The Roaring Jet Engines !

Who doesn’t love the sounds made by the roaring jet engine of a military air craft and hate the humming sound in the commercial counterpart? Having said so many things about the planes, let’s have a look at what is the source of power/energy for these machines that help them fly.

The answer is obviously the jet engine (as it is commonly called). The principle on which it works is similar to the 4stroke engine – which has 4 stages of fuel combustion.

The 4 stroke internal combustion engine uses – Intake, Compression, Ignition and power, and exhaust strokes to deliver power to the wheels. The turbojet/turbofan engines (as they are technically called) works along the same principle. The text book definition for a jet engine says that it is a reaction engine that discharges fast moving air which generates thrust by Jet Propulsion. A jet engine loosely refers to Turbofans, Turbojets, Rockets, Ram jets and Pulse jets.

Early air crafts used piston based engines to deliver power to the rotating blades. But as the speeds of air crafts increased, propeller blades approached the speeds of sound and the piston engines were not energy efficient at those speeds. A radical method for power delivery was needed, which led to the development of Jet Engines.

“Suck –Squeeze- Bang –Blow” is the typical flow of the fuel combustion. Atmospheric air is taken in and the speed is reduced (Intake) and is compressed in multiple stages (called Squeeze) and ignited with the fuel in the combustion chamber (called Bang) and then the hot exhaust gases are sent out (called Blow) that provide the thrust.

Turbo Jets

This is the simplest form of an air breathing engine. The intake consists of several huge blades that basically reduce the speed of air that enters the engine. The output of the intake section is fed to the compressor section, where a set of blades compress the air in ratios of 15:1 or more, which in turn increases the pressure and hence the temperature of the air. This highly compressed air is typically used in modern commercial air crafts to cool the turbines / air-conditioning / cabin pressurization or anti- icing for engine inlets.

The air that enters the combustion chamber is now lacking only the fuel to ignite. Once the fuel is sprayed, the mixture ignites to generate the energy needed to provide the thrust. The burning process is a bit different when compared to piston engines. Air temperature and pressure increases dramatically in a piston engine while air temperature increases while pressure decreases in case of jet engines. Only a small portion of the compressed air is actually used for combustion with the fuel. The rest of the air is used to absorb the tremendous amounts of heat generated in the combustion chamber. The engine section is so designed that there is a layer of unburned air between the engine core and the outer walls. This unburned air when mixed with the burning air fuel mixture brings down the temperature of the mixture to levels that the turbine can handle.

The air-fuel mixture is kept to a minimum to ensure that all the fuel is burnt completely providing the maximum amount of thrust as the flame of burning fuel exits the rear of the engine producing opposite force in the forward direction.

Another method that is generally used to provide additional thrust to the aircraft is to use after-burners where fuel is sprayed directly onto the hot exhaust gases to boost the thrust significantly. The only disadvantage is that the efficiency of these after-burners is too low and an F15 fighter aircraft with full fuel load can empty itself in 10 minutes of after-burner flight!!!

Turbo Fans

Turbofans are almost similar to the turbojets. Turbofans have a turbojet core and a big fan preceding the intake stage of the turbojet. There are two variants of the turbofans. Before they are discussed, Bypass Ratio has to be understood. The ratio of the amount of air that bypasses the engine core to the amount of air that passes through the core is known as the Bypass Ratio.

Depending on the ratio, turbo fans are classified into Low bypass turbo fans and high bypass turbofans.

Low Bypass Turbofans

A Low bypass turbofan typically has a multistage compressor instead of a single compressor as seen in turbojets. Since the core is huge and heavier, the airflow into the core needs to be larger and the engine needs to produce more thrust to drive the fans at the front to operate them at their designed limits of air flow and pressure ratios. Evidently, these type of turbofans produce more thrust as the core occupies a larger volume of the engine and is mainly used in modern military jets as speed and agility are advantageous than efficiency of operation.

After burner stages exist for the low bypass turbofans which facilitate the fighter jets to fly at supersonic speeds. Since fighter jets operate at all speeds (if not all, at least supersonic and subsonic flights), an afterburner duct followed by a variable geometry exit nozzle is used. When the after burners are not used, the nozzle is at its lowest opening diameter and the exhaust gases flow out to produce enough thrust. When fuel is sprayed to the exhaust gases in the afterburner duct, the temperature of the gases increases by a large degree and the exit nozzle must be at its widest to allow large volumes of gases to flow out which have higher exhaust velocity. This can be seen in the picture of a Sukhoi Su-35 shown above with a small exhaust opening for normal operations and wider opening to the right for after burner operations.

