SpaceX is building the Raptor engines at the company's headquarters in Hawthorne, California and testing them at the company's facilities in McGregor, Texas.

The Raptor engine is being built for SpaceX's Starship rocket, which is designed to be a fully-reusable launch system. Musk's company plans to use Starship to transport up to 100 people to Mars.

SpaceX founder and CEO Elon Musk showed off a photo and videos of the company test firing its new Raptor rocket engine in a series of tweets late on Sunday.

In a tweet, Musk said he was "so proud of great work" by the SpaceX team. Musk is the CEO of both privately-held SpaceX and electric automaker Tesla .

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.