Krishna Nguyen, an engineer at SpaceX, is designing a new engine system to feed water to a rocket’s core. He says an engine that uses the same principle could send astronauts to Mars in about three or four years.

This would have huge implications not just for technology, he says, but also for how we view space travel. “We are going to spend a tremendous amount of time talking about technologies that might be used in space,” he says. “So we’ll become increasingly reliant on technologies that will benefit our future generations.”

If the team can demonstrate the ability to use that water in the core of the Falcon rocket, it would dramatically improve its landing and re-entry capability. “That should enable me to get a lot closer to the point where I have to say, ‘What’s next?'” says Vandenberg.

To find out whether SpaceX is working on this, I wrote to the company, which declined to comment. In an email, Musk said:

Our reusable rocket tech demonstrator is at the center of our team’s attention. We have been designing, building, testing and operating it in a way that demonstrates a clear path to a successful Falcon 9 booster re-use landing, and then we will continue to iterate and improve, based on feedback from our customers, engineers, and the general public.

He pointed out that the technology demonstrator has completed tests of a number of the key components. “This system is now in the ‘doable’ phase,” said Musk. “We are preparing to demonstrate a landing at altitude using a liquid oxygen/liquid methane core stage in orbit.”

But Musk warned that there are still many details that will need to be worked out. At the moment, the demonstrator is focused on a specific flight plan, Musk said to give the designers “as much time as we need to find out the specifics of how to do the test we want to do with the core stage.” It is unclear whether the rocket can re-enter vertically, or at all.

SpaceX is continuing to work out the details of how best to demonstrate the technology. “We’re hoping as we do more tests with that hardware … that we’ll find the key path towards a reusable version of the rocket,” says Musk, who added: “We don’t have a schedule at this time.”