SpaceX is about to make its second attempt at launching the largest rocket the world has ever seen. The stainless steel monster, known as Starship, stands nearly 400 feet tall. Its massive first stage, known only as "Super Heavy," is powered by 33 Raptor engines that must fire in perfect synchrony to carry Starship into orbit.

SpaceX founder Elon Musk hopes that Starship can one day become a cheap, rapidly reusable system that will jumpstart human exploration of the moon and Mars.

Today's test flight is a small first step. If it works, Starship will launch from Texas, briefly enter space and then splash down in the Pacific Ocean near Hawaii. But even that might be a difficult goal to reach. Here's why.

Starship's first flight in April did not go according to plan

The first test flight of any rocket is going to be tough — and for its April 20 launch attempt, SpaceX tried to manage expectations. Somewhat tongue-in-cheek, the official countdown timeline promised "excitement guaranteed" after the launch.

The rocket lifted off shortly after 8:30 a.m. local time. Almost immediately it was clear that some of the 33 engines in the first stage had failed, and as it climbed into the sky, further engines flamed out.

Before the Starship could separate from its booster, the entire rocket began spinning out of control. It exploded roughly 4 minutes into flight.

In the aftermath, it emerged that Starship's flight termination system, which was designed to destroy the vehicle if it went out of control, had failed to do its job. On top of that, the rocket's first stage pulverized the concrete launch pad during liftoff, sending particulate dust and chunks of debris flying.

The failure of the pad in particular was embarrassing, says Jonathan McDowell, an astronomer at the Center for Astrophysics, Harvard & Smithsonian. "This enormous rocket basically blew the pad apart and showered concrete over miles of Texas," he says.

These rocketry goof-ups also caught the eye of government regulators. The Federal Aviation Administration grounded Starship pending a safety and environmental review. Earlier this week, the regulator cleared SpaceX for a second try, in part because of changes the company made to the design.

This time, SpaceX has made some major upgrades

First, engineers have added more oomph to Starship's self-destruct system. They've put in larger explosive charges that should be able to destroy the beefy rocket, if it strays off course as it did back in April.

The company has also created an entirely new system for attaching the Starship to its booster rocket. It will allow the spacecraft to use its engines to separate from the booster during flight, and continue its journey into orbit. That's assuming it works: This so-called "hot staging" strategy is new to SpaceX, and isn't used very often on American rockets.

Third, the Super Heavy booster rocket being used in this flight has some considerable improvements over the previous one, the company claims. Most importantly, it uses an electrical mechanism to control the thrust of its dozens of engines. That should make the spacecraft more robust if one or more engines fail on the way up.

Finally, there's a big upgrade to the launchpad, which got blasted in the first flight test. This time, SpaceX has installed a water deluge system that should keep the pad from getting too hot. Such systems are commonly used for other launch pads.

Starship is a big part of SpaceX's business plans

SpaceX is investing heavily in Starship. Musk has previously said that the company has spent $2 billion this year alone in development.

The company has focused on the mammoth rocket in part because Starship is central to Musk's dream of colonizing Mars. He hopes that a fleet of starships will one day be able to put enough supplies into orbit to carry the first settlers to the red planet.

The rocket is also a big part of SpaceX's business with NASA. The space agency has awarded around $4 billion in contracts to SpaceX so that it can develop Starship into a lunar lander. NASA plans on using a version of the rocket for some of its upcoming Artemis missions to the moon's surface, which could start as soon as 2025.

Finally, Starship has a very important role in SpaceX's business much closer to earth. The company's Starlink satellite internet system is awaiting a major upgrade, but SpaceX's current rockets aren't big enough to carry the newest, third generation of Starlink satellites into orbit, according to Chris Quilty, the president of Quilty Space, a private space analytics firm.

"Not only is the development of Starship burning a ton of cash, but it's also holding back their ability to launch these gen-3 satellites," Quilty says.

Whether it works is anyone's guess

In this test flight, SpaceX hopes to take off from their launch site in Brownsville, Texas. From there the Starship will shoot out over the Gulf of Mexico, separate from its heavy booster and enter what McDowell describes as a "marginal orbit" that will send it around the world. It will then splash down off the coast of Hawaii.

That's the plan on paper. What actually happens could look quite different.

SpaceX conducted two test fires of the new Super Heavy booster in August. The first, conducted on Aug. 6, ended prematurely after four engines failed to function properly. The second, conducted on Aug. 22, was successful, although two engines failed to run for the full duration of the six-second test.

In addition, the flight will be testing the rocket's "hot stage" separation system for the first time. And it remains to be seen whether the thermal protection system on Starship can stand up to the brutal heat of reentering the Earth's atmosphere.

McDowell says he thinks any scenario in which Starship separates from its booster and keeps flying should probably be considered a success, regardless of what happens to the spacecraft after that. But given the difficulty of getting the 33 first-stage Raptor engines to fire properly, he's not sure it'll get that far.

"I think the ignition reliability of the Raptor engines is the biggest question in my mind right now," he says.

Even if it ends in failure, Quilty believes that it won't have an immediate effect on SpaceX's business. The company is currently dominating the market for launching commercial satellites, thanks in part to past innovations, like a first stage that can land vertically on a barge. "They're doing absolutely fine without Starship," he says.

But McDowell adds that given the big ambitions of SpaceX, this monster rocket will have to work sooner or later.

"They need Starship to work eventually," he says. "The big question for me is: 'How much eventually can they get away with? How many failures can they tolerate?'"

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