Why are parts so heavy?

[-Velocity-]

The kerbal universe has the same gravitational constant as our universe, i'm pretty sure..

I've been using it to plan my orbits and intercepts, and the answers i get is exactly right (Depending on how many decimal places i use.)

i.e: if you want an orbit with a specific period and want to know what altitude you need.

cuberoot(("orbital period in sec"², times gravitational constant, times inertial mass of the planet) divided by (4pi²))

you get your desired semi-major axis. simply subtract planets radii and you have your orbital altitude for circular orbit.

And this holds true for all planets and moons as far as i've tested.

If you redefine G you would also need to redefine mass and distance to end up coming out with the same numbers.

If G is 10 times higher the inertal mass would be 10 times higher so either the mass number would have to be 1/100'th or 1m would have to be redefined cuberoot(1/100)

and if distances is redefined kN (thrusts) would have to be redefined, and probably alot of other things i can't come up with atm.

Redefining G has a lot of consequences if you still want your standard real world math formulas to apply.

So instead i believe they just said hm.. lets make it smaller and just say it's more dense.

That makes no sense. There is no big G in F = ma.

Redefining G has no consequences other than the force of gravity changes. The gravitational constant is, basically, how much gravitational force is exerted between two bodies of fixed masses. So saying that it would affect mass shows ignorance to what the gravitational constant is in the first place. It is impossible for it to affect mass, since mass is already included in its formulation, and is fixed.

Edited September 18, 2013 by |Velocity|

[Kerbart]

I don't mean to be aggressive, but i don't see how it's relevant to my post, you are the 21313th person to say it, and everybody knows that.

Useless post is useless.

Your post mentioned the reduced sizes of the Kerbal universe compared to "RL" whatever that is ("Real Life?" What is THAT?). So there is relevancy. As to what the relevance of this message is to the OP's text, well, yes, that got me stumped too.

[Kerbart]

That makes no sense. There is no big G in F = ma.

Redefining G has no consequences other than the force of gravity changes. How in the heck would it affect mass?

Because the game is developed around real world units and constants. Once you start messing with that, everything gets very complicated. Right now you can plug the numbers into any orbital calculator and it'll spew out the right numbers. Build a universe around a different gravitational constant and it messes everything up.

Today that might not be a problem as I think there are more Hohmann Transfer Calculators for KSP available than for our solar system (I'll admit this is an unsubstantiated claim but you get the idea) but when the game was developed and these kind of decisions were made that was not the case. It's a lot easier to get the game accepted when you refer to science books and wikipedia's without having to say "oh, but we changed the gravitational constant, the speed of light, and the value of pi"

In the end, KSP is a game. Certain decisions have been made to make the game more enjoyable (patched conics, a smaller solar system, life forms that can do without air, food and water, etc). Those were choices made. In hindsight, could other choices have been made? Maybe. Personally I think the decision to go for small planets with unrealistic masses was a smart one. Small sizes fix a lot of problems. Travel time, the amount of detail a 4MB texture can deliver in relation to kilometers when draped over a planet, etc. Yes, it removes "realism." But let's pause for a second here. You're playing a game that involves little green aliens named "Bob", "Bill" and "Jeb," once you accept that as reality all bets are off.

[-Velocity-]

Because the game is developed around real world units and constants. Once you start messing with that, everything gets very complicated. Right now you can plug the numbers into any orbital calculator and it'll spew out the right numbers. Build a universe around a different gravitational constant and it messes everything up.

Today that might not be a problem as I think there are more Hohmann Transfer Calculators for KSP available than for our solar system (I'll admit this is an unsubstantiated claim but you get the idea) but when the game was developed and these kind of decisions were made that was not the case. It's a lot easier to get the game accepted when you refer to science books and wikipedia's without having to say "oh, but we changed the gravitational constant, the speed of light, and the value of pi"

In the end, KSP is a game. Certain decisions have been made to make the game more enjoyable (patched conics, a smaller solar system, life forms that can do without air, food and water, etc). Those were choices made. In hindsight, could other choices have been made? Maybe. Personally I think the decision to go for small planets with unrealistic masses was a smart one. Small sizes fix a lot of problems. Travel time, the amount of detail a 4MB texture can deliver in relation to kilometers when draped over a planet, etc. Yes, it removes "realism." But let's pause for a second here. You're playing a game that involves little green aliens named "Bob", "Bill" and "Jeb," once you accept that as reality all bets are off.

