"They Fly!", Input/Output, ME Departments, February 2005

input/output

by Paul Sharke, Associate Editor

They Fly!

Two of the three X-43A research aircraft broke air-breathing engine speed records last year, first in March and then on November 16, when one flew at Mach 9.8. Knowing that scramjet secrets need to stay boxed up in the "sensitive" file, Mechanical Engineering asked NASA Dryden Flight Research Center's chief engineer, Laurie A. Marshall, to share what she could of the intricacies of hypersonic flight.

ME: Sketches of the X-43A show a deceptively simple device. Can you elaborate on its mechanical design?

What makes this hypersonic engine project unique is that the engine is airframe integrated. The entire lower surface of the vehicle is dedicated to making the engine work. The forebody and angles on the lower ram are designed to get the flow to the right speed for the engine to function.

ME: The scramjet burns hydrogen at supersonic speeds, taking in air as it flies through the atmosphere hypersonically. Can you explain the aerodynamics of the X-43A that make this happen?

It's all about geometry. A turbojet engine uses fans and compressors to supply air to its combustion process. A scramjet has no moving parts. It uses geometry to develop a shock train that reduces the speed of the airflow from hypersonic to supersonic velocities.

A scramjet is a ramjet that has supersonic combustion. We don't need to slow the flow down to subsonic speeds; we only slow the flow down from the hypersonic speed of the free stream to supersonic speed. The compression ram on the undersurface of the aircraft slows the flow before it reaches the engine.

ME: We're told the engine can be throttled to run at lower speeds, an advantage over rockets that must always run at full thrust. How?

All the engines for X-43A have been point-specific designs to function just at Mach 7 or just at Mach 10, or within a range around these Mach numbers. An engine that would work through a multispeed range would use different fuel mixtures to achieve those speeds. You'd use a series of fuel valves for the different mixtures.

The X-43A is a technology demonstrator. We demonstrated that you could design an engine for these speeds. You could expand on that and develop engines to go through different speed ranges. That's what the X-43C project was about, an engine that would accelerate from an air-launch Mach 5 to Mach 7. That project was canceled last year [2003], but the Defense Advanced Research Projects Agency and other organizations are continuing with aspects of it.

ME: Firing up the engine has been described as "lighting a match in a hurricane." How does one ignite a scramjet engine?

A silane tank on board supplies an ignitor to facilitate the combustion process. Another tank holds hydrogen fuel. Oxygen we gather from the atmosphere.

Pyrophoric silane ignites instantly when exposed to air. We start out with a mixture of silane and hydrogen, then back off on the silane and the hydrogen burns on its own.

A single oxygen molecule spends only one millisecond in the engine. In that time, fuel is injected, then combusted, producing thrust.

ME: Can you describe the exotic materials needed to handle high heat loads seen by the aircraft?

The nose is tungsten—the largest piece ever machined—with carbon-carbon leading edges. Carbon-carbon also lines the leading edges of the wings and the vertical tail. We have carbon-carbon chines along the sides of the nose, as well.

For the third flight, the Mach 10 flight, we had an additional hafnium-carbide coating on the leading edges that gave a bit more heat resistance.

Most of the exterior surfaces are black, because they are covered with the identical tiles used on the shuttle. The wings, tail, and rudders are steel.

The engine is a high-temperature copper alloy. Zirconia provides additional thermal protection.

ME: Any predictions for military or commercial applications?

I expect the technology to follow the natural order of things. You'll see it first in missiles, then in military aircraft, then in commercial aircraft. By starting out with missiles, which aren't usually that costly compared to other aircraft, scramjets can fill an immediate need.

Military vehicles are willing to take on elements of risk that commercial craft couldn't tolerate. Commercial aircraft is where the technology will impact the public, something I'm excited about. There'll be a developmental phase first; it's going to take time to actually get to that level.