Jet and Rocket Propulsion

Propulsion mechanism provides a necessary force which moves bodies that are initially at rest or overcomes retarding force when the body is propelled through medium. There are two essential elements in any propulsion mechanism: The energy source and energy conversion device to transform the energy into a form most suitable for propulsion purpose.

Jet propulsion is a means of locomotion in air medium whereby a reaction is imparted to a device by the momentum of ejected matter. In a jet propulsion system, also known as duct propulsion the surrounding air is ducted through the device and accelerated to a greater momentum by burning the fuel in the current of air prior to the ejection.

The idea of jet propulsion is old but only in last few decades, it has been found to be useful and practical. Different type of jet propulsion system have been developed and successfully tested for a large variety of applications.

Here, the output of jet turbine is only used to produce high velocity jet of gases and this jet is used to propel the vehicle by reaction, such systems are known as Jet Propulsion Systems.

Screw Propeller Engine:

In turbine propelled aircraft, compressor, combustion chamber and a gas turbine are used to drive a screw propeller. In this system, full expansion of hot gases takes place in the turbine (upto atmospheric pressure) and the net power developed by the turbine is used to drive the propeller as shown in the figure below. The propeller action is like a screw action which takes the vehicle in forward direction. The power supplied to the propeller is controlled by controlling the supply of fuel to the combustion chamber. The drive is due to the reaction on the craft caused by the acceleration of the gas stream passing through the propeller.

Turbo-Jet Engine:

In turbo jet unit, the turbine produces just sufficient power to drive the compressor by partial expansion of the hot gases in the turbine. The gases coming out of the turbine at high pressure are expanded in the nozzle producing a high velocity jet which gives the forward motion to the aircraft by the jet reaction.

The turbo jet unit gives high propulsion efficiency at higher speeds. The rate of increase of efficiency of screw propeller is higher at lower speeds but falls rapidly at higher speeds around the sonic velocity. The turbo jet units are best suited to aircrafts flying near to sonic velocities as it gives higher propulsion efficiency at higher speeds.

The method of increasing the thrust of a turbo jet engine by increasing mass flow rate of the outgoing jet is known as Thrust Augmentation. The thrust augmentation is employed for short period during take-off of the aircraft. It also helps for increasing the rate of climb of aircrafts particularly at higher altitudes.

Turbo-Prop Engine:

The turbo prop engine differs from turbo jet engine in that it uses a propeller mechanism to increase mass flow rate of air. The turbo prop engine used in aircrafts is shown in the figure given below.

In this system, gases are partially expanded in the turbine, and partially in the turbine. Nearly 80% of expansion takes place in the turbine and remaining of 20% in the nozzle. The power developed by the turbine is used to run the compressor as well as the propeller. The forward motion to the aircraft is given partially by the jet reaction produced by the hot gas.   

The turbo prop combines the advantages of turbo jet (low weight and simplicity in design) and merits of propeller (high power for take-off and high propulsion efficiency at low speeds). The overall efficiency is also improved by providing a diffuser before the compressor which helps to convert part of kinetic energy of incoming air into pressure energy.

At higher altitudes, the propeller has to do more work so at higher altitudes, jet engines work better requiring less power compared to propeller. At a speed of 500-700 km/hr, the efficiency of turbo prop combination is highest. Therefore these engines are more popular for civil purposes.

Ram Jet

The Ram jet is the simplest of all propulsive devices. There is neither a compressor nor a turbine. It consists of only three main components namely diffuser, combustion chamber and the exhaust nozzle. The pressure of the incoming air is increased to a sufficient level with the help of diffuser action. Once the flame is established by the spark, the fuel injected burns continuously.

The Ram jet engine commonly used in practice is shown in the figure below:

This air entering into Ram jet with supersonic speed is slowed down to sonic velocity in the supersonic diffuser, increasing air pressure.

The air pressure is further increased to the subsonic diffuser increasing also the temperature of the air. The diffuser section is designed to get desired ram effect. Its job is to decrease the velocity of incoming air and increasing pressure. The fuel injected into combustion chamber is ignited with the help of flame stabilizers. The high pressure and high temperature gases are passed through the nozzle converting pressure energy into kinetic energy. The high velocity gases leaving the nozzle provide required thrust to the ram jet.

The ram jet engines are used to fly at supersonic speeds (1.5 to 2 Mach number). The major advantages of ram jet engines are:

·        Light in weight

·        No moving parts

·        The possible use of wide variety of fuels

The major drawback of ram jet engine is that it cannot be started on its own from rest. It has to be accelerated to a certain flight velocity with some other devices. Therefore, ram jet engines are always equipped with a small turbo jet which helps in starting the ram jet or the vehicle has to be launched by rocket.

  

Pulse jet Engine:

A pulse jet engine resembles ram jet except that it has shutter valves which makes the process of induction intermittent. Air pressure is raised in the diffuser as done in the ram jet which is sufficient to open the shutter valves as shown in the figure below:

The high pressure air enters into the combustion chamber through shutter valves. The fuel injected into combustion chamber is burnt and consequent rise in the pressure in the combustion chamber closes the shutter valves and combustion takes place at constant volume. Thus the pulse jet cycle resembles otto cycle. Then the high pressure and high temperature gases expand through the valves producing the high exhaust velocity. Due to high degree of acceleration of gases, the pressure in the combustion chamber falls and the pressure difference between the air in diffuser and the gases in combustion chamber is sufficient to open the shutter valves again and fresh air at high pressure is supplied to the combustion chamber. The cycle is repeated at frequency of 100-200 cycles per second. The length of tail pipe determines the number of cycles per second, the shorter the tail pipe, greater the frequency of cycles.

The merits and demerits of pulse jet over ram jet is listed below:

·        The design of diffuser nozzle combination is very difficult in case of ram jet whereas the design of the pulse jet is simpler,

·        Valve shutter can cause loss of thrust incase of pulse jet engines,

·        Pulse jet engines require a lower launch speed and this is a great advantage. Both pulse jet and ram jet are not used for passenger or goods transport as controlled landing is difficult.

The pulse jet was first developed by Germans during world war II as Burr Bomb and it was used (Unmanned) for bombing with a speed of 650 km per hour.