Changing the Definition of a Meter (A journey into bad algebra and worse physics)

[Fel]

While wondering again about part balance, I figured why not just fix the whole mess and make Kerbin more earth-like.

Well, as many of you already know... it appears Kerbin is a near perfect clone of earth; increasing the radius by 10.6 changes the mass by 10.6^2, which then leaves gravity the same but Kerbin with very close numbers to our sweet planet (Although G isn't balanced).

So now, on a scrap of paper... 1 "kerbin meter" = 10.6 "earth meters" and all is right... except velocity... and acceleration... and everything else.

Since velocity can't really change (game setting), I guess saying that "kerbin time" is 10.6 x earth time fixes that...

And to balance G, "Kerbal Mass" has to be increased 10.6x earth mass (BOOO!!!!)

This imbalances g again, so Kerbin's mass is now 10.6 * Original * "Kerbal Mass Units" (but basically unchanged).

But what surprises me is that thrust remains unchanged.... and the isps decrease.

*grumble*and that mass increases*grumble*

In terms of what I was doing (Unit Conversions and multiplying by 1)... it kind of makes sense; but hardly any more than that.

[Spatzimaus]

Well, as many of you already know... it appears Kerbin is a near perfect clone of earth; increasing the radius by 10.6 changes the mass by 10.6^2

No, increasing the radius by 10.6 changes the mass by 10.6^3. Kerbin is ten times as dense as Earth, but has the same surface gravity because force goes as M/r^2.

KSP's basic Newtonian physics uses the same values as the real world, and the gravities are comparable; the distance scale is just far smaller, to make it easier to get to orbit.

[Fel]

We're ignoring the UNITS of G for now

G * m

------- =

r ^ 2

G * m * a ^ 2

--------------

(r * a) ^ 2

Balancing G's units ((a * m) ^ 3 / ((s * a) ^ 2 * (g * a)) fixes other problems (and yes, as I stated, mass has to be (m * a) * a)

Point was more to thrust (Kg * a) * (m * a) / (s * a) ^ 2 not changing but isp does.

Edited May 1, 2013 by Fel

[RoboRay]

The distance scale was shrunk down for gameplay reasons.

Maybe try Orbiter if you prefer the real-world distance scale and the correspondingly longer, slower events like launches?

[Fel]

I understand that the distance was shrunk...

But at the same time that created innumerable balance issues (RE: SSTO insanity). It also created many strange ways of balancing the insanity; some of which feel a little more restrictive than others.

Now, since Kerbin is 10.6 ^2 times less massive than earth, and 10.6 times less big... it is perfectly scaled which means going in reverse should be possible.

What surprises me is that TIME ALSO SCALES! It has to to balance many of these equations; and with time scaling, it results in:

So to go from Kerbin to Earth scale... ISP would increase x10.6 while Thrust remains the same. (And all mass would reduce by 10.6)

Does that sound like I'm balancing things? XD.

The converse is that to go from Earth to Kerbin scale, ISP would decrease, mass increase, thrust remains the same.

[Nexustrimean]

Kerbin is 10.6 ^2 times less massive than earth, and 10.6

Kerbin and earth have the same mass, Kerbin is just Very very dense.

[Spatzimaus]

Kerbin and earth have the same mass, Kerbin is just Very very dense.

No, Kerbin's mass is 5.29*10^22 kg, and Earth's is 5.97*10^24. This gives Kerbin a density 10.6 times that of Earth (a physically impossible value), but the same surface gravity.

[Fyrem]

No, Kerbin's mass is 5.29*10^22 kg, and Earth's is 5.97*10^24. This gives Kerbin a density 10.6 times that of Earth (a physically impossible value), but the same surface gravity.

As I understand it, that is the only parameter that is "changed" in the KSP universe. In order to make the Solar system small enough to be "fun" they filled the planets with a very Dense material (that yes, is unrealisiticly dense). but nothing about Time or space measurements were changed or messed with at all.

there is no 're-definition' of a meter, nor of G, nor of a second that needs to be done.

[Fel]

As I understand it, that is the only parameter that is "changed" in the KSP universe. In order to make the Solar system small enough to be "fun" they filled the planets with a very Dense material (that yes, is unrealisiticly dense). but nothing about Time or space measurements were changed or messed with at all.

there is no 're-definition' of a meter, nor of G, nor of a second that needs to be done.

Earth to Kerbin relationships are:

Thrust is reduced by ~10x, mass is haphazardly placed (Engines are often less massive, Tanks are more, SRBs are... something). ISPs are somewhat reduced but not significantly. (And yes, kerbal planetary distances / masses are much smaller than earth counterparts...)

Kerbal is scaled in many strange ways, I just found it interesting that to balance the equation TIME would have to be scaled as well.

[AngelLestat]

I see always many people mention that the kerbin has to be 10 times more dense than the earth just becoz is 10 times smaller and has the same gravity.

That is wrong...

No, increasing the radius by 10.6 changes the mass by 10.6^3. Kerbin is ten times as dense as Earth, but has the same surface gravity because force goes as M/r^2.

