[Spaceships] getting into orbit without superscience?

[RyanW]

Originally Posted by Anthony

Well, you can use hydrogen as reaction mass instead of combustion byproducts; that allows an ISp of around 1,400 without being any hotter than current rocket. The problem is that reactors can't generally run anywhere near as hot as a current rocket engine.

A ton of heat producing equipment (i.e. reactor) will heat a lot less reaction mass to those sorts of temperatures than a ton of chemical rocket engine can burn. In other words, you can have Isp or thrust (with current or near future technology), take your pick.