Kerbal Space Program

http://kerbalspaceprogram.com/

Someone's gone and made a fully functional rocket simulator. You can build them with decouplers, liquid and solid fuel boosters, perform orbits, multi-stage launches, etc.

It's got a very robust physics engine and all of the math is available to calculate stable orbits around Earth Kerth.

Currently not much more than a simulator, but the current version is Free (And will remain free). A future pay-for version is planned that will feature orbital and interplanetary missions, and a full on space program with budgets and cash to rake in for doing things successfully like getting satellites into a stable orbit.







And of course you can build some ridiculous things:



Here's a much friendly getting started tutorial:



Enjoy, I'm hooked. The Mod community has some interesting addons too, like mini boosters, a more robust selection of decouplers (The Applebee above used a few), and even some Ork Bitz! Strangely addicting just to mess around and play with different configs. Im still trying to get a satellite into orbit. It's hard.