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. (August 2008)
V2 rocket on launch pad, 1942
A launch pad is an above-ground facility from which a rocket-powered missile or space vehicle is vertically launched. A spaceport (or sometimes launch complex) is a facility which includes, and provides required support for, one or more launch pads; the term launch pad may sometimes be used to describe just the central launch platform. A launch pad typically includes a launch mount or launch platform—to support the vehicle and its service structure with umbilicals to provide propellants, cryogenic fluids, electrical power, communications and telemetry prior to launch—plus storage facilities for propellants and gases, equipment, access roads, drainage and all the requisite infrastructure to support rocket vehicle launches.
Some launch pads include service structures to provide one or more access platforms to inspect and maintain the vehicle and to allow access to the crew cabin for vehicles carrying humans. The pad may contain a flame deflection structure to prevent the intense heat of the rocket exhaust from damaging the vehicle or pad structures, and a sound suppression system spraying large quantities of water may be employed. The pad may also be protected by lightning arresters. A launch pad is distinct from a missile launch facility (or missile silo or missile complex), which also launches a missile vertically but is located underground in order to help harden it against enemy attack, or conceal it from surveillance.
Cryogenic propellants (liquid oxygen oxidizer, and liquid hydrogen or liquid methane fuel) need to be continuously topped off (i.e., boil-off replaced) during the launch sequence (countdown), as the vehicle awaits liftoff. This becomes particularly important as complex sequences may be interrupted by planned or unplanned holds to fix problems.
Most rockets need stable support for a few seconds after ignition while the engines build up to stable, full thrust. Therefore, the vehicle is commonly held on the pad by hold-down arms or explosive bolts, which are triggered when the vehicle is stable and ready to fly, at which point all umbilical connections with the pad are released.