Gas giants?

[Gordon Dry]

Evaluating some values of stock and planet packs and additional outer planet packs etc. it seems that so called "gas giants" always have a relative low dense / gravity.

Nothing that compares with Jupiter at all.

Actually I don't remember RSS values, but how is it regarding gas giants?

[OhioBob]

RSS uses real life values, so Jupiter is truly Jupiter-like, with a surface gravity of about 2.68 g.  All the scaled-down versions of RSS, like SSRSS, use the same values.

Outside of Jupiter, the other gas giants in our real life solar system all have surface gravities around 1 g, so it's really not abnormal that many of the planet packs would have gas giants with gravities and densities lower than Jupiter.

One way to estimate approximately where a gas giant should fall is to plot a graph of Log(radius) vs. Log(mass) for known real life gas giants.  Doing that we find that gas giants tend to cluster along a curve.

If we place a planet the size of Jool (assumed 10 times its stock radius) along that same curve, we find that its surface gravity should be closer to 1.2 g rather than its actual value of 0.8 g.  This tells us that Jool is less dense than it should be for its size.  If other planet packs have based their gas giants off of Jool, then this could help explain why densities/gravities are lower than you were expecting.
 

Edited July 31, 2017 by OhioBob

[OhioBob]

Below is what I use to determine the properties of a fictional gas giant.  It is a graph of Jupiter, Saturn, Uranus and Neptune fitted with a trendline.

From this we can estimate the radius and mass of any size gas giant, assuming that it falls on the trendline.  Below are the expected properties of gas giants at various sizes:

Mass	Radius	Gravity	Density
(kg)	(km)	(g)	(kg/m²)
1.898E+27	69,911	2.642	1,326
9.184E+26	65,000	1.479	798
6.353E+26	60,000	1.201	702
4.700E+26	55,000	1.057	674
3.580E+26	50,000	0.974	684
2.763E+26	45,000	0.928	724
2.141E+26	40,000	0.910	798
1.653E+26	35,000	0.918	920
1.263E+26	30,000	0.955	1,117
9.466E+25	25,000	1.030	1,446
6.874E+25	20,000	1.169	2,051

I always assume that KSP stock-sized planets are 1/10th the radius and 1/100th the mass of their real life counterparts.

 

Edited July 31, 2017 by OhioBob

[KerikBalm]

I changed Jool's surface G to 2.5 in my game because of this.

I also modified many other things to make them more "realistic" or at least closer analogues.

Another thing that bugs me is how massive the moons are relative to the mass of Jool. In our solar system, the largest moon of a gas giant relative to the mass of its planet is Titan, being about 1/4226 the mass of Saturn.

In contrast, Tylo is 1/100th the mass of Jool. This is approximately the mass ratio of the Earth and the Moon - the Earth and the Moon being the only known pair that satisfies one proposed definition of a double planet (but there are other proposed definitions, and no international agreement on what is or would be a double planet).

So, as part of my 3x rescal mod, I made a number of other changes, to implement a somewhat more realistic system (ignoring that I added a life bearing planet where duna is, and made duna and ike moons of that planet... because... fun)

Spoiler

***some lines omitted for brevity***

 

@Kopernicus:AFTER[Kopernicus]
{  
    @Body[Mun] 
    {
        @Orbit // Assign it the orbital parameters of Minmus
    }
    @Body[Minmus] 
    {
        @Orbit // Assign it the orbital parameters of Mun
    }
    @Body[Moho] 
    {
        @Properties
        {
            geeASL = 0.38 // Increase surface G to equal Mercury's
            radius = 230000 // decrease radius slightly
        }
    }
    @Body[Eve] 
    {
        @Properties
        {
            geeASL = 1.20 // Decrease surface G so density is similar to Kerbin
        }
        @Atmosphere
        {
            staticPressureASL = 1013.25
            pressureCurve
            {
...
            }
        }
    }
    @Body[Jool] 
    {
        @Properties
        {
            geeASL = 2.5 // Increase Surface G to 2.5
        }
    }
    @Body[Laythe] 
    {
        @Properties
        {
            geeASL = 0.366 //  based on density of Jupiter's moons, and a radius proportionately 2x bigger than Jupiter's moons
            radius = 343000
        }
        @Atmosphere
        {
        ...
        }
    }
    @Body[Vall] 
    {
        @Properties
        {
            geeASL = 0.268 //as with laythe and Tylo, proportional 2x radius of Jovian analogue
            radius = 294000
        }
    }
    @Body[Tylo] 
    {
        @Properties
        {
            geeASL = 0.292 //as with laythe and Vall, proportional 2x radius of Jovian analogue - yes its lower than Laythe, but you'll see that Io's surface gravity is higher than Ganymede's
            radius = 496000
        }
    }
    @Body[Dres] 
    {

