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Before takeoff, Ms. Gillis wished the crew good luck and a godspeed.

“It has been an absolute honor to prepare you for this historic flight,” she said.

In the Netflix documentary “Countdown: Inspiration4 Mission to Space,” Jared Isaacman, the mission’s commander paying for the trip, described her role as the equivalent of the “CapCom” on a NASA mission. That’s short for “Capsule Communication,” traditionally an astronaut on the ground who speaks with the crew in the spacecraft.

“The burden of ‘Will we execute well or not?’ will really fall on her,” Mr. Isaacman said in the documentary.

Ms. Gillis has been working for months with Mr. Isaacman and the other three astronauts to ensure they’re fully prepared for their trip, down to such details as how to operate a fire extinguisher on the flight.

“There’s two hats that you have to wear to be successful at that,” she said in the documentary. “One is the operational hat, where you need to understand exactly what actions you need to take to keep them safe. On the other aspect, I care very deeply about every single one of these people now.”

Ms. Gillis graduated from the University of Colorado Boulder with an engineering degree. In the Netflix documentary she said she was encouraged to pursue engineering by a high school mentor who was a former astronaut.

Ms. Gillis is also a classically trained violinist who started to learn how to play when she was 2 from her mother, a professional violinist.

“She certainly did not raise me to be an engineer,” she said.

This is not the first crew of astronauts riding in a SpaceX Crew Dragon capsule. In April, Ms. Gillis helped guide NASA’s Crew-2 mission to the International Space Station.

The flight was not without incident, as she had to warn the crew that a piece of space debris was about to come too close to their spacecraft for comfort. The crew was getting ready to go to sleep when she told them to perform a number of safety procedures, including putting their spacesuits back on.

The flight continued to the space station without incident and later analysis showed that it had been a false alarm, and no debris actually passed near the spacecraft.

They also made trips in a plane that flies in giant arcs that allow the occupants to feel as if they are in zero-gravity for about half a minute. (Gravity does not turn off; rather, the plane dives at the same rate as the people inside are falling, providing them the illusion that they are floating.)

The four went camping on Mount Rainier in Washington State, part of a team-building exercise organized by Mr. Isaacman.

While the Crew Dragon capsule is automated and usually flies itself, the Inspiration4 crew nonetheless underwent much of the same training as NASA astronauts to handle situations if something goes wrong. That included spending 30 continuous hours in a Crew Dragon simulator.

Mr. Isaacman said the hardest part was the deluge of technical information dumped on them.

“It was a little bit of death by PowerPoint for a couple weeks,” he said during Tuesday’s news conference. “But then it immediately went into kind of the more fun phase where now you’re taking all that knowledge that you’ve accumulated and you’re putting it to practical use.”

The 30-hour-long simulation turned out to be a highlight, not an ordeal.

In weekslong simulations of missions to Mars and the moon that Dr. Proctor had participated in previously, “Sometimes, you know, there’s a crew member you might want to kick out,” she said. “But in this case, there wasn’t any of that. We live together. We operate it together. We have fun together, and it really just made me want to do it again and it got me so excited for when we do it up in orbit.”

Three successful trips of a spacecraft also does not mean all potential problems have been discovered and fixed. There were 24 successful space shuttle missions before the loss of Challenger in 1986.

Even the astronauts aboard Crew Dragon have encountered risks. During the latest flight to the space station in April, mission controllers warned the crew that a piece of space debris was about to whiz past. The astronauts put on their spacesuits, got back in their seats and lowered their protective visors. The flight continued to the space station without incident and later analysis showed that it was a false alarm, that no debris actually passed near the spacecraft.

In the days in Florida leading up to the launch, Mr. Isaacman took advantage of his fighter jet experience.

“I also like to look at risk on a relative basis,” Mr. Issacman said during a news conference on Tuesday. “The last couple days, we’ve been tearing up the skies in fighter jets, which I put it relatively higher risk than this mission so that we’re nice and comfortable as we get strapped into Falcon.”

As commander for Inspiration4, Mr. Isaacman fills the leadership seat.

Mr. Isaacman gave two of the four seats to St. Jude. The hope seat was earmarked for a St. Jude health care worker, and hospital officials chose Ms. Arceneaux, who quickly said yes to the offer.

