Turbojet Development at The RAE - Early Designs

Early Designs

In 1936 ARC, now under the direction of Henry Tizard, returned to the turbine engine concept after learning that Whittle was going ahead with his designs at his new company, Power Jets. Tizard convinced Hayne Constant to return to the RAE from Imperial College to assist with the development of Griffith's designs. They set about building a 6-inch-diameter (150 mm) version of the inner portion of the Griffith engine, known as Anne, consisting of the hub and eight compressor stages without the outer turbine portions. On its first run a faulty seal allowed the oil to drain from the engine, and the blading was stripped off after only 30 seconds of running. In 1937, while Anne was being built, Griffith visited Jacob Ackeret of Brown Boveri, another turbine pioneer, and became convinced that the compressor/stator design was superior to his own contra-rotating "all compressor" concept. After it was damaged, Anne was rebuilt using the new layout and started running again in October 1939. It continued to be used in tests until it was destroyed in a German bombing raid by KG 54 on 13 August 1940, "Eagle Day".

At this point there was some debate as to how to proceed after Anne. The team, which included Griffith, Constant, Taffy Howell and D. Carter, studied a number of approaches to building a complete engine, as opposed to the compressor-only Anne. They decided that the only reasonable solution to low compressor efficiency was to use what would today be referred to as a "two-spool" design, with separate high and low-pressure compressors. However the team considered the concentric shafts needed for this layout to be too complex (although the reasons for this are not clear), and there was some consideration of using two completely separate compressor/turbine sections "side-by-side". Eventually they settled on building one of the two engines that would be used in such a layout, in order to study the mechanical problems.

The resulting Betty design consisted of a nine-stage compressor 1 ½ feet in diameter attached through a coupling to a four-stage turbine. A considerable amount of design effort went into various devices to relieve mechanical stress due to thermal expansion. For instance, the compressor and turbine blading was attached to large hollow rotors which they felt would expand and contract more like the outer engine casing than a series of solid disks as used in Anne. The ends of the turbine rotor were closed with double-cones, which had enough flexibility to expand with the rotor while still remaining solidly attached to the power shaft. The compressor and turbine were attached to each other through another rotor, allowing the two sections to be easily separated. The compressed air output was piped to an inlet cage on the turbine section. Finally the turbine was water-cooled, as it was believed that even the latest high-temperature alloys like Hadfield's ERA/ATV would eventually deform under constant operation.

Betty, also known as B.10, was first tested as separate compressor and turbine sections using steam to power them. In October 1940 they were run as a single complete engine for the first time. During testing it was decided that the water cooling was not needed, and was replaced by an air cooling system, and the turbine was allowed to run red hot at 675 C. Experiments with Betty convinced the team that any sort of piping between sections led to unacceptable losses, so the "distributed engine" concept Betty was built to test would likely be inefficient. At the same time it was decided that overall pressure ratios on the order of 5:1 would be sufficient for near-term engines, so it was decided to abandon the two-spool approach for the time being.