THE EVOLUTION OF THE TURBOJET ENGINE: 1945-1956

The first turbojet engines to achieve truly high performance (even by today’s standards) emerged at the end of the 1940s and in the early 1950s. These engines required several advances in technology, including better alloys, especially for blading, and higher turbine inlet temperatures. The most important advance, however, was to raise the overall compressor pressure-ratio from around 5 to 1 – as in General Electric’s J-47, the engine with by far the most flight hours as of 1952 – to more than 10 to 1. Because the average pressure-ratio per stage in axial compressors was then limited to around 1.15, this meant many more stages. It also meant a much smaller annulus area for the flow in the rear stages than in the forward stages. This reduction in annulus area required that a new, fundamental problem in compressor design had to be solved. When the rotational speed of the compressor was low, the front stages would not compress the flow enough to pass through the smaller annuli of the rear stages, causing these stages to stall and the compressor to go into a violent instability called surge. Consequently, some special provision was needed to enable the engine just to sustain operation at off-design conditions. A second factor exacerbated this problem. As the flow acquires a tangential com­ponent of velocity in a stage, a centrifugal force arises in it. A radial pressure gradient balances this force, resulting in radial equilibrium. Unless this pressure gradient is accounted for carefully in design, the flow can become so radially “distorted” by the time it reaches the rear stages that they are forced to operate far off the incidence angles for which they were intended and hence with high thermodynamic losses.

Accordingly, in order to design high pressure-ratio, multistage compressors, the engine companies had to find a solution to the problem of matching the rear stages with the front stages at off-design as well as at design operating conditions. The three engine companies that emerged as dominant in the U. S. and Britain by the early 1960s, Pratt & Whitney Aircraft, General Electric, and Rolls-Royce – solved this problem in three different ways.18

Pratt & Whitney – The Two-Spool Engine Pratt & Whitney, in spite of decades of experience with reciprocating aircraft engines, entered the turbojet business well behind GE and Rolls-Royce. During the late 1940s P&W invested heavily in jet engine technology, including extensive in­house tests of the performance of compressor airfoil profiles in cascade at off-design incidence angles. P&W received a study contract to design a high-thrust engine for a strategic bomber in 1947.19 They decided that the best way to achieve the requisite compressor pressure-ratio was, in effect, to divide the compressor into two separate compressors, powered by two separate turbines, turning at different speeds. This arrangement (displayed in Figure 1) is called a two-spool engine, with the front compressor serving as a low-pressure compressor and the rear one, as a high – pressure compressor. At off-design conditions the low-pressure spool rotates at much lower speed than the high-pressure spool; as a consequence the low-pressure compressor passes less flow into the high-pressure compressor at these conditions. Individually, each of the two compressors requires a comparatively modest number of stages, so that the cumulative effects of radial equilibrium on the back stages of each spool are not that severe.

The two-spool engine P&W designed under its 1947 study contract became the J – 57, powering the B-52 bomber, among other aircraft. It was a remarkable engine by any standards, all the more so considering that it was designed between 1947 and 1949, essentially using slide rule methods. The initial version was a 10,000-pound – thrust engine for subsonic flight; with afterburner added, it produced 15,000 pounds of thrust for low supersonic flight. It had an overall compressor pressure-ratio of 12.5 to 1, achieved in a 9-stage low-pressure compressor and a 7-stage high – pressure compressor (for an average pressure-ratio of 1.17 per stage). The J-57 went into service in 1953. Using basically the same design approach, P&W designed a somewhat larger two-spool engine in the early 1950s, the J-75, for Mach 2 flight.20