Doing a physics project - on KSP!

[whistlehead]

I'm currently at sixth form in the UK, and right now we have to do a short essay (~4 pages) on a physics-based topic of our choice - and I saw my opportunity and took it. My chosen topic is to compare the physics of Kerbal Space program to those of the real world.

I've already come up with a good shortlist of things to include in my essay. The two main ones (most relevant to my course) are:

- Spheres of influence vs multiple-point gravity (Ships not acting on planets or each other, planets on rails, lack of Lagrange points) and whether the difference is small enough to be negligible

- Planetary masses - do the planets have a ridiculous density or is the gravitational constant higher? (I vote the latter, as otherwise Kerbol just can't exist)

Other things that I might look at if I have space:

- Air resistance - calculating drag based on mass? Hmm...

- A comparison of the planetary bodies to our own - which planets have direct analogues, etc

- Temperature as a function of distance above sea level

This is what I've come up with so far. Obviously it's only a very short essay so I don't have room for much, but is there anything you more experienced Kerbonauts can think of that I've missed?

Thanks,

Chris

[Scotius]

Differences between Kerbol, and any real star. You can explain why such a small obiect can't be a star in our universe.

[Klingon Admiral]

Why orbits in real life do not remain stable like the ones in KSP.

[stupid_chris]

For the planet's mass, according to in game data from the knowledge, system, the planets really are supermassive and superdense, and it's not the gravitational constant acting up. Though you could explore that possibility

[jsfalconero]

So, caveat, I am a biologist not a physicist, BUT if you're limited to four pages (which is humorous to me as my students would see that as burdensomly long) you might try focusing on related topics like planets on rails, SOI vs multi point physics, and Lagrange points.

Why don't Lagrange points exist in KSP? Their model of orbital physics uses a SOI model rather than continuous multi-body interactions. That means that Lagrange points can't exist. How does that differ from current models of gravity? Why would they choose a simpler model that does not take into account complex interactions? Are there any other effects of the model? (I.E. gravitationally massless ships) Finally, how much would change if they integrated a more complex model of gravity (how subtle are the effects -what would it take to deorbit the mun if it became possible)?

Kudos for being ambitious!

Edited November 2, 2013 by jsfalconero
edited to add question about deorbiting the mun

[Seret]

If you're only looking at 4 pages (so about 1200-1500 words?) you won't be able to get into it too much. I'd say just pick the first two points and discuss them in as much depth as your space permits.

[Akinesis]

Differences between Kerbol, and any real star. You can explain why such a small obiect can't be a star in our universe.

I am sure that brown dwarfs can be Jupiter-size?

[Sirrobert]

So, caveat, I am a biologist not a physicist, BUT if you're limited to four pages (which is humorous to me as my students would see that as burdensomly long) you might try focusing on related topics like planets on rails, SOI vs multi point physics, and Lagrange points.

As a student, I think 4 pages is very short. It may seem like alot when you start, but once you really get going you'll hit the limit way before you have said everything you wanted to say.

I wanted to add a little something on the mass/gravity thing: In our own universe, Kerbol could not exist as a star. Stars have a certain minimum mass. Anything less than (if I recall correctly) 75 times the mass of Jupiter. Anything less and there would not be enough pressure inside (due to lack of gravity) to start fusion

[jebisanaggie]

I am sure that brown dwarfs can be Jupiter-size?

This is why I love this site. I just spent the past 30 minutes researching this.

Yes, they can be Jupiter size, albiet ALOT denser though and they would not be supporting a solar system the likes of Kerbol.

It would be interesting to look at the orbital dynamics of Kerbol and Jool in a true N-body sytem. I wonder if the actual orbital center would lie outside of the Kerbol's surface.

[Scotius]

I am sure that brown dwarfs can be Jupiter-size?

Yeah, Jupiter is too small to sustain low-level fusion process, needed to qualify as brown dwarf. Nonetheless Kerbol seems to be a fully functional, stable star - which is simply not possible for an obiect so small.

Edited November 2, 2013 by Scotius

[Luckless]

Yeah, Jupiter is too small to sustain low-level fusion process, needed to qualify as brown dwarf. Nonetheless Kerbol seems to be a fully functional, stable star - which is simply not possible for an obiect so small.

It would not be probable, not that it wouldn't be possible. Several years ago I read a neat little paper on the idea of a 'shell' star, which was simply a star like formation that forms around something else that acts as a super dense core. I'm a computer scientist, not a physics, and my memory is rather vague on the details. I can't remember what the requirements for the core were, but there was something neat going on that would keep the hydrogen 'cloud' around it from simply collapsing in as a neutron star like object or similar, but it would provide enough pressure to sustain active fusion on something smaller than the scale of a gas giant. I do remember that the writer had also included an expansion to the paper that demonstrated sustained gravity based fusion in a star like process on something as small as earth, but it was getting into some 'weird' science at that scale.

[whistlehead]

Looking at the task brief (It's 2000 words, BTW), it's looking like it might also be a good idea to compare propulsion in the game vs real life - would a Mainsail work the same in our world? And those jet engines with intakes all over the place...

[SpaceSphereOfDeath]

Looking at the task brief (It's 2000 words, BTW), it's looking like it might also be a good idea to compare propulsion in the game vs real life - would a Mainsail work the same in our world? And those jet engines with intakes all over the place...

On that topic, KSP engines have a very low TWR compared to real life engines, I believe the mainsail has 20 odd TWR and the saturn V's F-1 engines I think had a TWR of 147.

[Akinesis]

Yeah, Jupiter is too small to sustain low-level fusion process, needed to qualify as brown dwarf. Nonetheless Kerbol seems to be a fully functional, stable star - which is simply not possible for an obiect so small.

It's not that it's too small, it just not dense enough. The other guy mentioned Jupiter needs to be about 75 times more massive than it is to start fusion, which is right. Even so, Jupiter generates more heat from pressure than what the sun warms it, which is quite something!

Reference to the comment about the Kerbol/Jool relationship- in our own system, the barycenter between the Sun and Jupiter is 40k miles above the surface of the Sun!! So no reason why Kerbol/Jool wouldn't be the same if N-body was used However, because of the rail system everything rotates at the centre of it's own mass.

I, too, love these threads Hope this is all helping.

[macegee]

You should definitely check my project. Look for charts, especially temperature ones. I haven't finished writing a summary (I did write about this survey in our Russian blog), but most of the info is there in those charts.

https://picasaweb.google.com/113746879661497233526/KSPBasicPlanetarySurveyXXI

ADDED:

Oh, and here's spreadsheet with all the data:

https://docs.google.com/spreadsheet/ccc?key=0ArWRtW_GQue0dG9uM0lpMTV2WmU1VWItNzh1UUFuMlE&usp=drive_web

Edited November 2, 2013 by macegee