High Bypass Turbofans

The other variant of the turbo fans are the high bypass turbofans. They evolved from the low bypass turbo fans as commercial airliners wanted efficiency more than thrust.

As the name implies, the core is rather smaller comparatively, and produces lesser thrust. There is only one large fan at the input rather than a multistage fan as seen in low bypass engines.

The amount of noise produced by the engine mainly depends upon the difference between the exit velocity of the hot gases and that of the air surrounding it. Since high bypass turbofans mix the large amounts of low velocity bypassed air into the hot air stream at high velocities, the engine noise is considerably reduced when compared to pure turbojet engines. Needless to say, high bypass engines make your journey quieter than before.

Another interesting fact to be noted is the flight of heavy transport air crafts. The concept of thrust lapse rate comes into play. As altitude increases, the density of air decreases and the thrust produced by any engine depends upon the mass of air that enters into it. Hence high bypass ratio engines need to be bigger to produce enough thrust when climbing or in cruise at high altitudes. This means that because of the high thrust lapse rates, engines are bigger and the static thrust these engines produce are very high providing huge acceleration. This lone fact is responsible for the short take-offs performed by wide bodied heavy lifters!!

Turboprop

This is another form of air breathing engine where the core is a turbojet. However, the operation is a bit different. A propeller fan precedes the turbojet core. The operation of the turbojet remains the same, where compressor feeds the combustor that produces hot gases which drive the turbine. Some amount of power generated by the turbines is used to power the compressor section. The rest of the power is fed to the propeller blades through reduction gears. A propelling nozzle that exists at the rear of the engine expands the gases which are let out to the atmosphere. The nozzle relatively provides a small portion of the actual thrust generated by the engine. The propeller blades are huge and they produce most of the required thrust for the flight. Since propeller engines are costlier, these engines are mainly used for Short take off and landing (STOL) air crafts. Shown below is a comparison of all the three types of engines.

The concept of Ramjets, which is another type of engine, has been explained in the earlier post related to SR 71. Rockets on the other hand are simpler. It is not an air breathing engine. It does not take any atmospheric air to burn fuel. Air and fuel are both carried in tanks for the operation. Both the fuel and the oxidizer are mixed and burnt in a combustion chamber and the exhaust gases are propelled through a nozzle to produce tremendous amounts of thrust.

Scramjet

A Scramjet (Supersonic Combustion Ramjet) is an extension of the Ramjet. They both use the convergent input stage to “ram” the air into the engine for combustion and hence the name. Ramjet decelerates the air to subsonic speeds for its operation as the compressors do not work in the supersonic region. Scramjets on the other hand operate totally in the supersonic region. Though the speed of the air is reduced at the compressor stages, it would still be at supersonic velocities.

The Scramjets do not have any rotating mechanical components as seen in turbojets or turbofans. The engine has three stages: A converging inlet, where air is decelerated to lower supersonic speeds. A Combustor unit, which mixes air with the fuel to produce hot expanding gases and the last stage, is a convergent-divergent nozzle for the hot gases to flow out providing the huge thrust. This engine uses the energy in the air at transonic and supersonic speeds to compress and use for combustion. As they do not have any mechanical moving parts that help in compression, these engines cannot do a standing start and do not achieve any compression until supersonic flight. This means that there is a need for alternate methods of propulsion until the vehicle using a scramjet reaches supersonic speeds. Typically, rockets are used. Another way is to carry the vehicle under a heavy lift aircraft. Shown here is an experimental aircraft of NASA using Scramjet engine.

Pulse Jet engines and Pulse Detonation engines are some other types of jet engines where combustion occurs in pulses. They have been around in the design stages from a long time, but no major developments have been seen though.

The Legendary SR-71, Blackbird

For 24 years, from 1966 through the 1980s, US leaders from field commanders to the President of the United States relied on data gathered by SR 71 Blackbird reconnaissance aircraft. Flying missions around the globe at speeds above Mach 3 and altitudes of 85,000 feet (26,000 m) or more, Blackbirds were a vital tool of strategic military decisions as their advanced photographic and electronic sensor systems collected intelligence for the Air Force and other federal agencies.

The whole program of the SR 71 started as soon as the Lockheed U2 started to become vulnerable for the Soviet missiles. The U2 was more of a glider based design which flew at normal operating speeds and at heights up to 70,000 feet which were easy for enemy aircrafts to attack. This was an easy target for missiles too as the speeds of U2 was less too. Hence,a new aircraft was needed, that would fly faster and higher and would outrun every surface-to-air missile. This led to the birth of SR-71 Program. The SR-71 is a delta wing, twin engined, and high altitude reconnaissance aircraft with a maximum operating ceiling of 85000 feet at 3.2 Mach!

