Jet Propulsion/Jet engine types
Ramjet[edit | edit source]
A ramjet uses the open Brayton cycle. No rotating machinery is used and compression is achieved by the intake and diffuser. As such they require speed to compress air enough that good efficiency can be achieved. Ramjets are inefficient at subsonic speeds and their efficiency improves at supersonic speeds.
Fuel is injected into the compressed air and burnt using flameholders to stabilize the turbulent flame as in afterburners.
At hypersonic speeds the compression and dissociation processes make full diffusion unattractive and supersonic combustion is being researched. A Scramjet slows the air down to low supersonic speeds and then burn high flame velocity fuels such as hydrogen or methane to try to get net thrust.
Scramjet[edit | edit source]
As the velocity increases the total temperature of the gas stream rises above the dissociation temperature of the combustion products. This prevents efficient burning if the gas stream is diffused to subsonic speeds. To solve this, fuels with high propagation velocities such as hydrogen are used while diffusing the intake air to supersonic speeds without having a large rise in temperature of the gas stream. The challenges become one of obtaining stable flame fronts and a net thrust.
Turbojet[edit | edit source]
A turbojet adds a rotating compressor powered by a turbine. This allows increased compression beyond the stagnation pressure of the intake and improves the efficiency over a ramjet at lower speeds. The hot air after it leaves the turbine is accelerated by the nozzle and ejected. An afterburner can be used to augment the thrust.
Turbofan[edit | edit source]
A betlog shrouded fan allows a larger mass of air to be moved by a shrouded fan whose flow bypasses the core. The relative size of the fan compared to the core is identified by the bypass ratio.
The figure below shows the typical layout of a two shaft high bypass turbofan.
The ratio of turbofan bypass mass flow to the engine core mass flow in a turbofan engine.
As the name suggests, this is the ratio of the air which bypasses the engine core and flows round the outside of the engine and exits via the nozzle. In a modern, high bypass ratio engine, bypass ratios can be as high as 85%. Increasing the size of the fan and the bypass ratio causes a weight penalty. Unducted fans or Propfans have reduced weight penalty but the noise has not been acceptable in the west.
Geared Turbofan[edit | edit source]
Since the fan is much larger diameter than the turbine it must operate at much lower rpm. Traditionally this is achieved by multiple turbine stages. However this makes the turbine system unnecessarily complex and so gearboxes have been attempted to reduce the number of turbine stages required.. The power requirements have so far evaded application in larger size turbofans, but companies are still trying.
Example: Honeywell_ALF_502 used in the BAe 146.
Turboprop[edit | edit source]
Turboshaft with a gearbox and a propeller.
Turboshaft[edit | edit source]
Intake, Compressor, Combustor, and Turbine powering a shaft. Used in helicopters, APUs, as well as surface applications like tanks, ships, electricity generation.
Pulse jet engine[edit | edit source]
Uses pulse detonation to close off the intake without primary compression. Intake closure may be dynamic or with mechanical valves such as reed valves.
TYPES OF PROPULSION SYSTEMS (BASIC EXPLANATION LISTING)
I am an FAA Licensed Jet Engine, Piston Engine & Airframe Mechanic plus Skilled with Applied Physics and System Design. Here is how I would classify not so much Jet Engines per se, but Turbine Engines and all other Propulsion Powerplants:
1.) Turbine Propulsion:
A.) Turbojets Engines - No Bypass Cold Airflow Duct and large front Fan, just compressor-turbine spool(s) core. B.) Turbofan Engines - Large Fan in ahead of compressor-turbine spool(s) core with By-Pass Cold Airflow Duct around engine body and compressor-turbine spool(s) core.
2.) Turbine Torque:
A.) Turboshaft Engines - More Turbine Stages w/ Free Turbine to Gear Reduced Transmission.
B.) Turrboprop Engines - Same as Turboshaft Engine except Fuel Control Unit is linked to Blade Pitch System to prevent Windmilling of Propeller and RPM on a Turboprop Engine is controlled by Blade Pitch Control.
3.) Non-Turbine Propulsion Powerplants:
A.) Ramjet - Hollow convergent venturi tube, lit off when enough forward airspeed is present to provide the compression needed to light it off. Higher in efficiency and thrust than Propulsion Turbine Powerplants and is dependent on the ability of the fuel injection system pressure and fuel volume delivery limits. Ramjets are limited to speeds below where the Nitrogen and Oxygen in the air do not compress to such enormous pressures as to where the Oxygen and Nitrogen merge as one killing combustion. (Note: Air is 78% Nitrogen, 21% Oxygen & 1% various Inert Gases).
B.) Scramjet - Supersonic Scramjet or "scramjet". These propulsion powerplants are ramjets rated for much higher speeds from supersonic to into hypersonic speeds (more than 4000 MHP). The only limitation is what any ramjet needs to overcome: The merger of Oxygen and Nitrogen in the Air under enormous compression, when not controlled will merge the Oxygen and Nitrogen as one and kill combustion.