// whats missing here is in the SimgaDimensions config, Dres doesn't get scaled up like the other bodies, to make it a proper dwarf planet (I also add in Vesta and Pallas analogues)
        @Properties
        {
            geeASL = 0.03 // Decrease Surface G to 0.03
        }
    }
    @Body[Sun] 
    {
        @Properties
        {
            geeASL = 27.95 // Increase Surface G to that of Earth's sun
            radius = 65500000 // Decrease Radius to 1/4 that of stock
        }
    }
}

 

[Next_Star_Industries]

Again I ask why anyone scales anything or states it's an analogue of our solar system? It simply resembles a small solar system. Kerbin is Kerbin not Earth. The Kerbal solar system is the Kerbal solar system not our solar system. 1 meter is still 1 meter, 1 pound is still 1 pound, 1 newton is still 1 newton, etc. Kerbin is smaller then Earth so in order for it to have the same gravitational pull as Earth Kerbin itself would have to be denser then the Earth. Being denser doesn't change anything except the gravitational pull of Kerbin that would be why it requires the same delta v to do the same work. If Kerbin had the same density as Earth then everything would basically act like us being on the Moon.

 Are the orbits accurate for the densities IDK, we learn new things everyday in astronomy so they may just be correct. I do know the densities aren't accurate for the gravity in the Kerbal system, but again that has no effect in game on parts.

Edited July 31, 2017 by Next_Star_Industries

[KerikBalm]

They are clear analogues. The ones who made them even said so.

-but yes, its a fictional solar system, and the changes they made make it hard to compare it to real stuff (Regex would probably have a lot to say on the subject). To say that 1 meter is actually 10 meters doesn't work, unless you say that 800 Isp is actually 8000 real Isp... and even then the atmospheres don't scale the same 1:10 way.... so yea, its a fictional system quite different from ours.

But since its a game, and fiction, then there's nothing stopping us from changing it to be closer to reality. Some go as far as they can in that direction with RO/RSS, some do various rescales with mods like sigma dimensions, and I do something in between.

This planet I made has some basis in reality... but its far from realistic... especially once I made Duna and Ike its moon:

Spoiler

Or maybe it was worse when it was a very large moon of Kerbin:

If they want things to be more like our solar system, then so be it. You wouldn't complain about RSS being more like our solar system even if certain things about it are clearly wrong (like axial tilts, which is a game engine limitation)

[Next_Star_Industries]

@KerikBalm misunderstanding I'm talking about parts not planet sizes. I think it's awesome to have different systems at different sizes but the parts themselves would still be the same and not be scaled. You wouldn't use the same size rocket to reach the same orbit of a different size planet. It seems things are being scaled to do just this. You wouldn't give a motor more thrust and lower ISP, because those number are set in stone, to achieve the same orbit with the same rocket, no you would build things, like bigger motors, add more fuel tanks, etc. to accomplish these goals. So for stuff like RSS you wouldn't use the same rocket as you would in stock it wouldn't be big enough.

They aren't analogues if they were everything would be scaled including the densities and that would affect the gravitational pull of all the planets. So it's more realistic to view it as the density values aren't accurate and they don't really matter because it has no affect on the game or parts since this number can actually be anything you want it to be.

All in all the size of the planets don't matter 1+1 still equals 2 no matter what size it is.

In reality said density/mass of a body correlates to the gravitational pull of said body.

Edited July 31, 2017 by Next_Star_Industries

[OhioBob]

1 hour ago, KerikBalm said:

I changed Jool's surface G to 2.5 in my game because of this.

I also modified many other things to make them more "realistic" or at least closer analogues.

I've tinkered with similar changes to make the stock system a bit more realistic.  Many of my proposed changes are the same as yours.  Although this is not a complete list, these are some of the more egregious things that I think need changing: 

• Sun's radius and surface gravity are way off from what they should be for a main sequence star.
• Eve is too dense (surface gravity should be ~1.2).
• Jool is not dense enough (surface gravity should be ~1.2).
• Mun is too close to Kerbin (Kerbin would be tidally locked to Mun at current distance).

