Mach 20 or Bust
Weapons research may yet produce a true spaceplane.
- By Geoffrey Little
- Air & Space magazine, September 2007
(Page 2 of 3)
With four test flights over the Pacific Ocean slated to begin in 2009, the X-51A will ultimately attempt record-breaking engine burns lasting five minutes, which should propel the craft to about Mach 7. Like the X-43, the X-51A is a wave rider. After being boosted to high altitude, the vehicle will light its engine and surf its own shock wave, compressing the air in front of it and lowering drag. Though the immediate goal is to flight test a propulsion system for a superfast missile, the project received the X-plane designation in recognition of its potential to advance the field of hypersonics generally.
For Mark Lewis, the X-51A is all about the scramjet. “We want to see a scramjet engine work for more than 10 or 11 seconds,” he says, referring to the burn times of the two Hyper-X flights. Engine burns of several minutes would demonstrate to skeptics that long-duration scramjet-propelled flight is feasible.
Skeptics might be forgiven their doubts. Achieving combustion in an air-breathing engine moving at thousands of miles per hour has been compared to keeping a match lit in a hurricane. Hyper-X protected the precious flame in its combustion chamber behind carefully focused shock waves, but only for seconds. The X-51A engine will have to run at least 30 times longer.
To cover their bets, DARPA and the Air Force have two companies, Pratt & Whitney Rocketdyne and ATK, developing two kinds of hypersonic engines. One major difference from Hyper-X is that the X-51A will burn conventional jet fuel instead of the liquid hydrogen that very-high-performance rocket and scramjet engines normally use. It won’t be the first scramjet to do so: In December 2005, a DARPA-Navy project called HyFly launched a missile perched on a booster rocket from Wallops Island in Virginia. The missile’s air-breathing engine, which ran on JP-10 aviation fuel, flew for more than 15 seconds under scramjet power.
Pratt & Whitney’s engine is called the X-1. When flying at hypersonic speeds, JP-7 aviation fuel rushes into the X-1’s three-foot-long combustion chamber at 3,300 feet per second. A closed-loop system cycles the fuel around the engine, using it as coolant to draw heat and pressure off the combustion chamber. In the process, the extreme heat—more than 3,000 degrees Fahrenheit—“cracks” the fuel’s molecular structure. The cracking shortens the molecules and allows the fuel to burn more quickly, which is imperative. If the fuel doesn’t ignite in the microsecond in which it flows through the chamber, it will spew out uselessly, producing zero thrust—and a very fast falling object.
Over the past year, the X-1 engine has worked as advertised in Langley’s test chamber, culminating in a 50-second-plus, simulated X-51A flight at more than Mach 5 last April.
In less than two years, the X-51A will have a chance to prove itself in the atmosphere. Each test flight will begin with a B-52 taking off from Point Mugu, California. The airplane will carry the 14-foot vehicle up to 49,500 feet over the Pacific, where it will be released attached to a booster derived from an Army missile. The booster will get the demonstrator to over Mach 4, whereupon the scramjet engine will fire to propel it to full speed.
With the X-51A attempting to prove that hydrocarbon scramjets can propel hypersonic missiles, it’s up to other projects to sort out how to achieve higher Mach numbers. For some of those answers, Lewis and the Air Force made a long flight down under to work with the Australians who came up with HyShot.