How similar is the Kerbin Mun system to Earth and the Moon?

[johnnyhandsome]

I.e. is getting to Mun similar in difficulty as Apollo getting to the Moon? What the differences? Is the distance similar? Gravitational pull?

[Kimberly]

The size of both bodies and the distance between them is scaled down. As a result, Kerbin's atmosphere is much smaller than the real one, making it easier to get into orbit. But the gravitational pull of both Kerbin and the Mun are more or less the same as their real-world counterparts.

[johnnyhandsome]

Correct me if I'm wrong, but the gravitational pull of both is not active in the game, i.e. you're either in Kerbin's sphere of influence or Muns, but not both (as in real life) due to the processing power that would require on desktop system.

[Tw1]

Correct me if I'm wrong, but the gravitational pull of both is not active in the game, i.e. you're either in Kerbin's sphere of influence or Muns, but not both (as in real life) due to the processing power that would require on desktop system.

More or less true. Plus our Moon is not quite un a circular orbit, nor perfectly equatorial.

[urablahblah]

However both are tidally locked, which is a significant planetary feature that tends to get overlooked.

[Aphox]

Correct me if I'm wrong, but the gravitational pull of both is not active in the game, i.e. you're either in Kerbin's sphere of influence or Muns, but not both (as in real life) due to the processing power that would require on desktop system.

It's not really a question of power. KSP just doesn't use n-body physics. But that is true; you're either in one or the other.

Earth has a mean radius of about 6300km while Kerbin is pretty much exactly 600km. Earth's moon is roughly 384,400km give or take, depending on your source, from Earth. Kerbin's Mun is <insert number because I forgot>

In a nutshell, it is nothing similar because of the way the physics system works and the scale, as previously mentioned. Both bodies are much smaller. SoIs are much different. If you want to make the comparisons yourself, the wiki has some numbers.

http://wiki.kerbalspaceprogram.com/wiki/Kerbin

http://wiki.kerbalspaceprogram.com/wiki/Mun

Some of the information is a bit lacking. For Earth and its moon, I recommend WolframAlpha or wikipedia.

[Tw1]

11,400 Km.

Plus it's a lot easier to not have to worry about damage from lunar dust, temperatures, etc.

Edited July 21, 2013 by Tw1

[KSK]

You can navigate to the Moon using patched conics though - worked fine for the Apollo program

[Kimberly]

However both are tidally locked, which is a significant planetary feature that tends to get overlooked.

Isn't the moon tidally locked to Earth as well?

[Bloodbunny]

Well they are relatively similar, except that if Earth were in the game, it would be roughly the size of Jool. As far as missions go. It is somewhat easier to do an Apollo style mission in game, for the simple fact that the orbital speeds required are much lower.

Edited July 21, 2013 by Bloodbunny

[Sof]

Isn't the moon tidally locked to Earth as well?

Yes, hence why he said "both"

The sizes and distances have more or less the same ratios, however Earths moon has an inclination which the Mun doesn't.

[Superfluous J]

However both are tidally locked, which is a significant planetary feature that tends to get overlooked.

Does it? Anybody who's looked at the moon more than 2 or 3 times can't help but see that. It's one of those classic questions that every astronomy teacher has to answer, "Why does the moon rotate when we can see the same side all the time?"

[zarakon]

It is somewhat easier to do an Apollo style mission in game, for the simple fact that the orbital speeds required are much lower.

It's always difficult to compare this type of thing.

Yes, KSP requires much less delta-V. But KSP also has much less efficient parts. The engines in KSP have awful thrust:weight ratios compared to real engines, and a KSP fuel tank alone has a worse fuel:weight ratio than most entire rockets in the real world. These factors are evident in the difference in burn times. For example the second stage of the Saturn V burned for 6 minutes. In KSP you're unlikely to have an ascent stage last even 3 minutes.

[SkyRender]

The main reason why parts in KSP are so inefficient compared to real-world parts, of course, is because Kerbin is about 1/11th the size of our own Earth. To balance out the difficulty a bit, they upped the mass of everything. If they had kept real-world mass and ratios, getting off of Kerbin would be laughably easy.

[Ralathon]

The main reason why parts in KSP are so inefficient compared to real-world parts, of course, is because Kerbin is about 1/11th the size of our own Earth. To balance out the difficulty a bit, they upped the mass of everything. If they had kept real-world mass and ratios, getting off of Kerbin would be laughably easy.

