Making Rockets Like Airplanes

I'm thinking back to Elon Musk on Friday and some of his aircraft analogies for his rocket. He wants to build as much of the rocket in the factory instead of stacking it on the pad. This is akin to building an airplane in a factory instead of on the runway. This makes a lot of sense.

It makes less sense to run up the rocket's first stage engines before lifting off, as an airliner does before it takes off. This isn't a big deal with an airliner because it doesn't take a lot of fuel to run up the airliners engines momentarily before takeoff. Yet this is critical for a rocket because you're wasting the finite amount of oxidizer onboard (while the oxidizer is free in an aircraft.) The fuel expended prior to liftoff is wasted mass; the rocket works on the principle that about 90% of its mass is expended as useful propellant.