2022-07 #7 A Little History

To be able to make better sense of future chapters I am going to pause and interject some jet history that I only learned much later in my career. In 1982 I was still much a newbie on jet engines but was learning fast.

Original jet engines used a centrifugal compressor that was derived from turbo superchargers which needed to be very compact. However the fundamental rule of gas physics is how much air you can move through a pipe. The smallest diameter and area in a jet engine is between the compressor and the combustor and how much air you can push through that throat determines how powerful you can make the engine. The compressor, combustor and high pressure turbine make up what is called the gas generator or "core" engine. Efficiency and power come from compressing a lot of air tighter (compression ratio), lighting it and burning it efficiently and then extracting some of the heat energy through the turbine to drive the compressor.

The magic of a jet engine is not obvious unless one asks this question, "When you light off a bunch of jet fuel in a chamber, why doesn't it expand forward as well as backwards?" The answer is in the vane and blade designs. It's a lot more complicated than this but basically the area at the end of the compressor is smaller than the area at the end of the combustor and so the burning fuel takes the path of least resistance. Sometimes the airflow gets disturbed and the compressor will actually stall out, sometimes a fireball would come out the front. Compressor stalls are bad news.

The problem with the centrifugal compressor was that you could not get high pressure ratios and to induct more air you basically have to keep increasing the diameter of the impeller. Airplanes hate frontal area which means drag so the industry quickly went to an axial flow compressor. There are, of course, many successful centrifugal flow engines, primarily in the helicopter world where speeds are low and frontal area isn't as big a concern.

The initial jets were pure jets which is basically a gas generator accelerating a relatively small amount of air as quickly as possible. This gave the low frontal area needed and with after burner gave the thrust necessary to eventually break the sound barrier and beyond. But after-burning jets are terrible on fuel mileage so the idea came to add a second shaft, a second turbine and drive a small fan at the front that would bypass air around the engine. This is known as a low bypass engine. It gives acceptable fuel range at low speeds and can be put into afterburner for combat.

This is the configuration that is the standard today. You can see the original gas generator, it now has an axial compressor and a "fan turbine" (which is more commonly known as a Low Pressure Turbine, that has a concentric shaft going forward to drive a fan and low pressure compressor. How much air bypassing the engine is fixed and is known as the bypass ratio. Fighter jet engines are often in the 2:1 range.

Low bypass engines started in the 6,000 pounds of thrust area, quickly grew to 12-15,000 pounds and the latest low bypass engines are in the 25-30k range. Parallel to this was evolution to use turbojet engines on large transport aircraft. The epitome of piston technology is probably the Spruce Goose. It only weighed 300,000 pounds and was nearly as big as a B747 which amazing for it's time but it had 8 of the most complicated, and unreliable, piston engines in existence in 1947. Even WWII produced only the B29 which weighed in at 74,000 pounds.

Throughout the 50's and 60's there is a lot of large aircraft development. Primarily using pure jet and low bypass engines which resulted in a lot of weird airplanes. The most successful is probably the B52, first flown in 1955 and still flying today, it uses 8 Pratt and Whitney JT3 (later J57) engines in pods pods of 2. The B52 still only weighs about 180,000 pounds.

Pratt & Whitney, of course, was famous for its radial engines from WWII and quickly became the jet engine supplier of choice to the military, even though GE probably had more innovations than P&W ever did. GE was on some iconic helicopters, and of course the F86 but the High Bypass Fan changed the course of the game and especially commercial aviation.

GE decided that if a low bypass engine was good, why not use the gas generator to drive a huge fan and move lots of air at a slower speed, subsonic speeds. Thus was born the TF39, the first high bypass engine. The military chose the TF39 for the C5 Galaxy which first flew in 1969 and is still in service today. The TF39 had a 26:1 pressure ratio (high compression axial compressor), 8:1 bypass ratio (most air goes around the core engine) and produced 41,000 pounds of thrust.

The C5 has a gross weight of about 850,000 pounds which includes about 330,000 pounds of fuel and 240,000 pounds of cargo and can carry all that about 7,200 miles.

I forget the exact details but the core engine consisted of an axial compressor of probably 16 stages, each stage compressing the air more and more. There is an annular combustor replacing the can type combustors on early engines and a 2 stage high pressure turbine. The air is then expanded into a 6-stage low pressure turbine that drives the 1 1/2 stage fan. The short forward fan is also a low pressure compressor, delivering air into the high pressure compressor. The outer fan, which is behind the outer vanes drives a lot of air at a relatively low speed. Tip speeds are important as supersonic tip speeds are inefficient and aerodynamically unstable. Because there is no physical connection between the high and low pressure systems the core engine can run in the 16,000 rpm range while the fan and low pressure system is operating around 2,500 rpm.

With this evolution the stage is set for the next chapter in commercial aviation - The B747 and the great engine wars...