Ramjet Propulsion:
When the air is compressed to the required ignition pressure by the ram effect and the high velocity jet is produced without the aid of any machinery, so that propulsion unit moves with supersonic speed, then it is known as the Ramjet Propulsion. The ram effect is produced due to the inertia of the stream of entering air :
A diagrammatic view of the ram-jet is shown in figure above. The grid (1) faces the direction of motion. A stream of air with a high velocity equal to that of the jet enters the combustion chamber (2) through the apertures of the grid. The grid passages can be opened or closed by the shutter valves (3).
Petrol vapour is forced through a ring of fuel nozzles (4) by high pressure air from an air bottle (not shown) ; this mixes with air stream from grid. The pressure of the mixture of air and fuel is increased considerably due to deceleration caused by venturi-throat (5) on the mass of gases. The temperature of the mixture also rises. The hot mixture is now ignited automaticallyby the ignition bars (6) which have retained their heat from the previous explosion. The increased pressure due to combustion now closes the shutter valves (3). The burnt gases reach their maximum pressure after combustion and are expanded through the venturi-throat. Here and in the diverging portion (7) the pressure energy of the gases is converted into kinetic energy due to nozzle effect. The gases are then discharged out with a high velocity in the form of jet from the rear side of the unit. The flying bomb or ram-jet moves ahead in the direction opposite to this propulsion jet. The expansion of the burnt gases causes the pressure in the combustion chamber to fall below that of the atmosphere pressing on the grid; this causes the shutters to open and admit air for next cycle. The air space (8) helps to keep the combustion chamber hot.
The machine is started by compressed air from a high pressure air bottle (not shown); the mixture being fired electrically by an ignition plug (9). Light in weight per kg thrust and absence of moving components and supersonic speeds (550 to 650 kmph) are the merits of ram jet. Difficult starting, low thermodynamic efficiency and the high fuel consumption (4 times as that of turbo jets) are its main disadvantages.
ADVANTAGES AND DISADVANTAGES OF JET PROPULSION OVER OTHER SYSTEMS
Advantages
• Jet propulsion is simpler in construction as compared to piston engines.
• It is vibration free as the rotor is dynamically balanced.
• Its speed is as high as 300 kmph.
• Power production is continuous.
• Installation of power unit is easy as there are no radiators and other complicated parts.
• Power developed is as high as 1400 kW.
• No lubrication is required.
• Relatively lighter in construction.
• Less maintenance cost.
• High rate of climb.
Disadvantages
• Short life
• Construction cost is very high
• Noisy in operation
• Its compression ratio is not constant but varies with the square of its rotational speed
• Not so efficient at low speeds and at lower altitudes
• Its specific fuel consumption is very high
• Its thrust at take-off is less and hence thrust augmentation is required.
Note : The thrust of jet aircraft can be boosted by any one of the following methods.
• Tail pipe burning (Re-heat cycle)
• Bleed-Burn cycle
• Cooling the entering air (increasing mass flow)
ROCKET PROPULSION:
This is a form of jet propulsion, the term rocket propulsion being applied when the projectile carries its own supply of oxygen for combustion and does not utilise the surrounding atmosphere.
The rocket propulsion unit or 'rocket' when used as a weapon, is known as 'missile'.
Figure shows a simple rocket. The fuel is carried in two tanks, one containing alcohol and the other liquid oxygen. The later is used for combustion of alcohol.
These liquids are forced into the combustion chamber by pumps driven by the turbine. The turbine is driven by superheated steam which is produced by mixing a very concentrated hydrogen peroxide with calcium permanganate solution. After combustion the hot gases are expanded through a nozzle to form the propelling jet. The jet of burnt gases when strike the ground with a heavy thrust then the rocket is propelled up into the sky.
Application of Rockets : -
• Signaling
• long range artillery
• satellites
• lethal weapons
• space ships etc.
FUELS USED IN JET PROPULSION:
Light distillate type of fuels is commonly used in jet propulsion plants in order to over come limitations of combustion systems and varying climatic conditions. They must possess relatively very high calorific value, low freezing point and the least risk of fire. A few examples are:
• aviation petrol and kerosene
• gasolene
• paraffin
• ether
• amines
• alcohol
• natural gas etc.
The hydrocarbon fuels generally available at present are :
• mixture of paraffins
• isoparaffins
• olefins
• naphthalenes
• aromatics etc.