WASHINGTON — Virgin Galactic founder Sir Richard Branson said April 28 he remained optimistic that the company’s SpaceShipTwo suborbital vehicle will carry him and other customers into space, but declined to give a specific schedule about when those flights might begin.

Branson, in an on-stage interview at the headquarters of the Washington Post here, said the development of SpaceShipTwo, which has suffered years of delays and a fatal test flight accident in 2014, was harder than he originally imagined.

“It’s been tough. Space is tough. All of us who have been in it have found it tougher than we thought,” he said. He credited Virgin Galactic’s customers, who he said numbered 800, for encouraging him to continue the venture. “Their commitment has helped us keep our commitment to it.”

As he has done in recent interviews, though, he declined to give a schedule for beginning commercial flights of SpaceShipTwo. “I’ve made the mistake of giving dates before and being wrong,” he said.

When Jonathan Capehart, the member of the Post’s editorial board who interviewed Branson, mentioned the prospect of beginning commercial flights next year, he responded, “If you say so. That sounds good.”

Branson made similar comments in an interview published by the British newspaper The Telegraph April 2. “Well, we stopped giving dates,” he said. “But I think I’d be very disappointed if we’re not into space with a test flight by the end of the year and I’m not into space myself next year and the progam isn’t well underway by the end of next year.”

At a hearing of the space subcommittee of the Senate Commerce Committee April 26, Sen. Tom Udall (D-N.M.) asked George Whitesides, chief executive of Galactic Ventures, the Virgin business unit that includes Virgin Galactic, about whether Branson’s comments in that interview were accurate.

“We’re well into test flight now, and we’re looking forward to moving a fairly big transition of our staff to your state of New Mexico,” Whitesides responded. New Mexico is home to Spaceport America, a commercial spaceport built by the state to host SpaceShipTwo commercial flights.

“That’s great,” Udall responded. “As you know, New Mexico has invested, I think, $200 million in that spaceport, so we want to see you be a success there.”

The second SpaceShipTwo vehicle, known as VSS Unity, is in the middle of a test flight program. The vehicle has made three glide flights, where it is released from its WhiteKnightTwo aircraft to make an unpowered glide back to the runway at the Mojave Air and Space Port in California. The most recent glide flight was Feb. 24.

“We have started the glide flight program, and we will continue that for the next few months, and then we’ll get into powered flight over the course of the year,” Whitesides said at a February conference. “We aspire to push far into the test flight program during the course of 2017.”

In the interview, Branson aligned himself with two other wealthy individuals who have started space ventures: Jeff Bezos, founder of Blue Origin, and Elon Musk, founder of SpaceX. (Bezos’ holdings also include the Washington Post.) “Space is becoming a big new industry in America,” Branson said. “You’ve got three people who really are putting a lot of energy and time and effort into it. We’ll hopefully create some magic.”

Branson, in Washington for the People’s Climate March April 29, said that, unlike some of those other entrepreneurs, he was not motivated to go into space to escape any sort of ecological or other catastrophe on Earth. “Generally speaking, the Earth is a pretty good place to be,” he said.

“I don’t think we need to all go live on the moon or Mars,” he added. “They’re not very hospitable places.”