Ninety-nine percent of the mass of a rocket poised on the pad for launch is accounted for by the rocket itself. This mass consists mostly of propellant, but it also includes tanks, valves, communications and navigation instrumentation, stage separation mechanisms, and a fairing. The remaining 1 percent consists of the rocket's payload. Protected by the fairing from the supersonic airflow of rapid ascent, the payload reaches orbit altitude and velocity within one or two minutes of the launch initiation.

Many spacecraft are equipped to modify the orbit that the rocket carries them to. They might have propulsion onboard to raise their orbit or to trim it, or to escape Earth orbit altogether and head out to the planets or beyond the solar system. This onboard propulsion system—chemical, electric or even solar sail—is part of the launch vehicle payload, but in the design of the propulsion system, the...