Manufacturer claims novel design has weight advantage over conventional powerplants

A US firm claims a 5:1 thrust-to-weight advantage over conventional engines for a novel gas turbine in which the compressor, turbine and combustor are "nested" one inside the other, eliminating heavy shafts and disks. D-Star Engineering plans to run its new engine in July, under the US Defense Advanced Research Projects Agency's (DARPA) Small-Scale Propulsion System programme.

The first engine will produce just 20lb thrust (0.09kN), and is aimed at converting 70mm rockets into miniature cruise missiles. D-Star president Paul Dev says the company is already proposing a 200,000lb-thrust geared-fan engine using the same principles to power DARPA's planned Walrus airship.

In Shelton, Connecticut-based D-Star's patented design, the combustor is nested inside the turbine section and the turbine inside the compressor section, the result looking like an axial-flow gas turbine that has been telescoped into itself.

Instead of disks and shafts, the engine has bladed rings attached to filament-wound carbonfibre hoops supported by high-speed foil air bearings in the stationary casing. Compared with a conventional gas turbine of the same thrust, the engine is half the length and five times lighter, while efficiency is about the same, says Dev.

The initial engine is 70mm in diameter and 70mm long, weighs 0.45kg, and is the world's smallest two-spool gas turbine, says Dev. The first compressor and turbine stages are either side of the first rotor, behind which is the contra-rotating second rotor carrying the second compressor and turbine stages. Both rotors spin at 160,000RPM.

Air flows through the outermost compressor stages and is turned 180¡ to enter the combustion chamber, then turned another 180¡ to flow through the turbine stages to exhaust via a free-spinning nozzle. Because of the initial engine's small size, says Dev, fuel is injected at the compressor face and passes through a catalytic pre-burner before entering the central combustor. In larger engines fuel would be injected later.

The nested core design eliminates most, if not all, oil-lubricated bearings, easing assembly and improving reliability, says Dev. Clearance control and thermal matching of the nested rings will be achieved by materials selection and management of cooling airflow, he says.


Source: Flight International