There’s one person—Elon Musk—who is definitely keeping a close watch. Last week, he talked about how he and his spaceflight company, SpaceX, plan to eventually travel to Mars. 

The Starship mission’s inaugural orbital flight around Earth may come as early as March. But if his program’s long-term success is to be realized, Musk must make sure that the rocket’s engine, the Raptor 2, can generate enough thrust so that Starship can leave Earth’s orbit and, ultimately, reach Mars.

“It’s a spectacular piece of engineering, but extremely difficult to make and succeed at,” Musk said. “This engine has been mind-bogglingly difficult, but it is essential to making Starship work.”

The first version of the engine, the Raptor 1, had 180 metric tons of thrust, and a complex design that Musk joked looked like a “Christmas tree spaghetti pile.” The Raptor 2 is an upgraded version that is said to have 230 metric tons of thrust—with a goal of achieving 250 by the end of this year. 

The new engine burns fuel hotter than its predecessor, and its interior can hold much greater amounts of pressure. In a rocket engine, chamber pressure refers to the amount of fuel and oxidizing agent that can be mixed and ignited, which then generates thrust to lift the rocket. While increasing the chamber pressure creates a more powerful rocket, it also risks literally melting the engine itself.

“It’s got on the order of a gigawatt of heat,” Musk said. “A gigawatt is like what a nuclear power plant produces, so it really is desperately trying to melt at any point in time.”

The problems with the new engine are creating a headache for Musk. “Right now, of any technical problem, I’m spending the most time personally on Raptor 2,” he told reporters. He described the engine as one of his two most vexing challenges, alongside his role as Tesla’s CEO in trying to improve that company’s buggy Full Self-Driving software. 

As part of the effort to avoid melting the engine, SpaceX is working on applying heat shields that can absorb the immense energy. 

But heat isn’t the first problem Musk has had with the Raptor 2. After slow production of the engine late last year, website Space Explored and CNBC said Musk had warned staff of the “genuine risk of bankruptcy” if SpaceX didn’t get a grip on the production problems. He urged SpaceX employees to join him in working through the long Thanksgiving weekend.  

“We need all hands on deck to recover from what is, quite frankly, a disaster,” Musk wrote in an internal email cited by the news publications. 

While Musk said that his engineers are “very close to solving” the engine problems, he is notorious for overly optimistic predictions. 

In December, Musk said in a tweet that the Raptor 2 model would be the only one that SpaceX would produce going forward. This week, he said he expects SpaceX will manufacture five or six Raptor 2 engines, and, in March, ramp up that number to seven or more weekly. 

Musk has made it clear that an engine as powerful as Raptor 2 is necessary to supply any human-inhabited base on Mars.

“The critical threshold for Mars is to have a city that is self-sustaining,” he said. “If it’s missing any ingredients at all, however minor that is, then if the ships from Earth stop coming for any reason the city will die out.”