Qantas Airways’ proposed 20-hour nonstop flights between London and Sydney are going to be even less comfortable than expected — if the halfway-around-the-world service gets off the ground at all.

The Australian airline has dropped the notion of rolling out bunks, beds, a gym, or even a creche for passengers traveling on the world’s longest commercial flight. Instead, they’ll be given a space to have a stretch and a drink of water, Qantas chief executive officer Alan Joyce said this week in Seoul, South Korea, at an annual gathering of airline executives.

The more spartan comfort levels underscore Qantas's challenge as it tries to break through what it calls aviation's last frontier. There are other barriers, too: While Joyce said the planes proposed by Boeing and Airbus for the ultra long-haul flights can make the distance, neither can carry the weight that Qantas initially targeted.

Additionally, Joyce said Qantas pilots had not yet agreed to the longer working hours that the flights would entail.

"There are a significant number of hurdles to overcome, but we think we can make this work," he said. "There's still not full payload on each aircraft, but there's enough we think to make it commercially viable if the other parts of the business case get there."

But, he said, "we will kill the project" if the economics don't stack up.

Qantas expects to receive final aircraft proposals from Boeing and Airbus by August. That will include the price of the plane, as well as guarantees on fuel efficiency, maintenance costs, and reliability. Joyce said he'll order the jets by year-end if he decides to push ahead with the flights, which are known at Qantas as Project Sunrise.

Qantas has said it’s sizing up the long-range Airbus A350 against Boeing’s 777X. Boeing or Airbus would deliver the aircraft in 2022 and the first flights would be in 2023, Joyce said. The planes would have sections for first class, business, premium economy, and economy passengers.

The Sydney-London service won't be able to carry extra freight, Joyce said. Direct flights to New York from Sydney are more achievable, he said.

Qantas is trying to roll out a network of super-long direct services connecting Australia’s eastern seaboard with South America, South Africa, and North America.