For example you are mention that the gravity is reduced but the r^2 but you dont realize this mean it doesn´t need to be the same mass to have = surface gravity.

For example, lets compare Mars with Mercury.

Mars:

Surface gravity: 0,37 g

Mass: 6,4185 × 10^23 kg

Diameter: 6.794,4 km

Density: 3.9335 g/cm³

Mercury:

Surface Gravity: 0,38 g

Mass: 3,302×10^23 kg

Diameter: 4.879,4 Km

Density: 5,43 g/cm3

See? Mars has 2 times more mass than mercury and it has equal or less gravity.

This mean (I dont wanna make the calc right now) that kerbin only need to have aprox 20% of the earth mass to have the same surface gravity.

About the change of the Kerbin Meter, you are right, but it will be difficult to us if we wanna use the unit to compare.

Edited May 2, 2013 by AngelLestat

[foamyesque]

Angel: For Kerbin to have the same surface gravity as Earth, it needs to be denser in exact proportion to the radius differences. A Kerbin with 10x the density of the Earth but 1/10th the radius will have a surface gravity identical to Earth's but only 1/100th the mass.

[DeeZee]

It is just a game. Its not a die hard simulation.

[Spanier]

Talking about skales and their accuracy is kind of flawless, but more variety in fuel and it's mass wonld be cool.

[Splode]

It is just a game. Its not a die hard simulation.

Getting sick of these comments.

It's a game about physics, and orbital mechanics. It has to play by the rules. Nobody has a problem with Kerbin being dense, but people do mind if it mucks up other laws of physics.

[shand]

It is just a game. Its not a die hard simulation.

Yes and harry potter is a fictional novel, that doesnt stop people playing quiditch in the park. (the snitch tends to be a girl wearing yellow)

My point is there doesnt have to be a solid line between reality and fiction. let one spill into the other and see how it sits.

seconds, metres and kg are all pretty arbitary definitions (espeically kg) so in theoy we can "scale" them independently.

of course we have to start with the assumption that (for the laws of physics that apply) some approximation of the constants (Big G for example) are consistant. A simple solution is that Big G is 10x that of our universe. or something

[Ariellus]

...

... CGPM defined the metre in the new International System of Units (SI) as equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.

Why we should need a new definition of a metre ?

Same goes to a second

A Kerbal day is nearly 6 hours = 21600s or 60s/1min for each degree kerbal is turning fits perfect for me.

[AngelLestat]

foamyesque: Angel: For Kerbin to have the same surface gravity as Earth, it needs to be denser in exact proportion to the radius differences. A Kerbin with 10x the density of the Earth but 1/10th the radius will have a surface gravity identical to Earth's but only 1/100th the mass.

OMG, that was an unexpected turn.

Damm, you are right. I was thinking in the density like a linear relation with diameter, heh, that was so wrong, Is volume!

[Fel]

Well....

I spent a good while tweaking a small part set and finally got a decent Saturn V (Engine Thrust / Weight, Fuel Weight, and Empty Weight all approximately the same; ISP's tweaked so that it stages properly) to launch a 100Mg "payload" into a 90km orbit. And, yeah....

The Attachment Nodes are too weak and EVERYTHING needs to be held together with trusses / structs. The stages also move around an insane amount, which means trusses to keep things stable (and try to prevent resonant oscillations of death). The launch time is about 8 min (MET Elapsed) ... so probably 16 min due to how slow my computer is. Air resistance can actually destroy the ship (even going a few m/s over).

But, having to put together a 3,000Mg behemoth that takes ages to launch (although, I do feel that the launch feels a bit more spectacular)... not to mention that I ran out of room in the VAB several times and had to scale down a bit... for a mun landing... it does seem excessive for a game. (And the ISPs were really only cut in half [well, 2.17] so too much fiddling does drastically affect the game)

Well, at least I have a 100Mg launcher XD

[Delling]

seconds, metres and kg are all pretty arbitary definitions (espeically kg) so in theoy we can "scale" them independently.

Ehm, the definitions of meters and kg are connected - that's the whole point of the metric system. 1kg = 1 dm³ (aka 1 l) water mass at 20° C.

Current definition of the meter is 'Length of the path travelled by light in a vacuum in 1â„299792458 of a second', which joins time as well.

[RoboRay]

Yeah, 1/299792458 of a second isn't from an arbitrary source at all.

[NeoMorph]

This is the definition of a meter...

It's the Lunar Module Forum Reaction Control...

[shand]

Ehm, the definitions of meters and kg are connected - that's the whole point of the metric system. 1kg = 1 dm³ (aka 1 l) water mass at 20° C.

Current definition of the meter is 'Length of the path travelled by light in a vacuum in 1â„299792458 of a second', which joins time as well.

Actually the defination of a kg is "the mass of the international standard kg weights" of which there are two, one in the americas, one in europe. incidently these have lost mass over the years due to various processes and are now kept in a vaccum.

meter is arbitrary as you state.

and time i entirely arbitrary - "x number of decays of y"

All others rely on the definitions of the others in some vaguely sensible way (amp - current that causes 1m deflection in 1m long wires, or something, i forget)