Same-day round-trip flights around the globe may sound like science fiction, but engineers last week demonstrated a breakthrough that could make them possible.
The key technology is a so-called scramjet engine that can accelerate a aircraft to hypersonic velocity. That is, above 5 times the speed of sound. By comparison, the Concorde—which offered supersonic passenger service from 1976 to 2003—reached a top speed of about 2 times the speed of sound. The fastest jet ever built, the SR-71 Blackbird, reached 3 times the speed of sound.
After four decades of effort by researchers around the world, a US Department of Defense project has finally solved the challenge of keeping a scramjet running long enough to actually get somewhere. The flight by the unmanned X-51A, shown here in an Air Force illustration, took place over the Pacific Ocean off the California Coast on May 1. After dropping from the wing of a B-52 bomber and getting a boost to its supersonic operating speed from a rocket engine, the X-51A traveled 230 nautical miles under scramjet power.
The turbofans that power today’s jet engines are limited in the amount of thrust they can produce by their turning speed. Scramjets do away with this problem by eliminating moving parts altogether. In that respect, they resemble rocket engines. Unlike rockets, however, scramjets don’t bring their own oxygen supply for combustion, and that makes all the difference.
Fuel and oxidizer make up most of the weight and bulk of a rocket-powered craft. Recall the orange external fuel tank that the space shuttle had to lug with it on the way into orbit—it was larger than the shuttle itself. Scramjets are able to travel at rocket speed without all the extra tankage, making them much more practical for terrestrial passenger and cargo service.
Researchers at the University of Queensland in Australia first demonstrated scramjet operation in free flight, running for all of five seconds in 2002. Researchers at NASA in the US pushed the running time to 10 seconds in 2004. Now, the X-51A, built in a DARPA and US Air Force-funded project, has done what no scramjet has done before: stayed lit until its fuel supply ran out, more than six minutes after ignition.
Last week’s flight was the fourth and final test of the X-51A design. The previous flights, in 2010, 2011 and 2012, ended in failure. But the recent success should unlock more development money, both private and public, that could eventually make four-hour round-the-world flights possible from any major airport.