Planet pack home planets-are they Kerbin equivalents?

[Klapaucius]

I read somewhere that all home planets in planet packs have to be a Kerbin equivalent: atmosphere, size, gravity etc. Is this the case?  Is Gaia or Galileo etc analogous as far as the big parameters: gravity, atmospheric density etc goes?

The reason I am asking is because I submitted a time for @l0kki's the Race to the North Pole challenge and it was not great, so I am re-flying the route. To mix things up a bit, I thought of doing it with a different planet pack and wanted to know if mixing things up by taking off from a Kerbin equivalent will change things (other than the obvious terrain differences).

 

 

[OhioBob]

35 minutes ago, Klapaucius said:

I read somewhere that all home planets in planet packs have to be a Kerbin equivalent: atmosphere, size, gravity etc. Is this the case?  Is Gaia or Galileo etc analogous as far as the big parameters: gravity, atmospheric density etc goes?

Home worlds do not have to be a Kerbin equivalent.  I'm only familiar with GPP and GEP, so I don't know what other planet packs do.  In GPP, the home world, Gael, is equivalent to Kerbin.  But in GEP, with the GEP_Primary option install, the planet Nodens becomes the home world.  Nodens is not a Kerbin equivalent.  It has radius of 700 km, a surface gravity of 1.1 g, and an atmospheric pressure of 2 atm.

(edit)  I do recall there being some issue with home worlds that are smaller than Kerbin.  It causes some sort of physics glitch that makes the game unplayable (rocket launches from the home world are totally borked).   However, home worlds that are larger than Kerbin seem to work fine.
 

Edited September 26, 2018 by OhioBob

[Superfluous J]

The single most popular planet pack (to put it lightly) of them all, Real Solar System, breaks this "rule" because in that pack, Kerbin is Earth.

[Klapaucius]

@OhioBob  Thanks for the response.  

Edited September 26, 2018 by Klapaucius

[The Aziz]

Well, the toy solar system worked back in the day, and even Kerbin was 1/10th of its original size.

[OhioBob]

6 hours ago, The Aziz said:

Well, the toy solar system worked back in the day, and even Kerbin was 1/10th of its original size.

I think something has changed in one of the more recent versions of KSP.  A couple years ago I could downsize Kerbin with no problem.  But more recently I've tried it and things are totally screwed up.  When trying to launch a rocket it just won't build up any speed, like it's experiencing massive amounts of drag.  And when we cut the engine, instead of slowing down, the rocket continues to very slowly accelerate.  Also some of the numbers displayed in the AeroGUI are nonsense.  I'm not the only one who has experienced this; I've had conversations about it with somebody on Discord.  We ran some experiments and determined that the problem occurs every time we tried to reduce the size of Kerbin.

[KerikBalm]

I think there were issues with the Isp if the home planet's atmosphere is not 1 atm at sea level. This came up for mods like Duna space program - despite the pressure being 0.067 atmospheres, a LV-N got the Isp it would get at 1 atm on Kerbin (and rocket engine Isp on kerbin and Eve was completely borked).

This was fixed by making a band of atmosphere from 0 meters to 0.01 meters have an atmosphere pressure of 1 atm, before dropping to 0.067.

Some of the displayed surface pressures in map view were also wrong in some cases...

Other than that, I don't think its an issue.

[OhioBob]

10 hours ago, KerikBalm said:

This was fixed by making a band of atmosphere from 0 meters to 0.01 meters have an atmosphere pressure of 1 atm, before dropping to 0.067.

That fix use to work back in the old days of KSP, but not anymore.

The issue is that the lookup variable for the Isp curves is the atmospheric pressure measured in atmospheres, where the curves are calibrated on the presumption that 1 atm = 101.325 kPa.

But in KSP,

          1 atmosphere = sea level pressure of the home world.

So when the sea level pressure of the home world is not 101.325 kPa, the Isp curves will return incorrect numbers.  We have to make KSP think that the sea level pressure of the home world is 101.325 kPa even when it's not.

In the old days, KSP got the sea level pressure of the home world from the pressure curve.  So to trick KSP into thinking 1 atm = 101.325, we had to perform a little hocus pocus with the home world's pressure curve, doing just what you described.

But KSP no longer uses the pressure curve for this.  It now uses the parameter staticPressureASL. That is,

          1 atmosphere = staticPressureASL, home world.

So to get the Isp curves to return the correct values, we just have to do this,

          staticPressureASL, home world = 101.325

For other celestial bodies, the only use of staticPressureASL is for the planet data display in the Tracking Station.  The Tracking Station will always show the pressure of the home world as 1 atm.  So for other bodies we just have to set staticPressureASL so that it is in the correct ratio to the home world's staticPressureASL value.

 

Edited October 5, 2018 by OhioBob

[Ultimate Steve]

They certainly do not have to be Kerbin equivalent. I'm currently playing through Whirligig, where the homeworld has 6g gravity but rotates really fast so it bulges out at the equator, where the gravity is a bit over 1g. Orbital velocity is 8km/s but you start off with 4.5 from the rotation of the planet. There's only an atmosphere to fix the isp.

So basically there's no atmosphere whatsoever.

[Gordon Fecyk]

Alien Space Programs has seven or eight Kerbins (if you include Jool), each quite different from one another. Definitely the home world can have vastly different properties from the original Kerbin.

