Machine Design

ON THE DRAWING BOARD: Hybrid jet to combine turbine and ramjet technologies

The Revolutionary Turbine Accelerator (RTA), a new type of jet engine from General Electric and NASA, could take the next generation of cruise strike-fighters and longrange bombers from the runway to Mach 4 in less than 10 min.

ON THE DRAWING BOARD: Hybrid jet to combine turbine and ramjet technologies

NASA and General Electric's Revolutionary Turbine Accelerator could lead to engines seven years from now that have a 25% increase in Mach number, a 250% higher thrust-to-weight ratio, and twice the component life of current, state-of-the-art turbine engines.

It might also serve as the initial stage on two-stage spaceplanes that take off like conventional airplanes but reach space on their own.

The last time the U.S. was involved in developing a high-speed turbojet was in the late 1950s with the Pratt & Whitney J-58, the engine that powered the SR-71 Blackbird. But several hurdles stand in the way of a new design, most of them technical. Politics will likely play a role as well.

The RTA is envisioned as using turbine-based jet technology to reach Mach 3 and then transition to a ramjet for speeds to Mach 4 and higher, all in a single nacelle. During the turbine phase, a spinning compressor increases pressure of the incoming air. The high-pressure air mixes with fuel and burns in a combustor. The resulting gases spin a turbine linked to the compressor. Exhaust escaping from the turbine passes through a flow-accelerating augmenter and generates thrust. In the ramjet phase, an inlet cone compresses and slows incoming air to subsonic speeds.

The mixture ignites in the ramburner, a combustion chamber, and exhaust gases rush through a nozzle that accelerates flow, creating thrust.

The first engineering challenge is to construct a compressor, combustor, and turbine that withstand temperatures of 1,600°F generated by inlet air at Mach 4. Such materials are traditionally costly or heavy, penalties the RTA program can ill afford. New materials must also be developed and tested for the rotating components. Until now, rotating turbomachinery never had to operate above Mach 3.

NASA and GE engineers must perfect the augmenter/ramburner, as well. It boosts thrust during the turbine phase by about 50%. But between Mach 2 and 3, it must switch to being a ramburner, sending speed to over Mach 4.

The fuel control system, as well as the fuel, will also have to be totally new. Controls will adjust the air/fuel mixture and where it burns as the engine switches from turbine to ramjet, and eventually back again. There are also issues with the various nozzles and augmenter and how they will handle the different speeds and rates of exhaust gas flow. And the fuel will probably need to double as a heat sink in the overall thermal management scheme.

Currently, program engineers at NASA's Glenn Research Center in Cleveland, Ohio, are conducting air bypass tests in the Mach 3+ regime, checking augmenter performance, and building engine sections they hope to assemble into RTA-1 by summer 2006. They plan to produce a flight-test engine by 2011.

Unfortunately, the RTA program is on hold because of President Bush's initiative calling for manned missions to the Moon and Mars. Budgetary and manpower restrictions could eventually derail hypersonic engines in favor of low-cost expendable rockets.

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