If they just upped the mass of everything it shouldn't affect the wet/dry ratio for fuel tanks.

Kind of a shame actually. The immense mass of engines discourages staging. For the Apollo missions a Lunar Rendevouz mission was much cheaper on the dV than direct ascend. Due to the mass of engines the reverse is true for us.

Might've been nicer if they just upped the weight of fuel tanks a bit but nerfed the weight of engines. So it becomes a balance between complexity and dV budget, instead of our current situation of "simpler is better"

[urablahblah]

Does it? Anybody who's looked at the moon more than 2 or 3 times can't help but see that. It's one of those classic questions that every astronomy teacher has to answer, "Why does the moon rotate when we can see the same side all the time?"

Of course, but the original question was comparing the Moon to the Mun. It is easy to forget that both are tidally locked their parent body. The in game consequences of such a long dark/light cycle tend to get overlooked (at least that was my experience when I started playing Kerbal).

[maltesh]

Yes, hence why he said "both"

The sizes and distances have more or less the same ratios, however Earths moon has an inclination which the Mun doesn't.

The distance doesn't have the same ratio as the size. The Moon's about 60 earth radii from Earth. The Mun is exactly 20 Kerbin radii from Kerbin. Th reason for that is because, on the typical wide-view computer screen, a Mun 36,000 km from Kerbin would look tiny.

[Motokid600]

The big difference id say is the Earths 23 degree tilt that gives us the seasons. Kerbin has no tilt to make it easier. Infact... I don't think any of the planets have a seasonal tilt relative to their orbital plane. Correct me on that one.

[OUScooby]

Just the other day I learned about Cruithne, a captured asteroid sometimes call "Earth's second Moon." As soon as I heard that I exclaimed "Minmus!" Of course Cruithne doesn't orbit the Earth, it orbits the Sun in a elliptical orbit that takes it inside of Mercury's orbit and sometimes outside of Mar's but it's average distance from the Sun is close to that of Earth's and it's orbital period is 364 day. I know it's not all that similar to Kerbin and Minmus but I'm convinced Cruithne was the inspiration for Minmus.

[Alien1099]

Correct me if I'm wrong, but the gravitational pull of both is not active in the game, i.e. you're either in Kerbin's sphere of influence or Muns, but not both (as in real life) due to the processing power that would require on desktop system.

If I recall correctly, the three body "problem" has not been "solved" yet.

[Ralathon]

If I recall correctly, the three body "problem" has not been "solved" yet.

It hasn't been solved in the sense that they can tell you where body x will be at point t in time without calculating all the steps in between the initial state and time t.

It isn't actually all that expensive in a computational sense to figure all this out, your desktop could calculate the positions of all planets + 100's of ships in the kerbol system millions of years into the future in just a few seconds. However, since the position isn't given by a simple repeating equation you cannot put your spaceships and probes on rails. Your PC has no issues calculating all this, but you're going to hate it when your entire satellite constellation breaks apart when you timewarp a ship to Duna.

By limiting it to a simple 2 body problem you can ensure that your ships don't escape into Kerbol orbit or crash into the surface during a time warp. If Squad were to implement N body physics they'd also need to add some sort of RCS system that can do station keeping in the map view.

As it is though, I think it is a nice compromise. I would've loved to build stations in Lagrange points, but I'll gladly give it up if it means my constellations won't break/run out of RCS every time I travel to Jool.

[Dorsai!]

By limiting it to a simple 2 body problem you can ensure that your ships don't escape into Kerbol orbit or crash into the surface during a time warp. If Squad were to implement N body physics they'd also need to add some sort of RCS system that can do station keeping in the map view.

As it is though, I think it is a nice compromise. I would've loved to build stations in Lagrange points, but I'll gladly give it up if it means my constellations won't break/run out of RCS every time I travel to Jool.

Actually it wouldnt even even be necessary to have n-body physics for realistic gravitation, lagrange points and such. You can just as well reduce the calculations to a series of two body problems, one for each large body in the kerbal system.

All the gravitational vectors could then be added up to get the final vector on which the ship/station/probe is pulled.

There is really no need to calculate a real n-body system if you put the planets themselfes "on rails" and neglect the gravitational pull your ship has on everything else because its so infinitismaly small