Another seat, generosity, was raffled off to raise money for the hospital. Mr. Sembroski entered, donating $50, but he did not win the sweepstakes, which helped raise $13 million for St. Jude. A friend of his, though, did — an old college buddy from Embry-Riddle Aeronautical University in Florida. The friend, who remains anonymous, decided not to go to space but, knowing about Mr. Sembroski’s enthusiasm, transferred the prize to him.

“I think that just really puts me in a very special spot,” Mr. Sembroski said, “where not only do I feel very lucky to be here but I have a huge responsibility to pay that forward and show that generosity towards others, and to bring that message to everyone else.”

The last seat, prosperity, was the prize in a contest run by Mr. Isaacman’s company, Shift4 Payments. Contestants used the company’s software to design an online store and then tweeted videos describing their entrepreneurial and space dreams. (Using the software, Dr. Proctor started selling her space-related artwork, and in her video, she read a poem that she wrote.)

So far, more than $130 million has been raised including the $100 million that Mr. Isaacman is personally donating to St. Jude.

“We are elated with where we are from a fund-raising perspective,” said Richard C. Shadyac Jr., the president of ALSAC, the fund-raising organization for St. Jude. “I couldn’t be more pleased. We’ll continue to strive for that $200 million goal.”

Approximately two and a half hours before the flight — around 5:30 p.m. Eastern time — the crew will board the Crew Dragon capsule. SpaceX technicians will then complete a number of procedures before sealing them inside the spacecraft, a process that could take about an hour.

About 45 minutes before the scheduled launch time, SpaceX will start loading propellent into the rocket and begin making final checks of the spacecraft’s systems and the weather to decide whether it is safe for the mission to launch.

Once the rocket launches, the capsule will begin a series of steps to be lifted to orbit, including separating from the rocket’s first and second stages. In the hour or so that follows, the spacecraft will fire its thrusters, setting it on the course it will follow until the astronauts return to Earth on Saturday.

Mr. Isaacman started flying as a hobby, learning to pilot more and more advanced aircraft including military fighter jets. In 2012, he started a second company called Draken International, which owns fighter jets and provides training for pilots in the United States military. He has since sold Draken but still flies fighter jets for fun.

Last year, Mr. Isaacman wanted to invest in SpaceX, which remains a privately held company, but missed the latest investment offering by the company. Mr. Isaacman tried to convince SpaceX officials of his enthusiasm by telling them he wanted to buy a trip to orbit someday. That led to conversations that resulted in Mr. Isaacman undertaking the Inspiration4 mission. He is serving as the mission’s commander.

“When I grow up, I want to be a nurse at St. Jude,” she said in a video shown at the ceremony in 2003. “I want to be a mentor to patients. When they come in, I’ll say, ‘I had that when I was little, and I’m doing good.’”

Last year, Ms. Arceneaux was hired by St. Jude. She works with children with leukemia and lymphoma.

Ms. Arceneaux could become the youngest American ever to travel to orbit. She will also be the first person with a prosthetic body part to go to space. She will be the health officer for the mission.

Sian Proctor, 51, is a community college professor from Tempe, Ariz.

Dr. Proctor, who is African American and holds a doctorate in science education, had come close to becoming an astronaut the old-fashioned way. She said that in 2009, she was among 47 finalists whom NASA selected from 3,500 applications. The space agency chose nine new astronauts that year. Dr. Proctor was not one of them.

She applied twice more and was not even among the finalists.

She still pursued her space dreams in other ways. In 2013, Dr. Proctor was one of six people who lived for four months in a small building on the side of a Hawaiian volcano, part of an effort financed by NASA to study the isolation and stresses of a long trip to Mars.

She will be the pilot on the Inspiration4 mission, the first Black woman to serve as the pilot of a spacecraft.

Christopher Sembroski, 42, of Everett, Wash., works in data engineering for Lockheed Martin. During college, Mr. Sembroski worked as a counselor at Space Camp, an educational program in Huntsville, Ala., that offers children and families a taste of what life as an astronaut is like. He also volunteered for ProSpace, a nonprofit advocacy group that pushed to open space to more people.

Mr. Sembroski described himself as “that guy behind the scenes, that’s really helping other people accomplish their goals and to take center stage.”

He’ll be the mission specialist for Inspiration4, and responsible for certain tasks during the mission.