Jul 28th 2017

Hypersonics — The Far, Far Side of the Sound Barrier

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The world of travel may be on the verge of getting faster — a lot faster. Researchers and industry are putting a new emphasis on hypersonics, the technology of atmospheric flight at more than five times the speed of sound, or about a mile per second.

While the challenges of hypersonic flight are great, the potential is enormous, ranging from intercontinental passenger flights taking less than two hours, to achieving the dream of an aircraft that can fly into orbit and return to do it again.

Breaking the Speed Barrier … Again

Only one human-piloted airplane has ever achieved hypersonic flight. As Douglas Messier writes at Space.com, this was the legendary X-15 research plane, which made its last hypersonic flight 50 years ago in 1967.

All orbital spacecraft briefly enter the hypersonic regime during their climb into space, and again when returning to Earth’s atmosphere. The Space Shuttle operated as a hypersonic aircraft during re-entry, before slowing down to merely supersonic speeds.

But otherwise, since 1967, hypersonic flight has been limited to a handful of unpiloted research vehicles. This limited role has been due to the technology challenges of hypersonics, which center largely around heat.

Any object passing through the air at Mach 5 and above produces intense frictional heating as it pushes the air out of its path. Keeping this sheer heat from melting the airframe is the first great challenge of sustained hypersonic flight.

The second heat-related challenge is keeping the heat from melting critical parts of a jet engine. This is not a problem for rocket planes such as the X-15, but the intakes of conventional jet engines and even ramjets are limited to about Mach 6 — toward the “slow” (relatively speaking!) end of the hypersonic range. So-called scramjets (for supersonic combusting ramjet, a technology that limits heat buildup) have shown promise in sustaining higher speeds without burning up.

Hypersonics — Military and Civilian Potential

After half a century on the back burner, what has put hypersonics back in the technology spotlight? The most immediate concerns relate to national defense, as Patrick Tucker outlines at Defense One.

Like spacecraft, intercontinental ballistic missiles (ICBMs) reach hypersonic speeds in the upper atmosphere. But they cannot be steered, or change course beyond very narrow limits, rendering them inflexible. In contrast, a hypersonic aircraft could change course to evade enemy attack or in response to a changing mission requirement.

This is what makes hypersonic performance attractive to defense planners. What makes it critical for them is the growing capability of air defense systems that can target and shoot down merely supersonic aircraft, potentially even stealthy ones. The only way for U.S. aircraft to avoid these systems is to outrun them by reaching and sustaining hypersonic speeds.

But while the first operational hypersonic planes will be military, a new generation of hypersonic airliners could follow them through the speed barrier. Supersonic transports such as the now-retired Concorde did not sufficiently reduce intercontinental travel times enough to transform commercial flying. A hypersonic transport would be another matter — making a transcontinental flight take no longer than a regional hop does today.

And beyond the hypersonic transport lies the possibility of a true spaceplane, an aircraft that could take off and land like an ordinary jet, but capable of carrying payloads — and astronauts — into orbit. A hypersonic orbital spaceplane would make space travel as affordable as air travel is today.

(Artist Concept of DARPAs Experimental Spaceplane XS-1)

The technology of hypersonics calls for flying outside the box. This is something that Northrop Grumman has been doing since its pioneering Flying Wing took to the air in the 1940s. And as Darren Orf reports at Gizmodo, it continues to fly outside the box with proposals for hypersonic test aircraft such as the XS-1.

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