Assuming the balloon itself has zero mass, every cubic meter of hydrogen in your balloon can lift a maximum 30 grams of payload. For the 2000 ton space shuttle, that's sixty million cubic meters - a spherical balloon half a kilometer in diameter. There's a reason we don't launch rockets with balloons.

There are practical non-rocket ideas, though, such as the Verne Gun: http://nextbigfuture.com/2010/03/150-kiloton-nuclear-verne-g...