The aircraft remains a technological marvel. Practically every area of design required new approaches or breakthroughs in technology. To withstand high temperatures generated by friction in the upper atmosphere during sustained Mach 3 flight, the Blackbird required an array of specially developed materials including high temperature fuel, sealants, lubricants, wiring and other components. Ninety-three percent of the Blackbird's airframe consists of titanium alloy that allows the aircraft to operate in a regime where temperatures range from 230 degrees Celsius at its midsection to 510 degrees Celsius near the engine exhaust. The cockpit canopy, made of special heat resistant glass, must withstand surface temperatures as high as 340 degrees Celsius.

A new engine design was needed too. Turbo jet engines are in-efficient at very high speeds and Ramjet engines are not efficient at low speeds. So, both of these technologies were combined to allow for a "Variable Cycle" engine that works in all the three speed ranges. Two Pratt & Whitney J58 turbojet engines with afterburners, each supplying more than 35,000 pounds of thrust, are housed in wing with diameters larger than the fuselage itself. Virtually every part of these complex power plants had to be fabricated from special materials to meet the demands of triple-sonic flight.

A moveable spike in each inlet controls airflow, retracting at speeds above Mach 1.6 to capture more air for the engines. At sub-sonic speeds and below 30,000 feet, the spikes would be in locked position and the air needed for the turbines are fed through the bleed openings. Above the speeds of 1.6 Mach, the spikes open up and are in forward position and open up 1-5/8 inches per 0.1 Mach number. The air which is let in will be compressed by the compressors and then they split two ways. One stream is fed to the core, which is the turbine; the other stream is bypassed for the after burners. The excess air is dropped out of the system through the bleed outlets. As the speeds increase and it reaches speeds nearing 3.0 Mach, the acceleration of the aircraft itself would heat up the air too much and then compression of the air and another round of compression within the turbine would increase the air temperature tremendously. To compensate the high temperature of the air flowing through the turbines, the air: fuel mixture that is fed to the turbine is reduced to prevent the turbine blades from melting. Thus the SR-71's turbojet components produce far less thrust and the Blackbird flies most of the times with more than 80% of the thrust generated by the bypassed stream of air that is ignited in the afterburner stage and generating huge thrust as it expands through the nozzle.

This meant that the jet needed a new fuel too. The fuel not only was needed to act as a source of energy for the turbines, but also as a coolant/heat sink for the engine and aircraft accessories, air conditioning systems and TEB (Tri Ethyl Borane) tanks and control lines that actuated the afterburners. Engine oil is cooled by the main engine fuel through a heat exchanger and cooling pipes. The fuel was also used in most of the engine hydraulics too. This meant that the fuel needed to possess properties such as high thermal stability and should not break down at high temperatures and form coke deposits or damage the fuel tanks.

This criterion led to the development of JP-7(Jet Propellant 7) grade fuel. It had a huge thermal stability and a high flash point. The fuel was so stable at normal temperatures that, if you drop a lit match into a can of JP-7 fuel, the match would go off; but would not ignite the fuel. This led to another problem. The engines do not work without the fuel. This new fuel does not ignite at normal temperatures. Adding a servo motor to assist in air compression to ignite the mixture also would not solve the problem. Thus a new compound, Tri Ethyl Borane (TEB) was used. This chemical ignites at the first contact with air and it proved to be a good starter for the SR-71's advanced engines. Hence, every aircraft has a small tank of TEB containing up to 600cc to start the engines. This is the only limiting factor for the aircraft for long range operations because the SR71 has to turn off the after burners to refuel and the small size of the TEB tank limited its range as TEB was used for igniting the after burners too would last for 16 ignitions/re-ignitions.

Overall, the entire design of the plane was too far ahead in its technological achievement at that time and even new factories had to be built to manufacture them since normal tools could not be used with titanium based structures. Every aircraft that was built till this date was hand made and each aircraft had a unique response of its own. All this meant that the SR 71 which would be spying on an enemy had to just push its throttles full forward as soon as it sees a SAM alert.

Though the Blackbird was decommissioned from service in the 1990's, researchers are looking into using the SR-71 design along with aero spike engines to use this as an Reusable Launch Vehicle for space programs.

All these things remind me about the quote at the entrance to the SR-71 Hangar at Kadena Air Base in Japan.

"Though I fly through the valley of death, I shall fear no evil for I am at 80,000 feet and climbing"