Marble loop roller coaster launcher

[GNAR]

Help me here.

I need to figure out how to launch a marble so it navigates a vertical loop, then follows projectile motion at the end of the loop

The design must use elastic potential energy to do this and must consistantly launch it consistantly to a specifyied location.

I can't figure out how to launch the marble..

Any ideas other than a compression spring??

 

[JHamm]

Seems like you have a choice between spring and elastic band.