Page "Spaceplane" Paragraph 12

A difference between rocket based and air-breathing aerospace plane launch systems is that aerospace plane designs typically include minimal oxidizer storage for propulsion.

Air-breathing aerospace plane designs include engine inlets so they can use atmospheric oxygen for combustion.

Since the mass of the oxidizer is, at takeoff, the single largest mass of most rocket designs ( the Space Shuttle's liquid oxygen tank weighs 629, 340 kg, more than one of its solid rocket boosters ), this provides a huge potential weight savings benefit.

However, air breathing engines are usually very much heavier than rocket engines and the empty weight of the oxidiser tank, and since, unlike oxidiser, this extra weight must be carried into space it greatly offsets the overall system performance.