A terminal countdown demonstration test (TCDT) is a simulation of the final hours of a launch countdown and serves as a practice exercise in which both the launch team and flight crew rehearse launch day timelines and procedures. In the specific case of a TCDT for the Space Shuttle

Space Shuttle

The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

, the test culminates in a simulated ignition and RSLS Abort (automated shutdown of the orbiter's main engines). Following the simulated abort, the flight crew is briefed on emergency egress procedures and use of the fixed service structure slidewire system. On some earlier Shuttle missions, and Apollo missions, the test would conclude with the flight crew evacuating the launch pad by use of these emergency systems, however this is no longer part of the test.

Unmanned carrier rocket launches also undergo TCDTs, where countdown procedures are followed. These vary for specific rockets, for example solid-fuelled

Solid rocket

A solid rocket or a solid-fuel rocket is a rocket engine that uses solid propellants . The earliest rockets were solid-fuel rockets powered by gunpowder; they were used by the Chinese in warfare as early as the 13th century and later by the Mongols, Arabs, and Indians.All rockets used some form of...

rockets would not simulate an engine shutdown, as it is impossible to shut down a solid rocket after it has been lit.

TCDTs typically are carried out a few days before launch.