CHAPTER 1
Introduction: Propulsion in Sustainable Aviation

NASA-Boeing Transonic Truss-Braced Wing X-66A, Sustainable Flight Demonstrator (https://www.nasa.gov/image-article/nasa-boeing-provide-a-peak-at-what-the-x-66a-will-look-like/)

(Image Credit: Boeing)

1.1 History of the Airbreathing Jet Engine, a Twentieth-Century Invention-The Beginning

Powered flight is a twentieth-century invention. The era of powered flight began on December 17, 1903, with the Wright brothers who designed, fabricated, and flew "The Flyer" in Kitty Hawk, North Carolina. The power onboard The Flyer was a gas powered, 12-hp reciprocating intermittent combustion engine. This type of engine, with a propeller, provided power to all (manned) aircraft until the late 1930s. The history of aircraft gas turbine engines started in January 1930 with a patent issued to Frank Whittle in Great Britain. Figure 1.1 shows a p-v diagram and components of the Whittle engine as they appeared in the patent application. The flow pattern and engine assembly are shown in Figure 1.2. The performance of the W1 engine and the aircraft that flew it are shown in Figure 1.3. An engineer at work, Sir Frank Whittle, the inventor of the jet engine, with a slide rule is shown in Figure 1.4. For more details on the Whittle turbojet, see Meher-Homji (1).

The gas turbine engine in Figure 1.1 is based on the Brayton cycle. The compression in the Whittle engine is achieved via a double-sided centrifugal compressor. The axial compressor had not been developed due to aerodynamic stability complications. The combustion takes place in a reverse-flow burner that is very large relative to other engine components. The straight throughflow burner had posed problems with stable combustion and thus a reverse-flow combustor provided the needed flame stability in the burner. The compressor shaft power is delivered from a single-stage axial-flow turbine.

FIGURE 1.1 Patent drawings of Sir Frank Whittle jet engine

FIGURE 1.2 The assembly and flow pattern in Whittle jet engine

FIGURE 1.3 Performance testing of Whittle jet engine, known as W1, and the experimental aircraft, Gloster E28/39, that flew it in 1941.

(Source: Crown Publications)

FIGURE 1.4 Sir Frank Whittle with a slide rule.

(Source: Crown Publications)

FIGURE 1.5 The first historic meeting between the two inventors of the jet engine took place in WPAFB on May 3, 1978.

(Source: AFRL/AFMC)

In an independent effort, Hans-Joachim Pabst von Ohain invented a turbojet engine in Germany that was granted a patent in 1936. In 1937, von Ohain's engine, designated as the He S-1 turbojet engine with hydrogen fuel, was tested and produced a thrust of 250?lb at 10,000 rpm. Von Ohain's engine was the first to be developed ahead of the Whittle engine and flew on the first jet-powered aircraft, Heinkel 178, on August 27, 1939. Both Whittle and von Ohain are credited as the coinventors of airbreathing gas turbine engines. Figure 1.5 shows the two inventors of the jet engine, a historic meeting on May 3, 1978. A recent book by Eckardt (2) gives a fascinating account of early jet development stemming from research in the United Kingdom (UK), Germany, and Switzerland. This book is highly recommended for further reading.

The first production jet aircraft was the Messerschmitt Me 262, shown in Figure 1.6. Two Jumo 004B turbojet engines powered the Messerschmitt Me 262 jet fighter. The Me 262's first flight was on July 18, 1942. Dr. Anselm Franz of the Junkers Engine Company designed the Jumo 004, which was based on von Ohain's patent. The Jumo 004B engine cutaway is shown in Figure 1.7. This engine has many modern gas turbine features such as axial-flow compressor and a straight throughflow combustor with air cooling of the turbine and the nozzle. For more details, see Meher-Homji (3).