 

Edited July 31, 2017 by OhioBob

[Bill Phil]

3 hours ago, Next_Star_Industries said:

Again I ask why anyone scales anything or states it's an analogue of our solar system? It simply resembles a small solar system. Kerbin is Kerbin not Earth. The Kerbal solar system is the Kerbal solar system not our solar system. 1 meter is still 1 meter, 1 pound is still 1 pound, 1 newton is still 1 newton, etc. Kerbin is smaller then Earth so in order for it to have the same gravitational pull as Earth Kerbin itself would have to be denser then the Earth. Being denser doesn't change anything except the gravitational pull of Kerbin that would be why it requires the same delta v to do the same work. If Kerbin had the same density as Earth then everything would basically act like us being on the Moon.

 Are the orbits accurate for the densities IDK, we learn new things everyday in astronomy so they may just be correct. I do know the densities aren't accurate for the gravity in the Kerbal system, but again that has no effect in game on parts.

Everything in KSP, as a consequence of scaling down distance by 10, has a scaled down Delta-V of [sqrt(10)]/10.

[ProtoJeb21]

22 hours ago, OhioBob said:

RSS uses real life values, so Jupiter is truly Jupiter-like, with a surface gravity of about 2.68 g.  All the scaled-down versions of RSS, like SSRSS, use the same values.

Outside of Jupiter, the other gas giants in our real life solar system all have surface gravities around 1 g, so it's really not abnormal that many of the planet packs would have gas giants with gravities and densities lower than Jupiter.

One way to estimate approximately where a gas giant should fall is to plot a graph of Log(radius) vs. Log(mass) for known real life gas giants.  Doing that we find that gas giants tend to cluster along a curve.

If we place a planet the size of Jool (assumed 10 times its stock radius) along that same curve, we find that its surface gravity should be closer to 1.2 g rather than its actual value of 0.8 g.  This tells us that Jool is less dense than it should be for its size.  If other planet packs have based their gas giants off of Jool, then this could help explain why densities/gravities are lower than you were expecting.
 

Gas giants and other gas planets can also be incredibly low density. Take a look at Kepler-51c, with a density of 0.032 g/cm^3, and less gravity than Pluto. Planets like 51c and its two siblings are quite rare. Also, I have noticed before how low-density Jool and the OPM gas planets are. In real life, Jool would be less than 80 times the mass of Earth, less than the mass of Saturn!

4 hours ago, OhioBob said:

Below is what I use to determine the properties of a fictional gas giant.  It is a graph of Jupiter, Saturn, Uranus and Neptune fitted with a trendline.

From this we can estimate the radius and mass of any size gas giant, assuming that it falls on the trendline.  Below are the expected properties of gas giants at various sizes:

Mass	Radius	Gravity	Density
(kg)	(km)	(g)	(kg/m²)
1.898E+27	69,911	2.642	1,326
9.184E+26	65,000	1.479	798
6.353E+26	60,000	1.201	702
4.700E+26	55,000	1.057	674
3.580E+26	50,000	0.974	684
2.763E+26	45,000	0.928	724
2.141E+26	40,000	0.910	798
1.653E+26	35,000	0.918	920
1.263E+26	30,000	0.955	1,117
9.466E+25	25,000	1.030	1,446
6.874E+25	20,000	1.169	2,051

I always assume that KSP stock-sized planets are 1/10th the radius and 1/100th the mass of their real life counterparts.

 

Same here. I always consider KSP planets and moon with 10% real radius and 1% real mass. It is also the method I use for making Kopernicus planets, which I started for the Trappist-1 system. I would calculate mass, radius, density, and gravity values for an object I want to create before scaling it down to Kerbal-scale.

2 hours ago, OhioBob said:

I've tinkered with similar changes to make the stock system a bit more realistic.  Many of my proposed changes are the same as yours.  Although this is not a complete list, these are some of the more egregious things that I think need changing: 

• Sun's radius and surface gravity are way off from what they should be for a main sequence star.
• Eve is too dense (surface gravity should be ~1.2).
• Jool is not dense enough (surface gravity should be ~1.2).
• Mun is too close to Kerbin (Kerbin would be tidally locked to Mun at current distance).

 

Yes, Eve is dense (8.532 g/cm^3 when scaled to real-life sizes of 1.089 Re and 2.0532 Me), but that actually makes it analogous to Trappist-1c. The latter may have liquid metal seas of something like Mercury of Gallium due to its high metal content and temperatures. I also find Kerbol far too large for a 10% radius, 1% mass-scale yellow dwarf. It should actually be the size of Ciro from GPP.

Edited August 1, 2017 by ProtoJeb21

[KerikBalm]

@Next_Star_Industries to be honest, I'm not sure what you're trying to say.

Quote

misunderstanding I'm talking about parts not planet sizes.

