Scrambling for the scramjet

Boeing has several hypersonic projects that likely will depend on air-breathing engines

BY WILLIAM COLE

What is faster, potentially safer, better and more affordable — and gulps air at speeds of Mach 5 and more?

It's the scramjet engine, a new propulsion system that could propel Boeing into long-term hypersonic future. Such systems could provide significant advantages in mission time, weight, payload, mission profile flexibility and cost, compared with competing conventional propulsion systems.

Boeing Phantom Works and Boeing Rocketdyne are accelerating the race toward products linked by this one enabling super technology: scramjet engines that "breathe" and process it at supersonic speeds to produce the desired power. Ramjets use a similar process but at subsonic speeds.

Scramjet engines could power

• A hypersonic standoff missile that could attack time-critical and hardened, deeply buried targets;

• A reconnaissance and strike aircraft that is so fast that it could overwhelm an adversary's anti-access and counter-stealth assets;

• An air-breathing, reusable space launch vehicle that provides safer, more affordable access to space.

Scramjet engines also could be deployed in unmanned air vehicles. And scramjet power could, in the distant future, even lead to a commercial airplane that could fly at many times the speed of sound.

Boeing Rocketdyne is working on the Rocket-based Combined Cycle propulsion system that could be used to power reusable space vehicles. Common to all RBCC designs is the rocket-to-ramjet-to-scramjet-to-rocket sequence. Rocket thrusters with augmentation provide initial boost and operate up to about Mach 3.0, according to Boeing Rocketdyne. At that point sufficient compression in the flowpath has been attained to allow replacement of the rocket thrust with ramjet operation. Ram operation alone continues up to Mach 6.0, when scramjet operation takes over and continues driving the vehicle to hypersonic levels (Mach 12 to 15). At that point, the engine is transitioned to rocket mode, using its own onboard oxidizer, to accelerate the vehicle to orbital speed.

The scramjet concept has been around for some 30 years. In the 1970s, hypersonic work was being done at NASA's Langley Research Center, and prototype engines have been tested in Europe, Japan and Australia.

"Boeing is investigating future hypersonic applications related to weapons and missiles, aircraft and air-breathing launch vehicles," said George Orton, Phantom Works' program manager for Hypersonic Design and Application. Flight test programs, such as the Hyfly hypersonic missile demonstrator and the X-43 series of X-planes, could lead to such products, and scramjet engines are the key. But he cautioned, "The current status of scramjets is much like that of jet engines during the late 1930s. To move beyond curiosity, we must demonstrate, in flight, the ability of scramjets to power air vehicles. We have never actually flown a vehicle powered by a scramjet."

Gary Ratekin, Rocketdyne's program manager for the Boeing RBCC, said, "One of the realities that every propulsion builder the world over has to confront sooner or later is that conventional rocket systems have pretty much hit the theoretical limits. We have gone about as far as we can in both expendables and reusables.

"The RBCC is more than 'what if' territory. It's begun to take on the look of real and viable propulsion."