The drawing of the Jumo 004B turbojet engine in Figure 1.7 shows an air-cooling system that bleeds air from the compressor and cools the turbine and the exhaust nozzle. The engine produces ~2000?lbf of thrust at an airflow of 46.6?lbm/s. The engine pressure ratio is 3.14, turbine inlet temperature is 1427?°F, and the specific fuel consumption is 1.4?lbm/h/lbf-thrust. The engine dry weight is ~1650?lbf, and its diameter and length are ~30 and 152?in., respectively. Engine component efficiencies are reported to be 78% compressor, 95% combustor, and 79.5% turbine. We will put these numbers in perspective when we compare them with their modern counterparts.

FIGURE 1.6 The first production jet aircraft, Me 262

FIGURE 1.7 Jumo 004B engine cutaway features an axial-flow compressor, a straight through flow combustor, an air-cooled axial turbine, and an exhaust nozzle

FIGURE 1.8 The first US-produced aircraft gas turbine engine.

(Source: Courtesy of US Air Force Museum)

The jet engine came from Great Britain to the United States in 1941. The J-31 (also known by its company designation, I-16) was the first turbojet engine produced in quantity in the United States. It was developed from the General Electric I-A, which was a copy of the highly secret British "Whittle" engine. Figure 1.8 shows the J-31 gas turbine engine (Courtesy of Air Force Museum).

1.2 Innovations in Aircraft Gas Turbine Engines

In this section, we introduce the most significant innovations in the gas turbine industry since the introduction of the aircraft jet engine by Whittle and von Ohain. Dawson (4) and Wallace (5) as well as the NASA websites (6, 7) and NASA publication (8) should be consulted for further information.

1.2.1 Multi-spool Configuration

To achieve a high-pressure ratio gas turbine engine, two distinct and complementary approaches were invented in the United States. One is the multi-spool concept (developed by Pratt & Whitney) and the second is the variable stator (developed by GE). The multi-spool concept combines several compressor stages together in two or three groups, known as the low-pressure compressor (LPC), intermediate-pressure compressor (IPC), and high-pressure compressor (HPC). A different shaft that spins at different rotational speeds drives each group. Figure 1.9 shows the Trent 1000, a modern Rolls-Royce engine that employs three spools.

1.2.2 Variable Stator

The need to adjust the flow direction in a multistage high-pressure ratio compressor (in starting and off-design) prompted Gerhard Neumann of GE to invent the variable stator. By allowing the stators to rotate in pitch, compressors can operate at higher pressure ratios and away from stall. Modern gas turbine engines use variable stators in their LPC and IPC. The high-temperature environment of HPC has not been hospitable to variable stators.

1.2.3 Transonic Compressor

Better understanding of supersonic flow and the development of high strength-to-weight ratio titanium alloy allowed the development of supersonic tip fan blades. The transonic fan is borne at a high shaft speed that creates a relative supersonic flow at the tip and a subsonic flow at the hub. A modern transonic fan stage produces an average stage pressure ratio of ~1.6. The Jumo 004B produced a cycle pressure ratio of 3.14 with eight stages, which means an average stage pressure ratio of ~1.15. Therefore, to achieve a pressure ratio of 3.14, we need only two transonic fan stages instead of eight. The higher compression per stage has allowed a reduction in engine weight, size, and part count and has improved reliability. The advances in computational fluid dynamics (CFD) and nonintrusive testing techniques have paved the way for a better understanding of supersonic flow in compressors. A compressor flow simulation is shown in Figure 1.10(a). The rotor passage shock, boundary layer interaction, and subsequent flow separation are clearly visualized in Figure 1.10(a). An advanced transonic fan is shown in Figure 1.10(b) from Rolls-Royce.

FIGURE 1.9 Three-spool gas turbine engine as developed by Rolls-Royce.

(Source: The Jet Engine 2005. Reproduced by permission from The Jet Engine, Copyright Rolls-Royce plc 2005)

FIGURE 1.10 (a) CFD in transonic compressor rotor flowfield.

(Source: Courtesy of NASA);

(b) advanced transonic fan.

(Source: Reproduced with permission from Rolls-Royce plc)

1.2.4 Low-Emission Combustor

The gas turbine combustor has perhaps seen the most dramatic innovations/changes since the Whittle reverse-flow burner. A better understanding of the combustion process, from atomization and vaporization of the fuel to...