Jet Propulsion/Jet engine types

Ramjet Edit

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

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

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

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

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

Turboshaft with a gearbox and a propeller.

Turboshaft Edit

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

Uses pulse detonation to close off the intake without primary compression. Intake closure may be dynamic or with mechanical valves such as reed valves.


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
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  
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 
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].