# Jet Propulsion/Jet engine types
## Ramjet

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

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

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

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

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

Turboshaft with a gearbox and a propeller.

### Turboshaft

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

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:
2.) Turbine Torque: