Hypersonic flight

Less than a hundred years ago, the Wright Brothers made the first powered airplane flight. Next month, NASA will fly a whole new type of airplane, faster than anything that has flown to date: not just supersonic (faster than the speed of sound) but hypersonic (MUCH faster than the speed of sound).

Of course, NASA has a number of advantages over the Wright Brothers, not least the fact that nobody has to actually sit in the X-43A to test it–a good thing, it’s going to crash into the Pacific Ocean at the end of its flight.

The current speed record for flight, Mach 6.7 (6.7 times the speed of sound, or 7,230 kilometres per hour) was set by NASA’s X-15 rocket-powered research plane in 1967. The X-43A is expected to fly faster than Mach 7, or around 8,000 kph, and future versions are expected to reach Mach 10, or roughly 12,000 kph.

In a hypersonic aircraft, a flight from New York to Los Angeles, which currently takes six hours in a commercial plane, would only take half an hour–but commercial flight isn’t really the goal. Instead, NASA’s Hyper-X program, the six-year, $185 million research endeavor of which the X-43A is a part, is aimed at developing vehicles that can deliver payloads into space much more cheaply than traditional rockets.

Rockets have to lift not only their fuel and their payload off the ground, but also an oxidizer, which provides the oxygen necessary for the fuel to burn. A hypersonic spaceplane that could use air-breathing jets to climb to the edge of space, and only then ignite rockets to hurl itself into orbit, might weigh only half as much as a conventional rocket-powered vehicle because it wouldn’t need as much oxidizer.

Among the technical challenges are engines that can withstand the forces necessary to generate hypersonic speeds. Jet engines work according to Newton’s law that for every action there is an equal and opposite reaction. That means the faster you want a jet to travel, the faster exhaust has to shoot out of the engines. The turbines that accelerate the exhaust in conventional jet engines would fly apart at hypersonic speed.

To get around that, the X-43A will use a revolutionary type of jet engine called a supersonic-combustion ramjet, or scramjet.

In a ramjet, fuel is burned in a stream of air compressed by the forward movement of the aircraft itself–there are no moving parts. A scramjet is a ramjet that air flows through at supersonic speeds. Unfortunately, you can’t really test a scramjet on the ground, because there’s no way to maintain a sustained supersonic airflow.

Enter the X-43A. But before its hydrogen-fueled scramjet engine fires, it has to be accelerated to supersonic speeds. To do that, NASA will attach the four-metre-long, surfboard-shaped aircraft to a Pegasus rocket, which in turn will be loaded aboard a B-52 bomber. The B-52 will take off from Dryden Flight Research Center in the Mojave Desert, fly to 7,600 metres, and drop the rocket, which will then fire, lifting the airplane to 29,000 metres and a speed of Mach 7. At that point the X-43A will break free of the rocket and fire its scramjets.

NASA only expects about 10 seconds of powered flight, and maybe seven or eight minutes of data about the aerodynamics of hypersonic flight. The results will be factored into the design of the X-43B, the next plane in the test series–just as the Wright Brothers tried out their airplanes, then went back to the drawing board to build the next, even better one.

We’ll probably never see a commercial hypersonic jet. The high speed would mean high skin temperatures, uncomfortable acceleration, and low low maneuverability. But pushing the aeronautical envelope always generates unforeseen benefits that result in more efficient, safer and faster aircraft for all purposes.

More importantly, if we’re ever going to reach the point where ordinary people can travel to space hotels and orbiting cities, we need vehicles that can get there more cheaply and easily than our current rockets.

The X-43A’s flight is a step along the road to easier access to space, and if we can’t currently foresee exactly how the knowledge gained by it will change flight in the future, well, the Wright Brothers didn’t know that their brief initial flight would lead to jumbo jets, routine trans-Atlantic flights, or the Snowbirds.

Now if only NASA could conduct research into lost luggage…

Permanent link to this article: https://edwardwillett.com/2001/04/hypersonic-flight/

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