More than a decade after the demise of supersonic Concorde jets, the drive for easy and affordable access to space has inspired proposals for a new generation of superfast airliners able to streak across continents in minutes.

Recent advances in propulsion and spacecraft design—featuring lower-cost, reusable boosters and capsules—are transforming the way commercial and military entities view orbital missions. Elon Musk’s Space Exploration Technologies Corp. (SpaceX), Richard Branson’s Virgin Galactic LLC, and Jeff Bezos’ Blue Origin LLC are among the private companies developing space vehicles designed to launch multiple times with scant refurbishment.

Public references to Pentagon research, some classified, that describe how this new space technology could be used for spy planes, or to rapidly replace national-security satellites destroyed in a conflict, also have helped fuel talk of using similar concepts to revolutionize air travel.

Now, despite daunting technical and cost challenges, some entrepreneurs are betting they can do it.

By incorporating innovative engine designs, the benefits of 3-D printing and principles of using the same booster repeatedly, these entrepreneurs plan to transform commercial aircraft into so-called hypersonic space planes capable of carrying passengers. Powered by engines that burn hydrogen fuel with oxygen from the atmosphere, but without the piping and moving parts essential for today’s rockets, they would travel at least five times the speed of sound, or about 3,500 miles an hour, versus the 500 mph or so at which conventional jetliners typically cruise.

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Among those championing hypersonic travel for fare-paying passengers is Chris Milam, a Texas real-estate developer and technology investor who has pledged to invest as much as $20 million of his own funds over the next few years to design such a system. Along with Preston Carter, an expert formerly with the Pentagon’s advanced-research arm, Milam aims to develop a two-stage concept in which a rocket carries a hypersonic, winged vehicle and releases it to cruise at high altitude. The passenger-carrying craft would land at its destination like a conventional airliner.

“There has been lots of discussion about high-speed flight since the end of Concorde, but little has been realized,” Milam says. “The reality is systems like this are expensive.”

Preliminary designs call for the space plane to carry about 100 passengers and cruise at an altitude of 70,000 to 100,000 feet, with the goal of reaching any point on Earth within four hours. It would have a total takeoff weight of some 500,000 pounds, less than half of a fully loaded Airbus AIR, -3.54% A380 superjumbo.

The upshot, proponents argue, would provide options far beyond those promised by most of today’s civilian supersonic projects, which remain focused on building business jets or mini-airliners that essentially are updated versions of the Concorde on steroids. Closely held Boom Technology Inc., for example, is developing a three-engine, 45-passenger jet intended to fly 1,500 miles an hour—or roughly twice the speed of sound.

But if an entirely new category of airliner-like suborbital vehicles becomes reality in coming decades, they would fly many times faster and higher, traveling from London to New York in some 40 minutes—or 10 times as fast as current airline schedules. That is still less than the anticipated top speed of the Pentagon’s unmanned XS-1 experimental craft being developed to launch satellites (the Pentagon approved the design a month ago), or NASA’s earlier X-43 rocket ship demonstrator that hit a record-breaking velocity of more than 7,500 miles an hour without a pilot.

For a hypersonic passenger jet to become a reality, some major hurdles need to be cleared. The biggest one is that engineers so far have failed to produce materials or devise other means to safely handle the intense heat when airliners reach such hypersonic speeds. Aerospace giants including Boeing Co. BA, -3.81% and Airbus SE have had limited success, while Lockheed Martin Corp LMT, -0.20% only recently decided to pursue a scaled-down test vehicle for military purposes.

Skeptics such as consultant and entrepreneur Joel Sercel, a former government space researcher, consider that hurdle particularly challenging. “The problem of cooling any vehicle going that fast isn’t solvable,” he says. “These issues have been plowed before.”

But others in the aerospace industry are more optimistic. Julie Van Kleeck, vice president at engine maker Aerojet Rocketdyne Holdings Inc., AJRD, +0.50% says 3-D printing allows production of engine parts and other structures with unique shapes that allow them to stay cooler. Longer term, she says, “it can probably be broadened” to commercial applications.

Milam says there are other potential solutions to the overheating problem, some of which he is paying NASA experts to help develop. Entrepreneurs, he argues, “simply have never been willing to engineer” such fast airliners.

Part of the impetus for the renewed focus on hypersonic options stems from an initially little-noticed report prepared by Air Force planners and outside experts in late 2016. Titled “Fast Space,” the document spells out various strategies for “ultra low-cost,” entirely reusable launchers eventually able to whisk cargo, soldiers and weapons around the globe in a matter of minutes rather than many hours.

Citing a “confluence of government research and private sector innovation,” the report describes “a window of opportunity for the U.S. to shift its approach” to space access. Increasingly embraced by Air Force brass, parts of the document emphasize the long-term advantages of joining with industry to revamp commercial transportation.

Milam says his Austin, Texas, startup, Supernature LLC, needs substantial federal funding or additional private investors to build a prototype. On the upside, he says, interest among potential backers has been “bolstered by the success of commercial space.”