Elon Musk in 2017Mike Blake / Reuters

A roar pierced the air at Cape Canaveral in Florida for a few seconds Wednesday.

The sound was the cry of the engines on the Falcon Heavy, a massive launch vehicle developed by Elon Musk’s SpaceX. The Heavy had been hauled to the launchpad at Kennedy Space Center for a brief but crucial test that will determine whether the rocket is ready for its maiden flight, which Musk has said will take place later this month.

The milestone, known as a static-fire test, involved securely anchoring the Falcon Heavy to the launchpad, loading it with propellant, and then igniting its 27 engines at full thrust. For several seconds, plumes of thick smoke billowed out from the base of the launchpad and created a big white cloud. Musk tweeted shortly after the static fire that the test was “good.”

SpaceX did not broadcast the test online, but the action was livestreamed on social media by journalists staked out on a beach nearby. The action starts at about the 16:22 mark:

A static-fire test, designed to check the engines and various systems of a rocket, is the final step in preparations before a launch. The engines fired at full power Wednesday.

The Heavy was pulled upright on launchpad 39A, the site of launches during the Apollo and Space Shuttle programs, for the test the second week of January. Reporters and other observers clung to computer screens and waited. A six-hour window for the static-fire test that opened that week came and went, and the test was rescheduled again and again, until this Wednesday. For a few stressful days, it looked like the federal government threatened to delay the test for days or even weeks. The process resembled a precarious game of Jenga.

“We’re stepping through this carefully, it’s a beast of a vehicle,” explained Gwynne Shotwell, SpaceX’s president and chief operating officer, during an appearance January 11 at an aerospace conference in Texas.

And what a beast it is. The 230-foot-tall Falcon Heavy is essentially a monster mashup of three Falcon 9 cores—two of which have already flown solo. It can produce a maximum thrust at liftoff of more than 5 million pounds, which would make the rocket, if it flies, the most powerful rocket in operation, ahead of the Delta IV Heavy, manufactured by the United Launch Alliance, a joint venture of Boeing and Lockheed Martin. While the Heavy will carry only a Tesla car on its maiden launch, SpaceX says it will be able to lift 119,000 pounds of cargo into orbit. Musk hopes the Heavy will someday transport astronauts and hefty payloads to the moon and Mars.

For impatient watchers, Eric Berger at Ars Technica pointed out that false starts and days of delays are par for the course for high-stakes launches. “What’s up with these Falcon Heavy delays? Well, for perspective, the first Saturn V rocket launch required a 17-day countdown demonstration test to fuel and power up the rocket,” Berger tweeted. “These things take time if you want them not to go boom at the wrong time.”

Like the Falcon Heavy, the Saturn V rocket was a heavy-lift launch vehicle used by the U.S. government to boost astronauts to the moon and launch Skylab, the first American space station, in the 1960s and 1970s.

Engine tests in the rocket industry are routine and fairly quick affairs, and SpaceX’s tests of the Falcon 9 rarely make news. But there are no guarantees in rocketry. In September 2016, a Falcon 9 exploded on the launchpad while being fueled for the engine test. The incident destroyed the rocket and its $200 million payload, an Israeli communications satellite.

The Falcon Heavy went up on the launchpad just days after SpaceX launched a top-secret U.S. government payload that sparked rumors about whether it had failed. News reports citing anonymous sources suggest the payload, a satellite, failed or was lost. Northrop Grumman, the company hired to manufacture the satellite, said it could not comment on a classified mission. SpaceX said the Falcon 9 in the launch worked exactly as it should, and not much else. Spaceflight observers say the fault likely lies with Northrop Grumman, but the mystery briefly cast a pall over the highly anticipated Falcon Heavy preparations.

The roar of the Falcon Heavy’s engines on Wednesday will likely drown out speculation about the Zuma mishap, as space watchers anticipate the announcement of a launch date for the new rocket. SpaceX will, once more, fuel up the rocket, fire the engines, and hope the whole thing doesn’t blow up.

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