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An illustration of the "hopper" test-flight version of SpaceX's Starship, shared on Twitter by Elon Musk.

An illustration of the "hopper" test-flight version of SpaceX's Starship, shared on Twitter by Elon Musk. (Elon Musk)

In the latest sign of SpaceX's continuing progress on its Mars spaceship program, Chief Executive Elon Musk said the company has test-fired the Starship's Raptor flight engine for the first time.

The two-second test fire at SpaceX's McGregor, Texas, test facility, which Musk announced via Twitter on Sunday, marks a major coup for the Hawthorne company's rocket engine development. The Raptor engine uses a less conventional propellant than the Merlin engine SpaceX developed to power its workhorse Falcon 9 rocket. Development of this highly efficient engine is also more complex.

Starship, formerly known as BFR, is key to Musk's plans for Mars colonization. The two-stage Mars system includes the Super Heavy booster, which will propel the spaceship from Earth to space, and the Starship craft, which is designed to take 100 people to the Red Planet.

Musk tweeted late last month that as many as 31 Raptor engines could power Super Heavy. However, he couched that number, at least in the beginning, by saying it "will probably fly with fewer initially in case it blows up." Starship will be powered by seven Raptor engines for its journey to and from the Red Planet, according to SpaceX's website.

SpaceX said in an Instagram post Monday morning that the engine reached about 116 metric tons of force during the test, marking the highest-ever thrust from one of its engines. The company said the Raptor engine was at about 60 percent of its power during the test-fire.

"It's a major accomplishment for SpaceX and a major advancement in space engineering," said Richard Wirz, associate professor in aerospace engineering at UCLA and director of the university's plasma and space propulsion laboratory.

The Raptor engine will be powered by methane and liquid oxygen. Many other rocket engines, such as SpaceX's Merlin engines, use highly refined kerosene.

Engineering experts have said methane burns cleaner than kerosene and thus is less likely to clog fuel lines. SpaceX has also said methane could be made on Mars, which made it a better choice for its Starship spaceship. Methane also self-pressurizes, meaning tank-pressurization systems could be eliminated.

SpaceX isn't the only company interested in using methane. Jeff Bezos' Blue Origin is also developing its BE-4 engine to be powered by liquefied natural gas. That engine will propel the company's New Glenn rocket, as well as United Launch Alliance's future Vulcan Centaur rocket.

"It still requires testing and demonstration, but I think it's a very promising direction to go in," said Ann Karagozian, a UCLA professor in the department of mechanical and aerospace engineering.

SpaceX's Raptor engine uses a highly efficient technique known as full-flow staged combustion cycle. Although it is complicated and has a higher number of parts than other cycles, the result could be a highly reusable engine since its parts operate at a relatively low temperature and thus experience a lower level of stress, Wirz said.

Staged combustion cycles can also be more efficient, meaning rockets could launch heavier payloads with the same-size tanks as rockets using so-called gas generator cycle engines, said Paulo Lozano, director of the MIT Space Propulsion Lab.

The Hawthorne company previously conducted a number of tests on earlier versions of its Raptor engine.

SpaceX, whose full name is Space Exploration Technologies Corp., finished assembly of its Starship prototype last month, and Musk has said the company could conduct short flight tests - in which the spaceship will launch and briefly go up in the air before returning to Earth - as soon as this month.

The company is building and testing the Starship prototype at its Boca Chica facility in south Texas. SpaceX had previously planned to build Starship and the Super Heavy rocket booster on a 19-acre site at the Port of Los Angeles, but a company official told the port in a letter dated last month that SpaceX would terminate that lease agreement.

Musk has said the company would conduct development of Starship and the Raptor engine in Hawthorne but build the spaceship prototypes "locally at our launch site in Texas, as their size makes them very difficult to transport."

Development of the Mars spaceship and rocket booster system is one of SpaceX's main priorities. Last month, the company announced it would lay off about 10 percent of its more than 6,000-employee workforce, citing a need to get "leaner" to succeed at its plans to create the Mars spaceship and a global satellite internet network known as Starlink.

On an earnings call with analysts last week for Tesla Inc., where he also serves as CEO, Musk described development of Starship and Starlink as "two absolutely insane projects." SpaceX had previously said in a statement about the layoffs that "either of these developments, even when attempted separately, have bankrupted other organizations."

"SpaceX has to be incredibly spartan with expenditures until those programs reach fruition," Musk said.

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