Jet Engines

Jet Engines Classified

Jet Engines belong to the category of so called Reaction Engines because they generate thrust by expelling massive amounts of matter behind generating thrust forward according with Newton's Law of Motion. There are different kinds: turbojets, turbofans and open rotors (propfans), turboprops and turboshafts, ramjets and scramjets, rockets and ion thrusters. All jet engines have one principle in common: they generate thrust by discharging high velocity jet of exhaust gas, exhaust gas-air mix or ions in the direction opposite to the direction of motion. Turboprops and turboshafts work by discharging high velocity jet of gas into turbine fan, which in turn spins the shaft.

Briefly about Jet Engines

Thrust-to-weight or torque-to-weight ratio is a major factor that defines how powerful the motor  is comparatively to its size and weight. Higher the thrust is, lighter the motor is, more relatively powerful it is. Rocket engines usually have better thrust-to-weight ratio than most of other types because they use aggressive chemical components which explode on the contact with each other, producing extremely high expansions and velocities; they also do not have altitude limitations since they do not rely on atmospheric oxygen. But the necessity to carry oxidant on board along with the fuel affects the time machinery can operate without refuel (even if it is capable to be refueled, since most rockets are made for single use). Engines that use atmosphere oxygen for oxidant are called air-breathing, or duct engines since they must "breath-in" large amounts of air through intake ducts to be able to burn fuel. They are famous for long ranges (time in the air) and good TBOs. Fuel burn is another important characteristic of any jet and defines fuel consumption rate. Fuel burn rate is usually displayed in pounds per hour. The reason distance cannot be used here as a parameter because consumption varies depending on altitude and speed: higher the altitude is, thinner is the atmosphere, easier it is to go through it; and faster the plane or rocket goes higher the relative air resistance to the motion is, higher is the fuel burn per mile flown. Altitude and speed are another two factors to consider. Faster you try to compress the air coming in your ducts, greater it’s going to heat up making it hard to manage: rams and scrams have to constantly improve on their design and quality of alloys used to counteract the impact overheated air produces. These engines are designed to operate at very high altitudes and speeds, and are forced to deal with very cold and thin air suddenly becoming very hot and dense once compressed. Ram and Scram jets engines are however capable of reaching substantial speeds, over Mach 15 (Mach 1 is the speed of sound equal 768 miles per hour at sea level) and are irreplaceable in transcontinental defense applications. Let’s look at different types of jet engines now.

Turbojets

A turbojet operates by compressing intake atmospheric air inside the compression stage and then igniting it by mixing that compressed air in the combustor with the fuel; next, overheated and excited exhaust gas is directed towards turbine stage. Compressors can be axial, centrifugal or static. Most of turbines today use axial fan compression because it’s proven and technologically simple. Centrifugal compressors are not that common because they redirect the airflow under sharp angles causing the additional heat up. Most of ram and scramjets are examples of static compression stage; in fact these engines could not use much of spinning parts in the compression stage because of the high level of stresses and temperatures taking place at such high speeds and altitudes. Recently, there have been also discovered ways to avoid spinning parts at lower air-speeds, thus making it possible to simplify compression phase for average jet.

Turbofans and Open Rotors (Propfans)

The essential difference between turbojets and turbofans is in the percent of consumed airflow they designate for combustion. Turbojet directs all of it towards compression and combustion, while turbofan has much larger size intake fan which is designed to produce more airflow than needed - part it is being used to generate propulsion thrust. Effectively, turbofans and open rotors operate as turbojets that utilize excess torque to produce additional propeller thrust. Turbofans and open rotors are effectively combined, jet and propeller, propulsion motors.

Open rotors, or propfans, are turbofans with exposed air intake fans or turbine fans - they are effectively evolved turbofans! The idea behind propfans is this: why use heavy and expensive metal parts to encase this huge propeller which does not have a single reason to be enclosed, since its big purpose to create thrust, while giving up small portion of the airflow to the compressor-combustor to produce torque? Brilliant idea! Open rotors have been receiving lots of attention lately because it’s been discovered that they are capable of accelerating aircrafts pass turboprop speeds, (around 300 nautical miles per hour) all the way to upper subsonic speeds. They are noisy, however recent developments in shapes of fan blading promise big improvements toward noise reduction. French manufacturers have been experimenting with mid-size open rotors on commercial commuters market.

Turboprops and Turboshafts

It appears that the line dividing turboprops and turbofans has been slowly deteriorating, but future will show better. You see, originally, the turboprop was the piston motor prop plane adopted to be powered by more powerful jet producing torque instead of thrust. It’s been discovered, that jets produce enormous amount of overlooked shaft power on top of conventional jet thrust. By employing that shaft and hooking it up to the geared down conventional propeller aviators invented a thrust producing jet motor. The innovation became known as the turboprop. The problem with these designs is that turbine thrust now became wasted making this model not as energy-fuel efficient as turbofan and open rotor models which exploit both, jet thrust as well as shaft torque to produce combined thrust forward.

Turboprops generally have a fix ratio gear spinning a propeller, while turboshafts are hooked up to either electric generator to produce electricity, of clutched gear box to transfer torque. Turboprops power planes, and turboshaft power helicopters and tanks. Turboshaft is rarely structurally attached to the powerplant or to the vehicle itself. The first military use of turboshafts is attributed to Germans who installed one on one of their Panther tanks in 1944. Today, most of serious use helicopters are powered by turboshafts.