How can that be possible. You never mentioned a part once. No parts were mentioned at all in the posts preceding yours. All you mentioned were the solar system, and planets by name. How can you expect us to think that you're talking about part size and not planet size? Where exactly was the part of your post that we should think that you're talking about part size and not planet size?

11 hours ago, Next_Star_Industries said:

I think it's awesome to have different systems at different sizes but the parts themselves would still be the same and not be scaled.

Who said anything about scaling parts? In my 3x rescale gamplay where it takes about 5,500 m/s to reach LKO (with LKO orbital velocity being upwards of 4 km/sec), I didn't rescale any parts

Quote

You wouldn't use the same size rocket to reach the same orbit of a different size planet.

No, you wouldn't, you'd need something bigger.

Case in point for the last 2 quotes, this SSTO is 100% unmodified stock parts... its also much bigger than what I use for SSTOs in the stock system.

Quote

It seems things are being scaled to do just this. You wouldn't give a motor more thrust and lower ISP, because those number are set in stone, to achieve the same orbit with the same rocket, no you would build things, like bigger motors, add more fuel tanks, etc. to accomplish these goals. So for stuff like RSS you wouldn't use the same rocket as you would in stock it wouldn't be big enough

Ummmm... yes? We agree that you need a bigger rocket to reach orbit of bigger planets.... you say "It seems things are being scaled to just this" as if its bad or some sort of error. Higher dV requirements mean bigger rockets are needed for the same size payload... this is hardly a revelation.

Quote

They aren't analogues if they were everything would be scaled including the densities and that would affect the gravitational pull of all the planets

No, just No. I don't see any logic to this statement at all.

#1 - an analogue is not an exact replica, its just something that is similar.

"noun
1.
a person or thing seen as comparable to another."

#2) You wouldn't scale densities downward. I think you may mean that you would give them the same density... but that's not scaling it. As it is they did scale the densities... by increasing them, so give the planets comparable surface gravity, to make them analogues. So yes, they did scale everything including the densities (they didn't scale everything by the same factor), and they are analogues.

#3) the simulated gravitational "pull" of all planets is different, did you mean the surface gravity? Sure if they didn't scale the density, the surface gravity would be different - but they did scale the densities.

Quote

 So it's more realistic to view it as the density values aren't accurate

Yes, that is pretty much the consensus

Quote

and they don't really matter because it has no affect on the game or parts since this number can actually be anything you want it to be.

oh but it does matter. Sure its a game and the number can be anything you want it to be, but it has a huge effect on the dV requirements. Thus is has a huge effect on the game and the stats required of the parts that are needed to generate that dV.

Quote

All in all the size of the planets don't matter 1+1 still equals 2 no matter what size it is.

1+1 = 2 so the size of the planets doesn't matter??? what?!

The size of the planets does matter, a lot. A 600km radius planet with a surface G of 9.8 m/s/s has an orbital velocity of around 2,300 m/s. A 6000 km radius planet with a surface G of 9.8 m/s/s has an orbital velocity of around 8,000 m/s. How can you say a >3 fold increase in dV requirements doesn't matter?

Quote

In reality said density/mass of a body correlates to the gravitational pull of said body.

As it does in KSP - what are you talking about? What are you trying to say?

Edited August 1, 2017 by KerikBalm

[OhioBob]

On ‎7‎/‎31‎/‎2017 at 8:21 PM, ProtoJeb21 said:

I also find Kerbol far too large for a 10% radius, 1% mass-scale yellow dwarf. It should actually be the size of Ciro from GPP.

I'm the one who gave Ciro its properties.  It was sized specifically to place it on the curve of an H-R diagram.  I used the mass-radius and mass-luminosity equations found in this article,  i.e.

R / R_sun = 1.06 * (M / M_sun) ^0.945

L / L_sun = 1.02 * (M / M_sun) ^3.92

There were also properties of the home world (Gael) that I wanted to maintain.  Specifically, I wanted Gael to have an orbital period of exactly 426 days (to match KSP's built-in calendar), and a solar constant of 1360 W/m² (to have earthlike temperatures).  The problem had to be solved by iteration,

1. Assume an initial value for Gael's semimajor axis.
2. From Gael's semimajor axis and solar constant, compute Ciro's luminosity.
3. From Ciro's luminosity, compute its mass using the equation above.
4. From Ciro's mass and Gael's semimajor axis, compute Gael's orbital period.
5. If orbital period <426* days, increase SMA, and if >426* days decrease SMA.
6. Repeat steps 2-5 until orbital period equals exactly 426* days.
7. From Ciro's final mass, compute its radius using the equation above.
8. From Ciro's luminosity and radius, compute its surface temperature using Stefan-Boltzmann law.

