TDI and CN team on miniature turbojet propulsion system

  • 19-Mar-2010 02:39 EDT
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The low-cost TDI turbojet design incorporates a turbocharger impeller and eliminates the need for separate lubrication and cooling systems.

According to Technical Directions Inc. TDI Chief Engineer, Joseph Kovasity, when TDI was approached by a "major prime contractor" wanting a low-cost, miniature-munitions propulsion system, "we believed our turbojet technology could achieve the stringent performance and configuration specifications required for a mini cruise missile.”

Engine requirements included easy starting, sufficient power and range to complete a search and destroy mission, and operation without an oil lubrication system.

Developed during a U.S. Air Force SBIR Phase I and Phase II program, TDI’s resultant low-cost turbojet propulsion system, the TDI-J45, used inexpensive turbocharger components and could be scaled for a wide range of small expendable missiles. The patented TDI-J45 used low-inertia, radial compressor and turbine wheels and a low-speed ignition for easy starting, which also permits windmill cranking.

Fuel metered to the engine both cools and lubricates internal components, saving the space and weight of a separate oil-lubrication system. As a secondary benefit, the fuel flow is prewarmed for more rapid and complete combustion.

Throughout the development program, TDI used various engineering and manufacturing services from Concepts NREC (CN), including periodic design audits of the engine’s turbomachinery components.

Following the success of the TDI-J45 engine, TDI conducted a follow-up program for the U.S Army intended to improve overall engine efficiency by 8% and, thus, further increase missile payload and range.

In an attempt to decrease fuel consumption by modifying the turbocharger-based engine components, TDI asked CN to review the TDI-J45’s compressor and diffuser and to provide design modifications for reworking the parts. However, after making the recommended trim and profile modifications, engine efficiency improved by only 5%. To achieve higher efficiencies the missile engine would require a more advanced turbomachinery design.

The original TDI-J45 compressor impeller was an inexpensive investment casting with limited machine finishing. CN’s new impeller design incorporated lighter, stronger, and more efficient turbomachinery features to achieve optimum flow performance with minimal flow losses.

Manufacturing requirements of the advanced design necessitated replacing the investment casting with a fully machined compressor wheel. Therefore, while ensuring that performance requirements were met, it was also shown that the complex, precise shapes of the proposed new design could be easily manufactured at a relatively low cost.

TDI had CN provide five-axis machining toolpath instructions using CN's proprietary MAX-PAC CAM software for manufacturing turbomachinery components. The machine code generated by the specialized system resolved all machining issues, and all geometric features were held within their specified tolerances.

In tests of the TDI-J45 engine using the new compressor wheel and diffuser, gains in engine efficiency resulted in reduced specific fuel consumption up to 28%, and additional efficiency gains are expected through further development. TDI says recent wind-tunnel tests have also confirmed the self-starting capabilities of the engine system at all altitudes and vehicle speeds within the required flight envelope.

CN has since designed new rotating components for several other TDI engines that include high-efficiency compressors, diffusers, turbines, and nozzles. CN has also supplied the sophisticated machining toolpaths required to effectively manufacture their complex and precise shapes.

In addition to being used as an expendable turbojet propulsion system, TDI believes that numerous industrial and commercial applications can also benefit from its turbojet engine technology, including in power systems for electrical generation.

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