Static firing

A wet dress rehearsal (WDR), and a more extensive static fire, are system tests of a fully integrated space launch vehicle and its associated ground support equipment (GSE) prior to launch. The spacecraft or payload may or may not be attached to the launch vehicle during the WDR or static fire, but sufficient elements of the rocket are in place to help verify that the rocket is ready for flight, allowing problems to be seen prior to the actual launch.

A wet dress rehearsal is called "wet" because the liquid propellant components (such as liquid oxygen, liquid hydrogen, etc.) are loaded into the rocket during the test.

A static fire test comprises a wet dress rehearsal and goes one step farther: actually firing the engines at full thrust. The engine (or engines) is fired, while the launch vehicle is held firmly attached to the launch mount, with or without payload attached, for a few seconds in order to test engine startup while measuring pressure, temperature and propellant-flow gradients. The data gathered in such tests may be used to form a basis, upon analysis, for a unique (rocket- and engine-specific) set of criteria that will form a part of the go/nogo decision tree in the automated launch software that is used on the actual launch day, typically a few days later. Some static fire tests have fired the engines for up to seven seconds, although shorter firings are more typical.

Wet rehearsal and static fire tests can fail catastrophically, as with a pad explosion of a SpaceX Falcon 9 on 1 September 2016, which resulted from a major breach of the cryogenic helium system of the second stage of the rocket during the propellant-loading operations, well before the planned ignition of the engines for a static fire test sequence. The explosion destroyed the rocket and its payload - the Amos-6 satellite. Furthermore, due to extensive fire, the SLC-40 launch pad was heavily damaged and has to be rebuilt.

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