A launch pad is an above-ground platform from which a rocket-powered missile or space vehicle is vertically launched. A spaceport (or launch complex) is a facility which includes, and provides required support for, one or more launch pads. A launch pad generally contains a fixed or mobile service structure, which provides one or more access platforms to inspect and maintain the vehicle, and an umbilical structure which provides the vehicle with propellants, cryogenic fluids, electrical power, communications, and telemetry prior to launch. The service structure also provides 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 arrestors.
A launch pad is distinct from a missile launch facility (or missile silo), 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 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.
There are several different types of launch site, determined by the means by which the space vehicle gets to the pad.
The first large rocket, the V-2, travelled horizontally with its tail forward to the launch site at Peenemünde. This is the most common method of transport to the pad and was used for all large Soviet rockets, including the N1 and Energia.
In a similar manner, at the Soviet launch site near Volgograd, a silo used to launch test rockets would have its top opened and a second stage and payload would be driven in horizontally and tilted on top of a first stage already in the silo, the nose cone and some of the second stage remaining visible above ground. Hence no surface pad is used; Russian silos are reusable. This method was only used for the Cosmos series of small satellite launching vehicles.
In the 1920s, Hermann Oberth described a method in which the vehicle is assembled vertically on a floating barge, which he used in the movie Frau im Mond. This was seriously considered for use at Kennedy Space Center Launch Complex 39 for the Saturn V, but was rejected due to the instability of the top-heavy unfuelled rocket and gantry. However, the Sea Launch service uses the converted self-propelled oil drilling platform Ocean Odyssey to transport Zenit 3SL rockets horizontally to the Equator, and then to erect and launch them into geostationary transfer orbits.
Dnepr rockets are transported vertically and then inserted into a silo, as they are converted cold-launch R-36 Voevoda ICBMs.