Of course all these computations where made using real life sizes and masses.  After completing the computations, everything was reduced to stock-size proportions (0.1 radius/distance, and 0.01 mass).

(edit)

* 426 days is the orbital period at stock-size measured in 6-hour days.  When performing the calculations at life-size, the orbital period must be adjusted.  Scaling stock-size up to life-size we increase semimajor axis by 10 and mass by 100, so the orbital period is increased by SQRT(10^3/100) = 3.16227766.  And if we measure it in 24-hour days, we divide by 4.  So when performing the steps above, the actual orbital period that I'm trying to achieve is, 426 * 3.16227766 / 4 = 336.78257 days.  This will, of course, equal 426 six-hour days when scaled down to stock-size.

 

Edited August 2, 2017 by OhioBob

[kraden]

^ this is why I play with GPP

[OhioBob]

17 minutes ago, kraden said:

^ this is why I play with GPP

There are some things about GPP that are not particularly realistic, but where we could provide realism without detracting from the fun of the game, we at least made an effort to get it right.

[kraden]

Oh, I am aware of that. I installed Principia just to see how stable it was, and a favorite little body collided with it's parent. KSP is first and foremost a game, and I think that the GPP crew did a great job creating a level of beauty and realism while maintaining the fun of the game.

[Guest]

23 hours ago, Next_Star_Industries said:

I do know the densities aren't accurate for the gravity in the Kerbal system

The densities of bodies within KSP actually are accurate for the gravity, but this results in them being impossibly dense.

KSP has a lot of unrealistic things about it but if there's one thing they got realistic, that thing which allows one to play RSS without Principia and expect a fairly realistic representation of our own solar system, it's the gravity/orbit simulation. Even if it's a patched conics approximation it's still based on real, physical laws. That doesn't mean that things within the sim need be realistic but that they will behave in a fairly realistic manner within their stated parameters.

Which means that if you take a 600km radius body and make the surface gravity 9.8066m/s^2 it will have the correct density and mass to arrive at that surface gravity, no matter how unrealistic that may be.

Edited August 1, 2017 by regex

[Next_Star_Industries]

1 hour ago, regex said:

The densities of bodies within KSP actually are accurate for the gravity, but this results in them being impossible dense.

KSP has a lot of unrealistic things about it but if there's one thing they got realistic, that thing which allows one to play RSS without Principia and expect a fairly realistic representation of our own solar system, it's the gravity/orbit simulation. Even if it's a patched conics approximation it's still based on real, physical laws. That doesn't mean that things within the sim need be realistic but that they will behave in a fairly realistic manner within their stated parameters.

Which means that if you take a 600km radius body and make the surface gravity 9.8066m/s^2 it will have the correct density and mass to arrive at that surface gravity, no matter how unrealistic that may be.

That's exactly what I'm trying to say I just didn't get said this well. That's why Kerbin has the same gravity as Earth except it's way smaller. I get making bigger planets and things, but I don't get why one scales parts to fit this size, instead of building new parts like would be done in reality. I wouldn't build say the Saturn 5 and claim it's a replica and then still use all 3 stages to get to Kerbin low orbit. The thrust alone is set by the engine those numbers can't be changed so in reality I scaled down the rocket to work on Kerbin therefore it's no longer a Saturn 5 it's a weaker look alike of it.

Edited August 1, 2017 by Next_Star_Industries

[Guest]

28 minutes ago, Next_Star_Industries said:

That's exactly what I'm trying to say I just didn't get said this well. That's why Kerbin has the same gravity as Earth except it's way smaller. I get making bigger planets and things, but I don't get why one scales parts to fit this size, instead of building new parts like would be done in reality. I wouldn't build say the Saturn 5 and claim it's a replica and then still use all 3 stages to get to Kerbin low orbit. The thrust alone is set by the engine those numbers can't be changed so in reality I scaled down the rocket to work on Kerbin therefore it's no longer a Saturn 5 it's a weaker look alike of it.

Okay great, but what does that have to do with gas giant density?

[Next_Star_Industries]

Just now, regex said:

Okay great, but what does that have to do with gas giant density?

WOW my bad I thought I was on the other thread I started about why people scale parts. I really didn't notice I placed this in here. It has nothing to do with gas giants 

[KerikBalm]

@Next_Star_Industries  Ahh..... that makes a lot more sense now... I think I've accidentally posted on the wrong thread a time or two before, no worries.

Edited August 2, 2017 by KerikBalm