Ramjets and Scramjets

Both, ramjets and scramjets capitalize on extremely high velocity of an airflow coming in at little cost to the aircraft's aerodynamics. The aerodynamic of the aircraft allows it to go through the atmosphere with insignificant resistance and intakes are designed not to slow the airflow down but rather gently direct it into thinner inlet compression channels. These engines require substantial airspeed to operate and have very "slippery" shapes. The major differences between ramjet and scramjet construction: the first one slows the airflow down to subsonic speeds to compress it and the second one performs compression at supersonic speeds.

 

There are lot of advantages and disadvantages of doing it either way and much of technological details are not easily disclosed or discovered but what is important for us to know that ramjets are limited to speeds around Mach 5 exactly because the airflow gets slowed down prior to combustion and then exhaust has to accelerate all over again to supersonic speeds in order to create supersonic thrust. Scramjets conduct their compression and combustion without slowing down the airflow substantially by taking the advantage of the compression generated by shockwaves at supersonic speeds; there are capable of speeds exceeding Mach 15. One of the resent defense propositions is to use scramjet powered vehicles as a transcontinental carrier since their relatively low altitude put them under the anti-ballistic missile radars.

Static, or Organic-Flow Compressors

This model is new and is not commercially tested but promises very long Time Between Overhauls due to the low number of moving parts in it. It employs centrifugal and so called organic compression, similar to ramjets, except the extremely high air-speeds are not required for the turbine to start producing sufficient air compression; the static compression jet starts operating at speeds slightly above 100 knots which is not a possibility for ramjet which require higher speeds to start producing substantial thrust. Static Compressors almost do not have any moving components in compression or turbine stages, which make them easy and cheap to manufacture. Instead of piling up multi-level compression fans, sophisticated air-paths are created sending the air into intense spin, thus producing strong centrifugal force which causes air to be compressed around the circumference of the drum. Combustion stage is also different; it uses Multi-Burn method where Multiple Annular Combustors are placed behind one another digesting each other's oxygen-rich exhaust and adding mass-volume to the existing thrust. North American start-up Intradept Propulsion has been working on a prototype to prove that "organic compression" is sufficient to produce thrust to support low-mid air speeds.

Rockets

The rocket engines do not require atmospheric air for combustion since they carry the oxidant on board. This has its ups and downs: the advantage is that oxidant, being much more aggressive than atmospheric oxygen, is capable of producing better pressures and velocities than any jet could; the downside is of course the mass and the volume that oxidant demands - it takes away from the fuel and other components. Rocket engines are also more dangerous and less reusable than conventional jet engines.

Ion Thrusters

An ion thruster is a form of economical rocket propulsion used for spacecrafts in order to reach higher velocities. It can be called electric propulsion except that real ionized matter is being exhausted at very high velocities using electric magnets. Ion thrusters have two major advantages over rocket engines. For once, they are capable of reaching much higher speeds because the velocity of a vehicle is proportional to the velocity of the exhaust and since the jet of ions could potentially reach speeds close to the speed of light, the ion propelled rocket also could reach that speed. Second advantage is the fuel economy: ion thrusters use very little gas, instead, propelling little quantities to very high speeds. Essentially they operate like electric motors by converting electric energy into kinetic. However, because ion thrusters use such little amounts of propellant to produce the jet of ions, their thrust-to-weight ratio is very low; they can operate only in gravity free, open space environment.

Hybrid and Combined Cycle Engines

Combined cycle engines, or hybrid engines, use two or more propulsion methods at once to move the vehicle. Usually it is done to make the vehicle efficient at all altitudes, since one type of propulsion could be good for lower and the other for higher altitudes. There are turborockets, air-augmented rockets, precooled jets and reaction engines. Turborocket is a turbojet where additional oxidizer is added to the airstream to keep the jet running above atmosphere. They tend to be heavier than regular rockets and their airspeed does not exceed that of turbojet. Air-augmented rocket is a ramjet and a rocket in one. Mach 5 is possible but they are famous for their cooling and noise problems. Air-augmented rocket is a relatively new and undeveloped concept. Precooled jets and reaction engines use heat exchanger to cool the compressed air sending it into the combustor. Propulsion thrust is generated from two effects: the chemical expansion when fuel reacts with oxygen and heat expansion when the air expands from heat. Cooler the air is, more it’s going to expand when combusted with fuel. One of the problems of conventional ram and scramjets is that the air is already overheated from compression limiting the expansion to chemical one mostly. When the air is cooled it contracts, so when it reaches combustors and ignites, it experiences much more profound expansion and generates much higher pressures-velocities. Most of the engines of those types exist at the prototyping and concept stages.

Summary

The evolution of the jet engines has been enjoying rich and full of events history but if we look closer we notice that the core model has not evolved much since first jet left the ground. The great downsides of any jet has always been and always will be the massive amounts of fuel burned to produce the power, the space that fuel demands, the weight that fuel steals from other components and the speed of the aircraft being always limited to the speed of the exhaust produced by the burning of that fuel. No matter how durable and heat resistant our alloys will become jet engines will never afford us to reach near-speed-of-light velocities, neither will they ever allow us to travel far enough to explore our universe better. In order for the humanity to break the berrier of distance and gain superiority in the open space we would have to solve the dilemma of gravity and produce major discovery in one of the Other Than Air-gas Propulsion technologies (OTAP).

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