Many milestones have to be hit in order for all of the components and subsystems to be safely validated.
Cryogenic Propulsion team has conducted several Cold-Flow tests, where they rehearsed the firing of the engine with water and liquid Nitrogen to validate the system behavior, improve procedures, and gather data.
In parallel, research has been conducted in the design of a spark torch igniter, in an attempt to design a reliable ignition source for combustion. However, the project has been put on hold due to unpredicted results, and to prioritize resources within the team.