NASA picks an engine type

Cleveland, OH--In October of 1990, GE Aircraft Engines and Pratt & Whitney announced an agreement to jointly study the feasibility of developing a propulsion system for the next-generation, high speed civil transport (HSCT). Funding and overall program direction for the high risk technology development supporting industry's design of a propulsion system would come from NASA Lewis Research Center.

Now, after nearly six years of research, and the evaluation of as many different engine types, NASA and its industry partners have narrowed the field. Further study of the HSCT propulsion system will focus on only one engine cycle: a mixed-flow turbofan with a mixer-ejector nozzle.

Decreasing decibels. Since future supersonic airliners will operate from existing airports, stringent noise requirements rule out the single-flow-stream turbojet engines used in the Concorde, which is exempt from international community noise regulations. Consequently, the NASA team focused its attention on two different approaches to reducing noise while maintaining thrust. (Bear in mind that noise is a direct function of jet velocity, while thrust is the product of mass flow and jet velocity.)

One approach calls for an engine configura-tion that offers relatively low flow at takeoff, with a correspondingly high jet velocity. For example, a turbine bypass engine featuring a bypass stream similar to that found in conventional turbofan engines.

However, unlike a turbofan, bypass air bleeds off the primary flow stream at the compressor exit. Circumventing the combustor and turbine stages, it recombines with the primary stream before entering the nozzle. Benefit: modulating the bypass flow allows more optimum engine operation over the entire flight envelope when compared to a conventional turbojet. Drawback: lowering the noise signature to an acceptable level requires a complicated and unconventional nozzle with a 20 dB noise-reduction requirement for the nozzle design.

A second approach to reducing noise relies on the converse configuration to maintain thrust. That is, a relatively large amount of airflow at takeoff, and a correspondingly low jet velocity. This philosophy, incorporated in the Fan-on-Blade or FLADE engine, makes it easier to meet noise requirements with a more conventional nozzle design. However it requires much more complicated designs for the inlet and turbomachinery.

Specifically, FLADE employs an auxiliary fan to increase air flow and thereby lower noise during takeoff. Mounting this additional compression stage on the fan tip, in its own separate flow stream, provides extra air at takeoff--and an added benefit. The stream can be closed off to reduce drag during supersonic cruise. Shortcomings include the added weight of the auxiliary fan, and the less-than-optimum profile of the propulsion system resulting from the large fan diameter.

NASA's down-selected engine configuration--the Mixed Flow Turbofan--represents a compromise between these two approaches. It incorporates a low bypass ratio fan that entrains enough extra airflow to reduce the jet velocity when compared to a simple turbojet. Yet the engine maintains the turbojet's cruise characteristics.

Operation of the mixed flow turbofan resembles that of conventional subsonic engines. Air is compressed in a fan. Part of this flow splits off and bypasses the engine core. The remainder is compressed further, heated in a burner, and expanded through two turbines--the first driving the compressor, the second driving the fan. The bypassed air then mixes with the core air at the turbine exit and is expanded through the nozzle.

During takeoff, a mixer-ejector nozzle mixes low-energy air with the high-energy exhaust to further reduce jet velocity. After initial climb, the additional air entrained by the nozzle is closed off to decrease drag and improve propulsion system performance.

Targeting emissions. With the selection of a specific engine type, NASA and its industry team have passed the first tollgate. The next challenge, explains NASA's Joe Shaw, manager of theHigh Speed Research Propulsion Project Office, will be the refinement of engine cycle characteristics. In hardware terms, this means choosing the proper combination of inlet, combustor, and nozzle designs.

Combustor type, for instance, impacts emissions directly--one of the chief concerns regarding the High Speed Research program. Scientists worry that exhaust from a fleet of HSCTs would add significant nitrogen oxides (NOX) to the upper atmosphere, where they could react with, and remove, ozone. Combustor technology research, how-ever, has demonstrated the possibility of reducing en-gine emissions at supersonic cruise conditions to extremely low levels. In fact, NASA's present emissions goal of five grams of NOX per kilogram of fuel burned has already been met in small-scale laboratory tests.

This feat was accomplished with a "Lean Premixed Prevaporized" (LPP) combustion chamber under evaluation at GE Aircraft Engines. Adaptable to the mixed flow turbofan cycle, this combustor mixes fuel and air upstream of the burning zone, allowing sufficient time for the liquid fuel to vaporize completely. The mixture then enters the combustion system and ignites downstream of a flame stabilizer, where the mixture moves more slowly. Lean burning avoids the high flame temperatures that produce NOX at a high rate.

Pratt & Whitney's "Rich Burn-Quick Quench-Lean Burn" (RQL) combustor concept, also under consideration for the mixed flow turbofan, works in two stages. First, excess fuel is injected into a small amount of air. This "rich burn" environment generates chemical reactions that minimize NOX production. Downstream, air is added quickly and uniformly; burning is completed in a final fuel-lean stage. Like the LPP combustor, sufficiently high peak cycle temperatures are achieved with dramatic reductions in NOX emissions.

Both concepts, Shaw claims, support research that suggests a fleet of supersonic airliners, equipped with ultra-low NOX engine combustors, could have a negligible effect on stratospheric ozone.

Inlet selection affects propulsion system performance and noise signatures. Inlets for an HSCT require variable geometry to match inlet flow conditions with the engine's requirements over the entire flight regime, from takeoff to supersonic cruise. NASA and its industry team are considering two designs: a translating center-body (TCB) inlet, and a variable-diameter center-body (VDC) inlet.

The TCB features an axially translating center spike to change inlet throat area; the VDC uses a fixed center-body comprised of overlapping sleeves. As its diameter changes size, so does the throat area. The NASA/industry team is also evaluating two-dimensional inlet configurations that look promising.

Global competition. The U.S. is not alone in pursuing an HSCT propulsion system. Britain's Rolls-Royce and France's Snecma, producers of the Olympus engine that powers the Concorde, are also teamed up to study an engine type for the next-generation supersonic transport.

The Europeans, however, have placed their bet on high-flow engine cycles. Like the fan-on-blade concept, these designs entrain a large amount of extra airflow through the engine during takeoff. This extra flow, when mixed with the primary flow stream, results inlow jet velocity and therefore, a low noise signature. During cruise, the added flow is cut off.

The advantage of a hi-flow cycle is the use of a conventional light-weight, high-performance, convergent-divergent nozzle. The disadvantage is that passing the extra airflow requiresa physically larger engine, which increases weightand drag.

While NASA and its industry team believe the mixed flow turbofan results in a lighter, higher performance propulsion system more easily integrated to the airframe, they are quick to qualify this position. "I recently saw some charts from an Europeanpresentation in an open meeting. Their con-clusions were the same as ours, but for the high-flow engine concept. Which says weare still in the infancy stage of the technol-ogy development," says Shaw.

Meanwhile, there is a growing enthusiasm within Japan for a role in the HSCT. The country's Ministry of International Trade and Industry (MITI) runs a national engine programaimed at developing a Mach 5 "Super/Hyper-Sonic Transport Propulsion System." Many of the relevant technologies, however, would apply to a lower Mach number engine such as the mixed flow turbofan or high-flow concepts. Reports indicate that the HYPER program is on target for building a small demonstrator engine by 1998.

With industry projections indicating the HSCT market will support only one aircraft, the international economic stakes are high. NASA's program effort is to select those technologies and approaches that will give U.S. engine manufacturers a dominant role once a product launch decision is made. The next step in attaining that goal will be final configuration selections for critical propulsion components. These will lead to scale-up and validation tests throughout the 1997-2002 time frame.

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