American Airlines earlier this week joined United Airlines in announcing it would make an investment in the Boom Supersonic 2.0 Overture jets, the wave of the future for air travel.

American said it would order 20 of the Boom planes with an option for 40 more.

But the third member of the Big Three of U.S.-based carriers is not quite ready to make the same commitment to supersonic travel.

Delta Air Lines has not invested in Boom Supersonic.

In an interview with Fox Business News, as recounted by Business Insider, Delta CEO Ed Bastian said "has a lot more questions than answers still. Until we are confident that we could actually generate a reliable return from the aircraft, that's not where we're investing.”

It is not known how costly the 2.0 Overture jets are – by way of comparison, Business Insider noted that United has deposited $10 million for 100 of Archer Aviation's "Midnight" eVTOL jets – but it is known that they only carry about 65 to 80 passengers at a time. A profitable return on investment will likely mean a high cost in airfare.

Henry Harteveldt, travel analyst and president of Atmosphere Research Group, told Insider that Delta is to be credited for taking a cautious approach.

"There's an opportunity cost for an airline when they're buying airplanes, and it doesn't matter what type of aircraft because there's only so much money that a carrier has available to invest," he said. "Airlines want to invest wisely to get the best possible return, so I don't see any problem with Delta choosing not to order the Overture or eVTOLs."