C.) Pulsejet - Hollow convergent venturi tube or any type of differential diameter dual channel which doesn't need to be routed across in a linear manner, but in the same way hydraulic and pneumatic systems can route the master and actuator cylinders in series, any which way in terms of orientation. There are glass jars and dual plumbing pipes with differential diameters which can be made into pulse jets. The typical aviation-type Pulse Jet is a convergent venturi channel with spring-loaded shutters on the intake. During the combustion cycle, a vacuum develops in the aft exhaust section of the venturi causing the spring-loaded-opened shutters to pass airflow during combustion, until the vacuum increases more than the spring-loaded-open shutters can handle causing them to close. This allows a sealing-bias on the intake side as to maximize thrust in the aft rear section. But only for a limited dwell-time pulse cycle as to where combustion ceases and allows the shutters to open again to repeat the process. The aviation-type Pulse Jets run at a frequency around 250 to 600 PPS. As the frequency increases, the Pulse Jet comes online as running into the full thrust rating as fuel delivery increases to maximum limits. Aviation-grade Pulse Jets may utilize Heat-of-Compression on high compression Pulse Jet designs running on kerosene (same as "jet fuel") which allow the Pulse Jet to stay lit without additional spark plug or glow plug requirements after being lit off, which time the shutters to the fuel injection system. Or gasoline-models using shutter position to spark can be used if the compression is lower than needed for allowing a Heat-of-Compression combustion cycle to be utilized. Plumbing pipes of different diameters connected 180 degrees to each other on a 180 degree bend and can be turned into Pulse Jets too. This by using a fuel source in the smaller diameter pipe with spark plug and when it lights off, spark isn't needed anymore, because the compression rises high enough to keep the Pulse Jet lit. The 180 degree bend between the two different diameter pipes allow a high pulse on/off frequency to develop between combustion and fuel flow dwell when the combustion ceases. Something in the range of 1200 cycles per second. They're noisy for sure, but keep lit on their own. Also a glass jar with a cover at the top and small hole at the center with a little bit of gasoline in the bottom, carefully heated on a stove will light off at the top where the hole is and will light off around 1200 cycles per second as a Pulse Jet. Just be careful on adjusting the heat when using a glass jar.
D.) Rocket Propulsion - These Propulsion Powerplants utilize a tapering convergent channel within their airframe fuselage section in which a fuel which carries its own oxygen along with combustible compounds ignite. Escaping through the tapered convergent channel into thrust. Typical Rocket Fuels are both liquid and solid Oxy- Hydro Fuels which are Oxygen and Hydrogen mixed together and ignited. Other Rocket Fuels include Hydrogen Peroxide mixed with a catalyst in separate reservoirs injected into the taper convergent thrust channel at the correct ratios to support combustion. This also goes for the Space Shuttle as well, using liquid Oxygen and Hydrogen in separate reservoirs mixed with emulsified aluminum as to increase thrust output. Rocket Propulsion speeds are rated to over 30,000 MHP and are more practical for defense, high altitude weather/surveillance and space programs than passenger flight, at least for now this is the case.
E.) Missile Propulsion - Missile Propulsion is similar to Rocket Propulsion, except far more airframe system guidance is involved in the likes of airplanes flying on GPS or Radar-Guided systems which control Missile Airframe Flight Control Systems. This for pin-point guidance, steering and targeting/evasive actions to specific target and collision/avoidance from specific threats. Many missile designs either incorporate Rocket Propulsion in the same manner as Rockets or use small turbojet engines within their fuselage. More advanced missile designs are starting to use Ram Jets along with an initial Rocket Propulsion light-off to get them up to speed. When they reach a specific airspeed usually around 500 MHP, the Ram Jet will kick in using Oxy- Hydro fuel to keep them lit and can increase in speeds to around 2,200 MHP while also fly at low altitudes. Many of these types of missile designs also incorporate Microwave & Doppler Radar Tracking & Cancellation Systems. So they can't be tracked by most any type of Radar, even more advanced Radar Systems which track flying objects by their airflow pattern converted into a visual profile. Such missiles have been pioneered by Russia & India, such as the Moskit from Russia and the Bramos from India.
F.) Air Pressure Propulsion - This is none other than a larger diameter accumulator shaped like a cylinder with an enormous amount of air pressure pre-charged connected to a powerful air compression with high volume capacity tank. On the other end of this cylinder is a small diameter pipe which when opened up passing a high amount of air velocity, by the air pressure being stepped down at the end before the smaller tail pipe begins causing the air velocity to rise. These types of Propulsion Systems require a constant pressure to remain at a certain limit within the accumulator cylinder to be effective for any practical thrust. Experimental applications using this type of propulsion would also require a large enough accumulator cylinder to provide for enough constant pressure to stagnate while the air escapes through the small tail pipe into thrust. [Pressure = Force / Area; Force = Pressure x Area. This is the basic principle on how hydraulics, pneumatics, firearms, rockets, jet propulsion, piston engines and aircraft operate by].