It’s been 18 years since the fabled Concorde jet flew its last supersonic flight across the Atlantic. Now, United Airlines is taking a big step toward resuming ultra-fast commercial aviation by placing an order for 15 supersonic jets from startup company, Boom Supersonic.

The Denver-based aviation firm says its jets can fly faster than the speed of sound. United’s deal for the jets gives the carrier the option to up its order to as many as 35 planes. Boom Supersonic says it expects to start testing its plane, which is called the Overture, by 2026. The company says it aims to have its aircraft carrying passengers before 2030.

But Boom Supersonic has run into delays in the past when it comes to getting its supersonic jet off the ground. Even without factoring in the technology challenges of a modern supersonic jet, it’s exceedingly difficult for new planes to get clearance for commercial use. Securing approval from the FAA and regulators in other countries is one obstacle.

Even established manufacturers have run into major issues when introducing new or redesigned planes into the marketplace. Witness Boeing’s problems with the 737 Max, which was taken out of commission for nearly two years after two deadly crashes.

Boom Supersonic’s CEO says its jet engines would be more efficient than those used on the Concorde, and would rely on sustainable aviation fuel. But there remains concerns about emissions from a supersonic jet at a time when the aviation industry is taking steps to reduce its impact on the environment.

And while the appeal of cutting a flight from the East Coast to Paris down to less than four hours is certainly appealing to flyers, the tickets on such supersonic flights will no doubt run in the thousands. Can the market sustain those costs? A decrease in demand in part due to the high price of a ticket to board was part of the reason why the Concorde was ultimately shelved.

United’s attempt to revive supersonic air travel is another move the airline is making as it tries to position itself as an aerospace innovator. United announced in February it was investing $20 million in an electric air taxi start-up called Archer.