October 12, 2018, ©. Leeham News: In the last Corner we discussed the challenges of the nacelle outlet for an SST (SuperSonic Transport). Now we will discuss SST engines and what are the key technical challenges for these engines.
We start this week by looking at some design constraints for the engine which we don’t have in Subsonic airliner engines.
Some Corners ago we discussed the challenge of Ram drag for an SST engine. It will limit the Bypass ratio we can have for a fix cycle engine (engines which don’t have a variable Bypass ratio for take-off and cruise, called variable cycle engines). We will come back to the Ram drag and Bypass ratios when we discuss noise in a later Corner. Now we look at some other challenges in an SST engine.
For Subsonic airliner engines, the key hot area is the first turbine stage after the combustor. In Figure 1 it’s the T41 area, the Turbine Entry Temperature (TET) point. The higher this temperature is, the more power the core produces per kg air which passes the core. And the more powerful the core, the larger fan it can drive over the turbines.
The ideal is a small core which drives a large fan to give a high bypass ratio, which gives a high propulsive efficiency. At a speed of below M0.85, the resulting Ram drag is not a problem. This changes drastically for SST engines as we will see in later Corners. Now we focus what happens in the engine’s hot spots.
The limit of T41 for a small and efficient core is set by available materials but also how we can use cooling air in and around the turbine blades and stators. Suitable cooling air with a high enough pressure to feed the turbine section is available from the last stages of the compressor.
An SST engine can use the available turbine technology developed for the Subsonic airliner engines, the turbine section is therefore known technology and presents no special problems.
For airliner engines flying at M0.85 or less, the end temperature of the compressor (T3 in Figure 1) is not a problem. This is good because there is no cooling air available (the last stage of the compressor, which is the hottest, has also the highest pressure level in the engine, so no other air can cool it). The available materials (nickel-based alloys) can handle the temperatures of the last compressor stages without cooling.
For an SST engine, the maximum allowed T3 temperature is a problem. The faster we fly the higher the temperature at the entry of the engine and the higher the T3 will be with a certain level of compression. At high Mach, the speed decides the maximum compression ratio the engine can have.
The maximum temperature for the final stages of a compressor is around 700°C. With the air entering the engine at 115°C after our multi-chock inlet at Mach 2.2 ( compared with -25°C for a subsonic M0.85 airliner), we can have a total fan+compressor Overall Pressure Ratio (OPR) of 23 (compared with over 100 for an airliner engine).
With the fan section delivering around PR 3.5 (it must be at this level to have the bypass air match the core’s exit pressure in the mixing zone at Station 64 in Figure 1) we search for a core with a high-pressure compression of less than 7. Such airliner engines were designed in the 1970ies.
Today we have high-pressure compressors at more than PR 15, yesterday at 10 (the CFM56 for example). Cores with a high-pressure compressor at less than 7 is only found in military engines which are designed for flying at Mach 2 or above. The conclusion is existing cores of the right size suitable for use in a Mach 2.2 SST are very hard to find.
If we instead fly at Mach 1.4 we can have a pressure ratio of the high-pressure compressor of 10. The CFM56 core can then be used, which is what GE did for the Aerion SST engine. A suitable low-pressure system was designed for this core with a fan section pressure ratio of about 2.8.
We have now looked at the critical areas Ram drag, compressor exit temperature and allowed pressure ratios for high-speed SSTs and what this means for compressor designs and suitable existing cores.
Next week we will look at the noise problem which is intimately coupled to the Bypass ratio and therefore Ram drag.