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AN EXPERIMENTAL Nasa X-plane that soars "quietly" at supersonic speeds is on track to be built before the end of the year.

The stealthy jet's sonic boom is expected to sound no louder than the thump of a car door closing at ground level.

Nasa greenlit the project to be built by Lockheed Martin just last year.

The goal is to create a plane that can cruise at Mach 1.42 speeds, with a sonic boom one-thousand times quieter than previous supersonic aircraft.

It's been named X-59 QueSST by the US Air Force, and also has the potential to make supersonic passenger travel more viable.

Now a Lockheed Martin representative has told that the X-59 is due to be complete this year.

"It's moving very fast on the shop floor in terms of manufacturing and production," a Lockheed Martin rep told Space.com

The aviation giant plans to assemble the aircraft, proof test the frame and trial other systems – before rolling it out for test-flights in 2021.

"We're very confident. All kinds of modelling simulations and predictions align," the rep explained.

"So we believe, based on these models and simulations we've run, that it will achieve that low-boom sound once it reaches supersonic speeds."

The barrier for supersonic flight is 768mph – a speed known as Mach 1.

When flying at the speed, sound waves can't get out of the way fast enough, and so create an audio "wake".

This is known as a sonic boom, and is a loud and annoying giveaway that a supersonic plane is flying overhead.

Nasa's X-59 hopes to cruise at 937mph (with a top speed of 990mph, or Mach 1.42), with a "quiet" sonic boom.

A Concorde's "perceived" sound was 110dB – which is thousands of times louder than the X-59's planned noise level.

Air experts hope the shape of the X-59 will max its sonic boom out at between 60dB and 75dB for people on the ground.

That "thump" is somewhere between a car door closing and the sound of a vacuum cleaner.

If successful, Nasa's design could be rolled out for commercial flights – or possibly even the US military.

Here's what you need to know...

A sonic boom is a loud and powerful sound created by aircraft travelling faster than 768mph
These supersonic planes can be heard for miles around due to the sonic boom
One way of understanding sonic booms is to think about objects in water
If you toss a rock into water, waves will be released all around the point of impact
But if a boat is moving through water, the boat travels through the waves
When a boat travels fast enough, the waves can't get out of the way – creating a wake
Wakes are large waves made up of lots of little waves combining together
Similarly, airplanes create soundwaves as they travel
Once a plane travels faster than the speed of sound, the sound waves can't get out of the way enough
This creates a giant "wake" of sound waves – or a sonic boom
It's just like when a large boat goes past you on a lake
You won't experience anything immediately, but later you'll see the wake waves hit the shore
A sonic boom works in the exact same way

The cost of the program is $247.5million (£189.7million).

And the craft is expected to be 94 feet long with a wingspan of 29.5 feet.

It will boast a maximum take-off weight of 32,300lbs, and cruise at an altitude of 55,000 feet.

The craft will be propelled by a single General Electric F414 engine, and will sport a 4K camera at the front for added visibility.

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Supersonic 990mph Nasa X-plane as quiet as ‘thump of car door’ is nearly ready