King Airs and Hypersonics

King Airs and Hypersonics: "This year, at its annual media day, held at the company's Rocketdyne facility near West Palm Beach, Fla., at the end of March, Pratt & Whitney showcased its scramjet engine and the Boeing-built X-51A research vehicle it powers. For a business aviation user, the presentation may have seemed futuristic, but there was a message for today's operator as well.

Pratt & Whitney's Rocketdyne division was tasked by the U.S. Air Force to design a scramjet, or Supersonic Combustion Ramjet, engine using conventional materials and burning existing hydrocarbon fuels. Scramjet engines are not new, but most of them burn hydrogen, which is expensive to make and difficult to store. The Rocketdyne SJX61 engine that emerged was intended to propel NASA's X-43, but when that program was canceled, the engine was moved to the X-51. It burns JP-7, which, although meeting the requirement for an existing fuel, can hardly be considered conventional. JP-7 was used to power the Pratt & Whitney J58 engines on the SR-71 Blackbird, and it's exotic stuff, specially blended and carefully stored at separate facilities to ensure its integrity.

The engine is built of nothing more unusual than Inconel, a family of conventional high-temperature alloys made up of various mixtures of nickel, chromium, iron, molybdenum, cobalt and about a dozen metallic ingredients altogether. Inconel, along with titanium and ceramics, is a time-tested material used in aerospace where resistance to heat and corrosion are paramount requirements. But it is difficult to machine, to weld and to form into complex shapes.

Rocketdyne designed the X-51A engine, now designated the SJY61, with a two-dimensional inlet, meaning it has a linear lip that generates the initial inlet shock as a two-dimensional sheet. (Three-dimensional geometry is axisymmetric, or round.) The engine and vehicle interact to generate the proper airflow into the scramjet inlet, so the resulting vehicle resembles a fish with a long, flat nose and a square lip beneath it.



An X-51A test vehicle ready to depart from its B-52 mother ship over the Pacific Ocean on May 26, 2010. (courtesy Pratt & Whitney Rocketdyne)

The inside of the engine is a closely guarded secret, but its shape manages the inlet shock to preserve supersonic airflow through the engine and confine the combustion zone where fuel is added so that the engine does not unstart, or belch the flame out its inlet. The biggest challenge in hypersonics is heat, and the engine is the hottest part. Faced with two tough design hurdles, heat and liquid hydrocarbon fuel, the boys at Rocketdyne decided to use one to deal with the other.

The engine forms a squared pipe, and Rocketdyne elected to line all four walls with tiny buried channels through which the cold fuel is pumped to absorb the engine's excess heat. But the channels are also lined with a catalyst that helps the heat to 'crack' the large molecules of the fuel into smaller ones the way a petroleum refinery does in a cracking tower. At the end of the channels, lighter fractions and gases emerge to be injected into the combustion zone (there's no separate chamber) to create the hellishly hot flame and expand out the back to make thrust, all in a cycle that consumes about .001 second. The best part of all is that the designers also figured out how to fabricate the engine using standard brazing techniques that keep the cost of manufacture reasonable.

No, your King Air won't reach Mach 6 like the X-51A anytime soon, but it's instructive to know that the ingenious folks who figured out how to power the X-51A successfully have business cards with the same P&W logo that's on that PT6 in your airplane."