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Two Hours From D.C. To Australia? Sure, & In & A Hypersonic Space Plane

New combustor design could make air travel at Mach speeds a possibility.

The future of air travel ... two-hour flights from one side of the world to the other, including a half-hour of Space Shuttle-like views while in orbit, in a hypersonic space plane?

Although Astrox Corp., an aerospace development company, has invented a hypersonic space plane that travels as fast as Mach 25, 17,500 miles-per-hour, above the Earth’s atmosphere, the challenge has been the ability to mix fuel in the engine at such high speeds.

A solution might have been found, through two Maryland Industrial Partnerships Program projects, by A. James Clark School of Engineering faculty members Ashwani Gupta and Kenneth Yu, along with graduate student Ram Balar.

They have successfully designed and tested a combustor for the Astrox space plane, which uses an inward-turning scramjet engine.

“Hypersonic space planes could revolutionize the transportation industry, much like jet planes did for subsonic commercial aviation 50 years ago,” said Ajay Kothari, president of Astrox. “Seemingly remote parts of the world would be nearly as accessible as a two-hour drive.”

Mechanical engines will not operate at hypersonic speeds. According to Kothari, moving parts simply cannot work at 3,500 to 7,500 miles-per-hour, or 5 to 10 times faster than the speed of sound.

In contrast, hypersonic jet engines use rectangular, duct-like engines with no moving parts, also called scramjets. Air enters the engine inlet at hypersonic speeds and is compressed to supersonic speeds, after which it is mixed with fuel and ignited. The air leaves the engine traveling faster and at a higher pressure than when it came in – creating thrust.

But the large surface areas created by rectangular designs generate tremendous heat transfer into a vehicle, requiring extra fuel loads just to cool areas around the engine chamber.

These large surface areas are not only inefficient, but the extra fuel also adds significant volume and weight to the aircraft, explained Kothari, who holds a patent on an inward-turning vehicle design.

“Single-stage-to-orbit travel utilizing a rectangular-shaped engine design would be difficult,” noted Kothari.
Kothari’s engine is shaped like a funnel, where air comes in through a circular opening, increases in pressure as it passes through, then leaves with more thrust and less heating than through a rectangular design. The challenge is injecting fuel into the fast-moving air efficiently as it travels through the engine.

“Roughly speaking, you’re looking at the air flow traveling 1,000 meters-per-second inside the combustor,” said Yu. “The combustor is a meter long, so you have one millisecond for everything to happen – not just the fuel and air mixing – but the burning as well.

“The combustion is fast, so that’s not the problem,” explained Yu, “but before combustion can occur, you have to mix your fuel with the air quickly. This is more difficult when the air’s traveling at such high speeds.”

Kothari, Gupta, Yu and Balar designed an injector resembling a small, aerodynamic wing, which enters the engine at an angle in the same direction the air is flowing. Fuel is injected just at the wake where the air crosses the wing-shaped injector.

“You have to inject the fuel in the same direction as the air is traveling,” said Gupta. “That’s where the novelty comes in, as it gives you both high thrust and good mixing.”

The research team has tested the combustor at Mach 2, twice the speed of sound, in the university’s supersonic wind tunnel.

Kothari plans to test both his design and the combustor in a small, model space plane.

Astrox will market its vehicle design initially for military use, both as a stand-alone plane and as a weapons delivery system.

Eventually, Kothari envisions consumer planes taking off horizontally from large airports and reducing 20-hour flights to as little as 1.5 hours.