Try these:

[domtyler]

http://www.sciencemuseum.org.uk/launchpad/launchball/

 

[Sh4rkyBloke]

Hmmm. The Armadillo game was much better, building the structure to get your Armadillo from one place to the end (usually a space in mid-air).

 

[Aperitif]

I'm scientifically challenged and, once I got the hang of it, feel confident enough to build a rocket powered bike to take me from here to home in 3.456789 seconds!

Thanks for the link Dom

 

[red_tom]

Nice. Armadillo Run is more in depth but not free.