How long the rocket fuel could offset the force of gravity upon itself, thus suspending itself in-place. In the real world, you'd need the rocket itself (which

How long the rocket fuel could offset the force of gravity upon itself, thus suspending itself in-place. In the real world, you'd need the rocket itself (which in turn contains the rocket fuel) which adds weight, and you'd have a difficult time controlling power output so the rocket doesn't drift and lengthen/reduce the burn time -- but that's not the point. The idea is that we want a unit that relates the mass of the fuel with its total energy, and one way to approach that is to consider how long the fuel could offset the force of gravity on that fuel, which gives you a cute unit of measure in terms of time.