I think I've mentioned it before a few times but it's only recently that I've opened up my copy and started to play. (Things like another Deus Ex run and not having the mental energy to learn something new have got in the way).
What is the Kerbal Space Program ?
It's a rocket simulator. You build your own rockets out of snap together bits and pieces, take it to the launchpad and watch it explode. Most of the time. Some of the time, you can try for escaping the orbit of the home planet and attempt to reach its Mun. Or even build huge ships out of multiple component launches and attempt to reach the other planets.
There's a lot of scope in the "game". Shouldn't really call it a game actually. Something as high quality as this is a stellar teaching aid. The building of the rockets is highly simplified and they are very easy to control. You have 3 axes of freedom plus a throttle. There's a stabilisation module that quickly stops your rocket tumbling out of control, if you've added the right bits.
The complication is hidden under the skin, with full physics orbital mechanics modelling at play. Going from Earth to the Moon isn't just about pointing the rocket at the Moon and throttling up the engines. The Moon will have moved by the time you get there and the course will curve due to all the little bits of gravity in play. KSP fully models all that complication, it lets you know what's happening and there's a couple of tutorials that let you know what's going on.
Here's the first rocket ship attempt :
It's a fairly standard 2 stage rocket with a command module on top. The first stage has solid rocket boosters surrounding a main engine. The intention is that this stage gets your rocket to orbit. The second stage would then propel the command module to Mun orbit. This was early days so I was just looking to go Mun & back. Nothing fancy like an actual landing ...
How did this one go ? It got out of the atmosphere but never made the orbit. It needed more rockets. Actually needs another stage I think.
So - I add more rockets. And ...
I'm about to head back in again to see if I can make a working Mun rocket (without resorting to the wiki crib sheet!). But ... something like this can't really be described in words, so here's a Youtube video with someone talking you through the pictures.