Kermit to take Rolls for a spin

DSTO spin rig team member David Dyett installs an F-111 TF30 engine compressor disc in the test chamber nicknamed Kermit because of its colour.

By Flight Lieutenant Adam Grey

THE Air Force and the Defence Science and Technology Organisation have joined an international collaborative program with Rolls-Royce, South Africa, USA and Canada aimed at keeping C-130H and P-3 operations safe.

The aim of the program is to determine how long it takes for Rolls-Royce T56 engine turbine components to fail catastrophically.

In cooperation with Rolls-Royce, DSTO will test T56 turbine components in its high-speed “spin-pit”, nicknamed Kermit because of its vivid green colour.

Kermit can recreate the stress that T56 turbine components endure by spinning them to speeds in excess of 20,000 rpm at temperatures up to 1000 degrees Celsius.

The spin-pit repeatedly starts and stops the component to replicate the thermal and mechanical stresses created each time a T56 engine is started up and shut down or the aircraft performs touch and go landings.

It is anticipated that after 45,000 start and stop cycles the components will start to develop cracks and then, after another 10,000 cycles, those cracks will grow to the point of failure.

The forces involved when one of these components fails are so high that the spin-pit walls are made of 30cm thick lead-lined steel to contain debris.

DSTO is working on leading edge crack-monitoring methods to determine the optimal time to halt the test and avoid damage to Kermit’s innards.

Despite the T56 engine having millions of hours in service, Rolls-Royce decided five years ago that there were problems in the analyses carried out in the 1980s which determined the safe-life limits for rotating critical parts.
As a result, Rolls-Royce significantly reduced the life limits on some turbine components.

Owing to the age of the technology employed in the T56 engine design, original testing did not provide sufficient information to develop new life limits, which is why operators such as Australia, Canada and the US are getting involved.

To avoid disruptions to Air Force C-130H and P-3 operations, caused by the reduced life limits, engine structural integrity staff at the Directorate General Technical Airworthiness (DGTA) and DSTO have established that it is safe for the Air Force to continue applying the original limits until early 2004.

This will allow DGTA and DSTO to assess the results of the spin-pit testing to verify new safe-life limits for the crack growth lives in two turbine components.

.