X-57 team probes safety question that ended development without a flight
By Paul Brinkmann|July 20, 2023
Bearings considered as possible failure point under flight loads
NASA has declared the X-57 electric plane project a success even though the single-pilot aircraft was never flown, and the agency continues to analyze data to determine why its motors showed a potential to fail in flight.
NASA said on June 23 it will end the X-57 project in September, rather than extend the project’s timeline to achieve first flight. Some “project close out” work will continue through December, NASA added later.
The project began in 2016, when then-NASA Administrator Charles Bolden announced development of the X-57 as part of a broader experimental aircraft initiative. The proposed aircraft’s distributed propulsion concept was derived from an effort begun two years prior called LEAPTech, short for Leading Edge Asynchronous Propeller Technology.
In a news conference, Sean Clarke, NASA’s principal investigator for the X-57, identified the motors as the problem area that led to the decision to end the project.
“As we got into the detailed analysis and airworthiness assessment of the motors themselves, we found that there were some potential failure modes with the motors mechanically under flight loads that we hadn’t seen on the ground,” Clarke said, according to a recording.
Reached by phone Monday, Clarke told me he is still working on his analyses, but he added, “It was specifically the loading of the motors and the way the bearings interact with our vehicle loads. But I think that’s as much detail as I’ve been able to get published so far.”
The X-57 design called for a dozen propellers to be mounted under the forward edge of its wings plus a larger propeller on each wingtip. All 14 propellers would have turned during takeoff, and then the wingtip propellers would have taken over during cruise.
NASA had postponed the first flight several times. At one point in 2015, a lithium ion battery that was designed to contain a fire to one cell to prevent thermal runaway failed to do so during testing, resulting in the entire battery burning, Clarke told me. But a subsequent design fixed that problem.
The X-57 team had arrived at a fix for the motor-failure issue, but “it would take too long for us to go through and implement, to go through prototyping and then go through component testing and system integration. That’s just a little bit too much of a delay to fit in,” Clarke said in the June 23 press conference.
Bradley C. Flick, the director of NASA’s Armstrong Flight Research Center in California, said during the press conference that the X-57 was “a bold plan based on the belief that component technologies were commercially viable and available to be integrated into a fleet vehicle.”
But he added, “What we learned is that many of those necessary subsystems were not sufficiently mature for safe flight.”
Flick said the X-57 produced knowledge that “has elevated the state of the industry and has informed standards for current and future electrified aircraft.”
For example, electric air taxi developer Joby Aviation, of California, was involved in building electric motors for X-57 tests. Mark Moore, who led the LEAPTech research as principal investigator and did the same for the X-57 for a time, left NASA and started Whisper Aero of Tennessee, an electric propulsion company.