SpaceX has test fired prototype Raptor engines before but Sunday's test represented the first of a "flight ready" engine.
Raptor is a methane-fueled rocket engine, the first of its kind that SpaceX has developed. That is different than the engines powering SpaceX's Falcon series of rockets, which use a mixture of liquid oxygen and kerosene. Beyond its enormous size and added power, the Raptor engine is built to last longer, so that SpaceX can fly each rocket dozens, or possibly hundreds, of times.
SpaceX is currently building a prototype of the Starship rocket at its facility near Brownsville, Texas. The prototype Starship will use three Raptor engines for "hop" test flights. Musk has said the prototype Starship will be ready for short test flights as early as this year – although the company is facing a development setback after the top half of the rocket blew over in January. Musk said at the time that the damage will take "a few weeks to repair."
The final version of Starship will have seven Raptor engines on board, while the "Super Heavy" booster that launched it will have 31 Raptor engines.
The development of Raptor and Starship is an "absolutely insane" project, Musk said during Tesla's quarter investor call last week. Musk attributed recent SpaceX layoffs – which saw the company layoff more than 600 employees – to the company's need to be "incredibly spartan with expenditures" until its developing programs reach fruition.