While I agree with the spirit of your post (that we shouldn't take the physics of KSP too seriously), substantively, you are incorrect. Yes, a different gravitational constant will mess things up- BUT ONLY IF YOU KNOW THE MASSES OF THE PLANETS. We do not. The problem statement is this:

1) We know the gravitational forces that planetary bodies exert on other objects.

2) We know the size of planetary bodies.

3) Find the mass and density of the the planetary bodies.

You cannot do 3) UNLESS you know what the gravitational constant is. The gravitational constant could be anything, as this is a fictional universe, and thus, the mass and density of a planetary body could be anything.

In our problem statement, adjusting the strength of the gravitational constant does nothing except change the mass and density of planetary bodies, as the gravitational forces the planets exert are fixed. Since we cannot physically detach a portion of a planet and apply a known force to see what the mass and density of the planet is, we really have no idea what the gravitational constant or the mass of a planet is. We only know what the product of the two is.

Why does any of this matter? Well, in the context it was originally brought up in, the purported "high density" of planets was used as a justification to make overweight rocket parts.

Edited September 18, 2013 by |Velocity|

[Kerbart]

Why does any of this matter? Well, in the context it was originally brought up in, the purported "high density" of planets was used as a justification to make overweight rocket parts.

I fully agree. But that also assumes causality in that relationship. Unless one of the lead devs dives into the discussion we'll never know for sure, but this is, I think, basically what happens.

• Let's develop a space-going game with reasonable amounts of realism (thus using game technology modelled on real earth technology)
  
• Let's make everything smaller so we need less delta-v (because of smaller orbits and lower masses)
  
• Oh, but now we have only 1/10^th of earth's gravity. How to solve that?
  
• Scenario 1:Easy! Increase the mass of Kerbin!
  
• Scenario 2:Easy! Let's just tweak the gravitational constant!
  

For whatever reason, the devs went with scenario 1 (personally I'd too, I wouldn't be messing with universal constants that quickly) and initial weights were probably just eyeballed, based on fuel tank sizes (where scaling dry weight and wet weight is merely a matter of squaring and cubing, or the reciprocals of those) and going from there.

As I said before, I think a better reason Kerbal stuff is heavy is because the program isn't all that high-tech in the first place. As for the density of the planet... 99.999% (by volume) of Kerbin might have the same density as earth. It's just that they have a black hole in the core. Or something along those lines. Little green men. Anything goes

[Respawn]

sry for not staying quite on topic

Someone mentioned that the gravitational constant G might be different in the kerbal universe, i'm just saying it's not likely.

@velocity : all i'm saying is: that the formula works in-game.

So i can conclude that the most likely reason is that G(in-game) = G(irl)

If G(in-game) != G(irl) then some of the other units definitions in that formula would have to change. (since it does work)

Along with alot of other units used in other formulas due to the first change.

If you assume they changed G(in-game) by multiplying it by 10. (Using my formula would result in wrong altitudes, whitch it doesn't.)

There are however 2 ways you can modify unit definitions for the example formula in my previous post to still give me the correct answers:

1'st way: you could change the definition of (in-game time) from "1(kerbal)sec=1sec" too "1(kerbal)sec=0.316228sec"

but then you would also have to change the ISP definition to keep dV "m/s" consistent. Which sounds ridiculous.

2'nd way: the in-game mass-unit 1 could represent 0.1 tons instead of 1 ton. Which would give me the correct answers.

But if 1 mass-unit is 0.1 tons then 1 kN(ingame) units listed in engine descriptions would have to represent 0.1kN for "f=ma" to still be true. Which does not make sense.

@garek : in that example it would not cancel out but feed back on itself to give a bigger error.

[-Velocity-]

sry for not staying quite on topic

Someone mentioned that the gravitational constant G might be different in the kerbal universe, i'm just saying it's not likely.

@velocity : all i'm saying is: that the formula works in-game.

So i can conclude that the most likely reason is that G(in-game) = G(irl)

...

