We choose to do these things not because they are easy, but because they are hard.
Kerbal Space Program is a brilliant spaceship builder created by the guys at Squad. It was initially released to the public in 2011 and it was brought to my attention not long after that by a couple of friends. Although the game isn’t even out of alpha yet it has been steadily receiving updates and has been what I would consider a pretty compete game for quite a while. Up until this most recent 0.22 update it has only existed as a sandbox game; you have a huge array of possible parts and you just put them together however you want and see if your creation will fly/make orbit/land on the Mun depending on how ambitious you are. As well as rockets you can also build satellites, construct space stations in orbit and try your hand at space-planes.