Kerbal Space Program does an admirable job of modeling astrophysics and spaceflight, bu it does differ from the real thing in some significant ways. In the interests of promoting clarity, I've made a list of seven aspects where the Kerbal universe differs from our own.
1.) Orbital Decay
One headache the crew of the ISS has to deal with on a regular basis is a constant (albeit slow) drift toward the Earth. It's the result of friction caused by the ever so slight uppermost atmosphere, which (unlike Kerbal Space Program) doesn't cutoff at an exact altitude, but instead gradually decreases the higher you go without completely disappearing until a rather extreme distance from the planet. Even orbital bodies without any atmosphere are subject to solar winds that slowly change the flight paths of small objects such as satellites. Obviously the designers of KSP decided to leave this particular feature out since it adds nothing to the experience other than annoying "maintenance" in the form of orbital correction burns.
2.) Axial Tilt
Pretty much every planet in out solar system spins at an angle; Earth is titled 23 degrees, while Neptune is 30. Uranus is practically rolling around Sol. Unlike humans, Kerbals can enjoy planets that are universally on an even keel. Granted, some have rather off-kilter orbits, but players can (thankfully) always burn east to get a nice alignment. Much like the previous difference, this was done for convenience of play.
3.) Weight Distribution
Sadly, not every difference is there to facilitate enjoyment. Rockets in KSP have an annoying habit of flipping out of control shortly after launch. This is largely the result of changes in weight distribution. In KSP rockets drain propellant from top to bottom which causes an increasingly uneven allocation of mass throughout the vessel. Real life rockets circumvents this problem by having the oxidant and fuel stored in separate tanks, one atop the other. Unfortunately, the only feasible solution in KSP is to manually pump propellant around in order to sustain a relative equilibrium.
4.) Asparagus Staging
On the other hand, players do have a fairly useful engineering technique that has yet to work in reality. Specifically, on Earth feeding fuel from one booster to another introduces all sorts of problems. That said, the issues associated with asparagus staging are not insurmountable. In fact, SpaceX's Falcon Heavy Rocket is attempting to utilize a variant on the principle. Weather or not it can be done successfully has yet to be seen, but perhaps it will be a case of fiction turning into reality eventually.
5.) One-Tenth Scale
In large part due to the limitations of the game engine, the size of planets, and the celestial distances between them, are represented in miniature. While somewhat compensated by rocket performance characteristics, this design choice does cause some significant scientific oddities such as the density of Kerbin. How does the Kerbal's home world generate so much gravity? Is everything beneath the crust made of osmium? Kerbol (the star) is only big enough to be a brown dwarf. In our solar system the moon is more than twice the diameter of Kerbin, while the Earth is about the same volume as Jool. I thought it was a long way to Eeloo as is...can you imagine trying flying the distance between Earth and Pluto in KSP?
6.) Damage
A disturbing truth about spacecraft is the thickness of the hulls, which tend to be somewhere between tin cans and aluminium foil. In the case of damage, words like "tearing," "crumpling," "shearing," and "tattering," definitely apply. For better or worse this sort of thing doesn't happen in KSP. Craft are a collection of parts that are either fine or vaporized, depending on how much force they are subjected to. The result is crashes that look like a bunch of firecrackers mixed with toy blocks, rather than mangled heaps of twisted wreckage. Granted, accurate damage modeling is still something the video game industry (as a whole) is struggling with, so it's hard to blame KSP for being neglectful in this regard.
7.) The N-Body Problem
Figuring out how gravitational influences interact becomes exponentially complicated as the number of objects increases. Obviously a single object is trivially easy, and two translates into a rather elegant little mathematical equation. Go beyond that though and things start to become a real headache. So much so, astro-navigation is done in large part by customized formulas designed to approximately model the real thing with a regrettably slight degree of inaccuracy. Commonly called "patched conics," KSP opts not to use this and instead sets all planets and moons "on-rails," while using the aforementioned elegant equation when necessary. The result is a bit bizarre with spacecraft suddenly transitioning from one parent body to another rather than gradually being drawn in by gravitational influences.
Of course, I haven't even touched on life support, but since Kerbal physiology is still shrouded in mystery. It's impossible to say what their needs and tolerances are. Heck, only recently was it revealed that they weren't a mono-gender species. There are also mods that add in life support, and address many of the above mentioned differences in an attempt narrow the gap between game and reality. How you feel about it is entirely subjective, but personally, I play vanilla KSP since the game is still under active development. For now, I'm content to simply see where developers take us.
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