A Wet Dress Rehearsal tests out the systems of a spacecraft prior to spacecraft integration and launch. Enough elements are in place to verify that a rocket is ready for flight, allowing problems to be seen prior to the mating of a payload with the rocket. It is called “wet” because the liquid components (such as liquid oxygen, liquid hydrogen

Liquid hydrogen

Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...

, and cryogenics) are loaded into the rocket during the test.