I originally came up with the idea whilst building a non-BT AU where the current world powers, I think its the top 30 begin colonising and taking thier problems into space. The UN sponsored program gets the worlds closest to home, and although successful can never reach and effective consensus on how to act as a group and is outpaced by the single nation programs.
The WarShip designs were first based on the details from Janes Fighting Ships of World War I, in a double the mass and go from thier conversion, with the extra mass allowing for Hyper drives, Turret Belts etc. This meant that WarShips were never bigger than 200 000 tons and thier mass/volume was a little more realistic than BT's.
Space travel came about once humanity reached the Periodic Table's man made element's Island of Stability, where it is hypothosied that stable super-heavy elements will exisit see Wikipedia for a breakdown). Three elements are found which bring about the space race. Serenium, Gravitium and Hyperion. Serenuium (Atomic Number 176) is the elment that becomes the core of armour and warhead technology, know for its strength and ability to alloy with other metals. Gravitium (AN 186), when formed into a "cage" and chaged properly can create gravitational fields and Hyperium (AN 179), when placed in a Gravitium cage, can create a hyperspace window. There are several levels, or interior spheres of Hyperspace, allowing for all sorts "Jump Speeds" and for tactical use of Hyperspace in battle.
For there, humanity expands, build great WarFleets and Armies and has at it.