The Falcon 9 is a slim, slick rocket — a powerful beast, for sure, that unleashes a menacing roar at liftoff that reverberates across the Florida Space Coast. But it also provides a deceptively smooth ride, at least at first, when compared with the space shuttle.

The Falcon 9’s second stage, though, packs a punch.

That was the review of the first NASA astronauts to have flown on both Space X’s Falcon 9 and its predecessor, the space shuttle, manufactured for NASA by prime contractor Rockwell International. The Falcon 9 was smooth at first, then a bit of a rumble, “like driving fast over a gravel road,” said astronaut Doug Hurley, who with Bob Behnken offered thoughts of the rockets’ performance for reporters on Monday.

The mission was a test flight, and their job was not only to fly to the International Space Station — a goal they achieved Sunday — but to assess how the rocket and spacecraft, which had never before flown humans, performed. Now that SpaceX has shown it can safely deliver people to orbit, the question became how its system, the first developed entirely by a private company to send NASA astronauts to space, compared to the rockets and spacecraft NASA had engineered before.

Space enthusiasts finally have the beginnings of an answer to which system is better.

Clearly, SpaceX’s vehicles look different. Even the astronauts’ spacesuits were sleek and form-fitting departures from their clunky, at times dowdy predecessors. Inside the capsule, SpaceX founder Elon Musk and his engineers simplified the control panels, banishing the dozens of switches that made the shuttle such a complicated vehicle to fly, and instead opted for large touch screens. The Dragon capsule’s chairs were inspired by those in racecars, with custom foam molds.

Even the launch tower was revamped to look modern and sleek. And for this mission, NASA decided to bring back its “worm” logo from the 1970s, which was painted on the side of the Falcon 9 rocket.

Taken together, the changes were not only a nod to the science fiction that inspired Musk as a child but a deliberate attempt to create an aesthetic and evoke a mood.

“If you’re a member of the public … you don’t necessarily know that much about rocket design or how spacecraft work, but you know if it looks cool,” Musk said in a recent interview with The Washington Post. “And if it looks futuristic and, aesthetically, it seems like something new, that’s how people match the perception with the reality.”

Form is one thing. Function is another. And in spending nearly $3 billion in taxpayer money on the spacecraft, NASA cared far more about the latter, especially since it was the first launch of astronauts from United States soil since the space shuttle was retired in 2011.

So how did it fly? The reviews were overwhelmingly positive for a mission that appeared to go off without a hitch. But there were some surprises.

Behnken and Hurley, both of whom had flown on the space shuttle twice, were prepared for blastoff to be an exhilarating display of brute force — nine engines firing simultaneously for a total of 1.7 million pounds of thrust, or more than five 747s combined. Waves of fire thunder out in what is basically a controlled explosion that sends up a huge cloud and a sound you feel in your chest, even three miles away.

Given that, the astronauts were expecting a teeth-chattering thrill.

“We were surprised a little bit at how smooth things were off the pad,” Behnken said during one of several press calls after reaching the station. “The space shuttle is a pretty rough ride heading into orbit.”

One of the main differences, they said, was that the shuttle had two solid rocket motors that thunder on their way to orbit. While powerful, solid motors can’t be turned off once ignited. SpaceX’s Falcon 9 uses only liquid propellants, rocket-grade kerosene and liquid oxygen, and the result, the astronauts said, was a relatively fluid flight.

“It was a very smooth ride. You could see it on the webcast,” Musk said after the launch. “It looked quite smooth. In fact, a friend of mine who is a filmmaker said, ‘You need to put some shake into the camera to make it look more realistic.’ ”

But when the first and second stages separated, and the second stage engine ignited, it gave them a bit of a shimmy — like the flaming engine of the Batmobile, is how Benji Reed, SpaceX’s director of crew mission management, put it.

“Our expectation was as we continued with the flight into second stage that things would basically get a lot smoother than the space shuttle did,” Behnken said. “But Dragon was huffing and puffing all the way into orbit. And so it was not quite the same … smooth ride as the space shuttle was.”

Another new experience came at the very beginning of the mission, when Hurley and Behnken boarded the capsule and sat there through the fueling process. During the space shuttle era, the rocket’s tanks were fueled before the astronauts boarded.

SpaceX uses extremely cold propellant, so the rocket needs to be loaded just before liftoff to prevent it from boiling off. SpaceX “superchills” its propellant to make it denser, so more of its can be packed into the rocket, giving it more performance. It also means the astronauts can hear the fueling, a process full of grunts and hisses that makes the rocket seem like it’s stirring to life beneath them.

“Hearing the venting and the valve sounds and the little vibrations associated with that operation was a new experience for us,” Behnken said.

It was one they were prepared for because SpaceX had recorded audio of the sounds and played that for them during training exercises.

“We had heard all those sounds pretty much before, and that was extremely helpful,” Behnken said.

Perhaps one of the biggest surprises wasn’t the noise of the liftoff, or the staging, or the fueling but rather the complete silence when the spacecraft finally did reach the station, some 19 hours after liftoff.

The Dragon capsule glided in with such grace that “we didn’t feel the docking. It was just so smooth,” Hurley said. “That really, really surprised me.”