2 hours ago, OhioBob said:

But KSP no longer uses the pressure curve for [engine ISP calculations].  It now uses the parameter staticPressureASL. That is,

          1 atmosphere = staticPressureASL, home world.

When did this happen? I have a collection of configurations to fix if this is the case. I'm grateful if so, but it's curious to learn that this happened.

[OhioBob]

3 hours ago, Gordon Fecyk said:

When did this happen? I have a collection of configurations to fix if this is the case. I'm grateful if so, but it's curious to learn that this happened.

It was a long time ago, probably 3 years.  I think it was one of the early 1.x versions.

Just to clarify, KSP does use the pressureCurve to compute ISP, it just doesn't use it to define the value of 1 atm.

PressureCurve returns the pressure in units of kPa, while the ISP curve takes pressure in units of atmospheres.  So KSP has to convert kPa to atmospheres.  We have, atmospheres = kPa / X, where X is the conversion factor.  Years ago, the value of X was obtained from the home world's pressure curve, using the pressure at an altitude of 0.  But now X is equal to the home world's staticPressureASL.

[Klapaucius]

35 minutes ago, OhioBob said:

It was a long time ago, probably 3 years.  I think it was one of the early 1.x versions.

Just to clarify, KSP does use the pressureCurve to compute ISP, it just doesn't use it to define the value of 1 atm.

PressureCurve returns the pressure in units of kPa, while the ISP curve takes pressure in units of atmospheres.  So KSP has to convert kPa to atmospheres.  We have, atmospheres = kPa / X, where X is the conversion factor.  Years ago, the value of X was obtained from the home world's pressure curve, using the pressure at an altitude of 0.  But now X is equal to the home world's staticPressureASL.

So, I take it then that we cannot assume that Gaia, for example, is an exact analogue of Kerbin even though it has 1 atm pressure at sea level?  (Though I am pretty sure it is, my point is, it could still have those figures and be different if the modder defines the parameters differently?)

Does this apply to gravity as well?

Can I also assume that as long as the stock Kerbin is the homeworld, all stats in from the space center will be based on that figure?  Or, if I installed, for example, the Outer Planet Pack  in an altered system, would the stats change?   So, for example, if Kerbin has a pressure of 1 atm and some add-on mod planet we'll call Gargantua has a pressure of 10 atm, if we then install Gargantua in a planet pack where the home planet's pressure is half that of Kerbin, would Gargantua then be listed in the stats in the Space Center as:

A: 20 atm

B: 10 atm (but in actuality, half the pressure it would have if Kerbin were the homeworld) and its characteristics are altered based on the home planet?

I suppose I should go and test this....

Edited October 6, 2018 by Klapaucius

[OhioBob]

12 hours ago, Klapaucius said:

So, I take it then that we cannot assume that Gaia, for example, is an exact analogue of Kerbin even though it has 1 atm pressure at sea level?  (Though I am pretty sure it is, my point is, it could still have those figures and be different if the modder defines the parameters differently?)

If Gaia it the home world, then yes, that's correct.  The Tracking Station data will show it's atmospheric pressure as 1 atm regardless of what it is in kPa.
 

Quote

Does this apply to gravity as well?

Apparently not.  It looks like surface gravity is always in units of 1 g = 9.80665 m/s².
 

Quote

 

Can I also assume that as long as the stock Kerbin is the homeworld, all stats in from the space center will be based on that figure?  Or, if I installed, for example, the Outer Planet Pack  in an altered system, would the stats change?   So, for example, if Kerbin has a pressure of 1 atm and some add-on mod planet we'll call Gargantua has a pressure of 10 atm, if we then install Gargantua in a planet pack where the home planet's pressure is half that of Kerbin, would Gargantua then be listed in the stats in the Space Center as:

A: 20 atm

B: 10 atm (but in actuality, half the pressure it would have if Kerbin were the homeworld) and its characteristics are altered based on the home planet?

 

It depends.  Gargantua's atmospheric pressure will be displayed as,

          (staticPressreASL, Gargantua) / (staticPressureASL, home world)

So if for the home world staticPressureASL = 50.6625, then Gargantua's pressure will display as 20 atm, but if staticPressureASL = 101.325, then Gargantua's pressure will display as 10 atm.

While the logical thing to do for the home world is the set staticPressureASL equal to the actual sea level pressure, that's going to mess up the specific impulse of engines.  To get specific impulse to compute correctly, staticPressureASL for the home world must be set to equal to 101.325.

-------------------- (edit) --------------------

You've described a scenario in which we have a planet pack with a home world having an atmospheric pressure of 50.6625 kPa, into which other planet packs can be installed.  What I would probably do in that case is this,

• For the home world, staticPressureASL = 101.325.  This is necessary for specific impulse to be correctly computed.
• For other planets within my planet pack, staticPressureASL = 101.325 * (actual pressure, planet) / (actual pressure, home world).

For other planet packs that might be installed with my planet pack, I'd do something like this,

@Kopernicus:AFTER[otherPlanetPack]
{
	@Body[Gargantua]
	{
		@Atmosphere
		{
			@staticPressureASL *= 2
		}
	}
}

In other words, I'd change the staticPressureASL setting for the other planets so that their atmospheric pressure is displayed in the correct ratio to my home world's atmospheric pressure.

 

Edited October 6, 2018 by OhioBob

[Snark]

Moving to Add-on Discussions, since this isn't really a question about how to play the game.