To make rockets fully reusable, the recovery must include both stages and the payload fairing.
SpaceX is apparently working on a unique method to recover the upper stage of a rocket after it has delivered its payload to orbit.
SpaceX will try to bring rocket upper stage back from orbital velocity using a giant party balloon— Elon Musk (@elonmusk) April 15, 2018
The company has already recovered and relaunched the Falcon 9's first stage. After separating from the second stage, the booster performs a flip maneuver and heads back to Earth to land using its own rocket engines to slow descent and gently touch down on a solid landing pad.
The most recent launches have introduced an experimental system to catch the payload fairings in a giant net slung above the deck of a ship, with mixed results so far.
To make the entire launch system completely reusable, the upper stage must also be recovered. Traveling at a much higher orbital velocity, the second stage poses a bigger challenge.
SpaceX CEO Elon Musk suggests a helium-filled balloon is great for creating a giant object that retains its shape at high Mach speeds and drops the vehicle's ballistic coefficient by two orders of magnitude.
The second stage is already designed to shift its orbit to a certain point in the Pacific Ocean with no islands and few ships. Recovery will require a new target closer to shore, within range of a catcher ship.
Recovering the second stage is the next milestone in SpaceX's quest to take humans to Mars. The plan centers around a reusable heavy-lift launch vehicle with a second stage that integrates the spacecraft. Rather than deorbiting the second stage, its propellants will be refilled in orbit to provide more fuel for its journey between planets.