Differing engine architectures

An AH-64 Apache mechanic from the shops platoon, Bravo Company, 277th Aviation Support Battalion, rebuilds a T700 engine at Bagram Air Field, Afghanistan.
SSgt Todd Pouliot/10th Combat Aviation Brigade/US Army

By far the biggest difference between the two competitors’ ITEP engine proposals is the engine architecture each company has chosen: GE Aviation has chosen a single-spool architecture for the T901, whereas ATEC has chosen a dual-spool architecture.

GE’s reasoning is that by using a single-spool architecture – in which all of the rotating stages of the engine are on the same shaft and rotate at the same speed – its ITEP engine would be easier for US Army engine mechanics already used to the T700’s single-spool architecture to maintain.

A single-spool engine would also offer other benefits, in GE’s view: the engine would be more modular, less complex and would weigh less than a dual-spool engine, because it would have only one shaft and thus wouldn’t require additional bearings and frames. All of these factors would make the engine more reliable and easier to maintain, GE claims.

For its part, ATEC argues that a dual-spool architecture for the ITEP engine – an architecture used in most turboprop and jet fixed-wing aircraft – would provide better power, performance and fuelefficiency than would a single-spool engine, because it would offer better propulsive efficiency. ATEC also claims a dual-spool engine would be more reliable than a single-spool turboshaft and would offer lifecycle cost savings. However, when the ACC has all the ITEP test data in hand, it will be able to evaluate both companies’ claims for itself.

Engine weight is also a key factor in the US Army’s ITEP specification.

While it wants the ITEP turboshaft to be much more powerful than the T700 – a requirement which argues for a heavier, more robust engine – “the [US] Army wants to keep the weight as close to the T700 as possible,” said Sousa. “It allows the weight to go up a little bit,” but not by a great deal. He confirmed that GE’s T901 design is “a little bit heavier than the T700, but not significantly heavier – we’re well within the allowable limit specified by the US Army”.

The latest version of the T700, the T700-701D, weighs 456lb (207kg), measures 46 inches (1.17m) in length, and has a nominal diameter of 15.6 inches (396mm).