And how do you get the formula to work in game, if you don't know the mass of the planets?

Oh wait, you used the masses of the planets as given on the KSP Wiki?

Do you realize that those masses are derived by assuming that the gravitational constant in KSP is the same as in our universe?

Do you realize that there is no valid reason to assume this? And thus, the masses of the planets you are using are based on a faulty assumption?

Basically:

G*m = g*r^2

g*r^2 is known, it is the measured gravitational acceleration, g, times your distance from the gravitating body, r, squared.

However G, the gravitational constant, is unknown, as is the mass of the gravitating body, m.

ONE EQUATION. TWO UNKNOWNS. Remember algebra? It is impossible to solve this problem.

Thus, we can chose between an infinite number of combinations of G and m that simply must multiply to equal g*r^2. We can chose G to be WHATEVER we want. There is no reason to suppose that it is the same as in real life, especially when using the real-life value implies that planets are made out of super-dense materials.

Please elaborate on where you are getting confused about this.

Edited September 18, 2013 by |Velocity|

[-Velocity-]

I fully agree. But that also assumes causality in that relationship. Unless one of the lead devs dives into the discussion we'll never know for sure, but this is, I think, basically what happens.

• Let's develop a space-going game with reasonable amounts of realism (thus using game technology modelled on real earth technology)
  
• Let's make everything smaller so we need less delta-v (because of smaller orbits and lower masses)
  
• Oh, but now we have only 1/10^th of earth's gravity. How to solve that?
  
• Scenario 1:Easy! Increase the mass of Kerbin!
  
• Scenario 2:Easy! Let's just tweak the gravitational constant!
  

For whatever reason, the devs went with scenario 1 (personally I'd too, I wouldn't be messing with universal constants that quickly) and initial weights were probably just eyeballed, based on fuel tank sizes (where scaling dry weight and wet weight is merely a matter of squaring and cubing, or the reciprocals of those) and going from there.

As I said before, I think a better reason Kerbal stuff is heavy is because the program isn't all that high-tech in the first place. As for the density of the planet... 99.999% (by volume) of Kerbin might have the same density as earth. It's just that they have a black hole in the core. Or something along those lines. Little green men. Anything goes

Honestly, I wouldn't be surprised if it went along these lines. However, I just don't know how it's programmed internally. Plus, even if they just adjusted planet mass rather than gravitational constant, that doesn't mean the the official "cannon" of the KSP universe has to reflect the manner in which the game was programmed.

A super dense core for each planet is an interesting take on it. That way, the crust could be made of lighter, more realistic material. However, it still implies that the KSP universe follows a different set of the laws of physics.

As I see it, these are the two most simple interpretations:

A) Superdense planets:

1) The KSP universe follows different laws of physics, allowing for super-dense materials to exist at low pressure.

2) The Kerbals build their rockets out of materials that are well over an order of magnitude less dense than the most common elements they encounter on their planet, which is very unlike reality.

Gravitational Constant

1) The KSP universe follows different laws of physics- the gravitational constant is higher.

Occam's razor says that the simpler explanation is more likely to be correct, which is why I say that is the better explanation, especially when BOTH A) AND require that the laws of physics be different.

Edited September 18, 2013 by |Velocity|

[Respawn]

And how do you get the formula to work in game, if you don't know the mass of the planets?

Oh wait, you used the masses of the planets as given on the KSP Wiki?

I have it written down in my personal notebook along with alot of other ksp related numbers.

In my notes i've written down 5.29229x10²² which does not fit the wiki, but is the number that i've used.

Anyway we do know the mass of the planets, the numbers are available ingame.

In map view while focused on planet, there's a tab on the right displaying relevant information.

The mass of kerbin listed ingame is 5.292e+22kg

and GM is listed as 3.532e+12

Not knowing to what degree those numbers are rounded, only using Kerbin as a referance,

you can say that the gravitational constant ingame is somewhere between 6.6726e-11 and 6.6758e-11

G(irl) falls within those numbers and in either "extreme" would not make much difference.

so..

A) Superdense planets:

(for now at least, until the devs decide something else.)

[lajoswinkler]

Right, I've just checked out the masses. Indeed, the thermometer has 0.005 t which is 5 kg. Now that's a heavy thermometer...