Goals include showcasing how airliners can burn half the fuel and generate 75 per cent less pollution during each flight as compared to now, while also being much quieter than today's jets - perhaps even when flying supersonic.
NASA's renewed emphasis on X-planes is called "New Aviation Horizons".
The plan is to design, build and fly the series of X-planes during the next 10 years as a means to accelerate the adoption of advanced green aviation technologies by industry.
Design work already has begun on QueSST, short for Quiet Supersonic Technology.
A preliminary design contract was awarded in February to a team led by US-based aerospace company Lockheed Martin. If schedule and funding holds, this new supersonic X-plane could fly in the 2020 timeframe, NASA said.
QueSST aims to fix something the X-1 first introduced to the flying world nearly 70 years ago - the loud sonic boom.
The resulting sonic "boom" has variously been described as like distant thunder, the sound of your neighbour forcefully shutting his car door outside while you are inside, or as the thump of a "supersonic" heartbeat.
"We know the concept is going to work, but now the best way to continue our research is to demonstrate the capability to the public with an X-plane," said Peter Coen, NASA's supersonic project manager.
Meanwhile, other experimental aircraft also are under consideration, including those with novel shapes that break the mold of the traditional tube and wing airplane, and others that are propelled by hybrid electric power.
Exactly what these X-planes will look like, how they will be operated and where they will be flown all have yet to be precisely defined.
Despite these future test aircraft being referred to as X-planes, it is possible only some of them will actually get an official X-plane number designation - or perhaps none of them will.
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