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Space Agency is a realistic space sim that is free to play but offers in app purchases. It is made for people that want to build rockets and run their own space program but don't have a huge budget to do so. Build and launch your own rockets and satellites, then create a space station and explore your universe.
SpecsUpdate
Pros
Pro Challenging difficulty
Space Agency is not a particularly hard game, it's just difficult and takes time to master. Space Agency plays out in a series of tasks that must be accomplished within time, these tasks play as a series of mini-games that link to each other as you progress. If you make a mistake in any of these stages you will probably have to start over, but play enough and you will begin to complete tasks and progress through the levels of the game.
Pro Simplified rocket physics
Rocket Science can be tough, but Space Agency makes it fun. Using a simplified physics model for space, this game will help you understand the basic concepts of space travel. You will build a rocket powerful enough to lift out of Earth's gravity and then place that craft in an orbit around your favorite place in the solar system.