generated by the same big flat surfaces that generate lift, then my (speculative, uninformed) guess is that it would be common for that kind of part to set CoLOffset to equal CoPOffset, since that results in the most "intuitive" behavior.