Supersonic and hypersonic flight; the name alone evokes speed, and rightly so. Definitively, ‘Supersonic’ means aircraft capable of cruising speeds over Mach 1. ‘Hypersonic’ refers to cruising speeds in excess of Mach 5, or roughly 3,000 mph. To date, Hypersonic Aircraft has been pretty much wholly owned by the military industrial complex, NASA, and equivalent agencies from a select few other nations – Until now, that is. It appears that Airbus is poised to join the Hypersonic jet market.
That the speed of sound is referred to as Mach is done in deference to Physicist Ernst Mach, who’s work in the late 19th century focused on fluid dynamics – In that science, Mach refers to the velocity of the flow around an object as it approaches the speed of sound. In terms of aircraft flying though earth’s atmosphere, mach speed is not static; it varies predominantly with temperature, and to a lesser degree, with atmospheric pressure. Aircraft moving at such exceptional speed experience conditions that are, to say the least, brutal. How air flow impacts an aircraft as it approaches the speed of sound can be rather shocking, as General Chuck Yeager so famously found out when he took the Bell X-1 over that threshold for the first time in the history of flight, on October 14th, 1947.
So, hypersonic flights defined as that over Mach 5, but how fast can hypersonic craft go? Spacecraft in reentry experience speeds as high as Mach 10, and it’s believed that the U.S. Air Force’s Falcon Hypersonic Technology Vehicle, (HTV-2), reached a jaw dropping Mach 20, some 13,000 miles per hour, and maintained controlled flight at that speed for three minutes on August 11th, 2011, shortly before it crashed into the Pacific Ocean.
The engineering challenges for hypersonic flight are daunting, indeed. At these speeds, the temperature of the air flowing past the aircraft is so extreme that, as NASA researchers noted, “the chemistry of the diatomic molecules of the air must be considered. At low hypersonic speeds, the molecular bonds vibrate, which changes the magnitude of the forces generated by the air on the aircraft. At high hypersonic speeds, the molecules break apart producing an electrically charged plasma around the aircraft. Large variations in air density and pressure occur because of shock waves, and expansions.” In other words, the frictional heating of the airframe is so extreme that only certain high tech alloys can be used, and additional structural cooling methods may be required, like circulating fuel through the aircraft’s skin.
Given all these challenges, it is both fascinating and exciting that Airbus would wade in. Their patent, applied for in the U.S., is for an, “ultra-rapid air vehicle together with a method of aerial locomotion”, a hypersonic jet. French Airbus engineers Marco Prampolini and Yohann Coraboeuf, listed in the patent application as the inventors, have stated that they expect their craft to cruise at around 3,000 mph, or Mach 5. Comparatively, that speed would make this an aircraft flying at roughly the same speed as an SR-71 Blackbird, capable of flying from Tokyo to Los Angeles in around 3 hours. The jet is expected to fly at between 30,000 and 35,000 meters, roughly 20,000 meters higher than most commercial aircraft fly today, thereby avoiding the problem of other aircraft to worry about. Although the head of Airbus engineering has said that developing hypersonic jets is not a priority for the company, they plan to fly a prototype by 2019.
The Airbus patent application describes a stubby wing design, in keeping with most of the current hypersonic aircraft. The jet will be equipped with three distinct engine systems, designed to get the craft airborn, then to cruising altitude, and finally, to its 3,000 mph cruising speed. To achieve take off, the design employs a pair of under-fuselage turbofans and a center, rear mounted rocket motor. Those create an extremely steep angle of attack on take off, a “near-vertical ascendant flight”, pushing the jet past Mach 1. Once airborne, the turbofan engines are retracted into the fuselage, as is landing gear. The rocket motor takes the jet up to its 30,000 meter cruising altitude. Once there, the rocket motor shuts down and retracts into the fuselage. Then, wing mounted ram jets take over, boosting speed up to the planned Mach 5. Liquid hydrogen and oxygen tanks power the ramjet engines.
The inventors claim a much quieter and more efficient aircraft than the Concorde or the Tupolev Tu-144, and one considerably more environmentally friendly, due to the hydrogen fuel. Designed to carry about twenty passengers, Airbus perceives potential markets in both business and military sales. Certainly it’s cruising altitude and speed would put it in particularly good stead for the latter.