LAUNCH COMPLEX 39A, FLORIDA—Elon Musk stepped confidently from a black SUV on Monday afternoon, and his mouth spread into a wide grin as he surveyed the nearby launchpad. Just a quarter of mile away, his Falcon Heavy rocket loomed high in the sky, with sunlight glinting off its three white boosters.

“It’s small, don’t you think?” he quipped. “I think we need to step up our game.”

So began our interview with the founder of SpaceX on the eve of the launch of the most powerful rocket on Earth. During our time at the launchpad, we were most interested in the risks involved in such a test flight, with a brand new rocket packing the equivalent of four million pounds of TNT. And Musk was happy to oblige.

Biggest concerns

“One of my biggest concerns is booster-to-booster interaction,” Musk explained. “You’ve got a lot of dynamics going on there. Those rockets are very flexible; if they flex in unexpected ways they could potentially impact one another.”

With three Falcon 9 cores, the acoustical noise generated by the launch is three times greater than a single Falcon 9 launch. SpaceX engineers think they understand these interactions, but they haven’t tested them in flight. Some unexpected resonancy could cause a structural failure. These systems have all been tested extensively on the ground, but ultimately, nothing compares to an actual flight test.

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

Trevor Mahlmann for Ars Technica

“There’s a lot that could go wrong,” Musk admitted. “A really tremendous amount. I really like to emphasize that the odds of success are not super high. I don’t want to jinx it—I’m tempted to say. Because I feel super optimistic. But I feel as though that optimism has no basis in fact. I feel like we’ve got a two-thirds chance of success, but in reality we only have a 50-50 chance.”

Getting technical

Attired in a black blazer and a white t-shirt, with black designer jeans, Musk was in an ebullient mood Monday. He’d just driven up to the pad to see the rocket, and his family had joined him for the superlative experience of seeing such a large rocket within touching distance of the engines. Asked about the capability of the Falcon Heavy rocket to sustain an engine failure during the launch, he replied, “I can get technical, right?”

“This is for Ars Technica,” we replied.

After a gleeful bout of laugher, Musk said, “OK.”

For a typical Falcon 9 launch, the booster can sustain one engine failure out of nine engines, right off the pad. (A single Falcon 9 first stage engine has failed just once in flight). The first stage can even survive two failures if the payload isn’t too heavy or if it's going to geostationary transfer orbit. With the Falcon Heavy, the rocket could lose as many as six engines and still reach orbit—in theory.

“Something pretty horrible would have gone wrong,” Musk said. “The chance of us losing six engines and there not being something really bad that went wrong is pretty low.”

Because this is a test flight, Musk said it’s a “softball mission.” This means SpaceX won’t be pushing the envelope in terms of dynamic pressure during the launch. While the potential maximum dynamic pressure for a Falcon Heavy flight is “pretty high,” Musk said the peak dynamic pressure for this launch will be about 15- or 20-percent less than a Falcon 9 going to geostationary orbit.

Beyond GEO

Despite the mixed potential for success, this won’t just be a test flight with a splashy payload. Musk said Monday he hopes to demonstrate the capability to send payloads directly to geostationary orbit. This is one of the primary requests of the US Air Force, which sets requirements for national security launches. So with this mission, the upper stage will coast for six hours before relighting a final time to send the Tesla Roadster into deep space.

“The six-hour coast is needed for a lot of the big Air Force intel missions for direct injections to GEO,” Musk said. This six-hour period will be about twice as long as the longest coasts the Falcon 9 rocket has made.

After the launch and six-hour cruise, the Falcon Heavy’s upper stage will fire a third time to send the Tesla into a cycling orbit between Earth and Mars. This should take the Tesla out as far as the Martian orbit around the Sun, or even a bit further. Musk said the vehicle should get as far as 380 to 450 million km from Earth, depending on how the third burn goes.

So how close will the Roadster get to Mars? Musk did not offer a definitive answer. “Well, close in space terms. Most people don’t realize how far apart things are in space. In sci-fi movies everyone looks close as opposed to a bunch of distant, tiny, silent dots.”